A review of adaptive wall wind tunnels

Prog. Aerospace Sci. Vol. 22, pp. 81-111, 1985 Printed in Great Britain. All rights reserved.

03764)421/85 $0.00 +.50 Copyright © 1985 Pergamon Press Ltd.

A REVIEW OF ADAPTIVE WALL WIND TUNNELS U W E GANZER

Institut ff~r Luft- und Raumfahrt, Technische Universiti~t Berlin, Marchstr. 14, Sekr. F2 D-IO00, Berlin I0, FRG (Received 15 April 1985) Abstract--A brief outline is given of the adaptive wall concept. This is followed by a detailed description of the research facilities used to explore the concept. For tests of aerofoils, facilities with two adaptive walls are shown to yield results of very high quality. Wall adaptation was achieved even for fairly high subsonic Mach numbers. For tests of three-dimensional models, first results give hope that for subsonic flow the use of only two adaptive walls may be sufficient to reduce the wall interference to an acceptable level. On the other hand, test sections with three-dimensional adaptive walls have been employed successfully at subsonic main stream condition. Experimental results for low supersonic speeds are not yet available. From theoretical considerations, however, it can be expected that with three-dimensional adaptive walls one can achieve acceptably small wall interferences at low supersonic speeds.

CONTENTS 1. 2. 3.

INTRODUCTION THE PRINCIPLE OF THE ADAPTIVE WALL TECHNIQUE TEST SECTIONS FOR THE INVESTIGATION OF TWO-DIMENSIONAL FLOWS 3.1. Test sections with two ventilated wails 3.2. Test sections with two flexible walls ~ 3.3. Representative results obtained for two-dimensional flow 4. ON THE USE OF TEST SECTIONS WITH TWO ADAPTIVE WALLS FOR THREE-DIMENSIONAL MODEL TESTS 5. TEST SECTIONS WITH THREE-DIMENSIONAL ADAPTIVE WALLS 5.1. Survey on the three-dimensional test section designs 5.2. Representative results obtained in three-dimensional test sections 6. CONCLUDING REMARKS REFERENCES

81 82 84 84 88 93 95 98 98 106 109 110

1. I N T R O D U C T I O N Over the past 13 years a new approach to the problem of wind tunnel wall interference has been evolving. The method is known as the adaptive wall technique. It is based on the premise that if a streamline near the wind tunnel wall may be allowed to take its interference free shape, then the entire flow in the working section is free of wall interference and the forces and pressures on the model represent free-air data. Such a method is most desirable when configurations are to be tested at high lift or transonic speeds, because then wall interference effects are particularly severe, they lead to notable restrictions of model size in conventional wind tunnels to ensure at least correctable flow conditions. Early attempts to use adjustable wind tunnel walls for reducing wall interference effects to negligible were made during World War II in England, see Lock and Beavan (1944). A tunnel for aerofoil tests at high speeds was furnished with flexible top and bottom walls. The 'streamlined' wall shape was determined by first positioning the walls to a contour which gave constant pressure along their length with the model present. This corresponded to the condition in a free jet test section. The walls were then repositioned about midway between these contours and the straight wall location. A range of theoretical flow models was analysed for establishing these wall setting rules. 81 JPAS 2 2 : 2 - A

82

U. Ganzer

Flow field calculations involving a theoretical presentation of the model may be an acceptable procedure for determining the wall shape in simple two-dimensional flow. For more complicated flow conditions, such as three-dimensional flows or in the case of strong viscous effects, a sufficiently correct flow field calculation would be too laborious or even not feasible. In 1973, Ferri and Baronti as well as Sears independently suggested a much simpler method. The method is based on the measurement of flow conditions near the wind tunnel wall. Two independent flow variables--flow deflection and static pressure--have to be measured and then checked with regard to the condition that they have to fulfil a specific relationship consistent with an unrestricted flow field. The important fact is that only the flow conditions on a control surface close to the wall need to be considered. No knowledge about the model or the flow inside the control surface is required. This is the basis for the adaptive wall wind tunnel technique.

2. T H E P R I N C I P L E O F T H E A D A P T I V E W A L L T E C H N I Q U E The principle of the adaptive wall technique is most easily explained by referring to the flow around an aerofoil in a test section with flexible upper and lower walls. The adapted walls of the wind tunnel can be considered simply as substitutes for stream surfaces in an unrestricted flow field (neglecting for simplicity the wall boundary layer in the first approach). The test section flow is then the real part of such an unconfined flow field. In Fig. 1 a fictitious exterior part has been added to complete the picture.

.

.

.

.

.

.

.

.

.

FICTITIOUS FLOW ( OUTSIDE )

j PRESSURE , , ~ CALCULATED ~ PRESSURE MEASURED

REAL

FLOW (INSIDE)

WALL

FIG. 1. The principle of adaptive walls.

This picture of an unbounded flow field indicates the conditions which lead to the correct wall shape. If a wall is a substitute of a stream surface, the properties of streamlines must be applicable to this surface. The essential property of a stream surface is that it cannot sustain forces. Therefore, the pressure on both sides of the surface must be the same; there may be a pressure gradient across that surface but no pressure jump. This condition is used to check whether the wall shape corresponds to that of a stream surface in unrestricted flow. The pressure distribution along the wall inside the tunnel can be measured. Along the outer side of the tunnel the flow does not exist, it is fictitious, but one can calculate the pressure for the fictitious flow over the known wall shape taking the same main stream condition as exists for the tunnel flow. Only when both pressure distributions--the calculated and the measured o n e s - - a r e the same can the wall shape be considered as adapted.

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83

If there is a difference in the pressure distributions, this pressure difference can be used for an iterative adaptation procedure. Approximately a mean value between the two pressures is taken to calculate a new wall shape which would, in the fictitious flow, produce this mean pressure distribution. (More precisely a relaxation factor is used so that not an exact mean value is taken but more weight is given to the calculated (external) pressure distribution. In general the external pressure distribution is found to be less sensitive to changes in wall contour.) The wall is then deformed according to the calculated new shape and again the pressure distribution is measured and compared with the one just prescribed for the fictitious outside flow field. The procedure is repeated until the differences are within a prescribed order of smallness. A more general explanation of the adaptive wall principle may be deduced from the example above. In fact this technique requires (1) the measurement of two independent flow variables and (2) a check on whether the two variables satisfy a functional relationship which is consistent with interference free flow conditions. In the example discussed above the two measured flow variables were the pressure distribution and the flow direction along the wall (given by the measured wall shape). In terms of small disturbance theory the pressure coefficient relates to the u-disturbance of the flow, i.e. the deviation of the local velocity component in the main stream direction from the undisturbed value U~o. The flow direction on the other hand relates to the vdisturbance, i.e the local velocity component normal to the main stream direction. The relation between the two flow variables u and v under the assumption of an unconfined field for which the disturbances vanish at infinity is given by the solution of the Laplace equation. For two-dimensional flow the solution reads

u(x,h) =

,~/1

1 =

~ v(¢,h) J d¢. M~ -~ x-¢

It was this relation on which Ferri and Baronti (1973) as well as Sears (1973) based their suggestion to check interference free flow condition in a wind tunnel. A variation of the procedure was published later by Davis (1981). Instead of measuring two different flow quantities on one surface surrounding the model, he suggested the measurement of only one quantity but on two surfaces. For laser measurements, the vcomponent of the local flow velocity vector is the most convenient flow quantity. The decay of this quantity from one control surface to the other can again be described theoretically involving the assumption of an unbounded flow. This can then be used to check the existence of interference free flow condition. In the following, the various adaptive wall wind tunnel projects will be discussed. It will be shown that different constructional solutions have been chosen and different techniques for measuring the two flow variables are being applied. The state of the art in 1982 is fairly well documented in an AGARD CP (1982) which contains the papers presented during a Specialist's Meeting on 'Wall interference in wind tunnels'. This information was summarized and slightly extended by Ganzer (1983). A comprehensive literature review was made by Tuttle and Plentovich (1982). Very recent summaries of the current projects are available from a Euromech Colloquium. The summaries were edited by Hornung and Stanewsky (1984). Basically there are two different kinds of adaptive wall test sections. One type uses flexible impermeable walls, their shape being adjusted to the individual flow condition as described above. The other type employs ventilated (perforated or slotted) walls as common for conventional wind tunnels but with local control of the flow through the wall. The local flow control may be achieved either by dividing the plenum chamber into a number of subplena or compartments or by providing means for a local variation of the wall resistance (Fig. 2). Both types of walls--ventilated as well as flexible walls-- have been used for two- and for three-dimensional test sections.

U . Ganzer

84

JACKSYSTEM [FLEXIBLE WALL,SJ

DIVIDED PLENUM IVENTILATED WALLSJ [ l~=b'qb,B,~lb~ e eL,ebq,'~,,1 P0

VARIABLE POROSffY

FIG. 2. The different types of adaptive walls.

3. TEST SECTIONS

FOR THE INVESTIGATION FLOWS

OF T W O - D I M E N S I O N A L

A list of the main test facilities employed for the development of two-dimensional adaptive wall technique is given in Table 1. These test facilities and the related research work will be discussed in the following. TABLE 1. SURVEY OF T w o - D I M E N S I O N A L TEST SECTIONS

Test section size Institution

H x W× L(cm)

Typical model chord (cm)

Walls

Number of controls top/bottom

AEDC

l ft

30.5 x 30.5 x 95

15.2

Perforated

2 Subplena, global +

Calspan

1 ft

N A S A Ames I N A S A Ames II N A S A Langley O N E R A $4 L Ch O N E R A / C E R T T2 Southampton I Southampton II TU-Berlin I TU-Berlin II

13 ×25 cm 2 ft 0.3 m 18cm 40 cm 6 x 12 in. 6 in. 15 cm 15 cm

30.5 x 25.4 × 142 13x25x74 61 x 61 × 153 36 x 36 x 144 18x18x75 37 × 39 x 132 15.2 x 30.5 x 107 15.2 x 15.2 x 112 15 x 15 x 69 15 × 15 × 99

15.2 7.6 30.5 20.3 8 20 13.7 10.2 10 10

Perforated Slotted Slotted Flexible Flexible Flexible Flexible Flexible Flexible Flexible

10/8 10 16 19 10 16 15 19 8 13

3.1.

TEST SECTIONS

WITH

Two

VENTILATED

local var. porosity P1. Comp. P1, Comp. PI. Comp. Jacks Jacks Jacks Jacks Jacks Jacks Jacks

WALLS

Calspan 'self-correcting wind tunnel' was the first facility with adaptive ventilated walls. The project was initiated by the work of Sears. The tunnel involves a segmentation of the test section plenum and the application of active wall control by blowing or suction in the plenum segments (Fig. 3). A detailed description of the facility is given by Vidal et al. (1975) and Sears et al. (1976). The perforated top and bottom walls have 22.5% open area. The bottom plenum is divided into eight segments, the top one into ten. Each segment is connected to a pressure and a suction source with individual manually operated control valves. For the measurement of the two independent flow variables static pipes and flow angle probes are installed (Fig. 4). The two static pipes provide forty to fifty pressure readings at each control surface. The number of flow angle readings is much smaller. There are only as many flow angle probes as there are plenum segments, i.e. eight for the lower control surface and ten for the upper

Adaptive wall wind tunnels

FlG. 3. The Calspan self-correcting wind tunnel.

FIG. 4. Test section of Calspan self-correcting tunnel with the 6 ~ blockage model.

85

86

U. Ganzer

side. The flow angle sensors consist of two hypodermic tubes mounted side by side with the front faces chamfered at +45 °. Initially an attempt was made to measure the v-component of the flow near the wall by volumetric measurements of the flow through the porous walls. For this purpose flow meters were incorporated in the pipe system connected to the plenum compartments. However, comparison with pitch probe measurements revealed that the calibration was non-linear and that the wall boundary layer amplified the normal velocity in the inviscid stream by a factor ranging from about 1.15 to 6. Early experiments were made using a 6 in. chord NACA 0012 aerofoil. The objective of this work was first of all to demonstrate the feasibility of the adaptive wall concept and to develop practical modes for operating such a tunnel. At flow conditions up to M~ =0.725 and ~ = 2 ° wall adaptation led to nearly interference free results. Wall adjustment was made in simple sequences, since interaction between adjacent or opposite plena was found to be small. The attempt was then made to extend the operational range of the Calspan selfcorrecting tunnel to higher Mach number flows for which locally supersonic regions reach the control surfaces and the tunnel walls. A practical operational mode was found by adapting the walls first for a subcritical flow and then increasing the Mach number and readjusting the wall control sequentially. In this way wall adaptation was established for local Mach numbers of 1.05 at the wall and 1.30 on the aerofoil. However, in the course of these experiments it was found that the auxiliary compressor provided insufficient control of the flow at high Mach numbers. Therefore a smaller model of 4 in. chord was used for further tests. On the other hand the small number of flow angle probes per wall were found to be insufficient to define adequately the variation of the normal velocity component. A new static pipe was devised to measure both static pressure and its gradient normal to the control surface. (The gradient dp/dy is proportional to du/dy which equals dv/d,c thus yielding the normal component v by integration.) The new pipe had a diameter of 16 mm and 18 pairs of diametrically opposed orifices. It became known as the 'Calspan pipe'. With this pipe the definition of the normal velocity distribution near the model was greatly improved although it was felt that an even more detailed definition for regions of large pressure gradients, especially near a shock, was desirable. In the final report of this research project Erickson et al. (1981) concluded that the Calspan facility did in principle achieve a flow control for the interference free condition for Mach numbers up to M=0.9. For an improved design refined control near the model is recommended. AEDC investigated two-dimensional adaptive walls with different controls of wall ventilation. Three wall configurations were tested. One configuration featured global porosity control, a second configuration variable longitudinal control of the local hole angle. For a third configuration two subplena were attached to both the top and bottom walls which themselves had uniform porosity. Two subplena locations were investigated, the locations were obtained by prescribing either the flow angle or the pressure as target for the boundary value on the control surface. The tests were carried out in the 1 ft tunnel (IT) using a NACA 0012 aerofoil with 6 ')o solid blockage. The experiments were conducted for both lifting and nonlifting conditions including those conditions for which the supercritical flow regions extended to the test section boundary. Wall interference was significantly reduced by the wall control. It was considered important to match the pressure level upstream of the model and the minimum pressure in the vicinity of the model. One of the most effective means of matching these global parameters was the plenum pressure, which suggested the use of segmented plena control. The results of the experiments were published by Kraft and Parker in 1979 and Parker and Sickles in 1981. NASA Ames employed slotted top and bottom walls for their adaptive wall research. The walls of the 25 x 13 cm transonic test section had ten slots each providing an open area ratio of 12 3/o- Transverse inserts into the two plenum chambers were used to obtain

Adaptive wall wind tunnels

87

ten compartments on each side. The spacing, and thus the number of compartments, can be varied in steps of 2.54 cm. Such minimum spacing was applied in the region of the model. Laser velocimetry was used for the assessment of wall interference. The v-component of the flow velocity was measured at two different levels--as suggested by Davis (1981)both above and below the model. The velocimeter was traversed in the streamwise direction. Its motion and data acquisition were controlled by a dedicated minicomputer. Since the tunnel was operated at atmospheric stagnation condition the atmosphere could be used as the pressure source for the compartments while an air ejector provided suction. The auxiliary air system was designed to add or remove up to 10 ~ of the test section mass flow. The measured velocities were used to compute from linear flow theory the required changes in v-component. The plena pressure changes necessary to achieve the desired vdistribution were determined by means of a measured influence coefficient matrix. Convergence to interference free flow condition was achieved, as long as the supersonic zone of the 3 in. NACA 0012 remained below the second control level. More details of the tests are given by Satyanarayana et al. (1981). In their conclusion they point out that for routine testing the time to acquire LDV data must be decreased. The entire data acquisition sequence for a symmetric flow field took about 10 min of which 6 min were spent on acquiring and reducing laser data, 2.5 rain for traversing the laser and 1.5 min for the Scanivalve reading of the model pressure distribution. The research program is continuing at present with a new test section of slightly smaller height (13 x 11 cm). This is to obtain free-stream Mach numbers close to one. The test section is constructed such that the size and number of plenum chamber compartments can be changed rapidly to determine the optimum configuration. In the course of the current experimental program the following aspects are to be investigated. Three alternative methods for the assessment of wall interference will be compared: (1) one component LDV measurements at two levels, (2) two component LDV measurement

FZG. 5. NASA Ames 2 x 2 ft transonic test section with adaptive walls.

88

U. Ganzer

at one level and (3) side wall static pressure measurement at two levels. Also, the influence coefficients for the adjustment of plenum pressures will be determined by use of all three measurement techniques. In any case the determination will be made with and without the model. Furthermore a one-step-adaptation method is to be verified during the experimental program. Based on the experience with the small scale test section, a new adaptive wall test section is being constructed at NASA Ames for the 2 × 2 ft transonic wind tunnel (Fig. 5). The design incorporates several important improvements over the first test section. The flow through the slotted walls will be controlled by 64 slide valves, each driven by a stepping motor and all controlled by a small computer. The laser velocimetry will involve a very fast computer controlled traverse system of mirrors which will substantially reduce LV data acquisition time. A nonlinear outer-flow solver is being developed to cope with larger supersonic zones in the flow. A short note on the project was given in a paper by Schairer and Mendoza in 1982. Operation of this tunnel is now expected to begin in the fall of 1985. 3.2. TEST SECTIONS WITH T w o FLEXIBLE WALLS

The first projects with adaptive flexible walls were started in the early 1970s at O N E R A and at Southampton University. NASA Langley's test section with two flexible walls is a direct outcome of the work at Southampton University, which in fact was carried out under NASA grant. The test section is built for the 0.3 m Transonic Cryogenic Tunnel. It has a length of 142 cm with the top and bottom flexible walls extending over a range of 180 cm, the rear 40 cm providing a smooth transition between the end of the test section and the beginning of the fixed diffuser. The flexible walls are fixed at the upstream end of the test section whereas the downstream ends are fixed vertically but free to translate longitudinally as required by changes in wall shape. The 21 jacks on each wall are non-uniformly spaced so that the finest wall shape control is provided in the vicinity of the model. A stepper motor drives a 10 : 1 reduction gear box to turn each jackscrew. The jackscrew drives a block with two rods. The drive rods extend through the tunnel pressure shell and are connected to a flexure plate which holds the flexible wall. The flexible walls are milled from steel with integrated joints for the jacks. The wall thickness varies between 1/8 and 1/16 in. (Fig. 6). The picture of the test section with the side wall of the pressure casing removed, Fig. 7, shows those parts of the rods on which the flexible wall will be attached via the small flexure plates. The blocks with the screws cannot be seen. They are housed in an outer casing on top and below the test section. Motor and gearbox as well as the position reader (linear variable displacement transducer) are kept entirely outside the cryo and pressure

SCREW DRIVENBY TEPPER MOTOR

f

BLOCK

+7..5/-2.Scm MOVE

RODS

~__~

~ I L

FLEXUREPLATE

I=~i"1" r ' l - r " " "

FLEXIBLE WALL J

TEST

SECTION

'

SIDE

FXG. 6. Jack system for NASA Langley 0.3 m cryogenic tunnel test section with two flexible walls.

Adaptive wall wind tunnels

89

FIG. 7. NASA Langley0.3 m adaptive wall test section with the side wall removed.

environment (Fig. 8). At present (March 1985) the test section is being placed into the tunnel circuit. After calibration, tests with a NACA 0012 and a CAST 10 aerofoil are planned. The test section was first described by Ladson in 1979. A more recent report was given by Mineck (1984). ONERA's first experiments with adaptive flexible walls were carried out in the $4 LCH tunnel in Chalais-Mendon. It was considered basic exploratory research to provide experience for a larger test section for CERT T2 tunnel. A compliant plastic material was used for the walls, stiffened by longitudinal ribs. The ten jacks on each wall were operated manually. The tests were made with a NACA 64 A010 aerofoil at M~ =0.85. Interference free flow conditions were obtained after 3 to 5 iterations. It was found that departures from correct wall shapes by 5/10 mm (five times the wall setting tolerance) had only negligible effects on aerofoil pressure distribution. More details about this work were given by Chevallier (1975). C E R T / O N E R A wind tunnel T2 in Toulouse has a test section based on the experience gained with ONERA's first adaptive wall facility. The test section size is 37 x 39 cm. The tunnel can be operated at high pressure up to 5 bars and at cryogenic conditions. The fully automated adaptive wall test section has been in operation at normal temperatures for some time, see Gely (1979). Operation at cryogenic temperatures began in 1983. In this case the model has to be cooled outside the tunnel since tunnel running time is only 30 to

90

U. Ganzer

FIG. 8. NASA Langley adaptive wall test section for 0.3 m Transonic Cryogenic "Funnel ITCT).

60s, with 10s needed to establish steady flow condition. Provision is made for a quick model installation (Fig. 9). The flexible walls of the test section are made from steel 1.5 m m thick. Teflon is used to seal them against the side walls. Each wall is equipped with 91 pressure taps and can be moved by 16 jacks. M a x i m u m wall displacement is limited to 25 mm. Each jack is moved by a hydraulic actuator which is controlled by a stepping motor. One single step corresponds to a wall movement of 0.2 mm, thereby defining the wall setting tolerances. The wall position is controlled by a potentiometric displacement feeler with an accuracy of 5/100 m m (Fig. 10). Some sophisticated procedures are used in the tunnel operation. A starting wall configuration is calculated from a singularity representation of the model. The singularity strength is determined from an approximate estimate of aerodynamic characteristics of the model. For the sequential tests of a Mach or incidence sweep the last wall contour of one step is taken as the starting configuration of the next.

Adaptive wall wind tunnels

91

FIG. 9, The ONERA/CERT T2 test section.

~ TESTSECTION

~-L~ ~.FLEXIBLE WALL

~ T E F L O N SEAL

POIENIIOMEIER

I

FIG. 10. Jack system for wall displacement of CERT T2 tunnel.

The calculation of an improved wall configuration from the measured pressure distribution follows the classical line except that for the choice of the relaxation factor particular effort is made to ensure a fast convergence of the adaptation process, the estimated changes of the wall configurations are split into four changes of the test section stream tube: a change in width, divergence, centre line camber and centre line inclination. Different optimum relaxation factors have been found for these effects and are used for the adaptation.

92

U. Ganzer

Finally, special procedures are followed to calculate the correct main stream Mach number and to extrapolate the flow variables beyond the ends of the test section, all based on the data measured along the walls. More details about the T2 tunnel and its operational procedures are given by Archambaud and Chevallier (1982) and by Chevallier et al. (1983) as well as by Mignosi and Archambaud (1984). At Southampton University the work on adaptive flexible walls was initiated by the desire to provide improved conditions for magnetic balances. The power requirement and capital costs for magnetic balances are strongly influenced by size. The elimination of a plenum chamber by adaptive flexible walls allows the suspension electro-magnets to be placed closer to the model, thus reducing the size of the balance. The work in Southampton started in a low speed tunnel. G o o d y e r (1975) reported on the first tests on circular cylinders and a NACA 0012-64 aerofoil. The demonstration that interference free flow conditions were achieved by contouring the walls was particularly impressive because of the large blockage of the models. The circular cylinders had 25 and 30~o blockage and the aerofoil 10~o (tunnel height to aerofoil chord ratio 1.1). Streamlining the walls was very time-consuming in these early experiments. Around eight iterations were required and the computation of one pair of wall shapes took typically 1 h on a minicomputer. The subsequent design of a transonic test section with adaptive walls for Southampton University was based on detailed analytical work, as reported by Judd et al. (1976). The test section was to some extent a pilot facility for NASA Langley's 0.3 m TCT-test section. The test section has a nominal cross-section of 6 x 6 in. and flexible top and bottom walls of 44 in. (1.12 m) length. Each wall is fitted with 20 motorised screw jacks, of which the last one is used for Mach control. The link between the jacks and the wall is made by a metal flexure, similar to the one shown for the NASA Langley test section. Stepping motors and linear potentiometers are employed to drive and control the jacks. Wall adaptation is carried out fully automatically. The time to complete a streamlining cycle varies from about 1.5 to 5 min. The test section is shown in Fig. 11.

FrG. 11. Universityof Southampton transonic test section with flexiblewalls.

Adaptive wall wind tunnels

93

Tests were made with a 4 in. chord NACA 0012-64 and a 6 in. supercritical N P L 9510 aerofoil. They were particularly aimed at investigating the high Mach number range. The data gave good to fair agreement at high subsonic speeds with comparable interference free data but disparities became increasingly apparent at the higher level of Mach numbers (0.97) so far explored. More details are given by Goodyer and Wolf (1982), Wolf et al. (1982) and Goodyer (1984). At Berlin Technical University the first development of a test section with two flexible walls was made with a view to applying this technique to three-dimensional flow conditions. It was thus considered essential to use as few jacks as possible and to develop a fast, automatic wall control of high accuracy. It was found that eight jacks for each wall are sufficient provided stiff wall material is used (such as fibre glass) giving smooth bending lines. The wall control system involves DC electromotors to drive the jacks and high precision linear potentiometers which read the wall position by touching the wall. The fast control is achieved by supplying different voltages to each motor in proportion to the required local displacement. Wall setting for one iteration typically takes half a second. A number of tests were made at transonic speeds using a NACA 0012 and a supercritical CAST 7 aerofoil. The tests revealed that the supercritical aerofoil (in particular at its design condition) is much more sensitive to small changes in wall contour than the conventional aerofoil. The results are presented by Ganzer (1980, 1982). In order to provide better conditions for laser measurements in the flow field around the aerofoil the test section will be fitted with transparent side walls and at the same time extended to a length of 99 cm with the number of jacks increased to 13 for each wall. This change in test section configuration will reduce truncation effects and thus improve the test condition. The retrofitted test section will be operational at the end of 1985. 3.3.

REPRESENTATIVE RESULTS OBTAINED FOR T w o - D I M E N S I O N A L F L O W

Only a few test results will be presented here to illustrate the state of the art of twodimensional adaptive wall test sections. Figure 12 shows a subcritical pressure distribution for the NACA 0012 aerofoil. The tests from the TU-Berlin tunnel were made with a model of 89/0 solid blockage. With plane walls the pressure distribution exhibits large wall interference effects which are entirely eliminated by wall adaptation.

. . . .

~ - -

I

I

-0.6 O

I

I

O

o

o=

°v°o~e~o - °Ooo

LU 03 03 LU r~

oo0°0 o

,

O STRAIGHT WALL TU BERLIN Re=t0.10 s • ADAPTED WALL v INTERFER.FREE ARA BEDFORD Re=5.0.106

-0.4- • % ci~

~°~o - °°Oo U@ o°

•

Q-

e°~o Oo

v -0.2. o v

O@~z ° ° o o "@U °°o

°~ o

•

o °'~o o

0

o°

vo O o

V

0.2 J

°o

o •@o

'

I

0.2

,

I

0.4

L

I

0.6 CHORDWISE

,

I

0.8 POSITION x/c

FJo. 12. Surface pressure distribution for N A C A 0012 aerofoil at Mo~ =0.5 and ~ = 0 °.

94

U, G a n z e r

At transonic speeds interference from solid plane walls causes an even more severe distortion of the pressure distribution. Transonic blockage may occur, the main stream Mach number being limited when sonic speed is reached in the smallest cross-section between model and wall. There is ample evidence from numerous tests that transonic blockage can easily be avoided by adapting the solid walls. Figure 13 shows pressure distributions for a CAST 7 aerofoil. The model used for CERT T2 tunnel had 6~o solid blockage. The comparison of the T2 data with other GARTEur results obtained in conventional tunnels (ARA, NLR, ONERA $3 Ma) are indicative of the high standard reached with an adaptive wall test section. The T2 data

I

o ;,RA

¢' l! /

,.~! ~I

/ a.°

o S 3 Ma • CERT T2

.

a8

0

0.2

o./,

POSITION X/C

FIG. 13. Pressure d i s t r i b u t i o n for C A S T 7 aerofoil at M® = 0.76, C L = 0,73, R e = 6

x 106.

have been checked by calculating residual interferences with an RAE method using measured boundary conditions, see GARTEur (1983). The residual interferences were typically of the order of AM=0.002 and A~=0.03 °. Such accuracy is about the best presently available in aerofoil testing. It comes close to the stringent requirements laid down in an AGARD Conveners Report (see Steinte and Stanewsky, 1982). Finally, an example of tests at fairly high Mach number is given in Fig. 14. The example is taken from Goodyer and Wolf (1982). In this case the supersonic zone extends well up to the upper and lower wall. The maximum local Mach number at the wall was M = 1.1. If the local increase of wall boundary layer thickness due to the impinging shock is accounted for in the wall setting the aerofoil pressure distribution in the rear part coincides with the NASA data. In general, it may be concluded here that for testing two-dimensional models the adaptive wall technique is well established. The detrimental features of this technique are obvious,

-1 • •

NASA 19x6 REE

e ~ " ~ " ~

Re : 2.7 x 106

SOUTHAMPTON 1.5 x 106 /

"

LLI n,, ::) (/') tO LLI n - - .~ .1 .~ .

O; 0

,,,/'u

I

o20

.

.

t.p

I

o.40

~

h

CHORDWISE POSITION X/C

FIG. 14. Pressure d i s t r i b u t i o n for N A C A 0 0 1 2 - 6 4 aerofoil at M = = 0.89 a n d ~ = 4:'.

Adaptive wall wind tunnels

95

additional mechanical installations and additional wind tunnel time is required. On average two to three iterations are necessary. Each iteration requires at present 3 s (0.5 s for wall pressure measurements using a PSI* System, 2 s for the external flow field calculation and 0.5 s for the wall setting). However, these disadvantages are well balanced by a reduction of the necessary test section height (a factor of two) and an improved reliability of the test results. 4. O N T H E USE O F TEST SECTIONS W I T H T W O ADAPTIVE WALLS FOR T H R E E - D I M E N S I O N A L M O D E L TESTS The concept of using two adaptive walls for testing two-dimensional models is attractive because ideal test conditions are obtained while the additional complexity is acceptable. When the problem of testing three-dimensional models is addressed--which from a practical point of view is the more important--the situation turns out to be different. Ideal test conditions are obtained only at the expense of an extensive increase in mechanical complexity, i.e. a large number of jacks for the control of flexible solid walls, or a large number of plenum chamber compartments or perforated wall segments with their respective control devices for ventilated walls. In addition an ideal three-dimensional flow control requires that all test section walls have to be adaptable. However, this prevents optical access to the model, flow visualisation and optical measurement techniques like laser velocimetry are excluded. Therefore, any practical test section design must be a compromise between the desire to provide perfect interference free flow condition and the necessity to keep the mechanical complexity limited and it is desirable to allow some optical access to the flow. For subsonic flow it should, in principle, always be possible to reduce interference effects to a satisfactory level and to avoid transonic blockage with only two adaptive walls. A breakthrough in the use of two adaptable walls in three-dimensional model tests was achieved by the work of Wedemeyer (1982). He suggested an adaptation method which yields a cancellation of wall interference on the tunnel centre line. A brief outline of this method is given in the following. It is assumed that the model is concentrated on the centre line of the tunnel and it can be represented by a distribution of singularities along its axis. The displacement effect is described by a distribution of sources while the lift effect is given by horseshoe vortices of infinitesimal span. If the flow is confined in a rectangular test section of a wind tunnel with plane walls, the resulting flow field can be constructed by the 'image method', i.e. by using a double infinite set of singularities which produces a plane stream surface at the location of the walls. The total perturbation potential may then be written down which yields the perturbation at the tunnel centre line due to the walls only, as well as the total perturbations at the centre lines of the four test section walls. The equations contain the unknown strength of the singularities. At the wind tunnel walls the pressure distribution may be measured. This information from top and bottom wall can be used to determine the singularity distribution which in turn permits the wall induced perturbation to be calculated at the centre line of the tunnel. Since it is necessary to deduce two singularity distributions from the wall pressures-source and vortex strength--the information from upper and lower wall is split into symmetrical and asymmetrical disturbances. The source strength is linked with the symmetrical part, the vortex strength with the unsymmetrical part. Finally, with the wall induced pertubations at the tunnel centre line calculated, one then can calculate wall contours, which produce the same perturbation but of inverse sign. These wall contours are the desired adapted wall shapes. * PSI stands for Pressure SystemsInc. This computer controlled systemhas individual pressure transducers for each pressure part and allows fast scanning(up to 14000data per second).

96

U. Ganzer

The calculation of the adapted wall shapes involves integral equations for which Lamarche (1982) has developed a numerical scheme. In the calculation it was found that the computed singularity distribution showed very large oscillations which were traced back to the scatter contained in the measured pressure distribution; because of the damping of fluctuations over some larger distances an irregular wall pressure distribution can only be realized by singularity distributions on the tunnel centre line having large variations. On the other hand, when the oscillatory singularity distributions were used to calculate the wall interferences, it was f o u n d - - n o t surprisingly--that the induced perturbations were again smooth. Wedemeyer (1982) has proposed a method which avoids these difficulties. It consists of assuming a linear operator between the kernel functions involved in the computation of the wall pressures on the one hand and the interference effects on the tunnel centre line on the other. Apart from the advantage of avoiding oscillations in the computation, the integral relation between the kernel functions must be solved only once for a given test section geometry. This yields a much faster computational scheme. More details about that scheme are given by Lamarche and Wedemeyer (1984). The computational code of Lamarche was used by TU-Berlin for some tests in the TUBerlin two-dimensional test section and in the CERT T2 tunnel (in cooperation with ONERA/CERT). Test results obtained in the TU-Berlin test sections are shown in Fig. 15. Data points obtained in the three-dimensional test section with plane wails are compared with those

-.--'TNTERF.FREE ' o PLANE WALL

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o

20 TEST SECTION

•

3o

. . . .

k

.

.

.

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measured in the two-dimensional and three-dimensional test section with the walls adapted. The results from the two test sections with adapted walls are very close and compare well with the curve labelled 'interference free'. This curve is taken from Barche (1975) and was obtained for a different Reynolds number (Re= 15 × 106 against R e = 2 x 106). This may explain some of the discrepancies mainly for the pressure taps in the corners. In general, it seems that wall adaptation has yielded interference free results and the use of only two adaptable walls is sufficient. It also verifies the adaptation method suggested by Wedemeyer. The adaptation method has also been used for a lifting configuration in the CERT T2 test section with two adaptive walls. A result from surface pressure measurements on a high speed aircraft model is shown in Fig. 16. Wall adaptation has changed the surface Mach number distribution in the expected maner. Unfortunately, reference data are not yet available so that a conclusion about the residual interference effects cannot be drawn. There are three studies available which provide some information about residual wall interferences on three-dimensional configurations in a tunnel with two adapted walls.

Adaptive wall wind tunnels

97

Z "I"

o. ~

.~A- ~ ~ . A .

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b

ADAPTED WALL

aD

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Vert6 (1982) made a numerical study of which the main results are given by Wedemeyer (1982). He computed interference velocities for a 4.4 ~ blockage body of revolution. A comparison of interference effects for straight walls and two adapted walls at various positions in the test section was made. It showed that the residual interferences in terms of pressure deviation and variation of angle of incidence are very small, at least within 60 ~o of the distance from centre line to the wall. Another numerical study made by Smith (1984) showed similar results. It was found that by the use of two adapted walls the test quality requirements for the ETW (European Transonic Wind Tunnel) may be met at least for subsonic flow conditions. The main concern of this study was the spanwise variations of residual upwash. Some experimental evidence is available from a research project carried out at WPAFB (Wright-Patterson Air Force Base). Harney (1984) reports on the tests in the test section with plane solid side walls and flexible top and bottom walls amenable to three-dimensional wall contouring. The three-dimensional contouring was achieved by composing each flexible wall of nine cylindrical rods extending from the throat to the downstream end of the test section. Each rod may be contoured individually by ten electric jacks. Flexible followers back up the rods and act as seals against ventilation. For the experiments the walls were set to threedimensional contours as well as to two-dimensional contours. A suitable two-dimensional contour was obtained by setting all rods to the mid-semispan streamline shape calculated for a square test section. In any case the wall contours were determined from a theoretical flow field calculation with model representation by singularities. The singularity distribution was calculated from the model geometry and measured forces and moments. Thus, the test section was not operated in the manner of the adaptive wall technique although--by employing laser velocimetry--the tunnel could easily be operated that way. In the tests a fighter aircraft-like model of aspect ratio A = 1.5 was used with a blockage ratio of 2.5~o. Three component force measurements with three-dimensional contoured walls at M=0.9 yielded results very close to reference data from a much larger tunnel. For one incidence case (~ = 4 °) it was shown that virtually the same result could be achieved with an appropriate twodimensional wall contouring. 3PAS 2 2 : 2 - B

98

U. G a n z e r

In general, it may be concluded here that two-dimensional wall adaptation for threedimensional model tests is feasible and may yield acceptable residual wall interference. Further experimental results are necessary-in particular at higher Mach numbers and also for configurations with large aspect ratio wings--before a clearer judgement can be made. At least the evidence so far shows this concept as very promising for wind tunnel testing at subsonic speeds. 5. TEST SECTIONS WITH THREE-DIMENSIONAL ADAPTIVE WALLS The application of the adaptive wall technique to three-dimensional model tests has been investigated only during the last five or six years. It involved arrangements featuring different approaches to the problem of three-dimensional wall control. One arrangement was started in 1979 at the Technical University of Berlin and was based on the flexible wall concept. Eight flexible walls were employed to provide the desired approximation to a three-dimensional wall contour. Another arrangement was begun in 1980 at the AEDC, Tullahoma. It was based on the concept of ventilated walls with local control of the crossflow by means of a segmented, variable porosity wall. The design features and operational procedures of these two test sections will be discussed in the following. In addition, a third arrangement at the DFVLR G~Sttingen will be described. In principle, it is also a flexible wall type configuration but involves deformable rubber tube which allows a nearly perfect three-dimensional wall shaping. Finally, brief mention will be made of some arrangements with only partial three-dimensional wall control. They are listed in Table 2. TABLE 2. SURVEY OF THREE-DIMENSIONAL TEST SECTIONS

Institution AEDC/Calspan TU-Berlin DFVLR NASA Ames WPAFB University of Arizona Sverdrup

Cross-section

Test section Length

1 ft square 15 x 18 cm octagonal 80 cm diam. 13 x 25 cm rectangular 9 in. square 20 x 20 in. 12 ×24 in.

3 1/8 ft 80 cm 240 cm 74 cm 4 1/2 ft 35 in. 96in.

Walls

Number of controls

4 perforated 8 flexible solid rubber solid 2 slotted, solid side-walls 2 × 9 flexible rods solid side-walls venetian blinds, solid side-walls 6 + 3 + 3 slats, solid bottom

5.1. SURVEY ON THE THREE-DIMENSIONAL

TEST SECTION

64 wall segments 78 jacks 64 jacks 44 plenum comp. 186 jacks not specified 204 jacks

DESIGNS

AEDC's adaptive wall test section has a segmented, variable porosity wall configuration as shown in Fig. 17. The top and bottom wall each have 24 individually adjustable segments, the side walls have eight. Both the segments as well as the walls, have 60 °inclined holes, yielding a maximum porosity of 5 %. Stepping motors drive the segments through a rack and pinion mechanism. Linear potentiometers are utilized as position

sensing devices (Fig. 18).

FIG. 17. T h e A E D C s e g m e n t e d , v a r i a b l e p o r o s i t y wall test section.

Adaptive wall windtunnels

99

FIG. 18. Sidewallof AEDCtest sectionwith steppingmotordrive for slidingplate wallsegments.

The choice of this type of wall control was mainly determined by AEDC's aim to develop an adaptive wall configuration that could be applied to existing transonic test facilities with minimum modification. However, a detailed evaluation program was conducted as to the relative effectiveness of three different types of ventilated wall configurations. Besides the segmented, variable porosity wall the study included the possibility of segmenting the test section plenum and the use of slotted walls with baffles inserted in the slots. The spatial variation of wall control in the latter case was gained by varying the baffle angle. The evaluation was conducted through the Mach number range from 0.5 to 1.2 and is summarized qualitatively as follows. The segmented plenum configuration offered maximum control; the segmented, variable porosity configuration was superior to the variable baffle angle, slotted wall at Mach numbers above M~=0.7. The choice finally turned on the upper transonic Mach number capability and the relative ease of implementing the segmented, variable porosity in an adaptive wall test section. A segmented plenum was considered too complex because of the associated machinery and plumbing. One of the major problems in using ventilated walls for an adaptive wall test section arises from the requirement to measure two independent flow variables on a control surface surrounding the model. The measurement must be rapid and accurate at a sufficient number of locations to adequately define the distribution of both variables along the three-dimensional interface. Various techniques were examined including laser velocimetry, stationary and translating multiple probe arrangements and the two-velocity component static pipes (Calspan pipe). Laser velocimetry was ruled out because measurements cannot be made rapidly enough and because of difficulties in providing optical access. Multiple probes were eliminated because of the numbers required, their blockage effects, calibration requirements and the danger of their being inadvertently misaligned. Finally, the Calspan pipes were selected. A pair of two static pipes is arranged to rotate around the tunnel centre line, thus traversing a circular control surface (Fig. 19). The driving system consists of two stepping motors, one located in the settling chamber, the other in the diffuser. They drive a circular track which serves as a support for the pipes.

100

U. Ganzer

CALSPAN PIPE : VELOCITY MEASUREMENT SYSTEM

FIG. 19. AEDC measurement system for interface flow variables.

As discussed in Section 3.1, only the derivative of the flow angle in the longitudinal direction can be determined by the Calspan pipe (beside the static pressure). Therefore, it is necessary to measure the flow angle directly somewhere on the interface to serve as a constant of integration. For these measurements, two five-orifice, hemispherical-head, flow angle probes were mounted at the test section entrance. As far as the wall control procedure is concerned, one has to note that the relationship between the adjustment of a wall segment and the response of the flow variable is not a direct one. Therefore it was important to develop an automated technique to set the desired pressure distribution accurately and rapidly without a tunnel operator in the loop. The control procedure for the A E D C test section is as follows. Initial conditions for a test are selected and set in the tunnel so that the baseline mode interface measurements can be made. The resulting normal velocity component is used as a boundary condition for the exterior flow field calculation. This yields the desired interface pressure distribution for the first iterative step. Comparison of the desired and measured pressure distribution is made in terms of merit and constraint functions that are suitably defined. If these functions are not satisfied within specified adaptive wall tolerances, an incremental mode procedure is performed to determine the control parameter adjustments that are necessary to reduce these functions. If the constraints are not satisfied, a one-dimensional control parameter search is carried out until they are within tolerance. If the constraints are satisfied, the one-dimensional control parameter search is carried out to minimize the merit function. The overall procedure is performed successively until the adaptive wall iteration tolerances are met. The first automated experiments were conducted with an empty test section. The adaptive wall optimization procedure was successfully employed to establish a uniform longitudinal Mach number distribution from arbitrary initial conditions. After this, experiments were made with a wing/tail/body model. The swept lifting surfaces had a NACA 0012 section. The model was equipped with 134 pressure taps and its span was 70 ~o of the tunnel width in the 1 ft test section and its solid blockage was 2.5 j°~o. It was necessary to conduct the adaptive experiments at Mach numbers of 0.9 or higher in order to observe measurable interferences on the model pressure distribution. Repeatable, converged solutions were obtained in the adaptive mode of operation. However, the solutions did not agree exactly with interference free reference data. Extensive analysis of the results revealed that there was an error in the exterior computational code and that errors were being introduced by the interface flow-variable measurement system. The error sources have by now been corrected. A new Calspan pipe system has been designed and implemented. Adaptive wall experiments have been resumed since the beginning of 1985. A first description of the A E D C test section was given by Parker and Erickson in 1982. A more recent report of Parker and Erickson (1984) summarizes the results of the first test period with that test section.

Adaptive wall wind tunnels

101

The TU-Berlin test section has an octagon cross-section made up of eight flexible walls. All the walls are subject to a two-dimensional deformation, computer-controlled in an automatic mode. Spring steel lamellas are used to seal the corners between two adjacent walls. The flexible walls are made of steel 1.2 mm thick, the spring steel lamellas are 0.25 mm thick. The main dimensions of the test section are 15 x 18 cm (Fig. 20).

FIG. 20. TU-Berlin test section with eight flexiblewalls and spring steel lamellas to seal the comers.

The test section design is supposed to be applicable to cryogenic test facilities. A proposal has been made for an application to the ETW for which a rough preliminary sketch is presented in Fig. 21. More details about the TU-Berlin three-dimensional test section as well as some first test results were published by Ganzer and Igeta (1982). The operational procedure for these tests was, in principle, similar to that generally used for two-dimensional experiments in tunnels with two flexible walls. It will now be briefly described and will be followed by a discussion of a one-step-adaptation method which has been developed more recently. The adaptation procedure used so far was an iterative procedure. The pressure distributions measured along the test section walls yielded the longitudinal component of the disturbance velocity of the internal (real) flow ul. This is used to calculate the corresponding normal component vE under the assumption of an unrestricted flow field by means of an external flow field calculation code. In the case of the octagon test section a panel method was employed for this calculation (Fig. 22). On the other hand, the measured wall contour provides the information about the (real) internal v-component vl. An improved wall shape is determined by using the relation

•l(i 4-1)~---Vffi "Jr-g('l~Ei- vii ) Here K is a relaxation factor. The experiments revealed, and a theoretical simulation has confirmed, that the relaxation factor must for three-dimensional experiments be much smaller than in a two-

U. Ganzer

102 TEST SECTION CASING

ELECTRO MOTOR

r

i

FIG. 21. Octagon test section design suggested for ETW.

/0cm

l J

15cm

~,

18 cm

FIG. 22. Panel arrangement of control surface (26 × 16=416 panels).

dimensional case. Typically it has to be below 0.1 in order to ensure convergence of the process. The reason for this is the even more pronounced sensitivity of the internal flow in relation to the external flow to changes in wall shape. The small relaxation factor is linked with a slow rate of convergence. Ten iterations or more were necessary to arrive at a converged solution. This draw-back stimulated the development of a one-step method. Like classical wall interference theory the one-step method assumes the disturbance flow field in the tunnel to be a linear superposition of the model induced potential ~bM and the wall induced potential ~bw

The normal component of the velocity disturbance is obtained by differentiation with respect to n e, the direction taken normal to the control surface positive outwards. The

Adaptive wall wind tunnels

103

desired quantity is the disturbance vM produced by the model only which yields the wall shape for interference free flow. This can be obtained from

VM= ~n---~= ane

~n~ --vl

ane"

The quantity vz is again the normal velocity component as measured for the internal flow on the control surface (by reading the wall position). The task is now to determine the disturbance due to the wall. If the wall disturbance is represented by a source distribution on the control surface, its potential is then given by 1 t*

dpw(p) = - ~ f l

j a(q) g(p,q)dq

S' with the kernel g(p,g) denoting the compressible source potential

g(p,q)= [-~ (Xp--Xq)2q-(yp-- Vq)2-b(Zp--gq)2] -½. The source strength a(q) is given by the sum of the velocit3' component ve (computed with the external flow field code from the measured wall pressure distribution) and - v~

a(q) = vE(q) - vz(q). This stems from the definition of the source strength as the volume flux per surface area through a surface surrounding the source (outwards positive). In the present case the surface through which the volume flux is considered consists, in fact, of two surfaces, one just inside the other just outside the control surface on which the source distribution is located. With the above given source strength one may calculate the derivative drbw/dn e and finally the desired normal velocity disturbance due to the model alone

VM(p)= vI(p) + ~[v~(p) -- vz(P)I + ~--~ [v~q)-- vI(q)I J~eg(P,q)dq

S

with p E S. The second term in the equation is a consequence of the singularity which occurs when the source and control point coincide. The discretisation of the control surface S into rectangular elements A:, e.g. as shown in Fig. 22, and the assumption of piecewise constant source strength a(q) = a t = const., with q eat, finally yields

VM= VI + ( E - A) [rE - vt], with

E = Eke = identity (unit) matrix,

A = A~ =

1

0 f

4rcflOne

g(Pk,q) dq.

If one compares this result with the formula used in the earlier adaptation scheme it turns out that the constant relaxation factor K of the iterative relation is replaced in the onestep method by a matrix. This matrix depends on the free-stream Mach number and the shape of the control surface, i.e. the test section geometry. It is to be noted that the additional computational effort is very limited since the matrix .4 is the same as that used in the external flow field code. The method described above has been presented to a Euromech Colloquium by Rebstock (1984) and was also briefly described by Ganzer and Rebstock (1984). It has to be mentioned here that another one-step method was developed earlier by Wedemeyer (see Heddergott and Wedemeyer, 1984). He used a different procedure by representing the interference potential with Fourier series. This seems to involve more computational effort and moreover it is only applicable for a cylindrical control surface. For the TU-Berlin octagon test section it was required that the adaptation algorithm can cope with arbitrary

104

U. Ganzer

cross-sectional shapes of the control surface. Thus, a scheme based on the panel method was considered the most suitable. A flow chart of the wall adaptation procedure now used for TU-Berlin octagon test section is shown in Fig. 23. It includes the one-step formula a n d - - f o r the external flow field calculation--alternatively a panel or a TSP code, both in the analysis mode, linked with a special design programme. More details about this procedure are given by Rebstock (1984).

I MATRICESI 12D SPUNE INT. I I l REPRESENTATIONOF I MODELAND WAKE I BY SINGULARITIES j

J

I EXTRAPOLATION i AND SMOOTHING I

OF MEASUREDDATAI I

I ANALYSIS CODE~ IIPANEL O R TSP !L. . . . . . . . . . .J_-~-J

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IWALLADJUSTMENTIH=I'V=dx I FIG. 23. TU-Berlin wall adaptation procedure.

D F V L R has constructed an adaptive wall test section from a thick-walled rubber tube. The test section has a length of 240 cm and a circular cross-section of 80 cm diameter. The tube has a wall thickness of 6 cm. It is supported and can be deformed by 64 jacks. The jacks are connected to a rigid framework surrounding the test section. They are driven by stepping motors. Each jack acts via four support points on the rubber wall. The supports are vulcanized into the rubber and have a hinged connection to the jacks allowing for some small lateral displacement. The test section is shown in Fig. 24. The pressure taps in the rubber wall are located at the 1/4 and 3/4 positions between the support points in the longitudinal direction, and the mid-position between the supports in the circumferential direction. These have been calculated as positions of minimum error due to wall waviness effects. Wall pressure and wall displacement are measured at the 64 positions of the jacks. The measurement of the. wall data, the wall adaptation and the model testing are performed fully automatically. For the wall adaptation a one-step method is used based on the linearized potential equation. The theoretical framework takes advantage of the cylindrical geometry of the test section by expanding the perturbation potentials for internal and external flow in Fourier series. The method is described in some detail by Heddergott and Wedemeyer (1984). Experimental results obtained in the D F V L R rubber tube test section will be presented in the next section. The results provide a verification of the one-step method even in cases of large blockage and fairly high Mach numbers. This concludes the description of test sections with complete three-dimensional wall control. All three test sections discussed were specifically designed for investigating

Adaptive wall wind tunnels

105

FIG. 24. DFVLR deformable adaptive wall test section (DAM).

aircraft-like configurations at transonic speeds. In the following, four more facilities will be briefly described, which have one or two walls non-adaptable. One facility (NASA Ames) was designed to study a wing on the wall, like the WPAFB test section it has two walls adaptable in a three-dimensional mode. The Arizona AVT tunnel is meant for low speed tests of high lift configurations and the Sverdrup facility is made for testing road vehicles. NASA Ames 25 by 13 cm test section, originally used for two-dimensional tests with two adaptive walls, was modified for three-dimensional model tests to permit cross-stream control of flow conditions on upper and lower walls. The longitudinal arrangements of plenum chamber compartments was kept the same (see Section 3.1), but each of the six upper and lower compartments closest to the model was subdivided into three cross-stream compartments. Again laser velocimetry was used to measure normal velocity components on two control surfaces near the adaptive walls. For the three-dimensional experiments the velocimeter was also traversed in the spanwise direction. The test model was a semispan wing mounted on one side wall. In all the experiments at Moo =0.6 wall interference was substantially reduced by the adaptation procedure, although interference was not entirely eliminated. It was found that the use of linear influence coefficients was not fully adequate, the velocity changes required on the control surface could not be accurately produced. Also, wall control was, in some cases, insufficient. A report on the tests is given by Schairer (1983). Three-dimensional work is continuing at NASA Ames with theoretical studies, see Mendoza (1984) and Davis (1985). The WPAFB test section with two adaptive walls has already been described in Section 4 and it will be recalled that three-dimensional contouring of the top and bottom walls was achieved by individual deformation of rods which made up the walls. Wall shapes were calculated from measured model data. Three-dimensional wall contours yielded interference free test data for non-lifting and lifting configurations. Test results were reported by Harney (1984). The University of Arizona has a small low speed tunnel for demonstrating the use of adaptive walls for the test of very high-lift configurations. The 20 x 20 in. test section has a length of 35 in. Plane solid but transparent side walls permit the use of laser velocimetry to

106

U. Ganzer

measure the flow variables on the boundary of a 15 x 15 x 25 in. control volume. Venetian blinds of 1 cm chord are used to control the flow over the top and bottom walls. A division of the blinds along the centre line of the test section permits the study of configurations with non-symmetric down wash. At present initial measurements are being made in that tunnel with a small aspect ratio model having blown flaps. The tunnel has been briefly described by Sears (1982, 1983). Sverdrup has applied the adaptive wall concept to a wind tunnel suitable for testing automobiles. Experiments conducted in a subsonic ! x 2 ft test section have established the feasibility of the concept. The top wall and the two lateral walls of the test section were segmented into longitudinal flexible slats. The top wall consists of six slats, the side walls of three. Each slat is contourable by means of ! 7 manually actuated screw jacks. Polymer seals bounded to the slats and forced against wiper plates provide an adequate seal. The wiper plates which extend into the airstream were found to cause no adverse effects on the experimental results. The external flow field calculation required for the adaptation procedure was based on the panel method. A small relaxation factor of K = 0.075 was found to be adequate. The experiments demonstrated that in such a tunnel with adaptive walls model blockage in excess of 20 ~o is permissible as compared with about 5 ~o in conventional tunnels. The Sverdrup concept is described in some detail by Whitfield et al. (1982). Sverdrup also made some engineering evaluation of applying their adaptive wall concept to high-lift subsonic testing, see Starr and Varner (1984). 5.2. REPRESENTATIVE RESULTS OBTAINED IN THREE-DIMENSIONAL TEST SECTIONS A few test results are presented here in order to illustrate the present state of the art in adaptive wall wind tunnel technique. The results were obtained in the D F V L R rubber tube and in the TU-Berlin octagon test section. Tests in the D F V L R rubber tube comprised two bodies of revolution--ONERA C and a parabolic spindle-- as well as the AGARD calibration model B at zero lift and at lifting condition. In all cases good agreement was obtained with essentially interference reference data. Figure 25 illustrates the measurements with the FFA parabolic spindle. The fairly large blockage of the model results in pronounced wall interference effects evident in the surface

d~'.020 ,I,,U

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Adaptive wall wind tunnels

107

pressure distribution when the wall is not adapted. Wall adaptation reduces the interference so that the measured data come very close to the theoretical reference curve. The Mach number of the tests with this model approached M~ = 0.85. For 3-component force measurements two AGARD calibration models B were used, one gave a solid blockage of 3.5 ~o the other of 1 ~o. This calibration model is a simple wing-body combination namely a 60 ° swept delta wing of circular arc cross-section with a span four times the body diameter. The body is cylindrical with an ogive nose. An example of the test results for the AGARD calibration model is shown in Fig. 26. Again good agreement with interference free reference data is achieved when the rubber tube is adapted. A few data points at M~ = 0.5 for the 3.5 % blockage model and the rubber tube not adapted give an impressive demonstration of what interference effects the wall adaptation technique can cope with. More test results were published by Heddergott and Wedemeyer (1984).

Institub Wkndtunnel DFVLR DFVLR DFVLR AEDC DFVLR

o O •

Blockage

Rd)ber tube adapted

3.5% 1.0% 0.5"/0 001% 3.5%

Rubbertubeadapted TWG p~rforated PTWperf~rated R u i ~ tul~n~ od~ed

O.5 0.4

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MACH NUMBER M FIG. 26. Lift variation with Mach number for AGARD. Calibration model B.

A test result from the TU-Berlin octagon test section for a body of revolution has been presented in Section 4 together with results obtained in the two-dimensional test section (see Fig. 15). It was the first demonstration of the feasibility of the three-dimensional adaptation concept. In addition, tests at the TU-Berlin were made with an Airbus-like wing-body combination. This was chosen in order to include effects of spanwise variation of interference which are important particularly for large aspect ratio wings. Such models will, however, have only moderate solid blockage because wing span must be limited to say 70 % of the tunnel width. The blockage of the F4 model tested in the octagon test section is 1.2 %, the ratio of wing area to cross-sectional area of the test section is 6.7 %. Interference effects for this model are, therefore, small, but they appear even smaller in the measured data because of some sort of self-correcting effect resulting from changes of the relatively thick wall boundary layer. Lift, drag and pitching moment data for a moderate Mach number of M~ = 0.7 are presented in Fig. 27. It has to be noted that the discrepancies related to the data from the D F V L R 1 × 1 m tunnel obtained with the same model are at least partly due to the fact that the model surface was slightly affected in the course of the experiments by flow contamination concentrated on the centre line of that tunnel. The TU-B measurement

108

U. Ganzer I

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FIG. 27. 3-component measurements with F4-Model at M ~ =0.70.

points represent repeated tests showing scatter only within the size of the symbols. For these first tests the iterative adaptation procedure was used. Figure 28 shows the changes of wall contour during the iterations. The initial configuration was the aerodynamically straight wall. Obviously, the adaptation procedure converges and yields plausible wall contours. The contour for the side wall gives hope that this wall could be composed out of three plane portions which would permit the installation of windows. In Fig. 29 two sets of wall pressure distributions are shown, one set for the adapted and another for the plane wall and the measured (internal) pressure distributions are compared with the theoretical (external) ones. For the plane wall the theoretical (external) pressure distribution i s - - of course---cp = 0 = constant. Thus, the difference between the measurement points and the axis cp= 0 is a measure of the wall interference. For the adapted case the measured pressure distribution comes close to the one calculated for the exterior flow field. If one

~'2.

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FIG. 28. Wall contours during iteration. F4-model at M s =0.7, x =4.4 °.

Adaptive wall wind tunnels

109

PLANE WALL ~, -o.O6! v• ADAPTED WALLJ MEASURED ----ADAPT. W. CALCULATED

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POSITION ALONG THE WALL XIL FIG. 29. Pressure distribution along upper wall F4-model at M~ =0.70, x = 0 °.

compares these fairly pronounced changes of the flow situation at the wall with the rather small effects on the model (as shown in the force data) one can conclude that high quality model data can be achieved by three-dimensional wall adaptation. The test results presented here were published by Ganzer in 1983. Due to a breakdown of the wind tunnel driving system the adaptive wall experiments have been interrupted at TU-Berlin. They will be resumed this summer (1985). It may be concluded here that the use of three-dimensional wall adaptation is still in the demonstrational phase. Test results at present mainly cover only the lower part of the Mach number range of interest. However, they may be taken as providing good support of the feasibility of three-dimensional wall adaptation, at least for subsonic flow.

6. CONCLUDING REMARKS For the test of aerofoils at transonic speeds the advantages of the adaptive wall technique are clear. It probably will be used for any new test facility as well as being employed for improving existing ones. Whether ventilated or solid flexible walls are chosen high quality test results will be obtained. If a ventilated wall solution is chosen it will have the advantage that the boundary layer development on the two adaptive walls has no advance effect on the measurement of the two flow variables. The solid flexible wall solution, on the other hand, permits a much faster measurement of flow variables and it is also simpler. For the test of three-dimensional configurations the present state of the art calls for further investigation. Certainly, the possibility of using only two adaptive walls for three-dimensional model tests is very attractive. However, more experimental evidence is required to see how far the residual interference effects can be reliably assessed for the higher Mach number range. In any case, there can be no doubt that at low supersonic speeds the solid side walls will produce unacceptable interference effects. The possibility of adopting a hybrid technique was suggested by DFVLR. This requires the use of perforated flexible side walls with the perforation covered at subsonic speeds. Perforated walls have been shown to possess good wave cancellation properties. With the porosity non-adaptive, however, no improvement as compared into the present state of the art would be achieved at low supersonic speeds. Test sections with three-dimensional wall adaptation require fairly extensive control hardware. There is hope that some of the controls may prove unnecessary, in particular along the side walls. This would allow for windows in these wall regions thus avoiding one of the main drawbacks of three-dimensional adaptive test sections. As far as wall interference at subsonic speeds is concerned, experimental evidence verifies the general feasibility of the three-dimensional adaptation concept. So far, interference, free results have only been obtained in test sections with solid, flexible walls. However, it is to be expected that similar results will soon be available from ventilated wall test sections.

110

U. Ganzer

A d e c i s i v e q u e s t i o n is h o w d o the test s e c t i o n s p e r f o r m at l o w s u p e r s o n i c speeds. It can be a c c e p t e d t h a t a test section w i t h p e r f o r a t e d walls a n d l o c a l c o n t r o l o f p o r o s i t y will p r o v i d e a clear i m p r o v e m e n t a g a i n s t the c o n v e n t i o n a l v e n t i l a t e d walls w i t h c o n s t a n t p o r o s i t y . T h e p r o p e r t i e s o f test s e c t i o n s w i t h solid, flexible walls o n t h e o t h e r h a n d are w i d e l y u n k n o w n . T h e r e is o n l y a t h e o r e t i c a l s t u d y by G a n z e r et al. (1984) r e l a t e d to this subject. T h i s s t u d y is b a s e d on F F A flow field m e a s u r e m e n t s a n d d e m o n s t r a t e s t h a t the s u p e r s o n i c w a v e e q u a t i o n is a d e q u a t e for c a l c u l a t i n g t h e r e q u i r e d wall s h a p e f r o m m e a s u r e d wall pressures. T h e r e s u l t i n g wall c o n t o u r s e x h i b i t s t r o n g e r g r a d i e n t s t h a n at s u b s o n i c c o n d i t i o n but they still s e e m feasible. It has yet to be d e m o n s t r a t e d e x p e r i m e n t a l l y t h a t the s u g g e s t e d p r o c e d u r e yields i n t e r f e r e n c e free flow c o n d i t i o n . In c o n c l u s i o n , it has to be said t h a t for a n u m b e r of p r o b l e m s e x p e r i m e n t a l v e r i f i c a t i o n s are r e q u r e d b e f o r e a c o m p a r a t i v e e v a l u a t i o n of the t h r e e - d i m e n s i o n a l a d a p t i v e wall techn i q u e s can be made. Such e x p e r i m e n t s are p l a n n e d for the n e a r future. T h e results c a n be e x p e c t e d to b e c o m e a v a i l a b l e w i t h i n this decade.

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