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Comparison of road and wind-tunnel drag reductions for commercial vehicles
Journal of Wind Engineering and Industrial Aerodynamics, 49 (1993) 411-420
411
Elsevier
Comparison of Road and Wind-Tunnel Drag Reductions for Commercial Vehicles S. Watkins, J.W. Saunders and P.H. Hoffmann* Departments of Mechanical and *Aerospace Engineering, Royal Melbourne Institute of Technology, P.O. Box 2476V, Melbourne 3001, Australia
Abstract Drag coefficient reductions arising from streamlining devices to commercial vehicles are discussed. Road and wind-tunnel tests are briefly described, including road tests to measure on-road turbulence characteristics. Comparison of drag coefficient reductions at high yaw angles shows road results that are considerably lower than tunnel results, whereas at low yaw angles the road drag reductions can be higher than those measured in the tunnel. It is shown that the effects of atmospheric turbulence (which are always present during road tests when high yaw angles are measured due to the strong crosswinds) are significant. Tunnel data obtained in grid-generated turbulence are closer to results obtained on the road under high yaw angles.
1. INTRODUCTION Larger commercial vehicles, due to their function, have tended to be relatively unstreamlined and typically their shape is one bluffbody (the cabin) in front of a larger bluff body (a box-van trailer). The geometry usually gives a drag for the tandem combination that is less than the drag of the trailer alone. In order to further reduce the drag and fuel consumption, these shapes are frequently modified, consistent with keeping a maximum loading capacity in the trailer. Two methods of streamlining have traditionally been used; small deflector-type devices mounted on the cabin, Figure 1, and larger streamlined failings, Figure 2. The deflector devices (often little more than a flat plate) are sized and positioned such that their wake is just large enough to envelop the front face of the trailer, giving mutually beneficial drag reductions (i.e. giving both bodies lower drag in the tandem configuration) and thus lowering total vehicle drag. This has resulted in considerable commercial application, e.g. Saunders [1]. The effect on axi-symmetric bodies was investigated by Roshko and Koenig [2] who concluded that with optimum geometry the forebody drag could be reduced by almost two orders of magnitude. The deflector devices offer considerable commercial and Elsevier SciencePublishersB.V.
412 practical advantages over the streamlined fairings due to; a) the device wake can be "tuned" to suit lower height loads by arranging devices to hinge along the line of mounting to the cab roof and; b) reduced manufacturing and fitting costs.
Figure 1. Deflector - Type Drag Reducer.
Figure 2. Large Streamline Drag Reducer.
By far the most common method of measuring drag reductions arising from such modifications to commercial vehicles is scale-model tests in wind tunnels. Road tests conducted under a range of wind conditions are difficult and rare. Comparing results from the two testing methods, offering explanations for the differences found and suggesting improved wind-tunnel modelling techniques are the aims of the work reported here. 2. TRUCK ROAD TESTS A series of road tests based on a method by Buckley [3] on geometrically-similar commercial vehicles has been carried out. By measunng fuel flows on two vehicles, one of which incorporates an aerodynamic device, and also monitoring the wind strength and direction, the change in drag coefficient as a function of yaw angle can be evaluated. Parameters have generally been averaged over a 10 km test length. This procedure has been undertaken for approximately 30 different combinations of vehicles and aerodynamic modifications and each combination has been tested for typically between 100 and 200 km giving several different yaw angles for each combination, see for example Saunders et al. [4], Watkins et al. [5, 6]. For the majority of cases, comparisons have been drawn between the drag coefficient reductions measured by model tests and road tests. The most noticeable difference in results from the two test methods is at high yaw angles, see Figure 3. In order to investigate the lack of correlation at high yaw angles, an extended series of supporting tunnel and road tests has been carried out. The vehicle used was an Isuzu SBR22 fitted with a box van and four different aerodynamic devices as described in Section 3.
413
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Figure 3. Ratio of Road to Tunnel Drag Reductions.
Figure 4. Body Fairing.
3. TUNNEL TESTS Accurate one-eighth scale models of the vehicle and aerodynamic devices were made and tested in the 3m x 2m working section of the Industrial Wind Tunnel at the Royal Melbourne Institute of Technology (RMIT). Four devices were tested; these were: a) A commercially available deflector - Deflector A (as shown on the cab of the vehicle in Figure 1). b) An RMIT designed deflector - Deflector R (comprising of a flat plate with edge radii of of 100 mm). c) A streamlined fairing attached to the front of the box van face named a Body Fairing, shown in full size in Figure 4. d) A quadrant moulding fitted to the top and down the sides of the box van named Body Quadrants, shown in full-size in Figure 5. The blockage ratio of the one-eighth scale unyawed vehicle was 1.7% and the tunnel had a perforated roof of open area ratio 5% to reduce blockage effects. The floor boundary layer thickness was 40 mm and the typical model underbody clearance was 70 mm. The model was yawed in the range -15 to + 15 degrees and in addition the following parameters were varied:1. Reynolds number in the range 0.2 to 1.6 x 106 per metre whilst the vehicle was yawed at six degrees; 2. Mounting system; wheel mounts vs single central strut mounting; 3. Cooling systems; no radiator, compared with fully blocked radiator; 4. Turbulence characteristics were varied by the use of 4 different upstream grids giving five different values of turbulence intensities (i.e. four grids and tunnel baseline). Drag coefficients were measured and flow visualisation studies in the vicinity of the cabin roof and front face of the container were made using wool tufts.
414
/ i
Figure 5. Body Quadrants.
I
Figure 6. Reynolds Number Effects on Drag Reduction.
Figure 6 shows the effects of varying tunnel Reynolds number on the drag coefficient reduction obtained from the devices whilst the model was tested in an airstrearn of longitudinal turbulence intensity I. =3.4%. It appears that the drag coefficient reductions vary little with Reynolds numbers greater than 106 . The influence of mounting methods and the effect of radiator porosity were found to be negligible [5]. The lack of correct ground and wheel movement (the ground and wheel speeds should be suitably/scaled to the tunnel speed) has previously been shown to have a minor influence on drag, Blackmore [7]. Figures 7, 8, 9 and 10 show the road and tunnel data plotted as a function of yaw angle. It is apparent that for the deflector devices, the road drag savings at high yaw angles fall to zero, (and in some cases drag even increases) whereas the tunnel data show a worthwhile drag reduction. It is noticeable that with increasing longitudinal turbulence intensity the tunnel savings fall, with the exception of the high turbulence data for Deflector R where the drag savings were reduced when the longitudinal turbulence intensity was increased from 3.4% to 3.7%. (It is interesting to note that all turbulence intensities stated are longitudinal intensities but lateral intensities were also measured. The grid that gave 3.7% longitudinal intensity gave a lower lateral intensity Iv than the grid that gave the 3.4% longitudinal intensity.) Due to the sensitivity of the results to tunnel turbulence, it was decided that road turbulence intensities should be documented,
415
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Figure 7. Road and Tunnel Data Body Fairing
Figure 8. Road and Tunnel Data Body Quadrants
¢ .N
I Figure 9. Road and Tunnel Data Deflector A
Figure 10. Road and Tunnel Data Deflector R.
4. ON-ROAD TURBULENCE TESTS
Whilst the truck road tests involved measuring yaw angle and relative windspeed and other data required for drag coefficient computation, the data were not in a suitable form for examination of the turbulence characteristics To measure longitudinal and lateral intensities and spectral characteristics of on-road turbulence, tests were performed using mast-mounted hot-wire and propeller-vane anemometers. These were mounted above a vehicle which took measurements whilst stationary, and also while moving at 100 km/h along the same routes as the truck tests. Details can be found in Watkins [8]. From these data, a reasonable estimation of the turbulence characteristics encountered during (previous) truck testing could be made
416 5. P R E D I C T I O N OF ON-ROAD RESULTS F R O M TUNNEL DATA A knowledge of the reduction in drag coefficient, A C D a s a function of longitudinal intensity and yaw angle ~b from the tunnel enables comparison with on-road data at similar values of I u and ft. The information available from the truck test was; AC D, ~b and relative velocity, VR , with all parameters measured over 10 km stretches of road. From a knowledge of ~b and VR, the angle • between the ambient wind and road direction and also the ambient windspeed Vw could be found (i.e. average wind data relative to the ground could be computed from averaged moving vehicle data). One method to predict the turbulence experienced by a moving vehicle from the wind data relative to the ground would be to use • and Vw calculated from moving vehicle measurements and use a predictive model given in Watkins et al. [9]. Upon examining the relationship between stationary and moving-vehicle wind characteristics from the on-road turbulence tests, it is clear that:a) Vw calculated from moving vehicle data is frequently less than that measured at a ground site selected in open country. This reduction is due to the sheltering effect of trees that were present along much of the test route. The corollary of this is that yaw angles predicted from meteorological data will be too high, since most meteorological data is taken from open sites at 10 m and factored down to vehicle height with no allowance for local sheltering. b) I u and (particularly) Iv, calculated from measurements from a moving vehicle are often higher than predicted from the stationary wind measurements due to the wakes of the roadside obstructions.
To account for the above effects and also the fact that I u , I v (relative to the ground) are functions of Vw , corrections were applied to the data. Having estimated longitudinal and lateral turbulence intensities experienced by the trucks and knowing if, it is now possible to utilise tunnel data to see how it compares with data obtained on the road. The data obtained on the road were sorted into yaw angle categories as shown in Table 1 and for each corresponding discrete value of yaw angle in the tunnel, linear least-squares curve fits were found for AC D as a function of I u for the two deflectors. Any given road combination of ~b and I~ could then be selected from tunnel data and the corresponding tunnel AC D value found. This then enabled the tunnel data to be used in estimating the drag reduction on the road for each road data point. The results are shown in Figures 11 and 12.
417
Table 1. Yaw Angle Categories. Tunnel yaw angle (degrees)
Road yaw angle (degrees) 0 1.5 4.5 7.5 10.5
O.'*
----~ 1.5 ~ 4.5 ~ 7.5 ~ 10.5 *
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ROAD(VARIOUSTURBULENCE" ~
PREDICTED ROAD U S I N G T U N N E L DATA
I N T E N S t T I E S A N D SCALES) '
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Figure 11. Road Results Predicted from Tunnel Data - Deflector A.
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Figure 12. Road Results Predicted from Tunnel Data - Deflector R.
Wind-tunnel data obtained under the lowest turbulence intensity (I u = 1.3%) is shown as a dotted line and parabolic least-squares regression lines are shown for the measured onroad data and data from the predictive method above. Agreement between predicted and on-road data is now better than that for data obtained at any single turbulence level at the higher yaw angles, but the agreement at low yaw angles is not so good. 6. DISCUSSION AND CONSIDERATION OF REYNOLDS NUMBER EFFECTS
This method does not consider the effects of lateral turbulence intensity nor can it include the effects of longitudinal or lateral scale, or Reynolds number. It is useful in showing that the effects of longitudinal intensity, with other modelling parameters held constant, can considerably alter the aerodynamic characteristics of drag-reducing devices. It also indicates that the long-term road drag savings could differ considerably from predictions in characteristically smooth wind-tunnel flow. It is considered that the high lateral intensities encountered on the road during strong crosswinds due to wakes of roadside obstructions will be dominant in reducing the performance of deflector-type aerodynamic devices. The drag reduction they offer arises from a stable wake which does not appear to exist under high, turbulent, crosswinds.
418 The levels of turbulence measured on the road were in general considerably higher than those that could be easily generated in the tunnel hence the predictions from tunnel to road data were usually extrapolated. Further work is needed to generate higher intensities (typically up to 10% for longitudinal and lateral) and to improve modelling of the scales of turbulence (which were too short in the tunnel) consistent with a relatively fiat velocity profile and thin ground boundary layer. Although the blockage ratios were low when the relatively long vehicles were yawed, the drag coefficient would be influenced due to the rear of the vehicle approaching the wall which would affect the base pressure. However, the effect on the incremental (forebody) drag due to the fitting of an aerodynamic device should be negligible. To avoid differences in flow pattern between full and model scale (mainly low-Reynolds number flow separations that occur only on the models), it is frequently stated that the Reynolds number be above some critical value. For a complex body, requiring many dimensions to define its geometry, the Reynolds number should be based not on wheelbase, vehicle width or vehicle length (all of which have been suggested by other workers, e.g. the SAE Wind Tunnel Procedure for Trucks and Buses, [10]) but on some local length or radius. The critical value depends upon a variety of factors; these include local pressure gradients, surface roughness and free stream turbulence levels. One solution suitable for engineering uses may be basing the parameter on the smallest local dimension that has a significant influence on the flow, and to take into account local pressure gradients. For the case of zero yaw, the most significant dimension on which to base the parameter for commercial vehicles would seem to be the radii found on the cab top leading edge, see Cooper [11 ]. Flow separations in this area can have a considerable effect on the total vehicle drag, however under highly yawed conditions lee-side flows play a role in determining drag and hence vertical edge radii become important. To investigate the flow in the cab top area at zero yaw, flow visualisation studies were conducted at the top of the cab roof whilst varying the Reynolds number. The first series of tests was made with no box van behind the cab. The flow became attached to the roof at a local Reynolds number (based on the smallest local radii) of about 0.44 x 104. When the local pressure gradient was modified by placing the box-van trailer behind the cab, the flow was separated from the roof, even at the highest Reynolds number attainable. Investigation of the full-size roof flow on the same vehicle with a box-van trailer (by filming tug movements from a chase vehicle), travelling at 60 km/h (as opposed to 100 km/h for the road tests), showed a roof flow that was attached for about 70% of the time. For the remainder of the time the flow was separated. It appeared that the flow was influenced by small changes in wind conditions and required only minor disturbances to separate from the cab roof Thus for this case, the flow appeared strongly influenced by turbulence as well as Reynolds number. At higher speeds, greater levels of flow attachment should occur although this was not tested. It is difficult to see how this could be correctly modelled in a wind tunnel without having the correct Reynolds number and turbulence characteristics, and it is evident that the pressure field of the container considerably influences the cab roof flow.
419 At low Reynolds numbers, laminar separation and the associated wake from the cab front can lead to a lower total vehicle drag coefficient for certain geometries of commercial vehicles than for tests conducted at higher Reynolds numbers. This is due to the relatively large wake from a laminar separation shielding the front container face to a greater extent than would be the case for a smaller (or no) turbulent separation. This is a possible explanation why, at low Reynolds numbers (as tested in the tunnel), the drag reduction from an aerodynamic device was less than at higher Reynolds numbers (as tested on the road). This was generally found to be the case at low yaw angles, see Figures 7 to 10. This was evidently not the case for all the devices tested when higher yaw angles were encountered, thus incorrect Reynolds number similarity did not adequately account for the very large differences in drag coefficient reduction at higher yaw angles. 7. CONCLUSIONS Large differences were found when drag reductions measured in smooth wind-tunnel flow were compared to on-road results, particularly at high yaw angles. Utilising road measurements of the turbulent wind characteristics and drag reduction data obtained in turbulent flows in a wind tunnel gave better agreement with road results. However, turbulence characteristics in the tunnel differed from that experienced by a moving vehicle and also it was observed that the cab roof flow was influenced by Reynolds number effects. Since turbulence can strongly influence the drag savings (and hence fuel consumption), it is recommended that more effort is expended on understanding the turbulent environment of vehicles. 8. ACKNOWLEDGMENTS
The assistance of Arnotts-Brockhoff-Guest for providing vehicles and personnel and the financial assistance of the National Energy Research, Development and Demonstration Program are gratefully acknowledged. 9. REFERENCES
1. W.S. Saunders, Apparatus for Reducing Linear and Lateral Wind Resistance in a Tractor-Trailer Combination Vehicle, U.S. Patent Office, 3,241,876, 1966. 2. A. Roshko and K. Koenig, Interaction Effects on the Drag of BluffBodies in Tandem, in Aerodynamic Drag Mechanisms of Bluff Bodies and Road Vehicles, edited by Sovran G., Morel T and Mason W.T., Plenum Press, 1978. 3. F.T. Buckley, Jr., An Improved Over-the-Road Test Method for Determining the Fuel Savings Benefit of a Truck Aerodynamic-Drag-Reducing Device, SAE 850286, presented at the SAE International Congress and Exposition, Detroit, 1985.
420 4. J.W. Saunders, S. Watkins, P.H Hoffmann and F.T. Buckley, Jr., Comparison of OnRoad and Wind-Tunnel Tests for Tractor-Trailer Aerodynamic Devices, and Fuel Savings Predictions, SAE 850286, presented at the SAE International Congress and Exposition, Detroit, 1985. 5. S. Watkins, J.W. Saunders and PH. Hoffmann, Wind-Tunnel Modelling of Commercial Vehicle Drag-Reducing Devices: Three Case Studies, SAE 870717, presented at the Autotechnologies Conference, Monaco, 1987. 6. S. Watkins, P.H. Hoffmann and J.W. Saunders, "Comparison of On-Road and WindTunnel Tests for Rigid Truck Aerodynamic Devices, 9th Australasian Fluid Mechanics Conference, Auckland 8-12 Dec., 1986. 7. M. Blackmore, The Effect of Ground Simulation on a Slab-Sided Truck, M. Eng. Sc. Thesis, Dept. of Mech. Eng., University of Melbourne, 1984. 8.
S. Watkins, Wind-Tunnel Modelling of Vehicle Aerodynamics: with Emphasis on Turbulent Wind Effects on Commercial Vehicle Drag, Ph.D. Thesis, Mechanical Engineering, Department of Manufacturing and Process Engineering, Victorian University of Technology, RMIT Campus, November 1990.
9. S. Watkins, J.W. Saunders, P.H Hoffmann and J.D. Holmes, Measurements of Turbulence Experienced by Moving Vehicles, Part I Turbulence Intensity, submitted to the Journal of Wind Engineering and Industrial Aerodynamics. 10. SAE J1252 (Anon), SAE Wind-Tunnel Test Procedures for Trucks and Buses, SAE J1252 August 1979, Prepfint Version. 11. KR. Cooper, The Effect of Front-Edge Rounding and Rear-Edge Shaping on the Aerodynamic Drag of Bluff Vehicles in Ground Proximity, SAE 850288. Presented at the International Congress and Exposition, Detroit Feb. 25-March 1, 1985.
411
Elsevier
Comparison of Road and Wind-Tunnel Drag Reductions for Commercial Vehicles S. Watkins, J.W. Saunders and P.H. Hoffmann* Departments of Mechanical and *Aerospace Engineering, Royal Melbourne Institute of Technology, P.O. Box 2476V, Melbourne 3001, Australia
Abstract Drag coefficient reductions arising from streamlining devices to commercial vehicles are discussed. Road and wind-tunnel tests are briefly described, including road tests to measure on-road turbulence characteristics. Comparison of drag coefficient reductions at high yaw angles shows road results that are considerably lower than tunnel results, whereas at low yaw angles the road drag reductions can be higher than those measured in the tunnel. It is shown that the effects of atmospheric turbulence (which are always present during road tests when high yaw angles are measured due to the strong crosswinds) are significant. Tunnel data obtained in grid-generated turbulence are closer to results obtained on the road under high yaw angles.
1. INTRODUCTION Larger commercial vehicles, due to their function, have tended to be relatively unstreamlined and typically their shape is one bluffbody (the cabin) in front of a larger bluff body (a box-van trailer). The geometry usually gives a drag for the tandem combination that is less than the drag of the trailer alone. In order to further reduce the drag and fuel consumption, these shapes are frequently modified, consistent with keeping a maximum loading capacity in the trailer. Two methods of streamlining have traditionally been used; small deflector-type devices mounted on the cabin, Figure 1, and larger streamlined failings, Figure 2. The deflector devices (often little more than a flat plate) are sized and positioned such that their wake is just large enough to envelop the front face of the trailer, giving mutually beneficial drag reductions (i.e. giving both bodies lower drag in the tandem configuration) and thus lowering total vehicle drag. This has resulted in considerable commercial application, e.g. Saunders [1]. The effect on axi-symmetric bodies was investigated by Roshko and Koenig [2] who concluded that with optimum geometry the forebody drag could be reduced by almost two orders of magnitude. The deflector devices offer considerable commercial and Elsevier SciencePublishersB.V.
412 practical advantages over the streamlined fairings due to; a) the device wake can be "tuned" to suit lower height loads by arranging devices to hinge along the line of mounting to the cab roof and; b) reduced manufacturing and fitting costs.
Figure 1. Deflector - Type Drag Reducer.
Figure 2. Large Streamline Drag Reducer.
By far the most common method of measuring drag reductions arising from such modifications to commercial vehicles is scale-model tests in wind tunnels. Road tests conducted under a range of wind conditions are difficult and rare. Comparing results from the two testing methods, offering explanations for the differences found and suggesting improved wind-tunnel modelling techniques are the aims of the work reported here. 2. TRUCK ROAD TESTS A series of road tests based on a method by Buckley [3] on geometrically-similar commercial vehicles has been carried out. By measunng fuel flows on two vehicles, one of which incorporates an aerodynamic device, and also monitoring the wind strength and direction, the change in drag coefficient as a function of yaw angle can be evaluated. Parameters have generally been averaged over a 10 km test length. This procedure has been undertaken for approximately 30 different combinations of vehicles and aerodynamic modifications and each combination has been tested for typically between 100 and 200 km giving several different yaw angles for each combination, see for example Saunders et al. [4], Watkins et al. [5, 6]. For the majority of cases, comparisons have been drawn between the drag coefficient reductions measured by model tests and road tests. The most noticeable difference in results from the two test methods is at high yaw angles, see Figure 3. In order to investigate the lack of correlation at high yaw angles, an extended series of supporting tunnel and road tests has been carried out. The vehicle used was an Isuzu SBR22 fitted with a box van and four different aerodynamic devices as described in Section 3.
413
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Figure 3. Ratio of Road to Tunnel Drag Reductions.
Figure 4. Body Fairing.
3. TUNNEL TESTS Accurate one-eighth scale models of the vehicle and aerodynamic devices were made and tested in the 3m x 2m working section of the Industrial Wind Tunnel at the Royal Melbourne Institute of Technology (RMIT). Four devices were tested; these were: a) A commercially available deflector - Deflector A (as shown on the cab of the vehicle in Figure 1). b) An RMIT designed deflector - Deflector R (comprising of a flat plate with edge radii of of 100 mm). c) A streamlined fairing attached to the front of the box van face named a Body Fairing, shown in full size in Figure 4. d) A quadrant moulding fitted to the top and down the sides of the box van named Body Quadrants, shown in full-size in Figure 5. The blockage ratio of the one-eighth scale unyawed vehicle was 1.7% and the tunnel had a perforated roof of open area ratio 5% to reduce blockage effects. The floor boundary layer thickness was 40 mm and the typical model underbody clearance was 70 mm. The model was yawed in the range -15 to + 15 degrees and in addition the following parameters were varied:1. Reynolds number in the range 0.2 to 1.6 x 106 per metre whilst the vehicle was yawed at six degrees; 2. Mounting system; wheel mounts vs single central strut mounting; 3. Cooling systems; no radiator, compared with fully blocked radiator; 4. Turbulence characteristics were varied by the use of 4 different upstream grids giving five different values of turbulence intensities (i.e. four grids and tunnel baseline). Drag coefficients were measured and flow visualisation studies in the vicinity of the cabin roof and front face of the container were made using wool tufts.
414
/ i
Figure 5. Body Quadrants.
I
Figure 6. Reynolds Number Effects on Drag Reduction.
Figure 6 shows the effects of varying tunnel Reynolds number on the drag coefficient reduction obtained from the devices whilst the model was tested in an airstrearn of longitudinal turbulence intensity I. =3.4%. It appears that the drag coefficient reductions vary little with Reynolds numbers greater than 106 . The influence of mounting methods and the effect of radiator porosity were found to be negligible [5]. The lack of correct ground and wheel movement (the ground and wheel speeds should be suitably/scaled to the tunnel speed) has previously been shown to have a minor influence on drag, Blackmore [7]. Figures 7, 8, 9 and 10 show the road and tunnel data plotted as a function of yaw angle. It is apparent that for the deflector devices, the road drag savings at high yaw angles fall to zero, (and in some cases drag even increases) whereas the tunnel data show a worthwhile drag reduction. It is noticeable that with increasing longitudinal turbulence intensity the tunnel savings fall, with the exception of the high turbulence data for Deflector R where the drag savings were reduced when the longitudinal turbulence intensity was increased from 3.4% to 3.7%. (It is interesting to note that all turbulence intensities stated are longitudinal intensities but lateral intensities were also measured. The grid that gave 3.7% longitudinal intensity gave a lower lateral intensity Iv than the grid that gave the 3.4% longitudinal intensity.) Due to the sensitivity of the results to tunnel turbulence, it was decided that road turbulence intensities should be documented,
415
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Figure 7. Road and Tunnel Data Body Fairing
Figure 8. Road and Tunnel Data Body Quadrants
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I Figure 9. Road and Tunnel Data Deflector A
Figure 10. Road and Tunnel Data Deflector R.
4. ON-ROAD TURBULENCE TESTS
Whilst the truck road tests involved measuring yaw angle and relative windspeed and other data required for drag coefficient computation, the data were not in a suitable form for examination of the turbulence characteristics To measure longitudinal and lateral intensities and spectral characteristics of on-road turbulence, tests were performed using mast-mounted hot-wire and propeller-vane anemometers. These were mounted above a vehicle which took measurements whilst stationary, and also while moving at 100 km/h along the same routes as the truck tests. Details can be found in Watkins [8]. From these data, a reasonable estimation of the turbulence characteristics encountered during (previous) truck testing could be made
416 5. P R E D I C T I O N OF ON-ROAD RESULTS F R O M TUNNEL DATA A knowledge of the reduction in drag coefficient, A C D a s a function of longitudinal intensity and yaw angle ~b from the tunnel enables comparison with on-road data at similar values of I u and ft. The information available from the truck test was; AC D, ~b and relative velocity, VR , with all parameters measured over 10 km stretches of road. From a knowledge of ~b and VR, the angle • between the ambient wind and road direction and also the ambient windspeed Vw could be found (i.e. average wind data relative to the ground could be computed from averaged moving vehicle data). One method to predict the turbulence experienced by a moving vehicle from the wind data relative to the ground would be to use • and Vw calculated from moving vehicle measurements and use a predictive model given in Watkins et al. [9]. Upon examining the relationship between stationary and moving-vehicle wind characteristics from the on-road turbulence tests, it is clear that:a) Vw calculated from moving vehicle data is frequently less than that measured at a ground site selected in open country. This reduction is due to the sheltering effect of trees that were present along much of the test route. The corollary of this is that yaw angles predicted from meteorological data will be too high, since most meteorological data is taken from open sites at 10 m and factored down to vehicle height with no allowance for local sheltering. b) I u and (particularly) Iv, calculated from measurements from a moving vehicle are often higher than predicted from the stationary wind measurements due to the wakes of the roadside obstructions.
To account for the above effects and also the fact that I u , I v (relative to the ground) are functions of Vw , corrections were applied to the data. Having estimated longitudinal and lateral turbulence intensities experienced by the trucks and knowing if, it is now possible to utilise tunnel data to see how it compares with data obtained on the road. The data obtained on the road were sorted into yaw angle categories as shown in Table 1 and for each corresponding discrete value of yaw angle in the tunnel, linear least-squares curve fits were found for AC D as a function of I u for the two deflectors. Any given road combination of ~b and I~ could then be selected from tunnel data and the corresponding tunnel AC D value found. This then enabled the tunnel data to be used in estimating the drag reduction on the road for each road data point. The results are shown in Figures 11 and 12.
417
Table 1. Yaw Angle Categories. Tunnel yaw angle (degrees)
Road yaw angle (degrees) 0 1.5 4.5 7.5 10.5
O.'*
----~ 1.5 ~ 4.5 ~ 7.5 ~ 10.5 *
"0
3 6 9 12
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ROAD(VARIOUSTURBULENCE" ~
PREDICTED ROAD U S I N G T U N N E L DATA
I N T E N S t T I E S A N D SCALES) '
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Figure 11. Road Results Predicted from Tunnel Data - Deflector A.
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Figure 12. Road Results Predicted from Tunnel Data - Deflector R.
Wind-tunnel data obtained under the lowest turbulence intensity (I u = 1.3%) is shown as a dotted line and parabolic least-squares regression lines are shown for the measured onroad data and data from the predictive method above. Agreement between predicted and on-road data is now better than that for data obtained at any single turbulence level at the higher yaw angles, but the agreement at low yaw angles is not so good. 6. DISCUSSION AND CONSIDERATION OF REYNOLDS NUMBER EFFECTS
This method does not consider the effects of lateral turbulence intensity nor can it include the effects of longitudinal or lateral scale, or Reynolds number. It is useful in showing that the effects of longitudinal intensity, with other modelling parameters held constant, can considerably alter the aerodynamic characteristics of drag-reducing devices. It also indicates that the long-term road drag savings could differ considerably from predictions in characteristically smooth wind-tunnel flow. It is considered that the high lateral intensities encountered on the road during strong crosswinds due to wakes of roadside obstructions will be dominant in reducing the performance of deflector-type aerodynamic devices. The drag reduction they offer arises from a stable wake which does not appear to exist under high, turbulent, crosswinds.
418 The levels of turbulence measured on the road were in general considerably higher than those that could be easily generated in the tunnel hence the predictions from tunnel to road data were usually extrapolated. Further work is needed to generate higher intensities (typically up to 10% for longitudinal and lateral) and to improve modelling of the scales of turbulence (which were too short in the tunnel) consistent with a relatively fiat velocity profile and thin ground boundary layer. Although the blockage ratios were low when the relatively long vehicles were yawed, the drag coefficient would be influenced due to the rear of the vehicle approaching the wall which would affect the base pressure. However, the effect on the incremental (forebody) drag due to the fitting of an aerodynamic device should be negligible. To avoid differences in flow pattern between full and model scale (mainly low-Reynolds number flow separations that occur only on the models), it is frequently stated that the Reynolds number be above some critical value. For a complex body, requiring many dimensions to define its geometry, the Reynolds number should be based not on wheelbase, vehicle width or vehicle length (all of which have been suggested by other workers, e.g. the SAE Wind Tunnel Procedure for Trucks and Buses, [10]) but on some local length or radius. The critical value depends upon a variety of factors; these include local pressure gradients, surface roughness and free stream turbulence levels. One solution suitable for engineering uses may be basing the parameter on the smallest local dimension that has a significant influence on the flow, and to take into account local pressure gradients. For the case of zero yaw, the most significant dimension on which to base the parameter for commercial vehicles would seem to be the radii found on the cab top leading edge, see Cooper [11 ]. Flow separations in this area can have a considerable effect on the total vehicle drag, however under highly yawed conditions lee-side flows play a role in determining drag and hence vertical edge radii become important. To investigate the flow in the cab top area at zero yaw, flow visualisation studies were conducted at the top of the cab roof whilst varying the Reynolds number. The first series of tests was made with no box van behind the cab. The flow became attached to the roof at a local Reynolds number (based on the smallest local radii) of about 0.44 x 104. When the local pressure gradient was modified by placing the box-van trailer behind the cab, the flow was separated from the roof, even at the highest Reynolds number attainable. Investigation of the full-size roof flow on the same vehicle with a box-van trailer (by filming tug movements from a chase vehicle), travelling at 60 km/h (as opposed to 100 km/h for the road tests), showed a roof flow that was attached for about 70% of the time. For the remainder of the time the flow was separated. It appeared that the flow was influenced by small changes in wind conditions and required only minor disturbances to separate from the cab roof Thus for this case, the flow appeared strongly influenced by turbulence as well as Reynolds number. At higher speeds, greater levels of flow attachment should occur although this was not tested. It is difficult to see how this could be correctly modelled in a wind tunnel without having the correct Reynolds number and turbulence characteristics, and it is evident that the pressure field of the container considerably influences the cab roof flow.
419 At low Reynolds numbers, laminar separation and the associated wake from the cab front can lead to a lower total vehicle drag coefficient for certain geometries of commercial vehicles than for tests conducted at higher Reynolds numbers. This is due to the relatively large wake from a laminar separation shielding the front container face to a greater extent than would be the case for a smaller (or no) turbulent separation. This is a possible explanation why, at low Reynolds numbers (as tested in the tunnel), the drag reduction from an aerodynamic device was less than at higher Reynolds numbers (as tested on the road). This was generally found to be the case at low yaw angles, see Figures 7 to 10. This was evidently not the case for all the devices tested when higher yaw angles were encountered, thus incorrect Reynolds number similarity did not adequately account for the very large differences in drag coefficient reduction at higher yaw angles. 7. CONCLUSIONS Large differences were found when drag reductions measured in smooth wind-tunnel flow were compared to on-road results, particularly at high yaw angles. Utilising road measurements of the turbulent wind characteristics and drag reduction data obtained in turbulent flows in a wind tunnel gave better agreement with road results. However, turbulence characteristics in the tunnel differed from that experienced by a moving vehicle and also it was observed that the cab roof flow was influenced by Reynolds number effects. Since turbulence can strongly influence the drag savings (and hence fuel consumption), it is recommended that more effort is expended on understanding the turbulent environment of vehicles. 8. ACKNOWLEDGMENTS
The assistance of Arnotts-Brockhoff-Guest for providing vehicles and personnel and the financial assistance of the National Energy Research, Development and Demonstration Program are gratefully acknowledged. 9. REFERENCES
1. W.S. Saunders, Apparatus for Reducing Linear and Lateral Wind Resistance in a Tractor-Trailer Combination Vehicle, U.S. Patent Office, 3,241,876, 1966. 2. A. Roshko and K. Koenig, Interaction Effects on the Drag of BluffBodies in Tandem, in Aerodynamic Drag Mechanisms of Bluff Bodies and Road Vehicles, edited by Sovran G., Morel T and Mason W.T., Plenum Press, 1978. 3. F.T. Buckley, Jr., An Improved Over-the-Road Test Method for Determining the Fuel Savings Benefit of a Truck Aerodynamic-Drag-Reducing Device, SAE 850286, presented at the SAE International Congress and Exposition, Detroit, 1985.
420 4. J.W. Saunders, S. Watkins, P.H Hoffmann and F.T. Buckley, Jr., Comparison of OnRoad and Wind-Tunnel Tests for Tractor-Trailer Aerodynamic Devices, and Fuel Savings Predictions, SAE 850286, presented at the SAE International Congress and Exposition, Detroit, 1985. 5. S. Watkins, J.W. Saunders and PH. Hoffmann, Wind-Tunnel Modelling of Commercial Vehicle Drag-Reducing Devices: Three Case Studies, SAE 870717, presented at the Autotechnologies Conference, Monaco, 1987. 6. S. Watkins, P.H. Hoffmann and J.W. Saunders, "Comparison of On-Road and WindTunnel Tests for Rigid Truck Aerodynamic Devices, 9th Australasian Fluid Mechanics Conference, Auckland 8-12 Dec., 1986. 7. M. Blackmore, The Effect of Ground Simulation on a Slab-Sided Truck, M. Eng. Sc. Thesis, Dept. of Mech. Eng., University of Melbourne, 1984. 8.
S. Watkins, Wind-Tunnel Modelling of Vehicle Aerodynamics: with Emphasis on Turbulent Wind Effects on Commercial Vehicle Drag, Ph.D. Thesis, Mechanical Engineering, Department of Manufacturing and Process Engineering, Victorian University of Technology, RMIT Campus, November 1990.
9. S. Watkins, J.W. Saunders, P.H Hoffmann and J.D. Holmes, Measurements of Turbulence Experienced by Moving Vehicles, Part I Turbulence Intensity, submitted to the Journal of Wind Engineering and Industrial Aerodynamics. 10. SAE J1252 (Anon), SAE Wind-Tunnel Test Procedures for Trucks and Buses, SAE J1252 August 1979, Prepfint Version. 11. KR. Cooper, The Effect of Front-Edge Rounding and Rear-Edge Shaping on the Aerodynamic Drag of Bluff Vehicles in Ground Proximity, SAE 850288. Presented at the International Congress and Exposition, Detroit Feb. 25-March 1, 1985.