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Testing of road vehicles under cross wind conditions
Journal of Wind Engineering and Industrial Aerodynamics, 38 (1991) 59-69
59
Elsevier Science Publishers B.V., Amsterdam - - P r i n t e d in T h e Netherlands
Testing of road vehicles under cross wind conditions C. Kramer, R. Grundmann and H.J. Gerhardt Fachhochschule Aachen and Ingenieurgemeinschaft WSP, Welkenrather Strafie 120, W 5100 Aachen, Germany
Summary A brief description of the parameters influencing the cross wind behaviour of a car is given and the test procedures are explained for open loop testing, where the car runs through the cross wind with fixed wheels, a n d for closed loop testing, where the driver reacts on the cross wind disturbance. A function is derived describing the inter-relationship between the parameters which define the design of cross wind test facilities: the cross wind velocity, cross wind angle, driving speed and yaw angle. T h e evaluation of this function leads to the design of a facility which can realistically simulate the cross wind behaviour of a car. T h e design is illustrated by model tests with a 1 : 10 scale section model.
1. Introduction Modern car design leads to a low drag coefficient but increases the sensitivity of the car under cross wind. Therefore, large effort is spend on the wind tunnel testing of road vehicles at yawing flow conditions. It is common practice to execute six component measurements for yaw angles up to _+30 ° and most wind tunnels are carefully designed and optimized to deliver reliable results for this range. Compared with this extensive wind tunnel testing, the degree of verification of the cross wind aerodynamic data in real car testing is very poor for most cross wind test facilities. The usual cross wind testing method is the so-called open loop method. A car runs through a section with a cross wind with fixed wheels and the deviation of the car from the straight line is measured. The open loop method, however, only allows evaluation of the passive behaviour of the car without any participation of the driver which is not very relevant for real cross wind driving. Another disadvantage of this kind of simulation is that the ratio of cross wind velocity to driving velocity is close to unity at most cross wind test facilities. The resulting yaw angle is 45 ° and not very realistic. This paper describes some basic considerations for the execution of cross wind road tests and the design of a relevant facility.
0167-6105/91/$03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved.
60
2. Parameters influencing the cross wind behaviour of cars
On the road the resulting flow velocity is determined by the driving velocity and the cross wind velocity. Wallentowitz [ 1 ] measured the cross wind power density spectra in road tests as shown in Fig. 1. There is a strong decrease in the power density with increasing frequency and a significant difference between the two curves for different velocities. For a driving velocity of 130 km h-1 and wind velocities between 0 and 15 m s-1 the resulting yaw angle range is 0o-22.6 ° . Critical cross wind situations can occur at bridges, hedges, gaps, etc. or when passing a truck. For these cases the cross wind disturbance has a higher frequency and the cross wind test facility should also be able to simulate these situations. The cross wind action causes a yawing m o m e n t and a lateral force on the vehicle. By these actions the vehicle experiences a heading deviation. This leads to lateral forces at the tyres which counteract the yaw rotation due t o t h e cross wind. The a m o u n t of counter-rotating m o m e n t depends on the design of the wheel suspension and the centre of gravity position. The most important vehicle reaction is this yawing motion. T h e cross wind induced rolling motion is of minor importance. This is also the case for the pitching motion which can 10 e
v : 80
(deg s~) 2 Hz %
t\
G 10 7 "o
, I
.|
106
L__ i
......... co _ v 2
~
-b
10-I
10 0 -
autumn:C%v~{l summer:
F '~,
ILIh
frequency
[Hz]
Hz)
=
1.301
I0
(deg
m~/s2)2;
b =
1.27
~brv~(l HZ)
=
1.028
, I0
(deg
m a / s 2 ) 2;
b =
1.02
Fig. 1. Cross wind power spectra (from Wallentowitz [1 ] ).
61 ROAD TRACK (guidance variable)
WHEEL ANGLE (controller output)
DRIVER
P
(controller)
DYNAMIC DRIVING STATE (control variable)
VEHICLE
]
(controlled system)
I CROSS WIND (disturbance variable )
Fig. 2. Control circuit: driver-vehicle. %2
1,2
b
[2
.... ....
0,8 I
vehicle II vehic{eIII
>
I
I
0,6
0,6
0,44
O,& ~,~,v.'J"
012~ C :~ )
0,2-
, 0,5 op~,n
1,0
1,5
"\:,~
• ~~
2,0 2,5 frequency [Hz]
%.'2--...
0,5
loo D
b)
1,0
1,5
2,0 2,5 frequency [Hz]
closc'd loop
Fig. 3. Amplification factor (yaw velocity e/wind velocity) for three vehicles (from Uffelmann [2] ): (a) open loop; (b) closed loop. 40o v = 100 km/h w = 5 m/s
j
>
u
---4
L
i " ~ 10°
\
2 °
a
~
Z~
L
1° g,5 ~2
I 0o
= 0o
h
~
1 3 5 6 . lime [s] steady wind gust Fig. 4. Comparison of vehicle response for steady wind a n d / b r gust (from Gau£ [3 ] ).
62 -2
z"
.-
7(
gao
~JO
6C
E
~_50 °
~-70 o
/
09
,j
/
k, o
o .~;
/
60 *
/~c ~ j ~ ' - - . . ~ 50
/./"
-
2 ÷4
.
0,7
z.0
/
,0"6
/
vwz%h~ 0,5
30
~ 6 20
i
15
i
20
25
----
30
35
z+O .......
~,5
yaw cmgte I~[%
1 ~ ANF, Ectl.spon,Ninicars 2 2- OQimter-Benz 3 & AOAE ~;
VW
6 ~ Porsche
6 ; BNW Fig. 5. A e r o d y n a m i c q u a n t i t y
G/V2vehicle VS. y a w
angle.
be additionally influenced by changes in the front and rear lift coefficients under yawing flow conditions. The system driver + car corresponds to a control circuit (Fig. 2). T h e driver is the controller, the road the guidance variable, the wheel:angle the controller output and the cross wind the disturbance variable. The aim of cross wind testing must be to evaluate the response function for the vehicle'driver system. The response behaviour of a vehicle can be characterized by the magnification factor of the yaw velocity 4/v. This was investigated by Uffelmann [2] for three vehicles for open loop and closed loop test situations (Fig. 3 ). From these
63 59"
t
c, I o
i 8 AHF,Co[s~Gn t"iniC(;rs 2 ~ Doin
i
/
3 ~ ADAC L,:VW 5 : Porsche
o '~2]
I
v~ / v : 1 , 0 / /
/
/
5 ; BHW
*6
/ 32
] 3O
/
28
~
5
~
Y-s --6
26 ¸
24 ¸ J/
22 ¸
~
6
/ 20--
7
/
~6
~ J
/.8
52
56
60
6&
68
72
76
80
84
88 90
c r o s s wind t i n g l e
a
[%
Fig. 6. Yaw angle vs. cross wind angle. diagrams it is obvious that the residual time of the vehicle in the cross wind should be at least 1 s. Information on the evaluation of the track deviation in the open loop method can be taken from the early work of G a u l [3] (Fig. 4). He compared track deviation, track angle and the angle of deviation of the longitudinal axis of the car for steady cross wind and for wind gust. For the steady cross wind a steady state was obtained after less than 3 s. For the second gust this takes 5 s. Fuhrmann [4 ] proposed as a characterizing value for the cross wind response S A = ~max T4max (is
64
~ I00
',,,/v~eh, =
L~
1,0
CJ
-~ 80
j/
j .....
03
.......................
F. I
70 J
60
J so i i
.....
~1~f
0,5
¢0 t j
3o
':~,5
!
I
J 20 T [ 1 ~.5
50
$4
$8
62
66
70
7~,
75 -
-~
87
86
90
cross wind angle a [o]
q u a n t i t y G/vvehido 2 vs. c r o s s w i n d angle. ~max is the m a x i m u m yaw angle velocity, T~max is the time at which this maximum yaw velocity occurs and els is the yaw velocity after 1 s. This underlines that for closed loop tests, a residual time of at least 1 s for t h e vehicle in the cross wind section is necessary.
Fig. 7. A e r o d y n a m i c s
3. Requirements for a cross w i n d test facility
The parameters influencing the basic design of the cross wind test facility are the cross wind angle a the cross wind velocity vw, the driving velocity Vwhide and the resultant wind velocity v. For the yaw angle range of interest, _+ 20 ° , a linear relationship between the yaw angle and aerodynamic action can be assumed. This is also the case for the yawing moment and the side force. Aerodynamic quantity G (sideforce or yawing m o m e n t )
G = c P v2 A
c = k fl
where fl is the yaw angle. The wind velocity v resulting from cross wind velocity vw and driving speed Vvehic~eis derived from the velocity triangle
65
[l+2Vv°h'C'°c°s v2:v +(Vveh'°'e]lvwin 2
d
\
Vwind / -]
sin tan
~ - - COSOL "~- Vvehicle/Vwind
The aerodynamics quantity G is related to the square of the vehicle velocity
I(Vwind)2Fl+2VvehicosoL.k_(Vvehidt~ 1 ,)
G U2ehical ~
aretan ( \COSOL
sino~
"~-Vvehical/Vwind ,]
\Vvehical/
[_
Vwind
\ /)wind /
The evaluation of this function (Figs. 5-7) allows the selection of the necessary design parameters. A certain amount of the aerodynamic quantity G/vvehicae 2 is necessary in or-
Fig. 8. Model (scale 1: 10) of the three-fan section.
66 %,0 _
k
o z='Bm 8,0 o >
150
j
10 -
->..
0.8 ]
: _.
7---
06t
L.
O'h
--
- - ? -
~ ~
0.2
}0
120 150
-
X[m]
Fig. 9. Calculated velocity profile of a cross wind jet 1.5 m above the road surface for a j e t crosssection of 2.2 m (height) × 46 m (length) and cross wind angle of 55 °. "] ~ifh i
v, Ira/s:
h,}/l
y= variable
]
0'7
o,~
r~,5
04
0,3
0,2
o:
o
z= 110mm
:I
o:
02
o,~
o,~
o5
oA
o#
y[m!
Fig. 10. Measured and calculated horizontal cross wind velocity profiles ( 1 : l0 scale model, threefan section ).
67 der to obtain deviations of the vehicle which are sufficient for the evaluation and measurement procedure. The range of aerodynamic disturbances produced by some existing cross wind test facilities is obvious from the points indicated in the diagram in Fig. 5. All these facilities have a cross wind angle of 90 °. Therefore, the yaw angles are unrealistically high. In order to obtain a similar and obviously sufficiently strong disturbance at a yawing angle not larger than 20 °, the cross wind angle should be about 50 °-60 ° instead of 90°. This is more clearly shown in Fig. 6, where the yaw angle fl is plotted vs. the cross wind angle c~. The aerodynamic quantity 2 G/vvemcle is mainly determined by the velocity ratio Vwind/Vand depends only marginally on the cross wind angle ~ (Fig. 7). The requirement of a residual time of at least 1 s of a car driving at 130 km h - 1 means that the length of the cross wind test facility must be at least 40 m. The height of the cross wind jet for passenger car testing should be about 2 m. A typical spacing of the axial fans used to generate the cross wind is about 3 m because of the diameter required. If the hub wake of these axial fans is not carefully smoothened by appropriate means, the spacing of 3 m with a car velocity of about 30 m s - 1 leads to a disturbance frequency of 10 Hz which is typical for the frequency range of lateral tyre oscillations of passenger cars. z I with diffusor
% 30
×= 700ram y= Om,'~
25
z= vctriable 22 18
I
ih
1 10 i 5 2 3,2
O&
06
0,8
1,0
1,2
l&
1,6
1,8
2,0
2,2
2L
2,5
2,8 ×10 -1
Fig. 11. M e a s u r e d vertical cross w i n d profile ( 1 : 10 scale model, t h r e e - f a n section ).
z[m]
68
tsJ-
I
-i, t--!I ~dU s
16
m
f"
Fig. 12. Velocity profiles published for the Volkswagen cross wind facility (from Walloscheck and Horntrich [6] ).
69
Therefore, velocity inhomogeneities of more than 10% do not seem to be appropriate. 4. D e s i g n p r o p o s a l o f a cross w i n d test f a c i l i t y
A design study and some model tests were carried out with a 1 : 10 scale model of a three fan section (Fig. 8). The velocity profile calculated at a height of z = 1.5 m above the road is plotted for distances up to 15 m from the wind exit (Fig. 9). Even in the relatively large distance between 6 m and 12 m, the cross wind velocity is still sufficient. A large distance from the wind exit is required for safety reasons. In Fig. 10, results of hot wire measurements with the threefan section model are compared with the calculation which was executed according to Regenscheit [5]. Figure 11 shows the vertical velocity profile at a distance corresponding to 7 m at full size. The boundary layer on the road is astonishingly thin. It should be mentioned that the model was not equipped with any screens, flow straighteners, etc. In comparison with the cross wind velocity distribution which was published by Walloschek and Horntrich [6] for the Volkswagen cross wind facility (Fig. 12), progress obtained with the present study is reasonable. The full size facility would have the following specifications: total length
about 46 m
height of cross wind jet at nozzle exit
about 2.2 m
14 axial fans, approximate diameter
2.8 m
hub ratio
0.4
approximate total power input
1.800 k W
References 1 H. Wallentowitz, Fahrer-Fahrzeug-Seitenwind, Dissertation Technische Universit~it Braunschweig, 1979. 2 F. Uffelmann, Fahrverhalten von Personenwagen mit Fahrer bei Seitenwind-Anwendung neuer Berechnungsmethoden bei Fahrzeugentwicklung, VDI-Berichte 537, 1984, VDI-Verlag, D~isseldorf. 3 F. Gaul,, Das Verhalten von Kraftfahrzeugen bei Seitenwind, Automobiltech. Z., 54 (1952) 64-67. 4 K.-H. Fuhrmann, Neue Methoden zur Beurteilung der Seitenwindempfindlichkeit von Fahrzeugen, Volkswagen Transporteragung, Goslar, 1983. 5 B. Regenscheit, Jets in confined spaces, J. Wind Eng. Ind. Aerodyn., 16 (1984) 133-159. 6 P. Walloscheck and H. Horntrich, Seitenwindverhalten und Fahrzeugaerodynamik, Seminar Aerodynamik des Fahrzeuges, Haus der Technik, 1984, Essen.
59
Elsevier Science Publishers B.V., Amsterdam - - P r i n t e d in T h e Netherlands
Testing of road vehicles under cross wind conditions C. Kramer, R. Grundmann and H.J. Gerhardt Fachhochschule Aachen and Ingenieurgemeinschaft WSP, Welkenrather Strafie 120, W 5100 Aachen, Germany
Summary A brief description of the parameters influencing the cross wind behaviour of a car is given and the test procedures are explained for open loop testing, where the car runs through the cross wind with fixed wheels, a n d for closed loop testing, where the driver reacts on the cross wind disturbance. A function is derived describing the inter-relationship between the parameters which define the design of cross wind test facilities: the cross wind velocity, cross wind angle, driving speed and yaw angle. T h e evaluation of this function leads to the design of a facility which can realistically simulate the cross wind behaviour of a car. T h e design is illustrated by model tests with a 1 : 10 scale section model.
1. Introduction Modern car design leads to a low drag coefficient but increases the sensitivity of the car under cross wind. Therefore, large effort is spend on the wind tunnel testing of road vehicles at yawing flow conditions. It is common practice to execute six component measurements for yaw angles up to _+30 ° and most wind tunnels are carefully designed and optimized to deliver reliable results for this range. Compared with this extensive wind tunnel testing, the degree of verification of the cross wind aerodynamic data in real car testing is very poor for most cross wind test facilities. The usual cross wind testing method is the so-called open loop method. A car runs through a section with a cross wind with fixed wheels and the deviation of the car from the straight line is measured. The open loop method, however, only allows evaluation of the passive behaviour of the car without any participation of the driver which is not very relevant for real cross wind driving. Another disadvantage of this kind of simulation is that the ratio of cross wind velocity to driving velocity is close to unity at most cross wind test facilities. The resulting yaw angle is 45 ° and not very realistic. This paper describes some basic considerations for the execution of cross wind road tests and the design of a relevant facility.
0167-6105/91/$03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved.
60
2. Parameters influencing the cross wind behaviour of cars
On the road the resulting flow velocity is determined by the driving velocity and the cross wind velocity. Wallentowitz [ 1 ] measured the cross wind power density spectra in road tests as shown in Fig. 1. There is a strong decrease in the power density with increasing frequency and a significant difference between the two curves for different velocities. For a driving velocity of 130 km h-1 and wind velocities between 0 and 15 m s-1 the resulting yaw angle range is 0o-22.6 ° . Critical cross wind situations can occur at bridges, hedges, gaps, etc. or when passing a truck. For these cases the cross wind disturbance has a higher frequency and the cross wind test facility should also be able to simulate these situations. The cross wind action causes a yawing m o m e n t and a lateral force on the vehicle. By these actions the vehicle experiences a heading deviation. This leads to lateral forces at the tyres which counteract the yaw rotation due t o t h e cross wind. The a m o u n t of counter-rotating m o m e n t depends on the design of the wheel suspension and the centre of gravity position. The most important vehicle reaction is this yawing motion. T h e cross wind induced rolling motion is of minor importance. This is also the case for the pitching motion which can 10 e
v : 80
(deg s~) 2 Hz %
t\
G 10 7 "o
, I
.|
106
L__ i
......... co _ v 2
~
-b
10-I
10 0 -
autumn:C%v~{l summer:
F '~,
ILIh
frequency
[Hz]
Hz)
=
1.301
I0
(deg
m~/s2)2;
b =
1.27
~brv~(l HZ)
=
1.028
, I0
(deg
m a / s 2 ) 2;
b =
1.02
Fig. 1. Cross wind power spectra (from Wallentowitz [1 ] ).
61 ROAD TRACK (guidance variable)
WHEEL ANGLE (controller output)
DRIVER
P
(controller)
DYNAMIC DRIVING STATE (control variable)
VEHICLE
]
(controlled system)
I CROSS WIND (disturbance variable )
Fig. 2. Control circuit: driver-vehicle. %2
1,2
b
[2
.... ....
0,8 I
vehicle II vehic{eIII
>
I
I
0,6
0,6
0,44
O,& ~,~,v.'J"
012~ C :~ )
0,2-
, 0,5 op~,n
1,0
1,5
"\:,~
• ~~
2,0 2,5 frequency [Hz]
%.'2--...
0,5
loo D
b)
1,0
1,5
2,0 2,5 frequency [Hz]
closc'd loop
Fig. 3. Amplification factor (yaw velocity e/wind velocity) for three vehicles (from Uffelmann [2] ): (a) open loop; (b) closed loop. 40o v = 100 km/h w = 5 m/s
j
>
u
---4
L
i " ~ 10°
\
2 °
a
~
Z~
L
1° g,5 ~2
I 0o
= 0o
h
~
1 3 5 6 . lime [s] steady wind gust Fig. 4. Comparison of vehicle response for steady wind a n d / b r gust (from Gau£ [3 ] ).
62 -2
z"
.-
7(
gao
~JO
6C
E
~_50 °
~-70 o
/
09
,j
/
k, o
o .~;
/
60 *
/~c ~ j ~ ' - - . . ~ 50
/./"
-
2 ÷4
.
0,7
z.0
/
,0"6
/
vwz%h~ 0,5
30
~ 6 20
i
15
i
20
25
----
30
35
z+O .......
~,5
yaw cmgte I~[%
1 ~ ANF, Ectl.spon,Ninicars 2 2- OQimter-Benz 3 & AOAE ~;
VW
6 ~ Porsche
6 ; BNW Fig. 5. A e r o d y n a m i c q u a n t i t y
G/V2vehicle VS. y a w
angle.
be additionally influenced by changes in the front and rear lift coefficients under yawing flow conditions. The system driver + car corresponds to a control circuit (Fig. 2). T h e driver is the controller, the road the guidance variable, the wheel:angle the controller output and the cross wind the disturbance variable. The aim of cross wind testing must be to evaluate the response function for the vehicle'driver system. The response behaviour of a vehicle can be characterized by the magnification factor of the yaw velocity 4/v. This was investigated by Uffelmann [2] for three vehicles for open loop and closed loop test situations (Fig. 3 ). From these
63 59"
t
c, I o
i 8 AHF,Co[s~Gn t"iniC(;rs 2 ~ Doin
i
/
3 ~ ADAC L,:VW 5 : Porsche
o '~2]
I
v~ / v : 1 , 0 / /
/
/
5 ; BHW
*6
/ 32
] 3O
/
28
~
5
~
Y-s --6
26 ¸
24 ¸ J/
22 ¸
~
6
/ 20--
7
/
~6
~ J
/.8
52
56
60
6&
68
72
76
80
84
88 90
c r o s s wind t i n g l e
a
[%
Fig. 6. Yaw angle vs. cross wind angle. diagrams it is obvious that the residual time of the vehicle in the cross wind should be at least 1 s. Information on the evaluation of the track deviation in the open loop method can be taken from the early work of G a u l [3] (Fig. 4). He compared track deviation, track angle and the angle of deviation of the longitudinal axis of the car for steady cross wind and for wind gust. For the steady cross wind a steady state was obtained after less than 3 s. For the second gust this takes 5 s. Fuhrmann [4 ] proposed as a characterizing value for the cross wind response S A = ~max T4max (is
64
~ I00
',,,/v~eh, =
L~
1,0
CJ
-~ 80
j/
j .....
03
.......................
F. I
70 J
60
J so i i
.....
~1~f
0,5
¢0 t j
3o
':~,5
!
I
J 20 T [ 1 ~.5
50
$4
$8
62
66
70
7~,
75 -
-~
87
86
90
cross wind angle a [o]
q u a n t i t y G/vvehido 2 vs. c r o s s w i n d angle. ~max is the m a x i m u m yaw angle velocity, T~max is the time at which this maximum yaw velocity occurs and els is the yaw velocity after 1 s. This underlines that for closed loop tests, a residual time of at least 1 s for t h e vehicle in the cross wind section is necessary.
Fig. 7. A e r o d y n a m i c s
3. Requirements for a cross w i n d test facility
The parameters influencing the basic design of the cross wind test facility are the cross wind angle a the cross wind velocity vw, the driving velocity Vwhide and the resultant wind velocity v. For the yaw angle range of interest, _+ 20 ° , a linear relationship between the yaw angle and aerodynamic action can be assumed. This is also the case for the yawing moment and the side force. Aerodynamic quantity G (sideforce or yawing m o m e n t )
G = c P v2 A
c = k fl
where fl is the yaw angle. The wind velocity v resulting from cross wind velocity vw and driving speed Vvehic~eis derived from the velocity triangle
65
[l+2Vv°h'C'°c°s v2:v +(Vveh'°'e]lvwin 2
d
\
Vwind / -]
sin tan
~ - - COSOL "~- Vvehicle/Vwind
The aerodynamics quantity G is related to the square of the vehicle velocity
I(Vwind)2Fl+2VvehicosoL.k_(Vvehidt~ 1 ,)
G U2ehical ~
aretan ( \COSOL
sino~
"~-Vvehical/Vwind ,]
\Vvehical/
[_
Vwind
\ /)wind /
The evaluation of this function (Figs. 5-7) allows the selection of the necessary design parameters. A certain amount of the aerodynamic quantity G/vvehicae 2 is necessary in or-
Fig. 8. Model (scale 1: 10) of the three-fan section.
66 %,0 _
k
o z='Bm 8,0 o >
150
j
10 -
->..
0.8 ]
: _.
7---
06t
L.
O'h
--
- - ? -
~ ~
0.2
}0
120 150
-
X[m]
Fig. 9. Calculated velocity profile of a cross wind jet 1.5 m above the road surface for a j e t crosssection of 2.2 m (height) × 46 m (length) and cross wind angle of 55 °. "] ~ifh i
v, Ira/s:
h,}/l
y= variable
]
0'7
o,~
r~,5
04
0,3
0,2
o:
o
z= 110mm
:I
o:
02
o,~
o,~
o5
oA
o#
y[m!
Fig. 10. Measured and calculated horizontal cross wind velocity profiles ( 1 : l0 scale model, threefan section ).
67 der to obtain deviations of the vehicle which are sufficient for the evaluation and measurement procedure. The range of aerodynamic disturbances produced by some existing cross wind test facilities is obvious from the points indicated in the diagram in Fig. 5. All these facilities have a cross wind angle of 90 °. Therefore, the yaw angles are unrealistically high. In order to obtain a similar and obviously sufficiently strong disturbance at a yawing angle not larger than 20 °, the cross wind angle should be about 50 °-60 ° instead of 90°. This is more clearly shown in Fig. 6, where the yaw angle fl is plotted vs. the cross wind angle c~. The aerodynamic quantity 2 G/vvemcle is mainly determined by the velocity ratio Vwind/Vand depends only marginally on the cross wind angle ~ (Fig. 7). The requirement of a residual time of at least 1 s of a car driving at 130 km h - 1 means that the length of the cross wind test facility must be at least 40 m. The height of the cross wind jet for passenger car testing should be about 2 m. A typical spacing of the axial fans used to generate the cross wind is about 3 m because of the diameter required. If the hub wake of these axial fans is not carefully smoothened by appropriate means, the spacing of 3 m with a car velocity of about 30 m s - 1 leads to a disturbance frequency of 10 Hz which is typical for the frequency range of lateral tyre oscillations of passenger cars. z I with diffusor
% 30
×= 700ram y= Om,'~
25
z= vctriable 22 18
I
ih
1 10 i 5 2 3,2
O&
06
0,8
1,0
1,2
l&
1,6
1,8
2,0
2,2
2L
2,5
2,8 ×10 -1
Fig. 11. M e a s u r e d vertical cross w i n d profile ( 1 : 10 scale model, t h r e e - f a n section ).
z[m]
68
tsJ-
I
-i, t--!I ~dU s
16
m
f"
Fig. 12. Velocity profiles published for the Volkswagen cross wind facility (from Walloscheck and Horntrich [6] ).
69
Therefore, velocity inhomogeneities of more than 10% do not seem to be appropriate. 4. D e s i g n p r o p o s a l o f a cross w i n d test f a c i l i t y
A design study and some model tests were carried out with a 1 : 10 scale model of a three fan section (Fig. 8). The velocity profile calculated at a height of z = 1.5 m above the road is plotted for distances up to 15 m from the wind exit (Fig. 9). Even in the relatively large distance between 6 m and 12 m, the cross wind velocity is still sufficient. A large distance from the wind exit is required for safety reasons. In Fig. 10, results of hot wire measurements with the threefan section model are compared with the calculation which was executed according to Regenscheit [5]. Figure 11 shows the vertical velocity profile at a distance corresponding to 7 m at full size. The boundary layer on the road is astonishingly thin. It should be mentioned that the model was not equipped with any screens, flow straighteners, etc. In comparison with the cross wind velocity distribution which was published by Walloschek and Horntrich [6] for the Volkswagen cross wind facility (Fig. 12), progress obtained with the present study is reasonable. The full size facility would have the following specifications: total length
about 46 m
height of cross wind jet at nozzle exit
about 2.2 m
14 axial fans, approximate diameter
2.8 m
hub ratio
0.4
approximate total power input
1.800 k W
References 1 H. Wallentowitz, Fahrer-Fahrzeug-Seitenwind, Dissertation Technische Universit~it Braunschweig, 1979. 2 F. Uffelmann, Fahrverhalten von Personenwagen mit Fahrer bei Seitenwind-Anwendung neuer Berechnungsmethoden bei Fahrzeugentwicklung, VDI-Berichte 537, 1984, VDI-Verlag, D~isseldorf. 3 F. Gaul,, Das Verhalten von Kraftfahrzeugen bei Seitenwind, Automobiltech. Z., 54 (1952) 64-67. 4 K.-H. Fuhrmann, Neue Methoden zur Beurteilung der Seitenwindempfindlichkeit von Fahrzeugen, Volkswagen Transporteragung, Goslar, 1983. 5 B. Regenscheit, Jets in confined spaces, J. Wind Eng. Ind. Aerodyn., 16 (1984) 133-159. 6 P. Walloscheck and H. Horntrich, Seitenwindverhalten und Fahrzeugaerodynamik, Seminar Aerodynamik des Fahrzeuges, Haus der Technik, 1984, Essen.