Forces on vehicles in cross winds from moving model tests

Forces on vehicles in cross winds from moving model tests

Journal of Wind Engineering and Industrial Aerodynamics, 41-44 (1992) 2673 2684 2673 Elsevier F o r c e s on v e h i c l e s in cross w i n d s f r...

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Journal of Wind Engineering and Industrial Aerodynamics, 41-44 (1992) 2673 2684

2673

Elsevier

F o r c e s on v e h i c l e s in cross w i n d s f r o m m o v i n g m o d e l tests N D H u m p h r e y s a and C J B a k e r a a D e p a r t m e n t of Civil Engineering, N o t t i n g h a m University, U n i v e r s i t y Park, N o t t i n g h a m , NG7 2RD, United Kingdom Abstract T h i s p a p e r p r e s e n t s the r e s u l t s of a n i n v e s t i g a t i o n to m e a s u r e t h e a e r o d y n a m i c forces a n d m o m e n t s on v e h i c l e s in cross w i n d s u s i n g a n e x p e r i m e n t a l rig t h a t propels vehicle models across a n e n v i r o n m e n t a l wind t u n n e l in which a n a t m o s p h e r i c b o u n d a r y l a y e r h a s b e e n s i m u l a t e d . T h e r a t i o n a l e for these tests is p r e s e n t e d a n d the e x p e r i m e n t a l a p p a r a t u s is briefly described. T h e n e c e s s a r i l y r a t h e r complex m e t h o d s of d a t a a n a l y s i s a r e discussed a n d some r e s u l t s p r e s e n t e d for the a e r o d y n a m i c forces on a model lorry. T h e s e r e s u l t s highlight the i m p o r t a n c e of modelling b o t h a t m o s p h e r i c t u r b u l e n c e a n d vehicle motion w h e n a t t e m p t i n g to m e a s u r e cross wind forces on g r o u n d vehicles. 1. I N T R O D U C T I O N In a recent review of the a e r o d y n a m i c s of g r o u n d vehicles in cross winds [1],[2],[3] it w a s shown t h a t to a c c u r a t e l y obtain the a e r o d y n a m i c forces a n d m o m e n t s on these vehicles, which are n e c e s s a r y to predict accident risk, g r e a t c a r e m u s t be t a k e n w i t h the wind t u n n e l m o d e l l i n g . In p a r t i c u l a r the a t m o s p h e r i c turbulence needs to be modelled as accurately as possible with the correct relative velocities b e t w e e n the vehicle a n d the g r o u n d a n d the vehicle a n d the air. This can only be properly done by e x p e r i m e n t s t h a t m e a s u r e the forces on vehicle models t h a t are propelled across a wind t u n n e l in which an a t m o s p h e r i c b o u n d a r y l a y e r h a s b e e n s i m u l a t e d . A series of such t e s t s is r e p o r t e d in [4] for a high speed p a s s e n g e r train. T h e r e were h o w e v e r some p r o b l e m s associated with these tests. The tests were carried out at 1/50th scale, w h i c h w a s a c o m p r o m i s e b e t w e e n the l a r g e scales n e e d e d for R e y n o l d s n u m b e r s effects to be insignificant and the small scales needed for a n a d e q u a t e r e p r e s e n t a t i o n of the a t m o s p h e r i c b o u n d a r y layer. In fact n e i t h e r of these were a d e q u a t e l y a c c o u n t e d for; the r o u n d e d s h a p e of the vehicle t e s t e d left some d o u b t a b o u t Reynolds n u m b e r i n d e p e n d e n c e of the r e s u l t s a n d t h e r e w a s a severe m i s m a t c h b e t w e e n the s i m u l a t e d a t m o s p h e r i c b o u n d a r y l a y e r l e n g t h scales a n d t h e model scale. Also it w a s not clear if t h e c a l c u l a t e d force coefficients from the e x p e r i m e n t a l d a t a should be r e g a r d e d as m e a n values or e x t r e m e values. 0167-61(/5/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

2674 In this paper these points are addressed and a f u r t h e r series of moving model tests is reported for a sharp edged lorry model (which earlier work has shown is independent of Reynolds number effects at the wind tunnel speeds of interest [5],[6]) and in a much better simulation of the atmospheric boundary layer. In the n e xt section the experimental a p p a r a t u s is briefly described: Section 3 t h e n goes on to consider the r a t h e r complex analysis of the e x p e r i m e n t a l data. Section 4 p r e s e n t s some typical force and m o m e n t coefficient results for tests on a 1/50th scale lorry model, and discusses the different effects of turbulence simulation, and model motion on the mean and extreme values of these coefficients. Finally some conclusions are drawn in section 5 and suggestions made for further work. 2. EXPERIMENTAL APPARATUS The experimental rig that was used for these tests is similar to t hat used in [4]. A schematic plan of the wind tunnel and the moving model rig is shown in figure la. The vehicle model is attached to a trolley t h a t moves on a high quality guided track beneath the floor of the wind tunnel and perpendicular to the flow direction. This whole assembly can be catapulted across the wind t u n n el at speeds of up to 20m/s using an automatic retiring system. It is decelerated by using a similar catapult mechanism at the exit from the wind tunnel. The model used for the experiments described in this paper was a 1/50th scale articulated lorry model (described in detail in [5]). A photograph of the model in the wind tunnel is shown in figure lb. Forces and moments were measured by using a small 5 component internal strain gauge balance (which m e a s u r e d side and lift forces and rolling, pitching and yawing moments). This balance has a flat frequency response up to at least 100 Hz, well above the highest frequency of interest [5]. The outputs from this balance were recorded on a data logger that travelled with the trolley. The data was downloaded to a microcomputer every ten runs for analysis. For any particular flow or vehicle configuration 50 runs were carried out, to enable reliable statistical results to be obtained. Different yaw angles, ~/, (the flow direction relative to the vehicle direction of travel) were obtained by changing the relative magnitudes of wind tunnel and model speeds. Because of the high quality and high mass of the track, mechanical vibrations are insxgnificant below 30 to 50Hz, which allows aerodynamic force and moment spectra to be calculated from the force and m o men t data. Within the environmental wind tunnel an atmospheric boundary layer was simulated using b a r r i e r and roughness methods. The m e a s u r e d flow characteristics are shown in figure 2, where all the parameters are presented as the measured, model scale, values. The vertical velocity and intensity profiles at the tunnel centre are shown in figures 2a and 2b along with the curves taken from [7] for a 1/50th scale atmospheric boundary layer with a full scale r o u g h n e s s length, zo, of 0.03m. The a g r e e m e n t can be seen to be reasonable. Figure 2c shows a longitudinal velocity spectrum measured at a full scale equivalent height of 3m (6cm model scale). Again the expected curve for a roughness length of 0.03m is shown. It can be seen that the agreement is not particularly good - this spectrum is in fact scaled at about a 1/t00th scale,

2675 so there is a scale mismatch of about 2 to 1. However this is considerably better t h an has been achieved previously for such tests [4]. Finally figures 2d and 2e show the transverse profiles of mean velocity and turbulence intensity. It can be seen that these profiles are reasonably flat as is desirable, at least within the central 1.5 m of the wind tunnel. Experiments were also carried out with the model static and mounted on a t u rn tab le in the wind tunnel just upstream of the moving model track, and different yaw angles produced by simply rotating the model relative to the flow, in a man n er similar to [5] STAIRCASE )VINC,-~MODELRIG i

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3. THE ANALYSIS OF EXPERIMENTAL RESULTS Firstly let us consider how the results were analysed for the moving model tests. In what follows only side force (S) and lift force (L) m easurem ent s will be presented, although data for all the other channels was analysed in exactly the same way. However to begin with figure 3 shows a time series for all the force and moment channels from a typical run of the moving model rig. It can be seen t h a t the data appears to be very noisy, with much mechanical high frequency contamination of the signal. % PITCH i YAW ROLL

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and fi is the equivalent 3 second extreme value obtained from conventional Gumbel analysis of the wind tunnel velocity. It was originally hoped to calculate extreme values using the extreme value analysis set out in [2], which would have involved the determination of force power spectra. It will be seen however th at the wide spread of the results at some yaw angles makes this somewhat difficult to do on a routine basis (see section 4), and thus only results from the (admittedly inadequate) method set out above will be used. For the static model tests the analysis is substantially easier. Mean coefficients are simply found from the measured mean values of forces and velocities using equation 1 The extreme coefficients are calculated from the extreme values of forces and velocities obtained from conventional Gumbel data analysis of the wind tunnel data (in the same way as reported in [5]), and equations 3 and 4 with v=0.

4. EXPERIMENTAL RESULTS AND DISCUSSION Figures 4 to 7 show the following tests results for the 1/50th scale articulated lorry. a) Figures 4 a and b - mean of side and lift force coefficients for static tests, and a comparison with the results of [5] which were obtained from static model tests in a low turbulence flow and a grid turbulence flow with a length scale of 0.15m (7.5m full scale) and an intensity of 10.5%.

2680 b) Figures 5a and 5b - extreme side and lift force results for static tests (plotted as the ratio of the extreme to the mean coefficients) and a comparison with the results of [5]. c) Figures 6a and 6b - mean and standard deviation of side and lift force coefficients for moving model tests and a comparison with the results of (a) above~ d) Figures 7a and 7b - extreme side and lift force coefficients for moving model tests, plotted as in (b), and a comparison with the results of (b) above.

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2682 From figure 4a it can be seen that there is a definite difference between the static side force coefficient results of the present investigation and those of reference 5, which were obtained with the same model in low turbulence and grid t u r b u l e n c e flows, the r e s u l t s of the p r e s e n t i n v e s t i g a t i o n being consistently lower. Now in view of the fact t hat the results of [5] for the side force coefficient seem to be relatively insensitive to the presence of turbulence within the flow, then it would seem t hat the difference between these two sets of results is caused by the effect of shear in the present investigation. Indeed, because of this shear, it is difficult to know what to take as the reference velocity for forming the coefficients. The equivalent 3m high value has been t a k e n for the results shown here, a height which is close to the top of the vehicle. If the 1 or 2m velocity had been chosen the discrepancy between the two sets of results would have been substantially reduced. Figure 4b shows a substantial variation in mean lift force coefficients also. However the results of [5] have shown t h a t this param et er is very sensitive to p a r ameter s such as vehicle camber, and also to turbulence intensity, so these differences are not unexpected. Some of the difference may be due to the effect of shear just described, but it is not possible to quantify this. The ratios of the extreme to the mean force coefficients for the static tests are shown in figures 5a and 5b, and again compared with results of [5] as far as possible. Except at the lowest yaw angle range the side force coefficient ratio is around 1.0, which is what would be expected from the work of [2] if force fluctuations were caused chiefly by turbulence buffeting on the vehicle, and there is good agreement between these results and the results of [5]. The high value at a yaw angle of 15 degrees will be discussed further below. The lift force coefficient ratio tends to be in excess of 1.0, which implies t h a t lift force fluctuations are affected by factors other than turbulence buffeting, such as for example, vortex shedding. The agreement between the present results, and those of [5] is reasonable in view of the results shown in figure 4b Figure 6a shows a comparison between the static results for side force coefficient and the results of the moving model test. It can be seen that, the two sets of results are in agreement at all yaw angles The standard deviation of the moving model results are also shown on this figure. The general consistency of the values of standard deviation across the yaw angle range i~ perhaps an indication of the viability of the results from the moving mode] rig even at high model speeds where the mechanical noise could be expected to give the lowest signal to noise ratios. The mechanical characteristics of the rig are currently being investigated further. Figure 6b shows similar results for the lift force coefficients. Again substantial differences can be seen. For these tests the two sets of results do not compare so well at high yaw angles, where they should be identical, which d e m o n s t r a t e s the already mentioned sensitivity of the lift force coefficient results. Also the moving model coefficients have extremely high st andard deviations at the lower yaw angles. Figures 7a and b show the ratios of extreme to mean coefficients for side and lift force coefficients for the moving model tests, and compares them with the static values. The ratio for side force coefficient for the moving model tests is r a t h e r higher t h a n for the static tests and somewhat g r e a t e r t han 1.0 suggesting th at some vehicle induced unsteady flows are present. For the lift force coefficient ratio the two sets of results are similar at high yaw angles, but

2683 the moving model results are much higher at low yaw angles, suggesting substantial unsteady vehicle induced flows. Perhaps the main point that calls for comment from these results are the large discrepancies at low yaw angles. Examination of the individual results for moving model tests at these yaw angles shows an e x t r e m e l y broad distribution of results, with a suggestion of bipolar probability distributions. These distributions become much less broad, and much more "gaussian" at the higher yaw angles. This effect is a p p a r e n t in the very high st andard deviation values indicated in figure 6. It thus seems t hat at these yaw angles there might be a switching of flow pattern between two distinct regimes, which would result in the observed behaviour. This effect is, at the time of writing, still being investigated. 5. CONCLUSIONS From the results t h a t have been presented in the previous section the following conclusions can be drawn. a) The moving model technique for measuring vehicle forces and moments has been demonstrated to be viable. b) The effect of the simulation of atmospheric shear seems to be important. This implies that the position at which a reference velocity is defined needs to be carefully considered. c) The effect of model motion is very substantial, particularly, for the vehicle geometry tested, at the lower yaw angle range. d) There is a suggestion from the results that at the lower yaw angles there is a possible switching between two types of flow pattern, that leads to a very wide spread of experimental results. Clearly there is much scope for further investigation. The low yaw angle effect needs to be further investigated to determine the physical processes involved, and ways of analysing the data to enable better estimates of the e x tr eme force coefficients need to be derived. T h e r e a f t e r a subst ant i al experimental program needs to be carried out to investigate the effect of vehicle geometrical change, the use of wind fences, different atmospheric simulations etc. ACKNOWLEDGEMENTS During the course of the project described in this paper the first author was supported on an SERC grant GR/F 15388. The moving model rig described in the paper is owned by British Rail Research, and the help of the staff of that organisation is gratefully acknowledged

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REFERENCES 1. Baker C J "Ground vehicles in high winds - Part 1 Steady aerodynamic forces" Journal of Fluids and Structures 5, (1991), 69-90 2. Baker C J "Ground vehicles in high winds - Part 2 Unsteady aerodynamic forces" Journal of Fluids and Structures 5, (1991), 91-111 3. Baker C J "Ground vehicles in high winds - Part 3 The interaction of aerodynamic forces and the vehicle system" Journal of Fluids and Structures 5, (1991), 221-241 4.Baker C J "Train aerodynamic forces from moving model experiments" Journal of Wind Engineering and Industrial Aerodynamics 24,3, (1986) , 227252 5. Coleman S A "The aerodynamics of ground vehicles in cross winds", PhD Thesis, Nottingham University, (1990) 6. Coleman S A, Baker C J "High sided road vehicles in cross winds" Journal of wind engineering and industrial aerodynamics 36, (1990), 1383-1397 7. Cook N J "The designers guide to wind loading of building structures. Part 1", 1985