# Some Effects of Free-Stream Turbulence and the Presence of the Ground on the Flow Around Bluff Bodies

## Abstract

Certain aspects of Aeronautical and Building Aerodynamics research are shown to be applicable to the study of the flow around road vehicles. Road vehicles are exposed to the natural wind and the possible effects of free stream turbulence on the mean flow are discussed. A review is given of the effects of turbulence on the drag of bluff bodies with separation from sharp edges, and with separation from continuous surfaces. A road vehicle moves close to a ground plane and the influence that this may have on the flow is examined. Experimental results are presented of the forces acting on an idealised vehicle shape in “ground effect” and they demonstrate the importance of ground clearance. The results also expose differences between wind-tunnel measurements made with a moving wall and a fixed ground plane. Finally a two-dimensional, free-streamline theory is used to predict, qualitatively, the experimental results.

## Keywords

Wind Tunnel Drag Coefficient Drag Reduction Bluff Body Road Vehicle## Notation

- A
pLate frontal area

- C
_{D} drag coefficient

- C
_{L} lift coefficient

- C
_{p} pressure coefficient

- C
_{pb} base-pressure coefficient

- c
body length

- D
depth

- H
height

- h
ground clearance

- L
_{x} longitudinal scale of turbulence

- U
wind speed

- \(\sqrt{{{{\bar{u}}}^{2}}}\)
r.m.s. horizontal wind-speed fluctuations

- V
vehicle speed

- W
body width

- x
streamwise coordinate

- α
angle of incidence

- δ*
boundary-layer displacement thickness

- ∈
ground-clearance/body length;≡ h/c

- k
circulation

## Preview

Unable to display preview. Download preview PDF.

## References

- Bagley, J. A., 1960, Pressure distribution on two-dimensional wings near the ground, R.A.E. Rep. Aero 2625.Google Scholar
- Bearman, P. W., 1971, An investigation of the forces on flat plates normal to a turbulent flow, J. Fluid Mech. Vol. 46, pp. 177 - 198.CrossRefGoogle Scholar
- Bearman, P. W. &Fackrell, J. E, 1975, Calculation of two-dimensional and axisymmetric bluff body potential flow, J. Fluid Mech. Vol. 72, pp. 229 - 241.MATHCrossRefGoogle Scholar
- Bedi, A., 1971,. The effect of aspect ratio on the flow around bodies of rectangular section, MSc Thesis, Imperial College, London.Google Scholar
- Castro, I. P. &Robins, A. G., 1975, The effect of a thick incident boundary layer on the flow around a surface mounted cube, CEGB Marchwood Engineering Laboratories Report RIM/N795.Google Scholar
- Cermak, J. E., 1976, Aerodynamics of Buildings, Annual Review of Fluid Mechanics, VOL 8, pp. 75 - 105.Google Scholar
- Davenport, A. G., 1961, The application of statistical concepts to the wind loading of structures. Proc. Inst. Civ. Eng. London, Vol. 19, pp. 449 - 472.CrossRefGoogle Scholar
- East, L. E, 1970, The measurements of ground effect using a fixed ground board in a wind tunnel, R.A.E. Rep. TR-70123.Google Scholar
- Fackrell, J. K, 1975, The simulation and prediction of ground effect in car aerodynamics, Imperial College, Aeronautics Report 75 - 11.Google Scholar
- Fage, A. &Warsap, J. H., 1929, The effects of turbulence and surface roughness on the drag of a circular cylinder, R. &M. No. 1283Google Scholar
- Gartsh ore, I. S., 1973, The effects of free-stream turbulence on the drag of rectangular two-dimensional prisms, Ihiv. of W. Ontario, Faculty of Eng. Sci. Report BLWT-4-73.Google Scholar
- Harris, R. I., 1971, The nature of the wind, “The Modern Design of Wind-Sensitive Structures,” CIRIA, London, pp. 29 - 55.Google Scholar
- Hess, J. L. &Smith, A. M. 0., 1967, Calculation of potential flow about arbitrary bodies, Prog. Aero. Sci. Vol. 8, pp. 1 - 138.MATHGoogle Scholar
- Kavur, C 1971, An investigation of the effect of turbulence on the flow around cylinders of square cross-section, MSc Thesis, Imperial College, London.Google Scholar
- McLaren, F., Sherrat, A. &Morton, A. S., 1969, Effects of free-stream turbulence on the drag coefficient of bluff sharp-edged cylinders. Nature, Vol. 223, pp. 828-9, and Vol. 224, pp. 908 - 9.Google Scholar
- Nakaguchi, H., Hashimoto, K. &Mato, S., 1968, An experimental study on aerodynamic drag of rectangular cylinder, Journal of the Japan Society of Aeronautical and Space Sciences, Vol. 16, pp. 1 - 5.Google Scholar
- Parkinson, G. V. &Jandali, T., 1970, A wake source model for bluff body potential flow, J. Fluid Mech. Vol. 40, pp. 577 - 594.MATHCrossRefGoogle Scholar
- Stollery, J. L. &Burns, W. K., 1969, Forces on bodies in the presence of the ground, 1st Symposium on Road Vehicle Aerodynamics. City University, London, Paper 1.Google Scholar
- Tuck, E. 0., 1971, Irrotational flow past bodies close to a plane surface. J. Fluid Mech, Vol. 50, pp. 481 - 491.MATHGoogle Scholar
- Vickery, B. J., 1966, Fluctuating lift and drag on a long cylinder of square cross-section in a smooth and in a turbulent stream. J. Fluid Mech, Vol. 25, pp. 481 - 494.CrossRefGoogle Scholar
- Waters, D. M., 1973,. Thickness and camber effects of bodies in ground proximity, Advances in Road Vehicle Aerodynamics, BHRA Fluid Engineering.Google Scholar
- Werle, H., 1963 Simulation de l‘effet du sol au tunnel hydrodynamique, ONERA Report TP63.Google Scholar
- Widnall, S. E. &Barrows, T. M., 1970, An analytic solution for two and three-dimensional wings in ground effect, J. Fluid Mech, Vol. 41, pp. 769 - 792.MATHCrossRefGoogle Scholar