The ground effect experienced by transonic/supersonic projectiles with a diameter d flying close to solid surfaces has been the subject of several studies in recent years [1, 2, 3]. A projectile was chosen for these studies to simplify the analysis of the influence the ground has because a projectile at zero angle of attack would not be subjected to any force (or moment) other than drag, hence the occurrence of other forces and moments could directly be attributed to the presence of the ground. All investigations showed that the presence of the ground, indeed, altered the drag force and introduced previously non-existent lift and pitching moment.
One of the main observations in the aforementioned studies was that for high supersonic speeds such as M = 2.4, the ground-induced lift force was always directed away from the ground for all distances h of the projectile at which a ground effect was observed [1]. For transonic Mach numbers (M = 1.1 [2, 3]), however, this force could change sign depending on the ground clearance. The aim of the present study was to examine under which conditions this change will occur, i.e. to find at which Mach number a ground-plane-induced suction force on the projectile can no longer be established. Preliminary tests had indicated that this change would already be seen for mildly supersonic flows, and therefore the current study concentrates on the Mach number range M = 1.1-1.5. The investigation is conducted primarily by means of a numerical analysis and a number of wind-tunnel tests for both qualitative and quantitative validation.
The systematic change of Mach number M and ground clearance h/d reveals how different shock reflection configurations develop as M and h/d are varied. The ground effect is also found to have an appreciable influence on other important flow field features including the bow shock stand-off distance, the Mach stem height in the case of an irregular reflection, wake distortion/deflection, and the pressure distribution along the projectile’s surface. For the investigated projectile geometry it is established that for Mach numbers of 1.5 and higher, a projectile will only experience a lift force when flying in proximity to a solid surface. Experimental force measurements are seen to be in good agreement with the numerically predicted data.
References:
[1] G. Doig, T.J. Barber, E. Leonardi, A.J. Neely, H. Kleine: Aerodynamics of a supersonic projectile in proximity to a solid surface. AIAA J. 48(12):2916-2930 (2010)
[2] H. Kleine, J. Young, B. Oakes, K. Hiraki, H. Kusano, Y. Inatani: Aerodynamic ground effect for transonic projectiles. Proc. 28th Int. Symp. Shock Waves Vol.2, Manchester, UK, Springer, Heidelberg, ISBN 978-3-642-25684-4, pp. 519-524. (2012)
[3] K. Carriage, J. Young, H. Kleine, K. Hiraki: Reynolds-Averaged Navier-Stokes Computation of Transonic Projectiles in Ground Effect. Proc. 18th Australasian Fluid Mechanics Conference, Launceston, Australia, 3-7 Dec. 2012, ISBN 978-0-646-58373-0 (2012)
