Planar flow past two or more blades in ground effect

Abstract

Planar flow past a finite number of thin blades travelling in sequence, aligned or nearly aligned over flat ground, is examined analytically and numerically for large Reynolds numbers. The work is motivated by concerns with rotorcraft in ground effect. The representative blade length, wake length and height above ground are taken to be comparable, with each wake (except the final one) intersecting the next blade. Inner-outer interaction covers the whole system as all the wake shapes adjust to maintain pressure equality, forcing viscous-inviscid coupling through lateral offsets within each wake. Results are presented for several configurations, in the frame of the blades with the ground moving at freestream speed. The limits of small relative height and of many blades are then investigated. It is found, for the case of identical blade and wake lengths. that as the height is reduced the ground effect first becomes significant at heights just below one half of the blade length but then grows substantially; and that a virtually streamwise periodic motion ernerges for about three or more blades.