Skin friction behind isolated hemispheres and the formation of obstacle marks

We have measured the mean magnitude and direction, and the rms fluctuation intensity, of the skin friction behind plain hemispherical obstacles and behind hemispheres with tapered artificial tails one and four obstacle heights long. Downstream of reattachment, the mean skin-friction magnitude is about 20% greater than its free-stream value along the centre-line and comparably reduced to either side of it. The horizontal divergence of the skin-friction vector field is positive (divergent) along the centre-line and negative (convergent) to either side of it. Neither of these conditions favours development of a ridge longer than the separation length along the centre line. The development of ridges or tails many obstacle heights long, commonly observed in nature, requires considerable modification of the simple sediment-free wakes we have studied.