THE HORSESHOE VORTEX AND VORTEX SHEDDING AROUND A VERTICAL WALL-MOUNTED CYLINDER EXPOSED TO WAVES

This study concerns the flow around the base of a vertical, wall-mount ed cylinder - a pile - exposed to waves. The study comprises (i) flow visualization of horseshoe-vortex flow in front of and the lee-wake-vo rtex flow behind the pile and (ii) bed shear stress measurements aroun d the pile conducted in a wave flume, plus supplementary bed shear str ess measurements carried out in an oscillatory-flow water tunnel. The Reynolds number range of the flume experiments is Re-D = (2-9) x 10(3) and that of the tunnel experiments is Re-D = 10(3)-5 x 10(4), in whic h Re-D is based on the pile size. Steady-current tests were also carri ed out for reference. The horseshoe-vortex flow (like lee-wake-vortex flow) is governed primarily by the Keulegan-Carpenter number, KC. The range of KC was from 0 to about 25 in the flume experiments, and from 4 to 120 in the tunnel experiments. The experiments were conducted mai nly with circular piles. The results indicate that no horseshoe vortex exists for KC < 6. The size and lifespan of the horseshoe vortex incr ease with KC. The influence of the cross-sectional shape of the pile o n the horseshoe vortex was investigated. The results show that a squar e pile with 90 degrees orientation produces the largest horseshoe vort ex while that with 45 degrees orientation produces the smallest one, t he circular-pile result being between the two. The influence of a supe rimposed current on the horseshoe vortex was also investigated. The ra nge of the current-to-wave-induced-velocity ratio, U-c/U-m, was from 0 to about 0.8. The overall effect of the superimposed current is to in crease the size and lifespan of the horseshoe vortex. This effect incr eases with increasing U-c/U-m. Regarding the near-bed lee-wake flow, t he flow regimes observed for the two-dimensional free-cylinder case ex ist for the present case, too, but with one exception: in the present case, no transverse vortex street was observed in the so-called single -pair regime. The results show that the bed shear stress beneath the h orseshoe vortex and in the lee-wake area is heavily influenced by KC. The amplification of the bed shear stress with respect to its undistur bed value is maximum (O(4)) at the side edges of the pile, in contrast to what occurs in steady currents where the maximum occurs at an angl e of about 45 degrees from the upstream edge of the pile with an ampli fication of O(10).