SUPPRESSION OF FLUID FORCES ACTING ON TWO SQUARE PRISMS IN A TANDEM ARRANGEMENT BY PASSIVE CONTROL OF FLOW

Abstract

The suppression of fluid forces acting on two square prisms in a tandem arrangement in which a flow approaching the upstream prism was controlled by a thin flat plate was examined, with variation in spacing between the plate and the upstream prism. The width of the plate was one-ninth of the prism width. The position of the control plate was varied from the front surface of the upstream prism to 2·25 times the prism width in the upstream direction, and the position of the downstream prism was varied from the rear surface of the upstream prism to 10 times the prism width in the downstream direction. A dramatic decrease in fluid forces acting on both prisms was observed for a certain range of control plate positions. For such optimum positions of the control plate, the shear layers that separated from the control plate attached near the edges of the front surface of the upstream prism and each shear layer bifurcated into two layers, one part of the shear layers making a quasi-steady recirculating region between the control plate and the upstream prism, and the other part separating from the leading edges and attaching again to the side-surfaces of the upstream prism. When the control plate was placed with spacings S/W=1·50∼1·90 (S: spacing between the control plate and the upstream prism, W: width of the prism), the above-mentioned flow pattern appeared, and the fluid forces and vortex shedding of the upstream prism were almost completely suppressed. Also, the upstream prism was found to be insensitive to the existence of the downstream prism when the latter was located downstream, approximately six or more times the prism width.