Supercritical Flow in Chute Contraction

Abstract

Contractions with supercritical flow generate shockwaves requiring significantly larger wall heights than predicted by one-dimensional flow equations. Three distinct waves in a straight-walled contraction may be identified: wave 1 downstream from the contraction point, wave 2 in the chute axis, and wall wave 3 beyond the contraction end. The characteristics of these three waves were experimentally determined, based on the concept of the shock number. The effects of contraction ratio and bottom slope were investigated. Means to reduce shock waves in a contraction were also studied. A shock diffractor was introduced as a simple element that reduces shocks significantly. A design procedure is presented, and the optimum diffractor geometry with its optimum location is specified. Corresponding wave heights without and with diffractors were studied; a typical reduction of wave height with diffractors was 20–40. A discussion of off-design flow and choking flow is also included. Based on the present results, a complete hydraulic design of straight-walled chute contractions is possible.