INDIRECT VELOCITY MEASUREMENT IN STREAMS

Abstract

Streamgauging. normally requires the measurement of depth by sounders and determination of velocity using current meters. When the stream is swift, there are difficulties in making measurements with current meters. Further, accurate measurements of the depth as well as velocity are not possible in flows with pronounced unsteadiness, because of change in discharge that would take place during the time required for streaingauging a cross section. The dimensionless pressure distribution around circular cylinders is practically independent of the velocity over a wide range of Reynold's number. Obviously, therefore, measurement of pressure at any key points on the surface of such cylinders would enable determination of velocity if the pressure coefficient is known. Bridges with circular piers thus offer the possibility of an indirect method of determination of velocity and discharge, provided the pressures at certain key points can be measured by suitably instrumenting the pier. The pressure distribution around circular cylinders placed in an open channel have been measured in the laboratory, and the pressure coefficient found at a number of locations over the height of the cylinder. Similar data collected by Rana [10]. Pillai [9] and Gupta [3] are also used in the analysis to establish a relationship between the local pressure coefficient, blockage ratio, the ratio of cylinder diameter to the flow depth, and the location of the pressure tap relative to the flow depth. The graphical relationship for such a pressure coefficient at the 0° location with respect to the flow direction is found suitable for velocity computation. This relationship can be used alongwith the pressures measured at an actual bridge pier to compute the approach velocity.