FLOW OVER TRIANGULAR ROUGHNESSES IN OPEN CHANNELS

Abstract

This thesis presents the results of an experimental investigation concerning the flow over two-dimensional, sharp edged, triangular roughness elements placed in series on channel floor. The studies were carried out in a tilting flume with water and also in an open circuit wind tunnel. Experiments were carried out,to obtain the following information concerning the problem: (a) Variation of local and average skin shear coefficients with changes in the geometry of roughness element and flow conditions; (b) Variation of form drag coefficient of the element with the changes ir. the geometry of the roughness element and flow conditions; and (c) Velocity distribution at different locations along the length of the element. From these experimental results it was seen that the measured average skin shear decreases with the steepness of the elements. The measured average skin shear has been compared with the grain resistance computed from standard methods e. g. Manning-Strickler equation and Moody's diagram. The average skin shear is as low Ill as 0,25 times that obtained from Manning -Strickler equation for *L Qauai L . __1 *• g « Relationships have been obtained between the average skin shear coefficient, Cfl, Reynolds number, R , relative smoothness of the element Rb and the height-length ratio Jl. ks j^ for both smooth and sand coated elements and a method of computing the average skin shear on an undulated bed has been suggested. Foum drag coefficient, CD, is seen to be highest for-h— L equal to ~y^- in the range of •— values studied. Equations reliat*i•ng C^ , J=gL_ and JhJ" have been obtained which can be used for the estimation of form drag of undulated beds. Also the applicability of some of the relations derived for these artificial roughnesses to alluvial channels has been studied. A logarithmic velocity distribution law with a corrective velocity term in the lower region of flow is found to be suitable for these roughnesses. Making use of the graphical relations suggested, it is possible to predict velocity distribution at different locations on the element over the entire depth of flow. In addition to the aspects mentioned above, alluvial data of flumes, rivers and canals, available in literature, have been studied to determine the shear responsible for bed and total load transport in alluvial streams. This shear has been compared with the grain shear. It is seen that the shear responsible for total load transport is greater than the grain shear computed from Manning-Strickler equation and that for bed load transport is practically equal to the above grain shear. The conclusions derived on the basis of the analysis concerning the foregoing aspects afford a better insight into the problem of resistance to flow, velocity distribution and sediment transportation in alluvial strearns. iv