Determination of hydraulic characteristics of spillway structures with deflected flow

Abstract

At the present time, in the hydraulic engineering practice wide use is made of spillway structures with flow deflection in different portions of the flow route. They include whirl spillways, antiwhirl dissipators, shaft spillways with elbows in the taikace tunnel, and whirl gates.The integral flow characteristics required for design of spillways having bent portions, such as the discharge, the specific energy in the sections, and the averaged pressure at the limit, can be theoretically determined with sufficient accuracy for engineering analyses [1]. However, although there has been extensive study of helical flows in cylindrical water conduits, no methods are available for determining the flow velocity near the conduit lining required for predicting cavitation. This fact is a consequence of the attempt to approximate the velocity profile on the whole for the section.To solve specific engineering problems, search of the velocities in the entire flow section is not necessary. It is sufficient to know the local kinematic structure of the flow at the conduit walls for calculation of the cavitation numbers in its individual defects, such as projections from the formwork joints, uncut reinforcement, coarse aggregate of the concrete, etc.As a rule, the size of the above-mentioned defects does not exceed 2% with respect to the conduit diameter.Investigations intended to search for the mathematical relation and the velocity profile in the wall zone were performed on a shaft spillway model with deflected flow within the limits of the tailrace horizontal tunnel.The model was made of fiberglass with a wall roughness K e = 0.012 mm, a diameter of the tailrace tunnel with flow deflection d = 187 mm, and a head on the spillway model of about 1500 mm. If in a first approximation it is considered that for the helical flows the handbook data at the turbulent regime limits obtained for axial flows in pipelines are correct, the flow in the models can be classified as pertaining to the field of smooth pipes with respect to the hydraulic resistance.The velocity measurements were carried out by means of a laser Doppler anemometer installation which operated in accordance with the scheme of a backward light dispersion scheme. The system made it possible to perform measurements of the axial component of the flow velocity in the measuring elliptic volume, the linear dimension of which in a direction perpendicular to the flow direction was 0.10-0.15 mm.As kinematic characteristic after direct computer data processing, the time-average values of the axial components of the flow velocity at the measurement point v z and the turbulence intensity vz'/v 100%.The experiments reduced to demonstration of the simplest assumption, namely to the correctness, for the wall zone of the flows with deflection (zone thickness of no more than 0.02d), of the mathematical approximation of the velocity profile recommended in the reference literature for axial flows (smooth pipe region):