DECAY OF TURBULENCE BEHIND GRIDS IN OPEN CHANNEL

Abstract

Turbulence, which is normaly present in a flow of liquid, has great effects on many practical problems such as dissipation of energy, diffusion of heat, waste-waters and .pollutants, transport of sediments scouring and sedimentation of reservoirs as well as alluvial channels, tractive shear of sediment etc. In some cases turbulence has undesirable effects such as sedimentation in reservoirs, .scouring in alluvial channels, dissipation of energy etc and in such cases turbulente has to be controlled. On the otherhand' turbulence is,highly desirable such as in mixing process of Waters with different temperature salinity and oxygen contents, diffusion of pollutants etc. Most commonly grids are used either to, generate or to decay the turbulence level which is present in a -flow. Hence with this in view various investigators carried out experimental studies in wind tunnel to observe the effects of grid sizes and shapes on the decay of turbulence. They have put forward certain relationships to predict' the turbulence level at a distance downstream of a given size of grid. Literature review revealed that not much studies have been carried out on this aspect in case of open channel flow and there.is also need to verify existing relationships in this case . With this'in view an experimental programme to study the effect of grids' in open channel was taken up at the (iv) Hydraulic Laboratory of Civil Engineering ,Univerity Roorkee, Roorkee. The experimental programme includes -the,' basic measurements of turbulent fluctuating component: of velocity in axial direction along vertical. Four different grids of different sizes were used, whereas depth of flaw, discharge and slope of the flume were kept constant for all four runs. HBM inductance type pressure transducer was used to measure the fluctuating components of velocity at a point in combination with pitot tubes. The out put signal of this transducer was fed to a pen chart recorder through bridge box having other arrangement with amplifier. The analysis of experimental data suggests that (i) The intensity of turbulence varies linearly with width of the grid element for the same M/b and X. (ii) Intensity of turbulence is more for large M. (iii) The decay of turbulence intensity behind grid obeys the law proposed by Taylor and Harman-Howarth with modified values of%constants in these equations. (iv) The methodology as proposed by Naudascher and Farell (9) to predict the turbulence level at a distance from grid seems to be the most appealing because it has resulted. a systematic approach. The resutls are quite encouraging. Because of the limitation of experimental data the same could not be Verified over a wide range of data. Hence there is a need to carry out further studies in this direction before making any positive