EXPERIMENTAL STUDY OF TURBULENCE CHARACTERISTICS NEAR PIANO KEY WEIR

Abstract

Extensive laboratory investigations and then stringent analysis have been done to study the different aspects of turbulence hydraulics in open channel (with and without PKW) under multiple hydraulic conditions. Two types of flume, with different width have been utilized for the experimentation. The first flume of smaller width (50 cm) has been only utilized for head reduction study and to evaluate head discharge relationship. This study has been carried out to show the general utility of Piano Key Weir and appraisal for the estimation of approximate design discharge. The rest of the experimentation has been done in the second flume (260 cm) under different hydraulic conditions. The preliminary step of this study is to investigate hydraulic advantage of Piano Key Weir in comparison to rectangular weir in open channel. On the other hand, bed shear stress and velocity distribution around hydraulic structures such as Piano Key Weir can be an important aspect for the efficiency improvement and local mechanism development. An investigation on the turbulent kinetic energy and octant events distribution around Piano Key Weir is substantive to understand the mechanism of local transport and to determine which turbulence properties affect the local momentum transport. This dissertation examined in depth the bed shear stress methodology, velocity distribution and turbulent kinetic energy variations and bursting events in the approach flow domain of Piano Key Weir. The goal of the study is to investigate some alternative approaches for flow & turbulence characteristics and their significance in the upstream of Piano Key Weir. By using Piano Key Weir, up to 80% of head reduction has been achieved in comparison with rectangular weir of same width. Nonlinear empirical relationship has been established considering Q/LB as function of average head over Piano Key Weir crest. Selection of instrument using principal component analysis has been carried out. Acoustic Doppler Velocimeter has been qualified as better instrument under assumed conditions. Raw Acoustic Doppler Velocimeter data contain noise, which has been removed using available noise removal techniques. Noise removal using state space threshold has been applied. After application of noise removal technique on datasets, the occurrence of change in mean and mean velocity fluctuations (turbulence) has been estimated. It has been found that second order velocity (turbulence or mean velocity fluctuations) has higher correlation than first order (mean velocity) with available noise. Over hydraulically smooth boundary, local shear stress estimates using Turbulent Kinetic Energy and Modified Turbulent Kinetic Energy; has been found very similar at higher iv aspect ratio but more variations have observed for a smaller aspect ratio The Turbulent Kinetic Energy method has also been found to be the most suitable to estimate the local stresses near Piano Key Weir due to the presence of three dimensional local equilibrium conditions. Range of mean longitudinal velocity has happened to be increasing as moved away from the Piano Key Weir (0 to 3.5 cm/s @X=0 from 2 to 4.5 cm/s @X=30). The profiles of transverse velocity indicate that due to presence of wall; mean transverse velocity is at lower range near wall but at the same time also with higher uncertainty near wall. Velocity distribution in the YZ plane has been found to be highly nonlinear. Range of turbulent kinetic energy is increased below the 30% of the flow depth. Average standard deviation of turbulent kinetic energy has been estimated 0.65 cm2/s2/mass (z>0.3). Below that relative depth (z < 0.3) the average standard deviation increased more than four times i.e. 4.8 cm2/s2/mass. The average velocity and turbulent intensities near inlet key, in both directions (longitudinal & transverse), have also been investigated in a steady open channel flow with PKW. Peak value of turbulent kinetic energy is decreasing with decrease of Q/h (increase in flow resistance) and flow resistance is increased after introduction of PKW in channel. As flow resistance increased peak value is getting decreased by 22.18 to 0.83 cm2/s2 per unit mass. Irrespective of any condition with or without structure, position of Centroid lies below 0.3h, where h is the depth of flow. Region of occurring maximum turbulent kinetic energy is in between 0.15 h to 0.30 h, when the experiments have been done without Piano Key Weir. The average contribution of sweep and ejection displays increasing trend with high strength turbulence events. Probability of occurrence of the events (highly turbulent) H12 has been found maximum 5% in the entire flow domain, considering two dimensional events. Extended study for three dimensional octant events show that the probability of occurrence (H12) increased up to 10%. It shows the relative importance of three dimensional analyses to gather more precise information. In absence of three dimensional data, a new parameter turbulent ratio is defined and tested by author. This new parameter is capable of extract the turbulent character of flow in the upstream of PKW.