CRITICAL SUBMERGENCE FOR MULTIPLE HYDRAULIC INTAKES

Abstract

Hydraulic Intakes are frequently used in irrigation and drainage networks, hydropower, etc. for withdrawal of discharge. Insufficient water cover over the hydraulic intakes causes an air entraining vortex formation in the vicinity of the intake. Formation of such as air-entraining vortex results in significant hydraulic issues, structural damages, etc. Ensuring enough submergence for the intake during its operational period is necessary to evade such problems. The submergence at which the air-core tail of the surface vortex reaches the intake initiating airentrainment is called critical submergence (Sc). Critical submergence is an important parameter for design of intakes and is widely used as a parameter to determine an incipient state for which no air is entrained by intake vortices. Several studies have been reported in the literature for computation of critical submergence through experimental and analytical considerations. Many investigators have proposed empirical equations to calculate critical submergence. However, most of the previous studies are carried out on critical submergence for single lateral circular intakes (Ahmad et al., 2008, Hashid et al., 2021) and dual lateral square intakes (Hashid and Ahmad, 2022, Hashid and Eldho, 2023b). However, there is no study is available until date for computation of critical submergence for laterally placed rectangular intakes under a uniform approach in open channel flow. There is no study available until date on the discharge characteristics of rectangular intakes. The previous studies are focused on discharge characteristics of lateral circular intakes. However, there is a need to study the discharge characteristics of laterally placed rectangular intakes in open channel flow. In view of this, the present study was carried out to fill the knowledge gap in the literature regarding the determination of critical submergence and discharge characteristics at lateral intakes by conducting a series of experiments. Present study addresses an experimental investigation of critical submergence for single and dual lateral rectangular intakes in open channel flow. The influence of aspect ratio and other geometrical and flow parameters affecting the formation of an air-entraining vortices were studied. A rectangular intake geometry was chosen in this study due to its geometrical cross section applicability in the hydropower intakes over common circular or square cross-sections. The intake bottom clearance was kept equal to the height of the intake for its better performance. Moreover, an experimental investigation was conducted to examine the discharge characteristics of a single lateral rectangular intake in open channel flow. Functional relationships for the computation of critical submergence for single and dual intakes were also developed from the analysis of experimental data. An empirical relationship was also proposed to compute the coefficient of discharge for single lateral rectangular intakes. The scope of CFD based numerical modeling for simulating flow at the critical submergence of single and dual lateral intakes were also examined in this study. The CFD outcomes were validated with the experimental results. Experimental studies were performed on laterally placed single and dual rectangular intakes under a uniform approach flow in open channels. A separate experimental investigation was conducted to examine the critical submergence and discharge characteristics of single lateral rectangular intakes. The experiments were conducted on nine single intake model configurations by considering the intake bottom clearance equal to the intake height in each single intake model. An Acoustic Doppler Velocimeter (ADV) was used to measure the velocity field near the single intake., which was later used to validate the CFD results. In addition to this, an experimental study was also carried out to investigate the critical submergence for lateral rectangular dual identical intakes. For the dual intakes study, eleven identical rectangular intake model configurations were considered for conducting experiments, which were kept at three clear spacing between the intakes, and intake bottom clearance was considered equal to intake height for nine dual intakes models. The remaining two intake models were placed at two different bottom clearances from the bed in which one model was placed equal to the zero-bottom clearance, and the other was positioned at half of the intake height from the channel bed for evaluating the influence of intake bottom clearance for dual intakes. Additionally, dye tracer experiments for single and dual intake models were performed to analyse the flow pattern caused by flow diversion in the open channel. The dye tracer experiments observations enhanced the understanding of flow separation caused by flow withdrawal through single and dual intakes in a uniform approach flow. For a single intake, the flow can be divided into two distinct regions: one entering the intake and another continuing along its original path without entering the intake. For dual intakes, the flow can be divided into three distinct regions: the first region enters the Intake-1, which is placed at upstream side of approach flow, the second region is diverted by the Intake-2, which is located at downstream side, and the third region continues along its original open channel path without passing through any of the intakes. The observation made during the vortex formation at intakes revealed that the critical submergence was observed, when the different forms of the air entrainment vortex occurred progressively, and the final air-core tail of the vortex reached the intakes. An air-core vortex begins to yield its real form from a very small dip at the free surface. Observations during experimentation for dual intakes showed that when both the intakes carried the same discharge, the intake 2, reaches critical condition by entraining air through a free surface vortex. This observation persisted until the intake clear spacing (s) was less than double the intake height (2b). For clear spacing equal to the twice of the intake height the multiple vortices were observed and both intakes operates independently without mutual influence. Dimensional analysis was conducted for obtaining a functional relationship for the coefficient of discharge (Cd) of single intake, critical submergence of both single and dual intakes. This exhibited that coefficient of discharge as a function of approach Flow Froude number, intake flow Froude number, and intake aspect ratio. Analysis of data for discharge characteristic study of single intake revealed that coefficient of discharge decreased with increase in approach Flow Froude number, intake flow Froude number, and intake aspect ratio. An empirical relationship for the prediction of coefficient of discharge of single intake was developed. The comparison of computed discharge with observed data showed the error is within ±8% which is satisfactory for the computation of discharge. The remaining 45 data used for the validation of proposed equation showed error within the range of ±8%. The dimensional analysis of single intake showed that ratio of critical submergence to intake height as function of approach Flow Froude number, intake flow Froude number, intake aspect ratio, intake flow Reynolds number and Weber number. The critical submergence of single lateral intake increases with intake flow Froude number, intake aspect ratio, intake flow Reynolds number and intake flow Weber number and decreased with increase in approach flow Froude number. An empirical relationship for the prediction of critical submergence of single rectangular intake was developed with the 91 data set of present study using multiple regression analysis. The comparison of computed critical submergence with observed data showed the error is within ±20%, which is satisfactory for the computation of critical submergence. The remaining 27 data used for the validation of proposed equation showed error within the range of ±20%. Similarly, dimensional analysis of dual intakes showed that ratio of critical submergence to intake height as function of approach Flow Froude number, intake flow Froude number, intake aspect ratio, intake clear spacing ratio, intake flow Reynolds number, and intake flow Weber number. The critical submergence of dual intakes increases with intake flow Froude number, intake aspect ratio, intake flow Reynolds number and intake flow Weber number and decreased with increase in approach flow Froude number and intake clear spacing. An empirical relationship for the prediction of critical submergence of dual rectangular intakes was developed with the 107 data set of present study using multiple regression analysis. The comparison of computed critical submergence with observed data showed the error is within ±20% which is satisfactory for the computation of critical submergence. The remaining 27 data used for the validation of proposed equation showed error within the range of ±20%.Sensitivity analysis was conducted to examine the effect of various significant parameters on the coefficient of discharge and critical submergence of intakes. It was found that the approach flow Froude number is the most sensitive and significant parameter influencing the coefficient of discharge of lateral intakes. The outcome of the sensitivity analysis of critical submergence of intakes indicates that the intake Froude number is the most significant and sensitive input parameter influencing critical submergence. It was also found that intake aspect ratio can be considered the second most sensitive parameter affecting coefficient of discharge and critical submergence. Furthermore, the accuracy of the existing equations of critical submergence for single and dual intakes were checked using the data collected in present study. Although the existing equations are not applicable for lateral rectangular intake under uniform flow, however, their applicability are checked in this case also using the data collected in this study. A CFD-based numerical model was developed to simulate the flow at the critical submergence of single and dual lateral rectangular intakes under uniform approach flow conditions. A Multiphase 3D simulation was adopted for checking the critical submergence of single and dual lateral intakes. Volume of Fraction (VOF) model was used to simulate the airwater multiphase interaction. The simulations were performed using the SST k-ω turbulence model. A combined approach incorporating AVF analysis and surface streamline analysis for computing critical submergence for single and dual intakes proved to be effective and was successfully validated with the experimental results. The analysis of flow patterns through CFD simulations proven the capability of the CFD model to accurately replicate the physical behaviour of both single and dual lateral rectangular intakes under uniform approach flow, showing strong agreement with observed values. The SST k-ω model gave superior outcomes for flows in vicinity to the impervious boundaries. The velocity vector field obtained through CFD for a single intake closely matches the observed values obtained through ADV measurement. Specifically, it was noted that a bottom clearance equivalent to the intake height exhibited an improved flow withdrawal pattern with flow converging to the intake from all directions. The CFD model is observed to predict critical submergence with an error of less than ±10% and ±8% for single and dual intakes, respectively. The CFD model also indicated that the critical submergence of lateral single and dual rectangular intakes under open channel flow increases with an increase in intake Froude number and decreases with an increase in approach Froude number.