Energy Dissipation Mechanism of Draft Tube Vortex Rope Suppression by Self-Driven Jets from the Discharge Cone

This study investigates the redistribution of resolved flow structures and velocity-gradient-based entropy generation associated with axial water injection in a hydraulic-turbine draft tube at part load. Steady SST k–ω calculations screen six jet radii from 4 to 9 mm, followed by unsteady embedded large eddy simulation (ELES) for the no-jet and selected 7 mm cases. The 7 mm radius, corresponding to 0.175 times the cone radius and a nominal jet-flow fraction of 1.6%, is a candidate selected from steady-flow topology rather than a demonstrated thermodynamic optimum. Under the reported numerical settings, the selected jet removes the central reverse-flow region in the steady calculation and disrupts the coherent helical structure in the unsteady visualization. The reported pressure amplitudes at 1.6 Hz decrease from 249 to 167 Pa at DT1 and from 110 to 71 Pa at DT2, approximately 33% and 36%, respectively. For an assumed bypass-head loss of 10%, the jet kinetic-power flux is estimated as 1.44% of the turbine hydraulic input; this is not the measured net efficiency penalty. The velocity-gradient analysis indicates a change in the location of resolved dissipation, but excludes a separate subgrid-scale contribution and a wall-function entropy calculation. The results therefore support a conditional interpretation of the control mechanism, rather than a complete loss balance or a verified self-driven-system design.