Experimental analysis and proposal of semi-empirical prediction model on tidal stream turbine induced local scour subjected to wave-current loading

Abstract

Accurately predicting tidal stream turbine induced morphological evolution is crucial for layout and maintenance of turbine devices. However, the tidal stream turbine induced scour morphology under wave-current loading and corresponding prediction method remain relatively unexplored. Therefore, experimental tests were conducted in the wave-current flume to observe and analyze the mono-pile supported horizontal-axis tidal stream turbine (HATST) induced local scour subjected to wave-current loading. A semi-empirical model was proposed to predict the induced equilibrium scour depth. In this model, the Actuator Disc Theory and wave-current-scour correlations were introduced into Phenomenological Theory of Turbulence. Results indicate that the rotating rotor intensifies the scour around supporting mono-pile under wave-current loading. When waves superposed on current, the equilibrium scour time around HATST was shortened. The higher wave height, larger flow intensity, and longer wave period result in more intense scour around the HATST. The local scour is initially enhanced and then weakened with increasing water depth. The equilibrium scour depth around HATST foundation positively correlates with Keulegan-Carpenter (KC) number and relative wave-current velocity (U_cw). The derived semi-empirical model can appropriately predict the wave-current induced equilibrium scour depth around the HATST pile foundation when 0.699 ≤ KC ≤ 7.402 and 0.717 ≤ U_cw ≤ 0.932.