Experimental investigation of the flow-induced motion of a square-section cylinder

The flow-induced motion(FIM)of an elastically mounted square-section cylinder is experimentally investigated over a wide range of Reynolds numbers(1.5×10^(4)<Re<7.0×10^(4)).A 14 mlong towing tank water channel and a carrier are designed to facilitate the investigation of FIM at high velocities.The cylinder is limited to a transverse oscillation and is carried inside the water channel.The effect of cylinder orientation on FIM is studied by performing experiments for two angles of attack(45°and 0°).The experiments are performed for two different spring stiffness to consider the influence of the natural frequency on the response of the system.Since the water has been allowed to stay calm between the tests,experiments are conducted with zero disturbance of the fluid flow around the cylinder.The experimental setup has shown promising results for a circular cylinder in our previous studies.The results for the diamond cylinder(square-section cylinder with a 45°angle of attack)indicate that FIM only consists of vortex-induced vibration(VIV)and the oscillation in the upper branch occurs in a wider range of reduced velocities compared with the circular cylinder.It can be concluded that a diamond cylinder is a better option for having synchronization in a wider range of water velocities for the purpose of energy extraction in VIV-based ocean energy conversion devices.

Numerical prediction of hydrodynamic cavitating flow structures and their corresponding erosion

Hydrodynamic cavitating flows usually consist of 3-D intense vortical flows that are detached from solid boundaries.Detached vortical flows normally generate heaps of cavitating flow structures,which,in turn,govern the location of cavitation erosion before collapse.Thus,this study introduces a new numerical approach based on the improved delayed detached eddy simulation(IDDES)turbulence modeling for predicting cavitating flows.Then,the solution of compressible Eulerian-Eulerian two-phase flow and the IDDES turbulence model was linked to the microjet hypothesis and unsteady behavior of pressure and vapor volume to predict the corresponding erosion of cavitating flows.The method for cavitation erosion prediction,a modified version taken from previous studies,was applied as a post-processing tool.The validation of cavitating flow predictions was performed for the first time on the Grenoble axisymmetric nozzle by comparing them with 21 photos of cavitation from the previous experimental study.The results showed that the present numerical approach estimated various features of hydrodynamic cavitation well,including shedding processes and the length,shape,and collapse of cavitating structures.Using the numerical analysis,three main stages were detected for the present cavitating flow,and the vorticity-cavitation interactions were investigated by the vorticity transport equation.The streak-like and tube-like cavitating(STLIC and TULIC)structures were introduced in the second stage,initiated by flow instability,and entirely governed by corresponding turbulent flow structures.The collapse of these cavitating structures is one of the primary sources of cavitation erosion on lower and upper walls.The results of the numerical erosion predictions were compared with those of the previous erosion tests on the Grenoble axisymmetric nozzle.Satisfactory numerical performance was achieved in predicting the location and intensity of cavitation erosion.