Investigation on the dynamic behavior of cloud cavitation around a flexible hydrofoil

The objective of this paper is to investigate the dynamic behavior of cloud cavitating flow around a flexible hydrofoil with experimental and numerical methods.The tightly coupled fluid-structure interaction(FSI)modeling is applied and validated with the experimental data.The Q-criterion andω-criterion are applied to illustrate the interaction between the vortex structures and cavities.The flexibility is seen to result in nose-up twist deformation,causing a reduction of the shedding frequency from an increase in the attached cavity length.Due to the flexibility,the fluctuation of load coefficients of the flexible hydrofoil is larger than that of the rigid hydrofoil.Moreover,the re-entrant jet propagation speed of the flexible hydrofoil is greater than that of the rigid hydrofoil.The shed cloud cavity is observed to be uniform along the flexible hydrofoil span under the combined influence of the strong vibration and the gap flow.

THE FLEXIBLE HYDROFOIL WING FLOAT

The Flexible Hydrofoil Wing Float(Fhwf)is a new device for lifting the headline of a trawl.It is made from Superior Grade canvas.Part of the structure is cut away at the leading edge to form a water intake.The flow of seawater inflates the canvas into a shape similar to that of an aircraft wing,thus generating lift.Sea trials have been conducted on practical wing floats suitable for full scale fishing gear.Model tests have been conducted in a wind tunnel.At low attack angles,the lift coefficient(C_L)is 0.32,the lift to drag ratio(G_L/C_d)is 5 and the mo- ment ccefficient(C(?))is positive Better results are obtained when netting is placed below the model float,compared with the float in isolation.The wing float has many advantages compared with conventional lifting devices.It may be used at any depth.Large net mouth openings may be achieved at high speeds.It is cheap,simple to install and to handle.Spare wing floats may be folded for storage and do not require much space.

Numerical investigations of the transient cavitating vortical flow structures over a flexible NACA66 hydrofoil

In this paper,the cavitating flow over a flexible NACA66 hydrofoil is studied numerically by a modified fluid-structure interaction strategy with particular emphasis on understanding the flow-induced vibration and the cavitating vortical flow structures.The modified coupling approaches include(1)the hydrodynamic solution obtained by the large eddy simulation(LES)together with a homogenous cavitation model,(2)the structural deformation solved with a cantilever beam equation,(3)fluid-structural interpolation and volume mesh motion based on the radial basis functions and greedy algorithm.For the flexible hydrofoil,the dominant flow-induced vibration frequency is twice of the cavity shedding frequency.The cavity shedding frequency is same for the rigid and flexible hydrofoils,demonstrating that the structure vibration is not large enough to affect the cavitation evolution.The predicted cavitating behaviors are strongly three-dimensional,that is,the cavity is(a)of a triangular shape near the hydrofoil tip,(b)of a rectangular shape near the hydrofoil root,and(c)with a strong unsteadiness in the middle of the span,including the attached cavity growth,oscillation and shrinkage,break-off and collapse downstream.The unsteady hydroelastic response would strongly affect the cavitation shedding process with small-scale fragments at the cavity rear part.Furthermore,three vortex identification methods(i.e.,the vorticity,the Q-criteria and the Ω method)are adopted to investigate the cavitating vortex structures around the flexible hydrofoil.It is indicated that the cavity variation trend is consistent with the vortex evolution.The vortex structures are distributed near the foil trailing edge and in the cavitation region,especially at the cavity-liquid interface.With the transporting downstream the shedding cavities,the vortices gradually increase in the wake flows.