Cloud Cavitation Behavior on a Hydrofoil Due to Fluid-Structure Interaction

Abstract

Despite recent extensive research into fluid–structure interaction (FSI) of cavitating hydrofoils, there remain insufficient experimental data to explain many of the observed phenomena. The cloud cavitation behavior around a hydrofoil due to the effect of FSI is investigated, utilizing rigid and compliant three-dimensional (3D) hydrofoils held in a cantilevered configuration in a cavitation tunnel. The hydrofoils have identical undeformed geometry of tapered planform with a constant modified NACA0009 profile. The rigid model is made of stainless steel and the compliant model of a carbon and glass fiber-reinforced epoxy resin with the structural fibers aligned along the spanwise direction to avoid material bend-twist coupling. Tests were conducted at an incidence of 6 deg, a mean chord-based Reynolds number of 0.7 × 106 and cavitation number of 0.8. Force measurements were simultaneously acquired with high-speed imaging to enable correlation of forces with tip bending deformations and cavity physics. Hydrofoil compliance was seen to dampen the higher frequency force fluctuations while showing strong correlation between normal force and tip deflection. The 3D nature of the flow field was seen to cause complex cavitation behavior with two shedding modes observed on both models.

The Influence of Fluid-Structure Interaction on Cloud Cavitation About a Hydrofoil

“…With both hydrofoils exhibiting two shedding modes and power distributions showing they cover separate regions of the planform indicates the formation of two shedding events on the hydrofoil for a particular σ. The occurrence of the differing shedding events along the span is supported by the span-wise space-time plots in [8] and highspeed imaging. The formation of multiple span-wise shedding events is not observed at σ other than 0.8 as the cavity length, both stream-wise and span-wise, are incompatible with the hydrofoil span and, instead, result in an irregular break-off mechanism [22].…”

“…Two profiled plates were used to clamp the model within a housing that was attached to a 6-component force balance ( Figure 2) with estimated precision of less than 0.1% on all components. Further description of the mounting arrangement may be found in [8]. Data was obtained for cavitation numbers from 1.0 to 0.2 and at a Reynolds number of 0.7×10 6 and an incidence of 6°.…”

“…A Nikkor f/1.4 50 mm lens was used resulting in a magnification factor of 3.28 px/mm. High-speed images were recorded with a spatial resolution of 1024×1024 for 1 s at 7,000 Hz for the rigid hydrofoil while the flexible hydrofoil was recorded for 5 s at 1,000 Hz (due to synchronisation compatibility of the two cameras) to allow tip deflection measurements for data presented in [8]. The high-speed photography was synchronized with the force measurement acquisition by simultaneous triggering.…”

“…Depending on the flow conditions, several possible mechanisms have been identified as the primary instability causing periodic shedding. These include re-entrant jet formation [7][8][9][10], shockwave propagation [9][10][11][12][13][14] and growth of interfacial instabilities such as Kelvin-Helmholtz waves [15]. In a recent study on cloud cavitation about a sphere, all three mechanisms have been observed occurring either under varying flow conditions or as a complex coupled mechanism [13].…”

Morphing Hydrofoil Model Driven by Compliant Composite Structure and Internal Pressure

“…In [17], the cantilevered rigid and compliant three-dimensional hydrofoils were studied in a cavitation tunnel in order to analyse the cloud cavitation behavior. The rigid hydrofoil was made of stainless steel and the compliant one of a carbon and glass fiber-reinforced epoxy resin with the structural fibers aligned along the spanwise direction to avoid material bend-twist coupling.…”