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Unsteady loading on hydrofoils due to turbulence and cavitation


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Smith, SMH ORCID: 0000-0003-0545-1444 2020 , 'Unsteady loading on hydrofoils due to turbulence and cavitation', PhD thesis, University of Tasmania.

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There is an increasing demand in the operational requirements of a submarine for quieter, stealthier designs. For ships and submarines, constraints relating to control, serviceability and efficiency dictate that propulsion and control equipment be compact and located at the stern of the vessel. This equipment is then exposed to the turbulent flow about the vessel afterbody. At the stern, the hull boundary layer and embedded wakes have had the full vessel length to develop and may be further thickened due to afterbody adverse pressure gradients. Consequently, control surfaces and propulsion devices may be partially or fully immersed within the afterbody flow and be subject to unsteady loading and hence be a source of vibration and noise production. For these effects to be minimized, insight into the flow physics and excitation spectra are required. Such information would enable more rigorous analysis and design for optimisation of control surface and propeller structural response.
To further understand the flow physics, an experimental investigation was undertaken to analyze the steady and unsteady loads acting on a hydrofoil immersed in a turbulent boundary layer. Measurements were performed in a cavitation tunnel in which the hydrofoil was mounted from the test section ceiling, via a 6-component force balance. The turbulent boundary layer was artificially thickened via an array of transverse jets located upstream of the test section. The effect of boundary layer thickness was investigated, in which various thicknesses were generated to allow partial or full immersion of two hydrofoils of different aspect ratios. The effect of varying incidence and Reynolds number on the hydrodynamic loading was also investigated. Steady forces were found to be significantly affected by the relative scale of the boundary layer, particularly in the stall region. Identification of a broad peak in the unsteady force spectra, was made at a constant reduced frequency of 0.2. The amplitude of this peak was found to be dependant on boundary layer immersion and Reynolds number. Furthermore, a low-frequency stall component, superimposed over the existing broadband excitation of the boundary layer turbulence, was apparent in the spectra past stall.
Flow-induced vibrations occur when the motion of a structure is coupled with flow instabilities, resulting in amplification of vibrations and forces. This phenomenon is gaining importance as advances in material technology and ever-increasing optimisation have caused structures to be lighter, more flexible, and thus more susceptible to vibration. Flow induced vibration phenomena influence the performance of vast range of aerodynamic and hydrodynamic objects and are therefore significant in geometrical and structural design. Cavitation about a hydrofoil involves a range of complex dynamical phenomena including mass transfer via phase change and diffusion, shockwaves, large and small-scale instabilities and turbulence. These phenomena have the potential to cause significant and destructive vibrations. Understanding these fluid-structure interaction (FSI) phenomena is of interest as flow over a lifting body can significantly alter the performance of maritime propulsion and control systems.
The physics associated with various cavitation regimes about a hydrofoil are investigated in a variable-pressure water tunnel using high-speed photography and synchronized force measurements. Experiments were conducted on both a relatively stiff stainless steel and a flexible composite hydrofoil at Re = 0.8 × 106 for cavitation numbers, σ, ranging from 0.2 to 1.2. The flexible composite hydrofoil was manufactured as a carbon/glassepoxy hybrid structure with a lay-up sequence selected principally to consider spanwise bending deformations with no material-induced bend-twist coupling. The hydrofoils experienced a variety of cavitation regimes including sheet, cloud and super-cavitation. The NACA0009 model of tapered planform was vertically mounted in a cantilevered configuration to a six-component force balance at an incidence, α, of 6◦ to the oncoming flow. Tip deformations and cavitation behaviour were recorded with synchronized force measurements utilizing two high-speed cameras mounted underneath and to the side of the test section.
Break-up and shedding of an attached cavity was found to be due to either interfacial instabilities, re-entrant jet formation, shockwave propagation or a complex coupled mechanism, depending on σ. Three primary shedding modes, designated as Types I, IIa and IIb, are identified to occur on both hydrofoils. The Types IIa & IIb re-entrant jet driven oscillations exhibiting a linear dependence on σ, decreasing in frequency with decreasing σ due to growth in the cavity length. For the stiff hydrofoil, Type IIa shedding is observed to occur for 0.4 ≤ σ ≤ 1.0 with Type IIb shedding occuring for 0.7 ≤ σ ≤ 0.9. Shockwave-driven Type I shedding occurs for lower σ values (0.3-0.6) with the oscillation frequency being practically independent of σ. The Type IIa oscillations locked in to the first sub-harmonic of the stainless steel hydrofoil’s first bending mode in water which has been modulated due to the reduced added mass of the vapour cavity. The flexibility of the composite hydrofoil increased the FSI between the complex cavitation physics and structural deformations causing changes in the phenomena observed. Hydrodynamic bend-twist coupling is seen to result in nose-up twist deformations causing frequency modulation from the increase in cavity length. The lock-in phenomenon driven by re-entrant jet shedding observed on the stiff hydrofoil is also evident on the flexible hydrofoil at 0.70 ≤ σ ≤ 0.75, but occurs between different modes compared to the stiff hydrofoil. Flexibility is observed to accelerate cavitation regime transition with reducing σ. This is seen with the rapid growth and influence the shockwave instability has on the forces, deflections and cavitation behaviour on the flexible hydrofoil, suggesting structural behaviour plays a significant role in modifying cavity physics. The reduced stiffness causes secondary lock-in of the flexible hydrofoil’s one-quarter sub-harmonic, fn/4, at σ = 0.4. This leads to the most severe deflections observed in the conditions tested along with a shift in phase between normal force and tip deflection.

Item Type: Thesis - PhD
Authors/Creators:Smith, SMH
Keywords: Hydrofoils, Cavitation, Boundary layer, Fluid-Structure Interation
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Copyright 2020 the author

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