Bw Pearce

Numerical analysis of basic base-ventilated supercavitating hydrofoil sections

A numerical analysis of the inviscid flow over base-ventilated intercepted hydrofoils is presented. The low-order, non-linear boundary element formulation used is described along with the significant issues concerning the modelling of supercavities with this method. The use of transom-mounted interceptors is well established for the manoeuvring and trim control of high-speed vessels. The flow field over a forward-facing step at the trailing edge of a blunt-based hydrofoil section, with consequent cavity detachment from the outer edge of the step, is similar to that of the transom-mounted interceptor operating at high speed with free surface detachment from the outer edge. Due to this similarity, the term ‘intercepted’ hydrofoil is used to describe this arrangement. The results presented show that a number of geometric parameters, in particular thickness, leading-edge radius and trailing-edge slope, have a significant effect on the hydrodynamic performance of base-ventilated intercepted hydrofoils.

Ventilated cavity flow over a backward-facing step

Ventilated cavities detaching from a backward facing step (BFS) are investigated for a range of upstream boundary layer thicknesses in a cavitation tunnel. The upstream turbulent boundary layer thickness is varied by artificial thickening of the test section natural boundary layer using an array of transversely injected jets. Momentum thickness Reynolds numbers from 6.6 to 44 x 103 were tested giving boundary layer thickness to step height ratios from 1.25 to 3.8. A range of cavity lengths were obtained by variation of the ventilation flow rate for several freestream Reynolds numbers. Cavity length to step height ratios from 20 to 80 were achieved. Cavity length was found to be linearly dependent on ventilation rate and to decrease with increasing boundary layer thickness and/or Reynolds number. This result may have implications in the practical optimization of these flows which occur in applications such as drag reduction on marine hull forms.

Experimental investigation of a base-ventilated supercavitating hydrofoil with interceptor

Bryce W. PearceAustralian Maritime CollegeUniversity of TasmaniaLaunceston, AustraliaEmail: B.Pearce@amc.edu.auPaul A. BrandnerAustralian Maritime CollegeUniversity of TasmaniaLaunceston, AustraliaEmail: P.Brandner@amc.edu.auSUMMARYAn experimental capability for the investigation of ventilatedsupercavitating hydrofoils is described and preliminary resultsfrom the testing of a novel hydrofoil design are presented. Thetesting capability makes use of a cavitation tunnel in which largevolumes of incondensable gas may be continuously injected andremoved and a capability for rapid degassing. Instrumentationhas been developed for measurement of the total force generated,the cavitation number of the ventilated cavity and the mass fl owrate of ventilating gas. The hydrofoil investigated is symmetricand wedge shaped and makes use of a mechanism for producinga forward facing step on either upper or lower surfaces to inducesupercavitation and produce bi-directional lift. The device wastested at cavitation numbers typical of those for devices used formotion control of high-speedships. Results show that the concepthas potentialin his applicationalthoughcomplexinteractionsbe-tween the ventilated cavity and leading edge vapour cavities canoccur at high incidences.INTRODUCTIONTheperformanceof high-speedships andtheir crews arelim-ited by undesirablemotions necessitating the use of active motioncontrol systems to maximise the operating window of tolerableseastates. Due to the relatively shallow draft and high speed ofthese vessels cavitation numbers at which lift generating devicesmust operate are sufficiently low to create cavitation probl emson hydrofoils intended for non-cavitating operation although notlow enough for supercavitation to naturally develop. A classicalstrategy to address this problem is the introduction or venting ofincondensable gas about a hydrofoil to artificially induce s uper-cavitation.A novel design for a base-ventilated supercavitating hydro-foil was conceived by Australian Naval Architect Tony Elms asembodied in the patent application entitled “Improved HydrofoilDevice” [4]. The basis of this concept is the use of a symmetricalhydrofoil section from which a trailing supercavity is formed de-taching from geometric discontinuities, located between the mid-chord and trailing edge, on both the upper and lower surfaces, asshown in Figure 1. Deflection of the hydrofoil tail section cr eatesa forward-facing step (FFS) on one side and a backward-facingstep (BFS) on the other. The use of such a FFS on the trailingedgesoflifting surfacesor transomsofship hulls are oftentermedspoilers or interceptors. Flow asymmetry created by the discon-tinuities may thus be used to create bi-directional lift as requiredfor vessel motion control from a hydrofoil at nominally zero in-cidence. Various mechanisms for venting of the incondensablegas are possible including ducting of atmospheric air via strutssupporting the hydrofoil or via ports on the base of the leading ortrailing sections of the hydrofoil.cavitycenter of nose tailrotationθ

Wavelet analysis techniques in cavitating flows

Cavitating and bubbly flows involve a host of physical phenomena and processes ranging from nucleation, surface and interfacial effects, mass transfer via diffusion and phase change to macroscopic flow physics involving bubble dynamics, turbulent flow interactions and two-phase compressible effects. The complex physics that result from these phenomena and their interactions make for flows that are difficult to investigate and analyse. From an experimental perspective, evolving sensing technology and data processing provide opportunities for gaining new insight and understanding of these complex flows, and the continuous wavelet transform (CWT) is a powerful tool to aid in their elucidation. Five case studies are presented involving many of these phenomena in which the CWT was key to data analysis and interpretation. A diverse set of experiments are presented involving a range of physical and temporal scales and experimental techniques. Bubble turbulent break-up is investigated using hydroacoustics, bubble dynamics and high-speed imaging; microbubbles are sized using light scattering and ultrasonic sensing, and large-scale coherent shedding driven by various mechanisms are analysed using simultaneous high-speed imaging and physical measurement techniques. The experimental set-up, aspect of cavitation being addressed, how the wavelets were applied, their advantages over other techniques and key findings are presented for each case study. This paper is part of the theme issue ‘Redundancy rules: the continuous wavelet transform comes of age’.

Bubble breakup in a turbulent shear layer

The breakup of a millimetre size buoyantly rising bubble encountering a horizontal plane turbulent jet is experimentally investigated using high-speed shadowgraphy and acoustic techniques. The bubble diameter to jet height ratio is 0.75 and the jet height based Reynolds number is 4000. The high-speed imaging was recorded at 7 kHz simultaneous with hydrophone output at 100 kHz. Bubble breakup events were seen to produce simple binary divisions into products of similar size as well as three products where at least one was much smaller than the other products. Coalescence of products was also observed. In almost all cases time-frequency analysis of the acoustic emissions enabled the products to be identified and sized.

Cavitation due to an impacting sphere

Cavitation associated with the impact of a sphere on a flat surface is investigated using high-speed photography. The sphere, of diameter 15 or 45 mm and made from Ertacetal® or stainless steel, was fully submerged and accelerated using a spring-loaded mechanism to achieve Reynolds numbers based on impact velocity and sphere radius of up to 7.2×104. The static pressure and impact velocity were varied to achieve cavitation numbers ranging from 8.9 to 120.9. High-speed photography of the impacting sphere and induced cavitation bubble was filmed at 105-140 kHz. A log law relationship was found between the non-dimensional maximum bubble radius and the cavitation number. The relationship was modulated by the material properties. Interaction between the sphere and the bubble was also noted.