Pa Brandner

An experimental investigation of cloud cavitation about a sphere

Cloud cavitation occurrence about a sphere is investigated in a variable-pressure water tunnel using low- and high-speed photography. The model sphere, 0.15 m in diameter, was sting-mounted within a 0.6 m square test section and tested at a constant Reynolds number of 1.5×106 with cavitation numbers varying between 0.36 and 1.0. High-speed photographic recordings were made at 6 kHz for several cavitation numbers providing insight into cavity shedding and nucleation physics. Shedding phenomena and frequency content were investigated by means of pixel intensity time series data using wavelet analysis. Instantaneous cavity leading edge location was investigated using image processing and edge detection. The boundary layer at cavity separation is shown to be laminar for all cavitation numbers, with Kelvin–Helmholtz instability and transition to turbulence in the separated shear layer the main mechanism for cavity breakup and cloud formation at high cavitation numbers. At intermediate cavitation numbers, cavity lengths allow the development of re-entrant jet phenomena, providing a mechanism for shedding of large-scale K´arm´ an-type vortices similar to those for low-mode shedding in singlephase subcritical flow. This shedding mode, which exists at supercritical Reynolds numbers for single-phase flow, is eliminated at low cavitation numbers with the onset of supercavitation. Complex interactions between the separating laminar boundary layer and the cavity were observed. In all cases the cavity leading edge was structured in laminar cells separated by well-known ‘divots’. The initial laminar length and divot density were modulated by the unsteady cavity shedding process. At cavitation numbers where shedding was most energetic, with large portions of leading edge extinction, re-nucleation was seen to be circumferentially periodic and to consist of stretched streak-like bubbles that subsequently became fleck-like. This process appeared to be associated with laminar–turbulent transition of the attached boundary layer. Nucleation occurred periodically in time at these preferred sites and formed the characteristic cavity leading edge structure after sufficient accumulation of vapour had occurred. These observations suggest that three-dimensional instability of the decelerating boundary layer flow may have significantly influenced the developing structure of the cavity leading edge.

Hydrodynamic Performance of a Vortex Generator

The performance of a vortex generator intended for a marine application is investigated experimentally in a cavitation tunnel. Several tests were made as a preliminary study to gain basic knowledge of an initial design concept. Investigations included flow visualisation of local flow and wake trajectory, cavitation inception and occurrence, measurement of boundary layer profiles both upstream and downstream of the vortex generator, and measurement of forces and moments acting on the vortex generator. Boundary layers and forces and moments were measured both in cavitating and non-cavitating conditions. It was found that the influence of cavitation was not significant at moderate cavitation numbers and did not adversely affect the mixing effect or the lift to drag ratio (or efficiency) of the vortex generator.

Design and Calibration of a Water Tunnel for Skin Friction Research

Abstract A new water tunnel facility has been designed for application in skin friction and boundary layer research. The closed loop, recirculating facility with working section 200 χ 600 χ 2400 mm and test surface 600 χ 1000 mm has been designed to operate at working section flow speeds of up to 2 ms-1 (Reynolds number based on test surface length 2.2 χ 106). A force balance enables the direct measurement of drag on test surfaces. Hot film, pitot and multi-hole pressure probes are used to investigate the near wall flow.

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

The Influence of Viscous Effects and Physical Scale on Cavitation Tunnel Contraction Performance

The general performance of an asymmetric cavitation tunnel contraction is investigated using computational fluid dynamics (CFD) including the effects of fluid viscosity and physical scale. The horizontal and vertical profiles of the contraction geometry were chosen from a family of four-term sixth-order polynomials based on results from a CFD analysis and a consideration of the wall curvature distribution and its anticipated influence on boundary layer behavior. Inviscid and viscous CFD analyses were performed. The viscous predictions were validated against boundary layer measurements on existing full-scale cavitation tunnel test section ceiling and floor and for the chosen contraction geometry against model-scale wind tunnel tests. The viscous analysis showed the displacement effect of boundary layers to have a fairing effect on the contraction profile that reduced the magnitude of local pressure extrema at the entrance and exit. The maximum pressure gradients and minimum achievable test section cavitation numbers predicted by the viscous analysis are correspondingly less than those predicted by the inviscid analysis. The prediction of cavitation onset is discussed in detail. The minimum cavitation number is shown to be a function of the Froude number based on the test section velocity and height that incorporate the effects of physical scale on cavitation tunnel performance.

Development of a high pressure chamber for research into diesel spray dynamics

Abstract An optically accessible pressure vessel and ancillary instrumentation for studies of diesel spray dynamics has been designed and built at the Australian Maritime College. The requirement for large observation windows and internal pressures up to 10 MPa necessitated the use of finite element analysis in conjunction with first principles analysis. Strain gauge testing confirmed the calculated maximum stresses in the observation windows. The chamber is a registered pressure vessel conforming to AS1210. A purpose-built injection system capable of fuel pressures up to 120 MPa and injection pulse durations up to 30 ms has been implemented. Laser-based instrumentation is synchronised with individual injection events to measure spray droplet velocities and sizes.

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’.

Structural and acoustic responses of a fluid loaded shell due to propeller forces

The low frequency structural and acoustic responses of a fluid loaded shell to propeller induced fluid pressures are investigated. The propeller operates in the non-uniform wake field and produces fluctuating pressures on the blades of the propeller. This in turn generates acoustic waves and a near field that excites the surface of the shell. The resulting incident pressure is scattered and diffracted by the shell surface, and also excites structural vibration. A potential flow panel code is coupled with the Ffowcs-Williams and Hawkings acoustic analogy to predict the fluctuating propeller forces, blade pressures and the resulting incident field on the surface of the fluid loaded shell due to operation of the propeller in a non-uniform inflow. The propeller induced incident pressure field is then combined with a coupled three-dimensional finite element/boundary element model of the submerged shell to predict the vibro-acoustic and scattered field responses.