Parameters and techniques for detecting the location of three-dimensional crossflow separations are evaluated using several data sets. Several definitions of separations and the physics of the separation process are discussed along with descriptions of the separated flowfield. Measurement techniques that depend on each of these descriptions are then considered, and data are compared and contrasted. The data analyzed here represent a very rare combination of many different measurement techniques applied to the same geometry and apparatus from several different studies, including oil flow visualization, laser Doppler velocimetry, surface pressure, and surface hot-film skin-friction measurements (magnitude only and directional). Pressure is the least sensitive of the indicators of separation, although minima in rms pressure fluctuations correlate well with separation location. Hot-film skin-friction magnitude measurement is one of the easiest and most accurate techniques; local minima correlate well with separation location. Laser Doppler velocimeter measurements provide the most detail about the separation flowfield but at great expense and with the limitation of requiring knowledge of the separation line direction

Measurement of Three-Dimensional Crossflow Separation

: CFDSHIP-IOWA is a general-purpose unsteady Reynolds-averaged Navier-Stokes CFD code that has been developed, over the past 10 years, to handle a broad range of ship hydrodynamics problems. Originally designed to support both thesis and project research in the areas of resistance and propulsion, it has been successfully transitioned to Navy and university laboratories and industry, and has recently been extended to unsteady applications such as seakeeping and maneuvering. It was developed following a modern software-development philosophy, which was based upon open source, revision control, modular coding using Fortran 90/95, liberal use of comments, and an easy to understand architecture which enables model development by users.

http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA458092&Location=U2&doc=GetTRDoc.pdf

General-purpose parallel unsteady rans ship hydrodynamics code: cfdship-iowa

The purpose of this work is to describe the two-phase flow structure of cloud cavitation. The experimental study is performed in a cavitation tunnel equipped with a Venturi-type test section. The geometry studied is characterized by a convergent angle of 18° and a divergent angle of 8°. It leads to regular large vapour cloud shedding. The flow is investigated by means of an X-ray attenuation device. Twenty-four collimated detectors enable the instantaneous volume fraction of the vapour phase to be measured at different locations of the two-phase flow. The X-ray intensity measurement mode enables fast data acquisition (1,000 Hz). The results are compared with double optical probe measurements. The influences of the velocity and the size of the cavitation area on the time–space distribution of the vapour fraction are studied. The convection velocity of the vapour structures is estimated. The mean distribution of the volume fraction of the vapour phase during the shedding cycle is computed using the phase- averaging method.

X-ray measurements within unsteady cavitation

The growth, oscillation and collapse of vortex cavitation bubbles are examined using both two- and three-dimensional numerical models. As the bubble changes volume within the core of the vortex, the vorticity distribution of the surrounding flow is modified, which then changes the pressures at the bubble interface. This interaction can be complex. In the case of cylindrical cavitation bubbles, the bubble radius will oscillate as the bubble grows or collapses. The period of this oscillation is of the order of the vortex time scale, τV = 2πrc/uθ, max, where rc is the vortex core radius and uθ, max is its maximum tangential velocity. However, the period, oscillation amplitude and final bubble radius are sensitive to variations in the vortex properties and the rate and magnitude of the pressure reduction or increase. The growth and collapse of three-dimensional bubbles are reminiscent of the two-dimensional bubble dynamics. But, the axial and radial growth of the vortex bubbles are often strongly coupled, especially near the axial extents of the bubble. As an initially spherical nucleus grows into an elongated bubble, it may take on complex shapes and have volume oscillations that also scale with τV. Axial flow produced at the ends of the bubble can produce local pinching and fission of the elongated bubble. Again, small changes in flow parameters can result in substantial changes to the detailed volume history of the bubbles.

Growth, oscillation and collapse of vortex cavitation bubbles

The flowfield around a 6:1 prolate spheroid at angle of attack is predicted using solutions of the Reynolds-averaged Navier-Stokes (RANS) equations and detached-eddy simulation (DES). The calculations were performed at a Reynolds number of 4.2×10 6 , the flow is tripped at x/L=0.2, and the angle of attack a is varied from 10 to 20 deg. RANS calculations are performed using the Spalart-Allmaras one-equation model. The influence of corrections to the Spalart-Allmaras model accounting for streamline curvature and a nonlinear constitutive relation are also considered. DES predictions are evaluated against experimental measurements, RANS results, as well as calculations performed without an explicit turbulence model. In general, flowfield predictions of the mean properties from the RANS and DES are similar

Numerical Investigation of Flow Past a Prolate Spheroid

The flow in the crossflow separation region of a 6:1 prolate spheroid at 10- and 20-deg angle of attack, Re L = 4.20 x 10 6 , was investigated using a novel, miniature, three-dimensional, fiber-optic laser Doppler velocimeter (LDV). The probe was used to measure three simultaneous, orthogonal velocity components from within the model, and these measurements were simultaneous with wall-pressure measurements made just below the LDV probe volume. The LDV measurements extend from approximately y + = 7 out to beyond the boundary-layer edge. The design and operation of this LDV probe is summarized.

Detailed Investigation of the Three-Dimensional Separation About a 6:1 Prolate Spheroid

An extensive experimental investigation was carried out to examine tip-vortex induced cavitation on a ducted propulsor. The flowfield about a 3-bladed, ducted rotor operating in uniform inflow was measured in detail with three-dimensional LDV; cavitation inception was measured; and a correlated hydrophone/high-speed video system was used to identify and characterize the early, sub-visual cavitation events. Two geometrically-similar, ducted rotors were tested over a Reynolds number range from 1.4×106 to 9×106 in order to determine how the tip-vortex cavitation scales with Reynolds number. Analysis of the data shows that exponent for scaling tip-vortex cavitation with Reynolds number is smaller than for open rotors. It is shown that the parameters which are commonly accepted to control tip-vortex cavitation, vortex circulation and vortex core size, do not directly control cavitation inception on this ducted rotor. Rather it appears that cavitation is initiated by the stretching and deformation of secondary vortical structures resulting from the merger of the leakage and tip vortices.Copyright © 2003 by ASME

Tip-Vortex Induced Cavitation on a Ducted Propulsor

: Detailed velocity measurements were made of the velocity field behind Propeller 5168 in the Carderock Division, Naval Surface Warfare Center (CDNSWC) 36 inch water tunnel. The measurements were made in order to examine the behavior of the tip vortex flow. A first set of measurements was made with a three component Laser Doppler Velocimeter (LDV) system at four different advance coefficients and at four downstream stations. A second set of measurements was made with a three component, coincident LDV system at a single advance coefficient and downstream station. The coincident set of measurements was of higher accuracy and included data on the Reynolds shear stress terms. Plots are presented of the detailed tip vortex structure, and the variation of this structure with advance coefficient and location is examined. Data on the thrust, torque, and cavitation performance of the propeller are also included.

/pdf/cavitation-and-3-d-ldv-tip-flowfield-measurements-of-5hmuj64612.pdf

Cavitation and 3-D LDV Tip-Flowfield Measurements of Propeller 5168

The tip-leakage vortex occurring on a ducted rotor was examined using both three component Laser Doppler Velocimetry (LDV) and planar Particle Imaging Velocimetry (PIV). The vortex strength and core size were examined for different vortex cross sections downstream of the blade trailing edge. The variability of these quantities are observed with PIV and the average quantities are compared between LDV and PIV. Developed cavitation is also examined for the leakage vortex. The implication of vortex variability on cavitation inception is discussed.

Tip-Leakage Vortex Inception on a Ducted Rotor

: Propeller operation in crashback is technically very challenging, both computationally and experimentally. Propeller 4381 was evaluated experimentally in Carderock's 36-inch (0.91 m) water tunnel. Propeller 4381 was operated in the ahead condition for comparison. Details of the flow field were measured with 3-component laser Doppler velocimetry (LDV) and 2-component particle imaging velocimetry (PIV). Propulsion performance was measured with thrust and torque transducers. Cavitation was documented photographically with a strobe light, and flow visualization was conducted with a vertical laser light sheet for observation of the recirculation region of the propeller in crashback. Results showed that highly random cavitation occurred on the propeller in crashback on the downstream side at the leading edge, which is the trailing edge for ahead. A highly unstable recirculation zone occurred with a ring vortex near the propeller tip. The maximum variation in thrust and torque as computed from the standard deviation relative to the mean load with a value of 0.25 occurred at an advance ratio of J = -0.5. The high variation in loads appeared to correlate with the maximum total kinetic energy relative to tunnel velocity in the flow at the blade tip, which was measured by LDV as 1.5. These results indicate that vibration and wear may be reduced by minimizing operation near J = - 0.5 for this propeller.

Christopher J. Chesnakas

Papers

Detailed Investigation of the Three-Dimensional Separation About a 6:1 Prolate Spheroid

Tip-Vortex Induced Cavitation on a Ducted Propulsor

Cavitation and 3-D LDV Tip-Flowfield Measurements of Propeller 5168

Tip-Leakage Vortex Inception on a Ducted Rotor

Performance of Propeller 4381 in Crashback