Michael L. Billet

Thermodynamic Effects on Developed Cavitation

The results of an investigation of thermodynamic effects are presented. Distributions of temperature and pressure in a developed cavity were measured for zero- and quarter-caliber ogives. A semiempirical entrainment theory was developed to correlate the measured temperature depression in the cavity. This theory correlates the maximum temperature depression expressed in dimensionless form as the Jakob number in terms of the dimensionless numbers of Nusselt, Reynolds, Froude, and Peclet, and dimensionless cavity length, L/D. The results show that in general, the temperature depression increases with L/D and temperature and the cavitation number based on measured cavity pressure is a function of L/D for a given model contour, independent of the thermodynamic effect.

Correlations of thermodynamic effects for developed cavitation

The net positive suction head (NPSH) requirements for a pump are determined by the combined effects of cavitation, fluid properties, pump geometry, and pump operating point. An important part of this determination is the temperature depression (Delta T). Correlations are presented of the temperature depression for various degrees of developed cavitation on venturis and ogives. These correlations, based on a semi-empirical entrainment theory, express Delta T in terms of the dimensionless numbers of Nusselt, Reynolds, Froude, Weber, and Peclet, and dimensionless cavity length (L/D). The Delta T data were obtained in Freon 114, hydrogen and nitrogen for the venturis and in Freon 113 and water for the ogives.

Experimental Investigation of a Jet Impinging on a Ground Plane in Crossflow

An experimental investigation has been conducted in a wind tunnel to model the impingement of high-velocity jet exhaust flow on the ground, as encountered by V/STOL aircraft. A constant jet velocity was maintained while varying the wind tunnel crossflow velocity, upstream boundary-layer thickness, and height from the ground to the jet exit plane. The radial wall jet, when interacting with the crossflow, forms an oscillating horseshoe-shaped separation bubble, commonly referred to in the literature as a ground vortex. The streamwise distance of the separation point from the jet impingement point is documented here as a function of the flow parameters and geometry. Flow visualization of the flowfield and two-component laser Doppler velocimeter measurements taken through the separation bubble indicate that the separation bubble is highly unsteady and nonsymmetric. This unsteadiness may be related to shear-layer vortices shed from the lip of the jet. Thickening of the upstream boundary layer on the ground plane caused the wall jet to penetrate further upstream. The addition of a large plate flush-mounted to the jet exit caused the ground vortex to move downstream and also decreased the size of the ground vortex.

Gaussian beam effects in far-field in-line holography

Far-field in-line holography has been studied in detail when one or both of the beams used for recording and reconstruction are Gaussian. The contrast of the high frequency interference fringes and hence their recordability have been investigated with a specific example of the object having circular cross section. For reconstruction the effects of uniform as well as Gaussian beams are studied and compared.

High-Reynolds Number Liquid Flow Measurements

Liquid flow facilities are usually categorized as towing tanks, seakeeping and maneuvering basins, circulating water channels, blowdown facilities, and water tunnels. A very complete listing of such facilities has been compiled by the 16th International Towing Tank Conference (ITTC) (1985) and it includes most of the major water facilities in, the world. A description of new and planned cavitation facilities is given in Moll and Billet (1987). The intent of this chapter is to discuss the various experimental techniques used in hydrodynamic testing at high Reynolds numbers. This usually implies that water tunnels or tow tanks are used. Because Gad-el-Hak (1987) recently reviewed tow tank testing, our emphasis here is on water (or other liquid) tunnel measurements.

In-line Fraunhofer holography at a few far fields

The process of in-line Fraunhofer holography of particles is studied by an analysis of the four terms in the reconstructed field. Specific results are discussed for objects with circular cross section. The image shape and contrast are described in detail. The study is particularly useful when the recording is performed at a few far fields. It has been found that the recording at a higher far field is better due to a higher image-to-background irradiance ratio at the edges of the reconstructed image.

An experimental study of travelling-bubble cavitation noise

Abstract : In this investigation, high-frequency travelling-bubble cavitation noise generated on a Schiebe headform is analyzed with regard to noise theories which are based on the collapse of a single bubble. After normalizing the noise level by an event rate, -sigma C(p(min)) appears to provide a reasonable scaling factor for the noise level radiated per collapse, where sigma is the cavitation number and C(p(min)) is the minimum pressure coefficient on the headform. At frequencies above 10 kHz and free-stream flow velocities between 9.1 and 12.2 mps (30 and 40 fps), the noise level per collapse varied as roughly 1/sigma to the 8.4 power, and was constant with the flow velocity for constant -sigma/C(p(min)). The energy density spectra were flat from 10 to 100 kHz regardless of the cavitation number or free-stream velocity. (Author)

Volume magnification in particle field holography

An image-object volume relationship in holography for applications in quantitative particle fields is determined in terms of known physical parameters. The general relationships are described for collimated, point reference, or reconstruction sources. Some relevant methods for correlating the transverse magnification at different longitudinal distances are also described.

The Influence of Pressure Gradient on Desinent Cavitation From Isolated Surface Protrusions

An experimental investigation was conducted to study the desinent cavitation characteristics of various sizes of two-dimensional triangular and circular arc protrusions in a turbulent boundary layer for favorable, zero, and unfavorable pressure gradients. The roughness height (h ) varied from 0.025 cm (0.01 in.) to 0.762 cm (0.30 in.) and the relative height (h /δ) varied from 0.026 to 2.53. Desinent cavitation numbers (σd ) were obtained visually over a velocity range of 9.1 mps (30 fps) to 18.3 mps (60 fps) at an average total air content of 3.8 ppm (mole basis). The data for zero pressure gradient were in fair agreement with data obtained for the same protrusion shapes by Holl (1958). The cavitation number (σd ) was correlated with relative height (h /δ), Reynolds number (U δ/ν) and Clauser’s (1954) equilibrium boundary layer shape factor (G ) which includes the effect of pressure gradient. The data show that σd increases with pressure gradient. This result was not expected since it appears to contradict the trends implied by the so-called characteristic velocity theory developed by Holl (1958).

Reduction in size and unsteadiness of VTOL ground vortices by ground fences

A ground vortex, produced when a jet impinges on the ground in the presence of crossflow, is encountered by V/STOL aircraft hovering near the ground, and is known to be hazardous to the aircraft. The objective of this research was to identify a ground-based technique by which both the mean size and fluctuation in size of the ground vortex could be reduced. A simple passive method has been identified and examined in the laboratory. Specifically, one or two fine wire mesh screens (ground fences), bent in a horseshoe shape, located on the ground in front of the jet impingement point, proved to be very effective. The ground fences work by decreasing the momentum of the upstream-traveling wall jet, causing an effectively higher freestream-to-jet velocity ratio VJ Vj, and therefore, a ground vortex smaller in size and unsteadiness. At VjVj = 0.15, the addition of a single ground fence resulted in a 70% reduction in mean size of the ground vortex. With two ground fences, the mean size decreased by about 85%. Fluctuations in size decreased nearly in proportion to the mean size, for both the single and double fence configurations. These results were consistent over a wide range of jet Reynolds number (104 < Re]vi < 10s); further development and full-scale Reynolds number testing are required, however, to determine if this technique can be made practical for the case of actual VTOL aircraft. Nomenclature A = cross-sectional area of a slice through the centerline of the ground vortex dj = i.d. of jet h = distance from the nozzle exit plane to the ground