Particle image velocimetry ͑PIV͒ and high-speed photography are used to measure the flow structure at the closure region and downstream of sheet cavitation. The experiments are performed in a water tunnel of cross section 6.35ϫ5.08 cm 2 whose test area contains transparent nozzles with a prescribed pressure distribution. This study presents data on instantaneous and averaged velocity, vorticity and turbulence when the ambient pressure is reduced slightly below the cavitation inception level. The results demonstrate that the collapse of the vapor cavities in the closure region is the primary mechanism of vorticity production. When the cavity is thin there is no reverse flow downstream and below the cavitation, i.e., a reentrant flow does not occur. Instead, the cavities collapse as the vapor condenses, creating in the process hairpin-like vortices with microscopic bubbles in their cores. These hairpin vortices, some of which have sizes as much as three times the height of the stable cavity, dominate the flow downstream of the cavitating region. The averaged velocity distributions show that the unsteady collapse of the cavities in the closure region involves substantial increase in turbulence, momentum, and displacement thickness. Two series of tests performed at the same velocity and pressure, i.e., at the same hydrodynamic conditions, but at different water temperatures, 35°C and 45°C, show the effect of small changes in the cavitation index ͑ϭ4.69 vs. ϭ4.41͒. This small decrease causes only a slight increase in the size of the cavity, but has a significant impact on the turbulence level and momentum deficit in the boundary layer downstream. Ensemble averaging of the measured instantaneous velocity distributions is used for estimating the liquid void fraction, average velocities, Reynolds stresses, turbulent kinetic energy and pressure distributions. The results are used to examine the mass and momentum balance downstream of the cavitating region. It is shown that in dealing with the ensemble-averaged flow in the closure region of attached cavitation, one should account for the sharp ͑but still finite͒ gradients in the liquid void fraction. The 2-D continuity equation can only be satisfied when the gradients in void fraction are included in the analysis. Using the momentum equation it is possible to estimate the magnitude of the ''interaction term,'' i.e., the impact of the vapor phase on the liquid momentum. It is demonstrated that, at least for the present test conditions, the interaction term can be estimated as the local pressure multiplied by the gradient in void fraction.

Cavitation experiments performed in the near field of a 50 mm diameter (D) jet at ReD = 5 × 105, showed inception in the form of inclined ‘cylindrical’ bubbles at axial distances (x/D) less than 0.55, with indices of 2.5. On tripping the boundary layer, cavitation inception occurred at x/D ≈ 2, as distorted ‘spherical’ bubbles with inception indices of 1.7. To investigate these substantial differences, the near field of the jet was measured using PIV. Data on the primary flow, the strength distribution of the ‘streamwise’ vortices and the velocity profiles within the initial boundary layers were obtained. The untripped case showed a direct transition to three-dimensional flow in the near field (x/D < 0.7) even before rolling up to distinct vortex rings. Strong ‘streamwise’ vortices with strengths up to 25% of the jet velocity times the characteristic wavelength were seen. Cavitation inception occurred in the core of these vortices. In contrast, in the tripped jet the vortex sheet rolled up to the familiar Kelvin–Helmholtz vortex rings with weak secondary vortices. Using the measured nuclei distribution, strengths and straining of the ‘streamwise’ structures, the rates of cavitation events were estimated. The estimated results match very well the measured cavitation rates. Also, the Reynolds stresses in the near field of the jet show similar trends and magnitudes to those of Browand & Latigo (1979) and Bell & Mehta (1990) for a plane shear layer.

This paper examines in detail the flow structure and associated wall pressure fluctuations caused by the injection of a round, turbulent jet into a turbulent boundary layer. The velocity ratio, r, ratio of mean jet velocity to the mean cross flow, varies from 0.5 to 2.5 and the Reynolds number based on the cross flow speed and jet diameter is 1.9ϫ10 4 . Particle image velocimetry is used to measure the flow and flush mounted pressure sensors installed at several locations used to determine the wall pressure. The results consist of sample instantaneous flow structures, distributions of mean velocity, vorticity and turbulence intensity, as well as wall pressure spectra. The flow structure depends strongly on the velocity ratio and there are two distinctly different regions. At low velocity ratios, namely rϽ2, a semicylindrical vortical layer ͑''shell''͒ forms behind the jet, enclosing a domain with slow moving reverse flow. The vorticity in this semicylindrical shell originates from the jet shear layer. Conversely, at high velocity ratios, namely rϾ2, the near-wall flow behind the jet resembles a Karman vortex street and the wall-normal vortical structures contain cross flow boundary layer vorticity. Autospectra of the pressure signals show that the effect of the jet is mainly in the 15-100 Hz range. At rϽ2, the wall pressure fluctuation levels increase with r. At rϾ2, the wall pressure levels reach a plateau demonstrating the diminishing effect of the jet on the near-wall flow. Consistent with the flow structure, the highest wall pressure fluctuations occur off the jet centerline for rϽ2 and along the jet centerline for rϾ2. Also, the advection speed of near-wall vortical structures increase with r at rϽ2, while at rϾ2 it is a constant.

This paper focuses on the onset of tip-leakage cavitation on a fixed hydrofoil. The objectives are to investigate the effect of gap size on the flow structure, conditions of cavitation inception, the associated bubble dynamics and cavitation noise. The same hydrofoil with three tip gap sizes of 12%, 28%, and 52% of the maximum tip thickness are studied. Controlled cavitation tests are performed after de-aerating the water in the tunnel and using electrolysis to generate cavitation nuclei. The experiments consist of simultaneously detecting cavitation inception using a 2000 fps digital camera (visual) and two accelerometers (“acoustic”) mounted on the test section windows. Good agreement between these methods is achieved when the visual observations are performed carefully. To obtain the time-dependent noise spectra, portions of the signal containing cavitation noise are analyzed using Hilbert-Huang transforms. Rates of cavitation events as a function of the cavitation index (σ) for the three gap sizes are also measured. The cavitation inception index decreases with increasing gap sizes. The experiments demonstrate that high-amplitude noise spikes are generated when the bubbles are distorted and “shredded”—broken to several bubbles following their growth in the vortex core. Mere changes to bubble size and shape caused significantly lower noise. High-resolution particle image velocimetry (PIV) with a vector spacing of 180 μm is used to measure the flow, especially to capture the slender tip vortices where cavitation inception is observed. The instantaneous realizations are analyzed to obtain probability density functions of the circulation of the leakage vortex. The circulation decreases with increasing gap sizes and minimum pressure coefficients in the cores of these vortices are estimated using a Rankine model. The diameter of the vortex core varied between 540–720 μm. These coefficients show a very good agreement with the measured cavitation inception indices.

In hybrid electric vehicles (HEVs), the inverter is a critical component in the power module, which conditions the flow of electric power between the AC motor and the DC battery pack. The inverter includes a number of insulated gate bipolar transistors (IGBTs), which are high-frequency switches used in bi-directional DC-AC conversion. The heat generated in the IGBTs can result in degraded performance, reduced lifetime, and component failures. Heat fluxes as high as 250 W/cm 2 may occur, which makes the thermal management problem quite important. In this paper, the potential of self-oscillating jets to cool IGBTs in HEV power modules is investigated experimentally.A full factorial design of experiments was used to explore the impact of nozzle design, oscillation frequency, jet flow rate, nozzle-to-target distance, and jet configuration (freesurface or submerged) on heat transfer from a simulated electronic chip surface. In the free-surface configuration, selfoscillating jets yielded up to 18% enhancement in heat transfer over a steady jet with the same parasitic power consumption. An enhancement of up to 30% for the same flow rate (and velocity since all nozzles have the same exit area) was measured. However, in the submerged configuration, amongst the nozzle designs tested, the self-oscillating jets did not yield any enhancements in heat transfer over comparable steady jets. Results also suggest that oscillating jets provide a more uniform surface temperature distribution than steady jets.