Single-phase mixing through a narrow gap

Mixing through narrow gaps connecting adjacent flow paths is an important mass and heat transfer process for many thermo-hydraulic applications. Such flows are considered balanced when the inlet flow speeds of adjacent subchannels are matched. In the present work, experimental observations are presented for balanced and unbalanced flows including the mixing coefficients and flow visualization within the gap. The large coherent structures are identified, with frequency in general agreement with those reported by previous investigators. To utilize Proper Orthogonal Decomposition (POD) for the discrete data yielded by PIV, we employ method of Singular Value Decomposition (SVD). The bulk of the mixing is attributed to the dominant modes and demonstrate that mixing rates estimated from velocity measurements are in fair agreement with mixing coefficients based on tracer concentration measurements

Velocity field measurements of cavitating flows

A particle Image Velocimetry (PIV) system has been developed to study the microfluid mechanics of cavitating flows. Planar PIV was used to examine the non-cavitating flow in the thin boundary layer near a hydrofoil surface for the cases of a naturally developing boundary layer and a boundary layer stimulated to turbulence by roughness near the foil leading edge. PIV was also used to examine the flow near the surface of individual cavitation bubbles and incipient attached cavitation. A system was devised to create a single nucleus in the flow upstream of a hydrofoil, and planar PIV was used to study the flow around the resulting traveling cavitation bubble. Velocity vectors were determined close to the solid surfaces and the gas/liquid interfaces of the bubbles. Seeding of the flow with particles did not result in the addition of active cavitation nuclei.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47070/1/348_2004_Article_BF00189302.pd

Instantaneous and time-averaged flow fields of multiple vortices in the tip region of a ducted propulsor

The instantaneous and time-averaged flow fields in the tip region of a ducted marine propulsor are examined. In this flow, a primary tip-leakage vortex interacts with a secondary, co-rotating trailing edge vortex and other co- and counter-rotating vorticity found in the blade wake. Planar particle imaging velocimetry (PIV) is used to examine the flow in a plane approximately perpendicular to the mean axis of the primary vortex. An identification procedure is used to characterize multiple regions of compact vorticity in the flow fields as series of Gaussian vortices. Significant differences are found between the vortex properties from the time-averaged flow fields and the average vortex properties identified in the instantaneous flow fields. Variability in the vortical flow field results from spatial wandering of the vortices, correlated fluctuations of the vortex strength and core size, and both correlated and uncorrelated fluctuations in the relative positions of the vortices. This variability leads to pseudo-turbulent velocity fluctuations. Corrections for some of this variability are performed on the instantaneous flow fields. The resulting processed flow fields reveal a significant increase in flow variability in a region relatively far downstream of the blade trailing edge, a phenomenon that is masked through the process of simple averaging. This increased flow variability is also accompanied by the inception of discrete vortex cavitation bubbles, which is an unexpected result, since the mean flow pressures in the region of inception are much higher than the vapor pressure of the liquid. This suggests that unresolved fine-scale vortex interactions and stretching may be occurring in the region of increased flow variability.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47076/1/348_2005_Article_938.pd

On multi-point gas injection to form an air layer for frictional drag reduction

Air layer drag reduction has been shown to be a feasible drag reducing technique at the laboratory and at full ship scales. In most studies, the air layers have been generated via gas injection from two-dimensional spanwise slots. However, given ship's structural considerations, it would be preferable to use discrete holes. The present study expands on the work on single orifice gas injection to multi-hole injection. When compared with slot injection, multi-point injection lead to a reduced range of gas fluxes that formed an air layer. Gas injected from a series of discrete holes can exhibit complex flow patterns, including roll-up into the core of liquid vortices that form as part of the process of injecting gas into the liquid boundary layer. The finite span and length of the model utilized for the present experiments was modest. It remains to be shown if a larger model with similar scaled up geometry (and with more beanwise holes) would enable the formation of a stable air layer with a gas flux per unit span that is similar to that required for slot injection. Nevertheless, the results presented here illustrate the complexity associated with gas injection through multiple perforations in a hull

Experimental Methods for the Study of Hydrodynamic Cavitation

A review of traditional and novel experimental methods for the investigation of hydrodynamic cavitation is presented. The importance of water quality is discussed, along with its characterization and management. Methods for the direct and indirect experimental determination of cavitation inception are presented. Along with traditional optical visualization, methods of measuring developed cavitation are described, including point and surface electrical probes, optical bubble probes, acoustic measurements, and indirect measurements of noise and vibration. Recent developments in the use of ionizing radiation as a means to visualize cavitating flows are also discussed

Time resolved X-ray densitometry for cavitating and ventilated partial cavities

Gas cavities, natural and ventilated, can occur on ship propulsors, control surfaces and hulls. These cavities can be significant sources of small bubbles in the ship wake. We present a two-dimensional time resolved X-ray densitometry system developed for investigation of natural, ventilated and mixed cavities. First we consider the limitations and performance of our X-ray system, and compare the measured void fraction to stationary known void fraction, and to data for a ventilated cavity obtained using dual fiber optical probes. Second, we present preliminary data from time-resolved X-ray used to observe the overall dynamics and time dependent void fraction distribution of a cavitating backward facing step with and without gas injection. © 2013 - IOS Press and the authors. All rights reserved

Effect of Non-Condensable Gas Injection on Cavitation Dynamics of Partial Cavities

Partial cavities can undergo auto-oscillation causing large pressure pulsations, unsteady loading of machinery and generate significant noise. In the current experiments fully shedding cavities forming in the separated flow region downstream of a wedge were investigated. The Reynolds number based on hydraulic diameter was of the order of one million. The cavity dynamics were studied with and without injection of non-condensable gas into the cavity. Gas was injected directly into the cavitation region downstream of the wedge's apex, or into the recirculating region at mid cavity so that for the same amount of injected gas less ended up in the shear layer. It was found that relatively miniscule amounts of gas introduced into the shear layer at the cavity interface can reduce vapour production and dampen the auto oscillations, and the same amount of gas injected into the mid cavity would not have the same effect. The authors also examined whether the injected gas can switch the shedding mechanism from one dominated by condensation shock to one dominantly by reentrant jet

Interaction of a Compressible Bubbly Flow with an Obstacle Placed Within a Shedding Partial Cavity

Bubbly shocks act as the dominant mechanism of shedding of partial cavities under certain conditions. The compressible nature of low void fraction regions in a partial cavity can provide conditions necessary for the existence of such bubbly shocks. Using X-ray densitometry as a flow visualisation mechanism, the dynamics of bubbly shock fronts with and without the presence of an obstacle are presented. Different flow features associated with the shock dynamics are identified and the associated flow physics explained