scholarly journals Cavitation phenomena within regions of flow separation

1984 ◽  
Vol 140 ◽  
pp. 397-436 ◽  
Author(s):  
Joseph Katz
Keyword(s):  
Mixing Layer ◽  
Subsequent Development ◽  
The Body ◽  
Turbulent Shear ◽  
Laminar Separation ◽  
Cavitation Inception ◽  
Turbulent Shear Layer ◽  
Visualization Techniques ◽  
Axisymmetric Bodies ◽  
Onset Of Cavitation

The phenomenon of cavitation inception was studied on four axisymmetric bodies whose boundary layers underwent a laminar separation and subsequent turbulent reattachment. The non-cavitating flow was studied by holographic and schlieren flow-visualization techniques. Surface distributions on the mean and the fluctuating pressures were also measured. The conditions for cavitation inception and desinence were determined and holograms were recorded just prior to and at the onset of cavitation. The population of microbubbles and the subsequent development of visible cavitation was determined from the reconstructed image. In every case the appearance of visible cavitation was preceded by a cluster of microscopic bubbles in a small portion of the flow field providing clear evidence that cavitation is initiated from small nuclei. The inception zone was located within the turbulent shear layer downstream of transition and upstream of the reattachment region of the bodies with large separation regions. The location and the shape of this cavitation suggested a relation to the mixing-layer eddy structure. The inception region on the body with the smallest separation zone, a hemisphere-cylinder body, was located in the reattachment region, but the cavities were still detached from the surface. Instantaneous minimum-surface-pressure measurements do not account for observed cavitation-inception indices except for the hemisphere body, where the correlation is satisfactory. The rate of cavitation events was estimated from measurements of nuclei population, and fluctuating-pressure statistics in the portion of the flow susceptable to cavitation. It was demonstrated for the hemisphere body that at least one such cavitation event could occur every second. These findings are consistent with what is observed visually at the onset of cavitation and support the location of inception determined holographically.

Cavitation Inception Observations on Axisymmetric Bodies at Supercritical Reynolds Numbers

1976 ◽  
Vol 20 (01) ◽  
pp. 40-50
Author(s):  
V. H. Arakeri ◽  
A. J. Acosta
Keyword(s):  
Boundary Layer ◽  
Reynolds Numbers ◽  
The Body ◽  
Laminar Separation ◽  
Cavitation Inception ◽  
Minimum Pressure ◽  
Pressure Point ◽  
A Site ◽  
Axisymmetric Bodies

A laminar separation on a body provides a site for the inception of cavitation. The separated region disappears when the boundary layer upstream becomes turbulent; this may occur naturally or by stimulation. The consequences of this disappearance on the values of the cavitation inception index and the type and appearance of the cavitation at inception are investigated on three different axisymmetric bodies. On one of these bodies, a hemisphere-cylinder, a trip near the nose so energized the boundary layer that it was impossible for any form of cavitation to remain attached to the body even when a tension of about one half atm. existed at the minimum pressure point on the body.

Cavitation Inception on a Circular Cylinder at Critical and Supercritical Flow Range

1986 ◽  
Vol 108 (4) ◽  
pp. 421-427 ◽  
Author(s):  
A. Ihara ◽  
H. Murai
Keyword(s):  
Circular Cylinder ◽  
Separation Bubble ◽  
Circular Cylinders ◽  
Critical Flow ◽  
Laminar Separation Bubble ◽  
Supercritical Flow ◽  
Laminar Separation ◽  
Cavitation Inception ◽  
Rear Part ◽  
Axisymmetric Bodies

Cavitation tests were performed in the critical and supercritical flow range on circular cylinders with and without boundary layer trip. Mean and fluctuating static pressures were meausred on the smooth circular cylinder from θ = 0 to 180° and on the tripped surface at θ = 104 and 106° corresponding to tripping wire location α = 38 and 40 deg. Through these measurements it was found that cavitation that closely resembles bubble ring cavitation reported on axisymmetric bodies took palce in a reattachment region of the laminar separation bubble for the critical flow range where the laminar separation bubble was present. For the supercritical flow range where the laminar separation bubble disappeared, smooth cavitation with small irregular bubbles at its rear part took place at a location about 100° from the stagnation point.

The Initiation Of Gaseous Microbubble Growth In Laminar Separation Bubbles

1981 ◽  
Vol 103 (4) ◽  
pp. 543-549 ◽  
Author(s):  
B. R. Parkin
Keyword(s):  
Cavitation Bubble ◽  
Separation Zone ◽  
Separation Bubble ◽  
Practical Interest ◽  
The Body ◽  
Laminar Separation Bubble ◽  
Flow Conditions ◽  
Laminar Separation ◽  
Cavitation Inception ◽  
Air Diffusion

Flow conditions surrounding bubble-ring cavitation inception on hemispherical headforms are analyzed with respect to the initiation of air diffusion into microbubbles as is observed to occur at fixed positions in the boundary layer. Fairly recent observations have shown this phenomenon to occur in the laminar separation bubble on the body. The analysis shows, in agreement with the body of experimental evidence now available, that gaseous growth must be preceded by a period of vaporous growth starting in regions of low pressure upstream of the laminar separation bubble. It also appears that the most favorable condition for the initiation of gaseous growth should occur when a typical vapor bubble reaches its maximum radius as it enters the laminar separation bubble. The conditions for the initiation of subsequent gaseous growth, once the cavitation bubble is stabilized in the laminar separation zone, are more demanding. Nevertheless, it is found that the liquid in the water surrounding the bubble in the separation zone is definitely supersaturated for most flows of experimental or practical interest. Therefore, gaseous growth, as well as vaporous growth, is definitely to be associated with the onset of bubble-ring cavitation on both theoretical and experimental grounds.

Correlations Concerning the Process of Flow Deceleration

1975 ◽  
Vol 97 (2) ◽  
pp. 295-300 ◽  
Author(s):  
K. D. Papailiou
Keyword(s):  
Boundary Layer ◽  
Integral Equation ◽  
Separation Bubble ◽  
Turbulent Shear ◽  
Laminar Separation Bubble ◽  
Laminar Separation ◽  
Turbulent Shear Layer ◽  
Profile Losses ◽  
Flow Deceleration ◽  
Momentum Integral

Correlations are used by the engineer for design purposes and/or when a theoretical calculation is missing. The purpose of this paper is to show that, applying the momentum integral equation, it is possible to derive correlations for flow situations associated with a deceleration. Four different cases are presented in the paper: (a) The case of the profile losses in a cascade of blades; (b) the case of the secondary losses in a cascade of blades; (c) the case of the turbulent shear layer before reattachment in a laminar separation bubble; (d) the case of the interaction of a shock with a turbulent boundary layer. At the same time the important factors governing these flow situations are put into evidence.

scholarly journals Detecting vortical structures in time-resolved volumetric flow fields

2021 ◽  
Vol 1 (1) ◽  
Author(s):  
Karuna Agarwal ◽  
Omri Ram ◽  
Jin Wang ◽  
Yuhui Lu ◽  
Joseph Katz
Keyword(s):  
Near Field ◽  
Three Dimensional ◽  
Turbulent Shear ◽  
Time Resolved ◽  
Vortical Structures ◽  
Cavitation Inception ◽  
Turbulent Shear Layer ◽  
Velocity Gradients ◽  
Vortex Detection ◽  
Axial Stretching

The detection of three-dimensional coherent vortical structures that get advected as well as deformed with time is a challenge. However, it is critical for the statistical analysis of these vortices, for example, the quasi-streamwise vortices (QSVs) in the near field of a turbulent shear layer, where cavitation inception typically occurs. These structures exhibit underlying correlations among different properties that can be derived from the velocity gradients. Exploiting these correlations, a pseudo-Lagrangian vortex detection method is proposed that uses k-means clustering based on vorticity magnitude and direction, values of λ2, strain rate structure, axial stretching, and location. The method facilitates the finding that QSVs have pressure minima that are lower than those in the surrounding flow, including the primary spanwise vortices. These minima typically appear after a period of axial stretching and before contraction events.

Features of a reattaching turbulent shear layer in divergent channelflow

AIAA Journal ◽  
1985 ◽  
Vol 23 (2) ◽  
pp. 163-171 ◽  
Author(s):  
David M. Driver ◽  
H. Lee Seegmiller
Keyword(s):  
Shear Layer ◽  
Turbulent Shear ◽  
Turbulent Shear Layer

scholarly journals Optimal nonlinear eddy viscosity in Galerkin models of turbulent flows

2015 ◽  
Vol 766 ◽  
pp. 337-367 ◽  
Author(s):  
Bartosz Protas ◽  
Bernd R. Noack ◽  
Jan Östh
Keyword(s):  
Turbulent Flows ◽  
High Speed ◽  
Eddy Viscosity ◽  
Broad Class ◽  
Constitutive Relations ◽  
Body Height ◽  
Mixing Layer ◽  
The Body ◽  
Stream Velocity ◽  
Initial Vorticity

AbstractWe propose a variational approach to the identification of an optimal nonlinear eddy viscosity as a subscale turbulence representation for proper orthogonal decomposition (POD) models. The ansatz for the eddy viscosity is given in terms of an arbitrary function of the resolved fluctuation energy. This function is found as a minimizer of a cost functional measuring the difference between the target data coming from a resolved direct or large-eddy simulation of the flow and its reconstruction based on the POD model. The optimization is performed with a data-assimilation approach generalizing the 4D-VAR method. POD models with optimal eddy viscosities are presented for a 2D incompressible mixing layer at $\mathit{Re}=500$ (based on the initial vorticity thickness and the velocity of the high-speed stream) and a 3D Ahmed body wake at $\mathit{Re}=300\,000$ (based on the body height and the free-stream velocity). The variational optimization formulation elucidates a number of interesting physical insights concerning the eddy-viscosity ansatz used. The 20-dimensional model of the mixing-layer reveals a negative eddy-viscosity regime at low fluctuation levels which improves the transient times towards the attractor. The 100-dimensional wake model yields more accurate energy distributions as compared to the nonlinear modal eddy-viscosity benchmark proposed recently by Östh et al. (J. Fluid Mech., vol. 747, 2014, pp. 518–544). Our methodology can be applied to construct quite arbitrary closure relations and, more generally, constitutive relations optimizing statistical properties of a broad class of reduced-order models.

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