Transcritical Patterns of Cavitating Flow and Trends of Acoustic Level

2001 ◽  
Vol 123 (4) ◽  
pp. 850-856 ◽  
Author(s):  
Wei Gu ◽  
Yousheng He ◽  
Tianqun Hu
Keyword(s):  
Liquid Flow ◽  
High Speed ◽  
Cavity Flow ◽  
Periodic Flow ◽  
Cavitating Flows ◽  
Cavitation Noise ◽  
Cloud Cavitation ◽  
Three Stages ◽  
Cavity Shedding ◽  
Axisymmetric Bodies

Hydroacoustics of the transcritical cavitating flows on a NACA16012 hydrofoil at a 2/5/8-degree angle of attack and axisymmetric bodies with hemispherical and 45-degree conical headforms were studied, and the process of cloud cavitation shedding was observed by means of high-speed cinegraphy. By expressing the cavitation noise with partial acoustic level, it is found that the development of cavitation noise varies correspondingly with cavitation patterns. The instability of cavitation is a result of cavity-flow interaction, and is mainly affected by the liquid flow rather than by the cavitation bubbles. A periodic flow structure with a large cavitation vortex is observed and found to be responsible for inducing the reentrant-jet and consequent cavitation shedding, and explains the mechanism of periodic cavitation shedding from a new viewpoint. New terms for the three stages, growing, hatching and breaking, are used to describe the process of cavity shedding.

Combined Experimental and Computational Investigation of Unsteady Structure of Sheet/Cloud Cavitation

2013 ◽  
Vol 135 (7) ◽  
Author(s):  
Biao Huang ◽  
Yin L. Young ◽  
Guoyu Wang ◽  
Wei Shyy
Keyword(s):  
High Speed ◽  
Suction Side ◽  
Cavitating Flows ◽  
Cloud Cavitation ◽  
Image Velocimetry ◽  
Moderate Reynolds Number ◽  
Wake Patterns ◽  
Vorticity Dynamics ◽  
Cavity Shedding ◽  
Vorticity Production

The objective of this paper is to apply combined experimental and computational modeling to investigate unsteady sheet/cloud cavitating flows. In the numerical simulations, a filter-based density corrected model (FBDCM) is introduced to regulate the turbulent eddy viscosity in both the cavitation regions on the foil and in the wake, which is shown to be critical in accurately capturing the unsteady cavity shedding process, and the corresponding velocity and vorticity dynamics. In the experiments, high-speed video and particle image velocimetry (PIV) technique are used to measure the flow velocity and vorticity fields, as well as cavitation patterns. Results are presented for a Clark-Y hydrofoil fixed at an angle of attack of α = 8 deg at a moderate Reynolds number, Re = 7 × 105, for both subcavitating and sheet/cloud cavitating conditions. The results show that for the unsteady sheet/cloud cavitating case, the formation, breakup, shedding, and collapse of the sheet/cloud cavity lead to substantial increase in turbulent velocity fluctuations in the cavitating region around the foil and in the wake, and significantly modified the wake patterns. The turbulent boundary layer thickness is found to be much thicker, and the turbulent intensities are much higher in the sheet/cloud cavitating case. Compared to the wetted case, the wake region becomes much broader and is directed toward the suction side instead of the pressure side for the sheet/cloud cavitation case. The periodic formation, breakup, shedding, and collapse of the sheet/cloud cavities, and the associated baroclinic and viscoclinic torques, are shown to be important mechanisms for vorticity production and modification.

Mechanism and Control of Cloud Cavitation

1997 ◽  
Vol 119 (4) ◽  
pp. 788-794 ◽  
Author(s):  
Y. Kawanami ◽  
H. Kato ◽  
H. Yamaguchi ◽  
M. Tanimura ◽  
Y. Tagaya
Keyword(s):  
High Speed ◽  
Leading Edge ◽  
Generation Mechanism ◽  
Pressure Measurements ◽  
Foil Surface ◽  
Cavitation Noise ◽  
Cloud Cavitation ◽  
High Speed Video ◽  
And Control ◽  
Small Obstacle

Generation mechanism of cloud cavitation on a hydrofoil section was investigated in a sequence of experiments through observation of cloud cavitation by high-speed video and high-speed photo as well as pressure measurements by pressure pick-ups and a hydrophone. The mechanism was also investigated by controlling cloud cavitation with an obstacle fitted on the foil surface. From the results of these experiments, it was found that the collapse of a sheet cavity is triggered by a re-entrant jet rushing from the trailing edge to the leading edge of the sheet cavity, and consequently, the sheet cavity is shed in the vicinity of its leading edge and thrown downstream as a cluster of bubbles called cloud cavity. In other words, the re-entrant jet gives rise to cloud cavitation. Moreover, cloud cavitation could be controlled effectively by a small obstacle placed on the foil. It resulted in reduction of foil drag and cavitation noise.

Growth Process to Cloud-Like Cavitation on Separated Shear Layer

2003 ◽  
Author(s):  
Yasuhiro Saito ◽  
Keiichi Sato
Keyword(s):  
Shear Layer ◽  
Liquid Flow ◽  
High Speed ◽  
Separation Bubble ◽  
Laser Sheet ◽  
Video Camera ◽  
Cloud Cavitation ◽  
Separated Shear Layer ◽  
High Speed Video Camera ◽  
Divergent Channel

It is important to explain a high-speed liquid flow phenomenon because of the developments of various fluid machines. Cavitation is one of the most important phenomena in high-speed liquid flow. When cavitation appears in a liquid flow, it causes some problems such as performance deterioration, vibrations, noise, and cavitation damage. So, it is important to observe the cavity behavior to solve such problems. Especially, it is well known that a cloud cavitation has heavy unsteadiness and high impulse. In this study, a fundamental experimental research on the process of growing to the cloud-like cavitation was performed. The aspect of the cavitation in a circular cylindrical orifice and in a convergent-divergent channel was observed by the high-speed video camera. As a result, the unsteady shedding behavior of the vortex cavity was characteristically observed in a region of the separation bubble at an inlet of a circular cylindrical orifice. There are several vortex cavities on the separated shear layer. These cavities behave pairing and coalescence with each other repeatedly and grow up into a cloud-like cavity. The cloud-like cavity is shed downstream and then generates high impulses at the collapse. In the case of a circular cylindrical orifice, the cavities can be observed clearly by using a laser sheet method. Similar behaviors of the cavity are observed on the separated shear layer in a two dimensional convergent-divergent channel. This paper discusses how the behaviors of vortex cavity relate closely to the flow pattern at non-cavitating condition on the separated shear layer and that the process of the vortex cavities develop to the cloud-like cavity.

scholarly journals Bubbly shock propagation as a mechanism for sheet-to-cloud transition of partial cavities

2016 ◽  
Vol 802 ◽  
pp. 37-78 ◽  
Author(s):  
Harish Ganesh ◽  
Simo A. Mäkiharju ◽  
Steven L. Ceccio
Keyword(s):  
Mach Number ◽  
Void Fraction ◽  
High Speed ◽  
Large Scale ◽  
Dynamic Pressure ◽  
Cavity Flow ◽  
Laboratory Frame ◽  
Shock Propagation ◽  
Time Resolved ◽  
Cloud Cavitation

Partial cavitation in the separated region forming from the apex of a wedge is examined to reveal the flow mechanism responsible for the transition from stable sheet cavity to periodically shedding cloud cavitation. High-speed visualization and time-resolved X-ray densitometry measurements are used to examine the cavity dynamics, including the time-resolved void-fraction fields within the cavity. The experimentally observed time-averaged void-fraction profiles are compared to an analytical model employing free-streamline theory. From the instantaneous void-fraction flow fields, two distinct shedding mechanisms are identified. The classically described re-entrant flow in the cavity closure is confirmed as a mechanism for vapour entrainment and detachment that leads to intermittent shedding of smaller-scale cavities. But, with a sufficient reduction in cavitation number, large-scale periodic cloud shedding is associated with the formation and propagation of a bubbly shock within the high void-fraction bubbly mixture in the separated cavity flow. When the shock front impinges on flow at the wedge apex, a large cloud is pinched off. For periodic shedding, the speed of the front in the laboratory frame is of the order of half the free-stream speed. The features of the observed condensation shocks are related to the average and dynamic pressure and void fraction using classical one-dimensional jump conditions. The sound speed of the bubbly mixture is estimated to determine the Mach number of the cavity flow. The transition from intermittent to transitional to strongly periodic shedding occurs when the average Mach number of the cavity flow exceeds that required for the generation of strong shocks.

scholarly journals Observations of shock waves in cloud cavitation

1998 ◽  
Vol 355 ◽  
pp. 255-283 ◽  
Author(s):  
G. E. REISMAN ◽  
Y.-C. WANG ◽  
C. E. BRENNEN
Keyword(s):  
Shock Wave ◽  
Shock Waves ◽  
High Speed ◽  
Leading Edge ◽  
Wave Structure ◽  
Cavitating Flows ◽  
Cloud Cavitation ◽  
Radiated Noise ◽  
Wave Dynamics ◽  
New Perspective

This paper describes an investigation of the dynamics and acoustics of cloud cavitation, the structures which are often formed by the periodic breakup and collapse of a sheet or vortex cavity. This form of cavitation frequently causes severe noise and damage, though the precise mechanism responsible for the enhancement of these adverse effects is not fully understood. In this paper, we investigate the large impulsive surface pressures generated by this type of cavitation and correlate these with the images from high-speed motion pictures. This reveals that several types of propagating structures (shock waves) are formed in a collapsing cloud and dictate the dynamics and acoustics of collapse. One type of shock wave structure is associated with the coherent collapse of a well-defined and separate cloud when it is convected into a region of higher pressure. This type of global structure causes the largest impulsive pressures and radiated noise. But two other types of structure, termed ‘crescent-shaped regions’ and ‘leading-edge structures’ occur during the less-coherent collapse of clouds. These local events are smaller and therefore produce less radiated noise but the interior pressure pulse magnitudes are almost as large as those produced by the global events.The ubiquity and severity of these propagating shock wave structures provides a new perspective on the mechanisms reponsible for noise and damage in cavitating flows involving clouds of bubbles. It would appear that shock wave dynamics rather than the collapse dynamics of single bubbles determine the damage and noise in many cavitating flows.

The dynamics of partial cavity formation, shedding and the influence of dissolved and injected non-condensable gas

2017 ◽  
Vol 829 ◽  
pp. 420-458 ◽  
Author(s):  
Simo A. Mäkiharju ◽  
Harish Ganesh ◽  
Steven L. Ceccio
Keyword(s):  
Production Rate ◽  
Void Fraction ◽  
High Speed ◽  
Volume Fraction ◽  
Cavity Flow ◽  
Gas Content ◽  
Dissolved Gas ◽  
Cavity Shedding ◽  
Visual Imaging ◽  
Set Up

In the present study, the experimental set-up of Ganesh et al. (J. Fluid Mech., vol. 802, 2016, pp. 37–78) is used to examine the dynamics of a shedding cavity by examining the vapour production rate of the natural cavity and determining how minimal injection of non-condensable gas can substantially alter the vapour production rate, the resulting cavity flow and the cavity shedding process. The influence of the dissolved gas content on the shedding natural cavity flow is also examined. High-speed visual imaging and cinemagraphic X-ray densitometry were used to observe the void fraction dynamics of the cavity flow. Non-condensable gas is injected across the span of the cavity flow at two locations: immediately downstream of the cavity detachment location at the apex of the wedge or further downstream into mid-cavity. The gas injected near the apex is found to increase the pressure near the suction peak, which resulted in the suppression of vapour formation. Hence, the injection of gas could result in a substantial net reduction in the overall cavity void fraction. Injection at the mid-cavity did less to suppress the vapour production and resulted in less significant modification of both the mean cavity pressure and net volume fraction. Changes in the cavity void fraction, in turn, altered the dynamics of the bubbly shock formation. Variation of the dissolved gas content alone (i.e. without injection) did not significantly change the cavity dynamics.

scholarly journals Investigation of Two Mechanisms Governing Cloud Cavitation Shedding: Experimental Study and Numerical Highlight

2016 ◽  
Author(s):  
Kilian Croci ◽  
Petar Tomov ◽  
Florent Ravelet ◽  
Amélie Danlos ◽  
Sofiane Khelladi ◽  
...  
Keyword(s):  
High Speed ◽  
3D Model ◽  
Saturation Pressure ◽  
Image Sequences ◽  
Crucial Importance ◽  
Two Phase ◽  
Cloud Cavitation ◽  
X Ray ◽  
Cavity Shedding ◽  
Venturi Nozzle

Cavitation is a phenomenon of classical interest which can be observed in various applications. It consists in a transition of phase due to a pressure drop under the saturation pressure of a liquid. The unsteady behavior of this phenomenon leads to generate some issues such as erosion, noise or vibrations: as a result the comprehension of the cavity dynamics remains of crucial importance. Unsteady cavitation has been investigated in numerous studies and a mechanism of re-entrant jet has been firstly identified as responsible of the cavity shedding process. Recently, a second shedding mechanism, induced by a shock wave propagation due to the condensation of vapor structures, has been experimentally highlighted with X-ray measurements [1]. The present paper focuses on the experimental detection, with a wavelet method, of these two shedding features on 2D image sequences recorded with a high-speed camera about a double transparent horizontal Venturi nozzle with 18°/8° convergent/divergent angles respectively. A compressible two-phase flow numerical 3D model is performed in complement in order to illustrate some phenomena hardly perceptible experimentally.

An experimental investigation of cloud cavitation about a sphere

2010 ◽  
Vol 656 ◽  
pp. 147-176 ◽  
Author(s):  
P. A. BRANDNER ◽  
G. J. WALKER ◽  
P. N. NIEKAMP ◽  
B. ANDERSON
Keyword(s):  
Boundary Layer ◽  
High Speed ◽  
Single Phase ◽  
Leading Edge ◽  
Series Data ◽  
Variable Pressure ◽  
Cloud Cavitation ◽  
Pixel Intensity ◽  
Edge Structure ◽  
Cavity Shedding

Cloud cavitation occurrence about a sphere is investigated in a variable-pressure water tunnel using low- and high-speed photography. The model sphere, 0.15 m in diameter, was sting-mounted within a 0.6 m square test section and tested at a constant Reynolds number of 1.5 × 106 with cavitation numbers varying between 0.36 and 1.0. High-speed photographic recordings were made at 6 kHz for several cavitation numbers providing insight into cavity shedding and nucleation physics. Shedding phenomena and frequency content were investigated by means of pixel intensity time series data using wavelet analysis. Instantaneous cavity leading edge location was investigated using image processing and edge detection.The boundary layer at cavity separation is shown to be laminar for all cavitation numbers, with Kelvin–Helmholtz instability and transition to turbulence in the separated shear layer the main mechanism for cavity breakup and cloud formation at high cavitation numbers. At intermediate cavitation numbers, cavity lengths allow the development of re-entrant jet phenomena, providing a mechanism for shedding of large-scale Kármán-type vortices similar to those for low-mode shedding in single-phase subcritical flow. This shedding mode, which exists at supercritical Reynolds numbers for single-phase flow, is eliminated at low cavitation numbers with the onset of supercavitation.Complex interactions between the separating laminar boundary layer and the cavity were observed. In all cases the cavity leading edge was structured in laminar cells separated by well-known ‘divots’. The initial laminar length and divot density were modulated by the unsteady cavity shedding process. At cavitation numbers where shedding was most energetic, with large portions of leading edge extinction, re-nucleation was seen to be circumferentially periodic and to consist of stretched streak-like bubbles that subsequently became fleck-like. This process appeared to be associated with laminar–turbulent transition of the attached boundary layer. Nucleation occurred periodically in time at these preferred sites and formed the characteristic cavity leading edge structure after sufficient accumulation of vapour had occurred. These observations suggest that three-dimensional instability of the decelerating boundary layer flow may have significantly influenced the developing structure of the cavity leading edge.

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