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.

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.

scholarly journals Flow Features of the Near Wake of the Australian Boobook Owl (Ninox boobook) During Flapping Flight Suggest an Aerodynamic Mechanism of Sound Suppression for Stealthy Flight

2019 ◽  
Vol 1 (1) ◽  
Author(s):  
Jonathan Lawley ◽  
Hadar Ben-Gida ◽  
Krishnan Krishnamoorthy ◽  
Erin E Hackett ◽  
Gregory A Kopp ◽  
...  
Keyword(s):  
High Speed ◽  
Pressure Field ◽  
Near Wake ◽  
Calidris Mauri ◽  
Long Duration ◽  
Image Velocimetry ◽  
Western Sandpiper ◽  
Wake Patterns ◽  
Flow Features ◽  
The Subject

Synopsis The mechanisms associated with the ability of owls to fly silently have been the subject of scientific interest for many decades and may be relevant to bio-inspired design to reduce noise of flapping and non-flapping flying devices. Here, we characterize the near wake dynamics and the associated flow structures produced during flight of the Australian boobook owl (Ninox boobook). Three individual owls were flown at 8 ms−1 in a climatic avian wind tunnel. The velocity field in the wake was sampled at 500 Hz using long-duration high-speed particle image velocimetry (PIV) while the wing kinematics were imaged simultaneously using high speed video. The time series of velocity maps that were acquired over several consecutive wingbeat cycles enabled us to characterize the wake patterns and to associate them with the phases of the wingbeat cycle. We found that the owl wake was dramatically different from other birds measured under the same flow conditions (i.e., western sandpiper, Calidris mauri and European starling, Sturnus vulgaris). The near wake of the owl did not exhibit any apparent shedding of organized vortices. Instead, a more chaotic wake pattern was observed, in which the characteristic scales of vorticity (associated with turbulence) are substantially smaller in comparison to other birds. Estimating the pressure field developed in the wake shows that owls reduce the pressure Hessian (i.e., the pressure distribution) to approximately zero. We hypothesize that owls manipulate the near wake to suppress the aeroacoustic signal by controlling the size of vortices generated in the wake, which are associated with noise reduction through suppression of the pressure field. Understanding how specialized feather structures, wing morphology, or flight kinematics of owls contribute to this effect remains a challenge for additional study.

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.

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.

Experimental and Numerical Analysis of Flow Instabilities in a Radial Fan

2014 ◽  
Author(s):  
Andreas Lucius ◽  
Andreas Lehwald ◽  
Dominique Thévenin ◽  
Gunther Brenner
Keyword(s):  
High Speed ◽  
Transient Flow ◽  
Suction Side ◽  
Computational Techniques ◽  
Unsteady Forces ◽  
Operation Conditions ◽  
Image Velocimetry ◽  
Rotor Stator Interaction ◽  
Open Issue ◽  
Computational Fluid Dynamics Cfd

Dynamic loads in turbomachines may lead to severe fluid induced vibrations, in particular, if resonance conditions are matched. The major sources of such unsteadiness are rotor/stator interaction, flow separation on the blade suction side at part-load and separation due to the curvature of the shroud. In the present work, some of these phenomena are investigated using computational techniques (Computational Fluid Dynamics - CFD) as well as novel measurement methods (High-Speed Particle Image velocimetry - PIV). The measurements provide a unique database of the velocity fields in an industrial impeller at various operation conditions. They are the base for validation of the computational methods. In view of the complexity of the separated, transient and turbulent flow field, this is still an open issue. Besides that, the analysis of the transient flow fields allows the determination of the “modes” of unsteady forces und thus, to shed some light on the sources of unsteadiness in the flow.

Experimental Investigation of Boundary Layer Instabilities on the Free Surface of Non-Turbulent Jet

2012 ◽  
Author(s):  
Matthieu A. Andre ◽  
Philippe M. Bardet
Keyword(s):  
Boundary Layer ◽  
Free Surface ◽  
High Speed ◽  
Particle Tracking Velocimetry ◽  
Capillary Waves ◽  
Wave Growth ◽  
Image Velocimetry ◽  
Planar Laser ◽  
Surface Visualization ◽  
The Waves

Shear instabilities induced by the relaxation of laminar boundary layer at the free surface of a high speed liquid jet are investigated experimentally. Physical insights into these instabilities and the resulting capillary wave growth are gained by performing non-intrusive measurements of flow structure in the direct vicinity of the surface. The experimental results are a combination of surface visualization, planar laser induced fluorescence (PLIF), particle image velocimetry (PIV), and particle tracking velocimetry (PTV). They suggest that 2D spanwise vortices in the shear layer play a major role in these instabilities by triggering 2D waves on the free surface as predicted by linear stability analysis. These vortices, however, are found to travel at a different speed than the capillary waves they initially created resulting in interference with the waves and wave growth. A new experimental facility was built; it consists of a 20.3 × 146.mm rectangular water wall jet with Reynolds number based on channel depth between 3.13 × 104 to 1.65 × 105 and 115. to 264. based on boundary layer momentum thickness.

scholarly journals The Influence of Carrier Air Preheating on Autoignition of Inline-Injected Hydrogen–Nitrogen Mixtures in Vitiated Air of High Temperature

2017 ◽  
Vol 140 (3) ◽  
Author(s):  
Christoph A. Schmalhofer ◽  
Peter Griebel ◽  
Manfred Aigner
Keyword(s):  
Hydrogen Content ◽  
Gas Turbines ◽  
High Speed ◽  
Flame Stabilization ◽  
Operating Conditions ◽  
Promoting Effect ◽  
Experimental Conditions ◽  
Lean Premixed Combustion ◽  
Image Velocimetry ◽  
Fuel Mixtures

The use of highly reactive hydrogen-rich fuels in lean premixed combustion systems strongly affects the operability of stationary gas turbines (GT) resulting in higher autoignition and flashback risks. The present study investigates the autoignition behavior and ignition kernel evolution of hydrogen–nitrogen fuel mixtures in an inline co-flow injector configuration at relevant reheat combustor operating conditions. High-speed luminosity and particle image velocimetry (PIV) measurements in an optically accessible reheat combustor are employed. Autoignition and flame stabilization limits strongly depend on temperatures of vitiated air and carrier preheating. Higher hydrogen content significantly promotes the formation and development of different types of autoignition kernels: More autoignition kernels evolve with higher hydrogen content showing the promoting effect of equivalence ratio on local ignition events. Autoignition kernels develop downstream a certain distance from the injector, indicating the influence of ignition delay on kernel development. The development of autoignition kernels is linked to the shear layer development derived from global experimental conditions.

Experimental investigation of bubble formation and motion mechanism near the moonpool

2021 ◽  
pp. 147509022110449
Author(s):  
Xiongliang Yao ◽  
Xianghong Huang ◽  
Zeyu Shi ◽  
Wei Xiao ◽  
Kainan Huang
Keyword(s):  
High Speed ◽  
Bubble Formation ◽  
Similarity Criteria ◽  
Dominant Factor ◽  
Acoustic Detection ◽  
Prototype Test ◽  
Tracer Particles ◽  
Research Ship ◽  
Image Velocimetry ◽  
Ventilation Method

When a research ship sails at a high speed, there is relative motion between the ship and fluid. The ship is slammed by the fluid. To reduce the direct impact of the fluid, sonar is installed in the moonpool, and acoustic detection equipment is installed along the research ship bottom behind the moonpool. However, during high-speed sailing, a large number of bubbles form in the moonpool. Some bubbles escape from the moonpool and flow backward along the bottom of the ship. When a large number of bubbles are around the sonar and acoustic detection equipment, the equipment malfunctions. However, there have been few studies on bubble formation in the moonpool with sonar and distribution along the ship bottom behind the moonpool. Therefore, a related model was developed and prototype tests were carried out in this study. The appropriate similarity criteria were selected and verified to ensure the reliability of the experiment. Considering the influences of speed, sonar, moonpool shape, and draft, the reason and mechanism of bubble formation in a sonar moonpool were studied. An artificial ventilation method was used to simulate a real navigation environment. Because the bubbles are in a bright state under laser irradiation, the bubbles can be used as tracer particles. A high-speed camera captured illuminated bubbles. The distribution mechanism of bubbles along the ship bottom behind the moonpool was investigated using particle image velocimetry under the influence of the moonpool shape and sailing speed. The model experimental results agreed well with those of the prototype test. The air sucked into the water was the dominant factor in bubble formation in the moonpool. The bubbles were distributed in a W shape under the ship bottom.

Velocity and Volume Fraction Measurements of Granular Flows in a Steep Flume

2021 ◽  
Author(s):  
Luca Sarno ◽  
Maria Nicolina Papa ◽  
Luigi Carleo ◽  
Paolo Villani
Keyword(s):  
High Speed ◽  
Volume Fraction ◽  
Solid Volume Fraction ◽  
Granular Flows ◽  
Indirect Measurement ◽  
Measuring Instruments ◽  
Second Order Statistics ◽  
Granular Dynamics ◽  
Image Velocimetry ◽  
Near Wall

ABSTRACT Laboratory experiments on granular flows remain essential tools for gaining insight into several aspects of granular dynamics that are inaccessible from field-scale investigations. Here, we report an experimental campaign on steady dry granular flows in a flume with inclination of 35°. Different flow rates are investigated by adjusting an inflow gate, while various kinematic boundary conditions are observed by varying the basal roughness. The flume is instrumented with high-speed cameras and a no-flicker LED lamp to get reliable particle image velocimetry measurements in terms of both time averages and second-order statistics (i.e., granular temperature). The same measuring instruments are also used to obtain concurrent estimations of the solid volume fraction at the sidewall by employing the stochastic-optical method (SOM). This innovative approach uses a measurable quantity, called two-dimensional volume fraction, which is correlated with the near-wall volume fraction and is obtainable from digital images under controlled illumination conditions. The knowledge of this quantity allows the indirect measurement of the near-wall volume fraction thanks to a stochastic transfer function previously obtained from numerical simulations of distributions of randomly dispersed spheres. The combined measurements of velocity and volume fraction allow a better understanding of the flow dynamics and reveal the superposition of different flow regimes along the flow depth, where frictional and collisional mechanisms exhibit varying relative magnitudes.

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