An Unstructured RANS Study of Tip-Leakage Vortex Cavitation Inception

2003 ◽  
Author(s):  
W. H. Brewer ◽  
D. L. Marcum ◽  
S. D. Jessup ◽  
C. Chesnakas ◽  
D. G. Hyams ◽  
...  
Keyword(s):  
Reynolds Number ◽  
Vortex Core ◽  
Experimental Measurements ◽  
Scaling Effects ◽  
Trailing Edge ◽  
Cavitation Inception ◽  
Tip Leakage ◽  
Edge Vortex ◽  
Good Agreement ◽  
Made In

To study the physics of cavitation inception, a ducted propulsor simulation is developed and extensively validated with experimental results. The numerical method is shown to be in good agreement with experimental measurements made in the vortex. The simulation is used as a tool for investigating the minimum pressure, circulation, and axial/tangential velocities in the vortex core. Additionally, the tool is used to study Reynolds number scaling effects of cavitation inception. The simulation reveals that the leakage vortex exhibits little dependence on Reynolds number, while the trailing edge vortex appears to exhibit classical trends. Moreover, the trailing edge, albeit the weaker vortex, appears to be causing inception.

Effect of Gap Size on Tip Leakage Cavitation Inception, Associated Noise and Flow Structure

2002 ◽  
Vol 124 (4) ◽  
pp. 994-1004 ◽  
Author(s):  
Shridhar Gopalan ◽  
Joseph Katz ◽  
Han L. Liu
Keyword(s):  
Flow Structure ◽  
Vortex Core ◽  
Bubble Dynamics ◽  
Digital Camera ◽  
High Amplitude ◽  
Gap Size ◽  
Cavitation Noise ◽  
Cavitation Inception ◽  
Tip Leakage ◽  
Good Agreement

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.

Unsteady Pressure Field Due to Interactions Among Tip Leakage Vortex, Trailing Edge Vortex, and Vortex Shedding in a Ducted Propeller

2007 ◽  
Author(s):  
Chunill Hah ◽  
Yu-Tai Lee
Keyword(s):  
Unsteady Flow ◽  
Vortex Shedding ◽  
Static Pressure ◽  
Flow Analysis ◽  
Trailing Edge ◽  
Tip Leakage Vortex ◽  
Cavitation Inception ◽  
Tip Leakage ◽  
Ducted Propeller ◽  
Edge Vortex

A detailed numerical study was performed to investigate the unsteady flow field near the tip of a ducted propeller blade. The primary objective was to understand the formation of the point of minimum static pressure, where cavitation inception occurs, in a ducted propeller. An experimental study showed that cavitation inception (minimum static pressure) occurs at about 50% blade chord downstream of the rotor trailing edge, while conventional estimation predicts it about 10% blade chord downstream in the tip leakage vortex. Steady flow analysis, which indicates that the minimum static pressure occurs about 15% axial chord downstream in the tip leakage vortex, does not calculate measured cavitation inception correctly. The flow field near the tip section is unsteady due to interactions among the tip leakage vortex, the trailing edge vortex, and vortex shedding in the wake. Because the steady flow analysis does not reproduce the measured minimum static pressure location in the current rotor, it was suspected that the observed phenomenon was due to some unsteady flow phenomenon in the tip region. To capture relevant unsteady flow physics as much as possible, a large eddy simulation (LES) was applied to the current investigation. The present study reveals that periodic interaction between the tip leakage vortex and the trailing edge vortex initially creates a local low pressure point about 15% blade chord downstream. As the tip leakage vortex flows downstream, it is bent and stretched by the interaction between the shed trailing edge vortex and the tip leakage vortex originating from the adjacent blade interaction. This stretching of the tip leakage vortex creates a new lower local pressure core in the tip leakage vortex. The current unsteady flow simulation shows that the minimum pressure point, where cavitation inception occurs, is observed intermittently at about 50% blade chord downstream of the trailing edge, as the measurement shows.

Measurements of the Tip Clearance Flow for a High-Reynolds-Number Axial-Flow Rotor

1995 ◽  
Vol 117 (4) ◽  
pp. 522-532 ◽  
Author(s):  
W. C. Zierke ◽  
K. J. Farrell ◽  
W. A. Straka
Keyword(s):  
Reynolds Number ◽  
Flow Visualization ◽  
Vortex Core ◽  
Rotor Blade ◽  
High Reynolds Number ◽  
Tip Clearance ◽  
Tip Leakage Vortex ◽  
Tip Leakage ◽  
Blade Tip ◽  
High Reynolds

A high-Reynolds-number pump (HIREP) facility has been used to acquire flow measurements in the rotor blade tip clearance region, with blade chord Reynolds numbers of 3,900,000 and 5,500,000. The initial experiment involved rotor blades with varying tip clearances, while a second experiment involved a more detailed investigation of a rotor blade row with a single tip clearance. The flow visualization on the blade surface and within the flow field indicate the existence of a trailing-edge separation vortex, a vortex that migrates radially upward along the trailing edge and then turns in the circumferential direction near the casing, moving in the opposite direction of blade rotation. Flow visualization also helps in establishing the trajectory of the tip leakage vortex core and shows the unsteadiness of the vortex. Detailed measurements show the effects of tip clearance size and downstream distance on the structure of the rotor tip leakage vortex. The character of the velocity profile along the vortex core changes from a jetlike profile to a wakelike profile as the tip clearance becomes smaller. Also, for small clearances, the presence and proximity of the casing endwall affects the roll-up, shape, dissipation, and unsteadiness of the tip leakage vortex. Measurements also show how much circulation is retained by the blade tip and how much is shed into the vortex, a vortex associated with high losses.

RANS Simulation of Ducted Marine Propulsor Flow Including Subvisual Cavitation and Acoustic Modeling

2005 ◽  
Vol 128 (4) ◽  
pp. 799-810 ◽  
Author(s):  
Jin Kim ◽  
Eric G. Paterson ◽  
Frederick Stern
Keyword(s):  
Vortex Core ◽  
Bubble Dynamics ◽  
Leading Edge ◽  
Suction Side ◽  
Interaction Region ◽  
Trailing Edge ◽  
Experimental Fluid Dynamics ◽  
Tip Leakage Vortex ◽  
Tip Leakage ◽  
Marine Propulsor

High-fidelity Reynolds-averaged Navier Stokes (RANS) simulations are presented for the ducted marine propulsor P5206, including verification and validation (V&V) using available experimental fluid dynamics data, and subvisual cavitation, and acoustics analysis using the modified Rayleigh-Plesset equation along the bubble trajectories with a far-field form of the acoustic pressure for a collapsing spherical bubble. CFDSHIP-IOWA is used with the blended k−ω∕k−ε turbulence model and extensions for a relative rotating coordinate system and overset grids. The intervals of V&V analysis for thrust, torque, and profile averaged radial velocity just downstream of rotor tip are reasonable in comparison with previous results. The flow pattern displays the interaction and merging of the tip-leakage and trailing edge vortices. In the interaction region, multiple peaks and vorticity are smaller, whereas in the merging region, there is better agreement with the experiment. The tip-leakage vortex core position, size, circulation, and cavitation patterns for σi=5 also show good agreement with the experiment, although the vortex core size is larger and the circulation in the interaction region is smaller. The simulations indicate globally minimum Cp=−σi=−8.8 on the suction side of the rotor tip at 84% chord from the leading edge and locally minimum Cp=−6.4 in the tip-leakage vortex at 8% chord downstream of the trailing edge, whereas EFD indicates σi=11 and the location in the tip-leakage vortex core 50% chord downstream of the trailing edge. Subvisual cavitation and acoustics analysis show that bubble dynamics may partly explain these discrepancies.

Effect of Hydrofoil Planform on Tip Vortex Roll-Up and Cavitation

1995 ◽  
Vol 117 (1) ◽  
pp. 162-169 ◽  
Author(s):  
D. H. Fruman ◽  
P. Cerrutti ◽  
T. Pichon ◽  
P. Dupont
Keyword(s):  
Reynolds Number ◽  
Incidence Angle ◽  
Pressure Coefficient ◽  
Tangential Velocity ◽  
Leading Edge ◽  
Velocity Profiles ◽  
The Other ◽  
Tip Vortex ◽  
Trailing Edge ◽  
Cavitation Inception

The effect of the planform of hydrofoils on tip vortex roll-up and cavitation has been investigated by testing three foils having the same NACA 16020 cross section but different shapes. One foil has an elliptical shape while the other two are shaped like quarters of ellipses; one with a straight leading edge and the other with a straight trailing edge. Experiments were conducted in the ENSTA, Ecole Navale and IMHEF cavitation tunnels with homologous foils of different sizes to investigate Reynolds number effects. Hydrodynamic forces as well as cavitation inception and desinence performance were measured as a function of Reynolds number and foil incidence angle. Laser Doppler measurements of the tangential and axial velocity profiles in the region immediately downstream of the tip were also performed. At equal incidence angle and Reynolds number, the three foils show different critical cavitation conditions and the maximum tangential velocity near the tip increases as the hydrofoil tip is moved from a forward to a rear position. However, the velocity profiles become more similar with increasing downstream distance, and at downstream distances greater than one chord aft of the tip, the differences between the foils disappear. The rate of tip vortex roll-up is much faster for the straight leading edge than for the straight trailing edge foil and, in the latter case, a significant portion of the roll-up occurs along the foil curved leading edge. The minimum of the pressure coefficient on the axis of the vortex was estimated from the velocity measurements and correlated with the desinent cavitation number for the largest free stream velocities. The correlation of data is very satisfactory. At the highest Reynolds number tested and at equal lift coefficients, the straight leading edge foil displays the most favorable cavitation desinent numbers.

Cavitation Influence on von Kármán Vortex Shedding and Induced Hydrofoil Vibrations

2007 ◽  
Vol 129 (8) ◽  
pp. 966-973 ◽  
Author(s):  
Philippe Ausoni ◽  
Mohamed Farhat ◽  
Xavier Escaler ◽  
Eduard Egusquiza ◽  
François Avellan
Keyword(s):  
Reynolds Number ◽  
Vortex Shedding ◽  
Trailing Edge ◽  
Cavitation Inception ◽  
Structure Interaction ◽  
New Correlation ◽  
Von Karman ◽  
Vortex Shedding Frequency ◽  
Shedding Frequency ◽  
Von Kármán

The present study deals with the shedding process of the von Kármán vortices at the trailing edge of a 2D hydrofoil at high Reynolds number Reh=25×103–65×103. This research focuses mainly on the effects of cavitation and fluid-structure interaction on the mechanism of the vortex generation. The vortex shedding frequency, derived from the flow-induced vibration measurement, is found to follow the Strouhal law provided that no hydrofoil resonance frequencies are excited, i.e., lock-off. For such a regime, the von Kármán vortices exhibit strong spanwise 3D instabilities and the cavitation inception index is linearly dependent on the square root of the Reynolds number. In the case of resonance, the vortex shedding frequency is locked onto the hydrofoil eigenfrequency and the spatial coherence is enhanced with a quasi-2D shape. The measurements of the hydrofoil wall velocity amplitude and phase reveal the first torsion eigenmotion. In this case, the cavitation inception index is found to be significantly increased compared to lock-off conditions. It makes clear that the vortex roll-up is amplified by the phase locked vibrations of the trailing edge. For the cavitation inception index, a new correlation relationship that encompasses the entire range of Reynolds numbers, including both the lock-off and the lock-in cases, is proposed and validated. In contrast to the earlier models, the new correlation takes into account the trailing edge displacement velocity. In addition, it is found that the transverse velocity of the trailing edge increases the vortex strength linearly. This effect is important in the context of the fluid-structure interaction, since it implies that the velocity of the hydrofoil trailing edge increases the fluctuating forces on the body. It is also demonstrated that cavitation developing in the vortex street cannot be considered as a passive agent for the turbulent wake flow. In fact, for fully developed cavitation, the vortex shedding frequency increases up to 15%, which is accompanied by the increase of the vortex advection velocity and reduction of the streamwise vortex spacing. In addition, a significant increase of the vortex-induced vibration level is found at cavitation onset. These effects are addressed and thought to be a result of the increase of the vorticity by cavitation.

scholarly journals The leading-edge vortex of swift wing-shaped delta wings

2017 ◽  
Vol 4 (8) ◽  
pp. 170077 ◽  
Author(s):  
Rowan Eveline Muir ◽  
Abel Arredondo-Galeana ◽  
Ignazio Maria Viola
Keyword(s):  
Reynolds Number ◽  
Delta Wing ◽  
Leading Edge ◽  
Trailing Edge ◽  
Delta Wings ◽  
Leading Edge Vortex ◽  
Wing Model ◽  
Image Velocimetry ◽  
Edge Vortex ◽  
First Time

Recent investigations on the aerodynamics of natural fliers have illuminated the significance of the leading-edge vortex (LEV) for lift generation in a variety of flight conditions. A well-documented example of an LEV is that generated by aircraft with highly swept, delta-shaped wings. While the wing aerodynamics of a manoeuvring aircraft, a bird gliding and a bird in flapping flight vary significantly, it is believed that this existing knowledge can serve to add understanding to the complex aerodynamics of natural fliers. In this investigation, a model non-slender delta-shaped wing with a sharp leading edge is tested at low Reynolds number, along with a delta wing of the same design, but with a modified trailing edge inspired by the wing of a common swift Apus apus . The effect of the tapering swift wing on LEV development and stability is compared with the flow structure over the unmodified delta wing model through particle image velocimetry. For the first time, a leading-edge vortex system consisting of a dual or triple LEV is recorded on a swift wing-shaped delta wing, where such a system is found across all tested conditions. It is shown that the spanwise location of LEV breakdown is governed by the local chord rather than Reynolds number or angle of attack. These findings suggest that the trailing-edge geometry of the swift wing alone does not prevent the common swift from generating an LEV system comparable with that of a delta-shaped wing.

Theory of Scale Effects on Cavitation Inception on Axially Symmetric Bodies

1961 ◽  
Vol 83 (3) ◽  
pp. 379-383 ◽  
Author(s):  
R. O¯shima
Keyword(s):  
Reynolds Number ◽  
Critical Reynolds Number ◽  
Scale Effects ◽  
Present Theory ◽  
Axially Symmetric ◽  
Cavitation Inception ◽  
Spherical Bubbles ◽  
Similarity Law ◽  
Special Case ◽  
Good Agreement

On the assumption that the motion of incipient cavitation bubbles on geometrically similar bodies is dynamically similar, the relation between incipient cavitation number kdi and Reynolds number Re has been obtained from the dynamical similarity law deduced from the equation of the motion of spherical bubbles. A comparison of calculated values based on this theory with experimental data obtained by R. W. Kermeen and others shows good agreement at values of Reynolds number greater than the critical Reynolds number. Also, by comparing this theory with the formula by R. T. Knapp, concerning scale effects on cavitation inception, it is shown that Knapp’s formula is a special case of the present theory.

scholarly journals Simulation of Trailing Edge Vortex Shedding in a Transonic Turbine Cascade

1996 ◽  
Author(s):  
Tom C. Currie ◽  
William E. Carscallen
Keyword(s):  
Vortex Shedding ◽  
Navier Stokes ◽  
Numerical Dissipation ◽  
Turbine Cascade ◽  
Trailing Edge ◽  
Edge Vortex ◽  
Transonic Turbine ◽  
Exit Mach Number ◽  
Good Agreement

Mid-span losses in the NRC transonic turbine cascade peak at an exit Mach number (M2) of ∼1.0 and then decrease by ∼40% as M2 is increased to the design value of 1.16. Since recent experimental results suggest that the decrease may be related to a reduction in the intensity of trailing edge vortex shedding, both steady and unsteady quasi-3D Navier-Stokes simulations have been performed with a highly refined (unstructured) grid to determine the role of shedding. Predicted shedding frequencies are in good agreement with experiment, indicating the blade boundary layers and trailing edge separated free shear layers have been modelled satisfactorily, but the agreement for base pressures is relatively poor, probably due largely to false entropy created downstream of the trailing edge by numerical dissipation. The results emphasize the importance of accounting for the effect of vortex shedding on base pressure and loss.

Measurements of the Tip Clearance Flow for a High Reynolds Number Axial-Flow Rotor: Part 1 — Flow Visualization

1994 ◽  
Author(s):  
W. C. Zierke ◽  
K. J. Farrell ◽  
W. A. Straka
Keyword(s):  
Reynolds Number ◽  
Flow Visualization ◽  
Rotor Blade ◽  
High Reynolds Number ◽  
Rotor Blades ◽  
Tip Clearance ◽  
Trailing Edge ◽  
Tip Leakage ◽  
High Reynolds ◽  
Edge Separation

A high Reynolds number pump (HIREP) facility has been used to acquire flow measurements in the rotor blade tip clearance region-with blade chord Reynolds numbers of 3,900,000 and 5,500,000. The initial experiment involved rotor blades with varying tip clearances, while a second experiment involved a more detailed investigation of a rotor blade row with a single tip clearance. This paper focuses on flow visualization, employing techniques unique for use in water. The flow visualization on the blade surface and within the flow field indicate that the combination of centripetal acceleration and separation near the trailing edge of the rotor blade suction surface results in the formation of a trailing-edge separation vortex-a vortex which migrates radially upwards along the trailing edge and then turns in the circumferential direction near the casing, moving in the opposite direction of blade rotation. Flow visualization also helps in establishing the trajectory of the tip leakage vortex core. The trailing-edge separation vortex, which lies closer to the endwall than the tip leakage vortex, seems to have an influence on this trajectory. Finally, the periodic interaction of the rotor blades with wakes from the upstream inlet guide vanes-as well as freestream turbulence and vortex structure instabilities-affects the unsteadiness of the vortex.

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