Active control of vortex shedding from a blunt trailing edge using oscillating piezoelectric flaps

Vortex cavitation can appear in the wake flow of hydrofoils, inducing unwanted consequences such as vibrations or unstable behaviors in hydraulic machinery and systems. To investigate the cavitation effects on hydrofoil vortex shedding, a numerical investigation of the flow around a 2D NACA0009 with a blunt trailing edge at free caviation conditions and at two degrees of cavitation developments has been carried out by means of the Zwart cavitation model and the LES WALE turbulence model which permits predicting the laminar to turbulent transition of the boundary layers. To analyze the dynamic behavior of the vortex shedding process and the coherent structures, two identification methods based on the Eulerian and Lagrangian reference frames have been applied to the simulated unsteady flow field. It is found that the cavitation occurrence in the wake significantly changes the main vortex shedding characteristics including the morphology of the vortices, the vortex formation length, the effective height of the near wake flow and the shedding frequency. The numerical results predict that the circular shape of the vortices changes to an elliptical one and that the vortex shedding frequency is significantly increased under cavitation conditions. The main reason for the frequency increase seems to be the reduction in the transverse separation between the upper and lower rows of vortices induced by the increase in the vortex formation length.

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A study of three-dimensional aspects of vortex shedding from a bluff body with a mild geometric disturbance

Journal of Fluid Mechanics ◽

10.1017/s0022112096003631 ◽

1997 ◽

Vol 330 ◽

pp. 85-112 ◽

Cited By ~ 104

Author(s):

N. TOMBAZIS ◽

P. W. BEARMAN

Keyword(s):

Reynolds Number ◽

Vortex Shedding ◽

Bluff Body ◽

Vortex Formation ◽

Three Dimensional ◽

Base Pressure ◽

Trailing Edge ◽

Blunt Trailing Edge ◽

Trailing Edges ◽

Mode Transitions

Experiments have been carried out to study the three-dimensional characteristics of vortex shedding from a half-ellipse shape with a blunt trailing edge. In order to control the occurrence of vortex dislocations, the trailing edges of the models used were constructed with a series of periodic waves across their spans. Flow visualization was carried out in a water tunnel at a Reynolds number of 2500, based on trailing-edge thickness. A number of shedding modes were observed and the sequence of mode transitions recorded. Quantitative data were obtained from wind tunnel measurements performed at a Reynolds number of 40000. Two shedding frequencies were recorded with the higher frequency occurring at spanwise positions coinciding with minima in the chord. At these same positions the base pressure was lowest and the vortex formation length longest. Arguments are put forward to explain these observations. It is shown that the concept of a universal Strouhal number holds, even when the flow is three-dimensional. The spanwise variation in time-average base pressure is predicted using the estimated amount of time the flow spends at the two shedding frequencies.

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The effect of base bleed on vortex shedding and base pressure in compressible flow

Journal of Fluid Mechanics ◽

10.1017/s0022112081000748 ◽

1981 ◽

Vol 110 ◽

pp. 273-292 ◽

Cited By ~ 26

Author(s):

F. Motallebi ◽

J. F. Norbury

Keyword(s):

Supersonic Flow ◽

Compressible Flow ◽

Vortex Shedding ◽

Transonic Flow ◽

Vortex Formation ◽

Base Pressure ◽

Trailing Edge ◽

Blunt Trailing Edge ◽

Base Bleed ◽

The Relationship

Experiments have been carried out to investigate the phenomenon of vortex shedding from the blunt trailing edge of an aerodynamic body in transonic and supersonic flow. The effect of a discharge of bleed air from a slot in the trailing edge has been included and the relationship between the vortex formation and base pressure has been considered.In transonic flow a small amount of bleed air was found to produce a rearward shift in the point of origin of the vortices with a consequent substantial increase in base pressure. The effect was less marked in supersonic flow. At higher rates of bleed two different regimes of vortex shedding were identified and increase in bleed rate caused a reduction in base pressure. For bleed rates giving near-maximum base pressure no vortex shedding occurred.

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Vortex shedding from a blunt trailing edge with equal and unequal external mean velocities

Journal of Fluid Mechanics ◽

10.1017/s0022112076000499 ◽

1976 ◽

Vol 75 (4) ◽

pp. 721-735 ◽

Cited By ~ 25

Author(s):

D. R. Boldman ◽

P. F. Brinich ◽

M. E. Goldstein

Keyword(s):

Steady State ◽

Flow Visualization ◽

Strouhal Number ◽

Vortex Shedding ◽

Free Stream ◽

Theoretical Calculations ◽

Vortex Formation ◽

Trailing Edge ◽

Blunt Trailing Edge ◽

Good Agreement

A flow-visualization study has shown that strong Kármán vortices develop behind the blunt trailing edge of a plate when the free-stream velocities over both surfaces are equal and that the vortices tend to disappear when the surface velocities are unequal. This observation provides an explanation for the occurrence and disappearance of certain discrete tones often found to be present in the noise spectra of coaxial jets. Both the vortex formation and the tones occur at a Strouhal number based on the lip thickness and the average of the external steady-state velocities of about 0.2.Results from theoretical calculations of the vortex formation, based on an inviscid incompressible analysis of the motion of point vortices, were in good agreement with the experimental observations.

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