Active Control Methods for Drag Reduction in Flow Over Bluff Bodies (Keynote)

2002 ◽  
Author(s):  
Haecheon Choi
Keyword(s):  
Circular Cylinder ◽  
Drag Reduction ◽  
Active Control ◽  
High Frequency ◽  
Free Stream ◽  
Free Stream Velocity ◽  
Stream Velocity ◽  
Control Methods ◽  
Periodic Forcing ◽  
Time Periodic

In this paper, we present two successful results from active controls of flows over a circular cylinder and a sphere for drag reduction. The Reynolds number range considered for the flow over a circular cylinder is 40∼3900 based on the free-stream velocity and cylinder diameter, whereas for the flow over a sphere it is 105 based on the free-stream velocity and sphere diameter. The successful active control methods are a distributed (spatially periodic) forcing and a high-frequency (time periodic) forcing. With these control methods, the mean drag and lift fluctuations decrease and vortical structures are significantly modified. For example, the time-periodic forcing with a high frequency (larger than 20 times the vortex shedding frequency) produces 50% drag reduction for the flow over a sphere at Re = 105. The distributed forcing applied to the flow over a circular cylinder results in a significant drag reduction at all the Reynolds numbers investigated.

Reducing the drag on a circular cylinder by upstream installation of an I-type bluff body as passive control

2009 ◽  
Vol 223 (10) ◽  
pp. 2291-2296 ◽  
Author(s):  
Y Triyogi ◽  
D Suprayogi ◽  
E Spirda
Keyword(s):  
Circular Cylinder ◽  
Velocity Vector ◽  
Bluff Body ◽  
Passive Control ◽  
Free Stream ◽  
Free Stream Velocity ◽  
Stream Velocity ◽  
Bluff Bodies ◽  
Subsonic Wind Tunnel ◽  
Large Cylinder

The bluff body cut from a small circular cylinder that is cut at both sides parallel to the y-axis was used as passive control to reduce the drag of a larger circular cylinder. The small bluff body cut is called an I-type bluff body, which interacts with a larger one downstream. I-type bluff bodies with different cutting angles of θs = 0°(circular), 10°, 20°, 30°, 45°, 53°, and 65° were located in front and at the line axis of the circular cylinder at a spacing S/ d = 1.375, where their cutting surfaces are perpendicular to the free stream velocity vector. The tandem arrangement was tested in a subsonic wind tunnel at a Reynolds number (based on the diameter d of the circular cylinder and free stream velocity) of Re = 5.3×104. The results show that installing the bluff bodies (circular or sliced) as a passive control in front of the large circular cylinder effectively reduces the drag of the large cylinder. The passive control with cutting angle θs = 65° gives the highest drag reduction on the large circular cylinder situated downstream. It gives about 0.52 times the drag of a single cylinder.

Contribution to the Study of a Flow Controlled by a Synthetic Jet

2012 ◽  
Vol 152-154 ◽  
pp. 1522-1525 ◽  
Author(s):  
Islem Benhegouga ◽  
Adel Boukehili
Keyword(s):  
Natural Frequency ◽  
Active Control ◽  
Free Stream ◽  
Synthetic Jet ◽  
Free Stream Velocity ◽  
Stream Velocity ◽  
Cylinder Diameter ◽  
Piston System ◽  
Control Of Flow ◽  
Lock In

Active control of flow over a cylinder at Re=1800, based on the free-stream velocity U∞=0.67m/s, and cylinder diameter d=40mm, is carried out using a synthetic jet. The slot injectors are positioned at θinj = ± 70°. An actuator was used to produce a synthetic jet by exciting a cavity with an alternative piston system. The results show that for the frequency of control located around the natural frequency of the vortex emission a phenomenon of lock-in appear.

Effect of an Oscillating Free Stream on the Unsteady Pressure on a Circular Cylinder

1973 ◽  
Vol 95 (2) ◽  
pp. 249-252 ◽  
Author(s):  
H. M. Hatfield ◽  
M. V. Morkovin
Keyword(s):  
Circular Cylinder ◽  
High Frequency ◽  
Free Stream ◽  
Vortex Street ◽  
Stream Velocity ◽  
Mean Velocity ◽  
Resonant Coupling ◽  
Unsteady Pressure ◽  
The Mean ◽  
Coupling Mechanisms

An experimental investigation was made of the unsteady pressure on a circular cylinder in a free stream of constant and oscillating velocity. Consideration of possible resonant coupling mechanisms between the Karman vortex street fluctuations and free-stream velocity oscillations showed such coupling to, be insignificant. The frequency of the vortex street fluctuations scales with the mean velocity for high frequency (30 to 50 Hz at mean velocity of 40 fps) oscillation of the free stream.

scholarly journals Added mass of a circular cylinder oscillating in a free stream

2013 ◽  
Vol 469 (2156) ◽  
pp. 20130135 ◽  
Author(s):  
Efstathios Konstantinidis
Keyword(s):  
Circular Cylinder ◽  
Free Stream ◽  
Transverse Velocity ◽  
Inertial Force ◽  
Added Mass ◽  
Stream Velocity ◽  
Virtual Experiment ◽  
Classical Formulation ◽  
Fundamental Understanding ◽  
Two Dimensional Flow

The fundamental understanding of the added mass phenomenon associated with the motion of a solid body relative to a fluid is revisited. This paper focuses on the two-dimensional flow around a circular cylinder oscillating transversely in a free stream. A virtual experiment reveals that the classical approach to this problem leads to a paradox. The inertial force is derived afresh based on analysis of the motion in a frame of reference attached to the cylinder centroid, which overcomes the paradox in the classical formulation. It is shown that the inertial force depends not only on the acceleration of the cylinder per se , but also on the relative motion between body and fluid embodied in a parameter called alpha, α , which represents the ratio of the maximum transverse velocity of the cylinder to the free-stream velocity; the induced inertial force is directionally varying and non-harmonic in time depended on the alpha parameter. It is further shown that the component of the inertial force in the transverse direction is negligible for α <0.1, increases quadratically for α <0.5, and tends asymptotically to the classical result as , i.e. in still fluid.

scholarly journals Boundary-Layer Type Solutions for Initial Platelet Activation and Deposition

2002 ◽  
Vol 4 (2) ◽  
pp. 95-108 ◽  
Author(s):  
T. David ◽  
P. G. de Groot ◽  
P. G. Walker
Keyword(s):  
Platelet Activation ◽  
Peclet Number ◽  
Free Stream ◽  
Free Stream Velocity ◽  
Stream Velocity ◽  
Bulk Fluid ◽  
Péclet Number ◽  
Activated Platelets ◽  
Activation Rate ◽  
Initial Adhesion

This paper presents, on the basis of high Peclet number, a mathematical model for the activation and initial adhesion of flowing platelets onto a surface. In contrast to past work, the model is applicable to general 2D and axi-symmetric flows where the wall shear stress is knowna priori. Results indicate that for high activation reaction rates there exist two layers, one containing only activated platelets and the other both activated and non-activated platelets. Fundamental relationships are proposed between the adhesion rate of platelets to the surface and the characteristic parameters of Peclet number and Reynolds number. Activation in the bulk fluid (blood) is characterised by the Damkohler number, which is a function of activation rate and the free-stream velocity. It is shown that, as the free-stream velocity varies, there exists a maximum of activated platelet flux to the wall for particular values of the velocity. These values, at which the maximum occur, are themselves functions of the platelet activation rate. As the free-stream velocity increases the activation of platelets ceases altogether and adhesion is reduced to a very small value strengthening the hypothesis of the correlation between atherogenesis/thrombogenesis and areas of low shear.

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