Unsteady mixed convection about a rotating circular cylinder with small fluctuations in the free-stream velocity

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.

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

Proceedings of the Institution of Mechanical Engineers Part C Journal of Mechanical Engineering Science ◽

10.1243/09544062jmes1543 ◽

2009 ◽

Vol 223 (10) ◽

pp. 2291-2296 ◽

Cited By ~ 10

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.

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Measurements of Turbulent Mixed Convection Flow Over a Vertical Forward-Facing Step

Heat Transfer: Volume 1 ◽

10.1115/ht2003-47088 ◽

2003 ◽

Author(s):

H. I. Abu-Mulaweh ◽

B. F. Armaly ◽

T. S. Chen

Keyword(s):

Mixed Convection ◽

Free Stream ◽

Free Stream Velocity ◽

Heated Wall ◽

Laser Doppler Velocimeter ◽

Convection Flow ◽

Stream Velocity ◽

Convection Boundary Layer ◽

Turbulent Natural Convection ◽

Forward Facing Step

Measurements of turbulent mixed convection boundary-layer air flow over a two-dimensional, vertical forward-facing step are presented. The upstream and downstream walls and the step itself were heated to a uniform and constant temperature. Air velocity and temperature distributions and their turbulent fluctuations are measured simultaneously by using a two-component laser-Doppler velocimeter (LDV) and a cold wire anemometer were used, respectively. The present study treats buoyancy-dominated mixed convection over a vertical forward-facing step and examines the effect of a small free stream velocity on turbulent natural convection. It was found that the reattachment length increases while the heat transfer rate from the downstream heated wall decreases as the small free stream velocity increases.

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