Effects of Periodic Excitation on the Flow Around a D-Shaped Cylinder at Low Reynolds Numbers |
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Authors: | Vitali Palei Avi Seifert |
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Institution: | (1) Faculty of Engineering, School of Mechanical Engineering, Tel-Aviv University, Tel-Aviv, Israel |
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Abstract: | The baseline and forced flow around a bluff body with semi-elliptical D-shape was investigated by solving the 2D Navier–Stokes
equations at low Reynolds numbers. A D-shape rather than the canonic circular-cylinder was selected due to the fixed separation
points in the latter, enabling to study a pure wake rather than boundary-layer control. The correlation between Strouhal and
Reynolds numbers, the mean drag, the lift and drag oscillations vs. the Reynolds number and wake structure were investigated
and compared to experimental and numerical data. Effects of open-loop forcing, resulting from the influence of zero-mass-flux
actuators located at the fixed separation points, were studied at a Reynolds number of 150. Fluidic rather than body motion
or volume forcing was selected due to applicability considerations. The motivation for the study was to quantify the changes
in the flow field features, as captured by Proper Orthogonal Decomposition (POD) analysis, due to open-loop forcing, inside
and outside the “lock-in” regime. This is done in order to evaluate the suitability of low-order-models based on POD modes
of this changing flow field, for future feed-back flow control studies. The evolution of the natural and the excited vortices
in the Kármán wake were also investigated. The formation and convection regions of the vortex evolution were documented. It
was found that the forcing causes an earlier detachment of the vortices from the boundary-layers, but does not affect their
circulation or convection speeds. The results of the POD analysis of the near-wake flow show that the influence of the bluff
body shape (“D”-shaped versus circular cylinder) on the baseline POD wake modes is small. It was found that the eigenfunctions
(mode-shapes) of the POD velocity modes are less sensitive to slot excitation than the vorticity modes. As a result of the
open-loop excitation, two types of mode-shape-change were observed: a mode can be exchanged with a lower-energy mode or shifted
to a low energy level. In the latter case, the most energetic mode becomes the “actuator” mode. The evolution of one-slot
excitation on still fluid (“Synthetic jet”) was studied and compared to published data and to “actuator” modes with external
flow present. Based on the current findings, it is hypothesized that the cross-flow velocity POD modes are suitable for feedback
control of wake flow using periodic excitation, due to their low sensitivity to the excitation as compared to the streamwise
velocity or vorticity modes. |
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Keywords: | D Shape Cylinder Wake Shedding Flow control Actuators POD |
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