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Aerodynamics and dynamics of bluff bodies in presence of the moving surface boundary-layer control Munshi, Sandeep R.


Moving Surface Boundary-layer Control (MSBC) was applied to several two dimensional bluff bodies using a high speed rotating cylinder as a momentum injecting device. Flow past a symmetric airfoil; a D-section; as well as square and rectangular prisms, representing a family of shapes with progressively increasing bluffness were studied in presence of the MSBC. In the case of the airfoil, the leading edge was replaced by a rotating cylinder; while the cylindrical element formed the top and bottom upstream corners of the D-section, square and rectangular prisms. Extensive wind tunnel investigation gave data about the effect of system parameters like rate of the momentum injection, angle of attack and the surface condition of the cylinder on steady and fluctuating components of the pressure distribution around the body, vortex shedding frequency (Strouhal number), and the lift and drag coefficients. A gain in the Strouhal number with increasing momentum injection suggest a decrease in the effective bluffness of the body. A significant reduction in the drag (up to 80%) was observed for the prisms at a maximum rate of momentum injection, Uc/U = 4 (Uc = cylinder surface speed, U = freestream wind speed). In the case of the airfoil, the lift coefficient increased by 160% and the stall angle was delayed from 110° to 48°. A rough criterion in terms of the location of the stagnation point was established to help decide the reversal in the direction of momentum injection as a function of angle of attack to ensure continued benefit. Effect of momentum injection in suppressing the vortex resonance and galloping type of instabilities were studied by mounting the bluff prism models on a dynamic-test rig inside the wind tunnel test section. The measurement of amplitude and frequency of the transverse oscillations over a range of wind speeds showed complete vibration suppression for momentum injection rates Uc/U < 2. Asymmetric momentum injection (e.g. top cylinder rotating, bottom cylinder stationary), was also found to be effective in disrupting the vortex shedding process and thereby inhibiting vibrations. The suppression of galloping instability in presence of the MSBC was also predicted by the quasisteady analysis. A numerical panel method was developed to simulate bluff body fluid dynamnics in presence of the MSBC. The body is descretized into a large number of panels (100 - 150) with each panel comprising of a continuous distribution of linearly varying vorticity and a constant source strength. A set of linear algebraic equations approximates the Fredholm type integral equation derived from ideal fluid flow assumption. The wake is modelled by upper and lower 'free vortex layers' emanating from the separation points on the body. Vorticity is allowed to be shed and dissipated as it is convected downstream along the panels on the 'free vortex layers'. An analytical expression relates the point vortex modelling a.rotating cylinder to the rate of momentum injection. The panel method is capable of treating multielement configurations (e.g. a rotating cylinder and the truncated airfoil). An iterative scheme based on the convergence of the wake shape is used to obtain the final solution. The numerically obtained pressure distribution, the lift and drag coefficients agree well with the experimental results. Flow visualization studies in a water channel were performed to obtain better physical insight into the MSBC process. Plexiglas models with rotating cylinders in conjunction with a fine suspension of polyvinyl chloride particles and slit lighting were used to visualize the streaklines. The still photographs and video movie recorded, rather dramatically, the effectiveness of the MSBC in suppressing separation and vortex shedding, making the flow approach the potential character. Overall, the present research firmly establishes potential of the MSBC as a versatile tool for lift augmentation, drag reduction and vibration suppression of several bluff bodies encountered in industrial engineering practice.

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