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Aerodynamics of several slender and bluff bodies in presence of momentum injection Deshpande, Vijay S.

Abstract

The concept of Moving Surface Boundary-layer Control (MSBC), as applied to a two-dimensional Joukowski airfoil as well as three-dimensional cube, water-tank and building models, is investigated through a planned wind tunnel test-program at a subcritical Reynolds number of 2.5 x 10[superscript 5]. High speed rotating cylinders served as momentum injection elements and controlled the key parameter Uc/U, where Uc is the cylinder surface velocity and U represents the free-stream velocity. In case of the two-dimensional airfoil, a single rotating cylinder replaced the nose of the airfoil. Results suggest that the concept is quite promising leading to an increase in lift by around 100 % and the delay in stall from 10° to 35°. It led to the rise of lift to drag ratio by 167 %. The momentum injection also resulted in an increase in the Strouhal number, at all angles of attack (α), thus rendering the airfoil to behave as an effectively more slender body, even at a high α. In general, effect of the cylinder surface roughness was to further increase the Strouhal number by a small amount. The three-dimensional cube model, with an edge length of W, carried two momentum-injecting elements at the vertical edges of the front face. The study with basic cube in presence of the MSBC provided, for the first time, the fundamental information concerning pressure distribution and forces which should serve as a reference in future. At α = 0 and Uc/U = 4, a reduction in drag by around 67% is indeed impressive. The Den Hartog criterion for galloping showed an improvement in stability with an increase in the momentum injection. The cube model when supported by a pillar served as a water-tank to assess the effect of height (H). Two heights were considered: H = 2W and H = 3W. A t the lower height, the effect of Uc/U was to reduce the drag at virtually all angles of attack, however, the decrease was substantially less compared to that observed for the basic cube. This is primarily due to the lateral flow created on the side faces of the tank because of the gap formed by the proximity of the ground. This adversely affects reattachment and separation of the boundary-layer. However, at the higher height (H = 3W), the trend reverses as expected. Now the reduction in drag is significantly higher even compared to that for the basic cube case. Both the tank models were found to be susceptible to galloping instability for 75° < α < 90°, even in presence of the momentum injection. Tests with the building models assesses the effect of aspect ratios (A.R. = 2, 3) on the pressure distribution and forces, using the basic cube as the top element. In general, irrespective of the A.R., the influence of momentum injection is to reduce the drag, almost at all α. However, the decrease in CD is less at a higher aspect ratio. Perhaps the most important effect of the higher A.R. is the building's susceptibility to galloping. This can be eliviated by injecting momentum over greater height of the building compared to 33% in the present case (H = 3W). The fundamental information of long range importance presented in the thesis should serve as a reference and prove useful to industrial aerodynamicists as well as practicing engineers. [Scientific formulae used in this abstract could not be reproduced.]

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