| Summary: | 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.] === Applied Science, Faculty of === Mechanical Engineering, Department of === Graduate
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