Effect of humps on the stability of boundary layers over an airfoil
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The effect of humps on the stability of subsonic boundary layers over an airfoil is investigated. The mean flow is calculated by using an interacting boundary-layer solver which accounts for strong viscous/inviscid interaction and separation bubbles. The code is capable of solving compressible as well as incompressible flows. Then, the two-dimensional mean flow is fed into a stability program which is capable of doing two-and three-dimensional analysis. The output of this stability program is the growth rates which are integrated along a prescribed path to yield the amplification factor (i.e., N-factor), which is used to predict transition from laminar to turbulent flow. The analysis is performed for different heights and locations of the hump and for different Mach numbers. The results show that compressibility stabilizes the flow and that the most dangerous frequency decreases as the Mach number increases for a fixed location of the hump. Also this most dangerous frequency decreases as the hump is moved downstream. Moreover, the amplification factor increases as the hump height increases and as the hump is moved downstream.
The influence of suction and heat-transfer strips on controlling the destabilizing influence of the hump is investigated. The results show that cooling and suction strips stabilize the flow and therefore delay transition from laminar to turbulent flow. Moreover, a heating strip destabilizes the flow in the presence of a hump. Applying suction through multiple strips can be as effective as continuous suction. Also the total flow rate required using multiple strips is less than that required using a single strip. We optimize the locations of these strips for a certain hump location. Moreover, cooling through multiple strips is as effective as cooling through a single strip. We optimize the locations and levels of these cooling strips for a certain hump location.
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