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Licentiate seminar

Stability and transition of three-dimensional boundary layers


Defendant Main Advisor Extra Advisor Date
Mohammad Hosseini Ardeshir Hanifi Dan Henningson 2013-06-13

Opponent
Vassilis Theofilis, Universidad Politécnica de Madrid

Evaluation committee

Abstract

A focus has been put on the stability characteristics of different flow types existing on air vehicles. Flow passing over wings and different junctions on an aircraft face numerous local features, ranging from different pressure gradients, to interacting boundary layers. Primarily, stability characteristics of flow over a wing subject to negative pressure gradient is studied. The current numerical study conforms to an experimental study conducted by Saric and coworkers, in their Arizona State University wind tunnel experiments. Within that framework, a passive control mechanism has been tested to delay transition of flow from laminar to turbulence. The same control approach has been studied here, in addition to underling mechanisms playing major roles in flow transition, such as nonlinear effects and secondary instabilities. The same flow type has been considered to study the receptivity of three-dimensional boundary layers to freestream turbulence perturbations. Similarly, the numerical configuration, follows the experiments performed in the same group by Downs (2012). The experiments entail investigation of the effect of low freestream turbulence on crossflow instability. A well-documented experiment enables the numerical studies to properly reproduce the experimental environments. Another common three-dimensional flow feature arises as a result of stream- lines passing through a junction, the so called corner-flow. For instance, this flow can be formed in the junction between the wing and fuselage on a plane. A series of direct numerical simulations using linear Navier-Stokes equations have been performed to determine the optimal initial perturbation. Optimal refers to a perturbation which can gain the maximum energy from the flow over a period of time. Power iterations between direct and adjoint Navier-Stokes equations determine the optimal initial perturbation. In other words this method seeks to determine the worst case scenario in terms of perturbation growth. Determining the optimal initial condition can help improve the design of such surfaces in addition to possible control mechanisms.
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