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978-3-8439-2308-8, Reihe Ingenieurwissenschaften
Experimental investigations on roughness initiated instability and transition in airfoil boundary layers
161 Seiten, Dissertation Universität Stuttgart (2015), Softcover, A5
Roughness-induced boundary layer transition is a key factor contributing to performance losses of airfoils. The present work is, therefore, devoted to the stability and transition of an airfoil boundary layer which is disturbed by an isolated, cylindrical roughness of low aspect ratio. The experimental investigations are performed in a low-speed wind tunnel and aligned with operational conditions of large wind turbine blades. The roughness is located in the airfoil leading edge region and the inflow turbulence level is modeled by a controlled excitation of perturbation modes, which allow for phase-locked perturbation tracking with hot-wire anemometry and Particle Image Velocimetry (PIV).
The present thesis has two main emphases. The first part is devoted to medium height, sub-critical roughness elements. Downstream of these elements the transition process is governed by wave-type instability modes. Three stages of nonlinearity, which depend mainly on the mean flow distortion, are identified: At the lower limit of the medium height range, a weak nonlinear growth of fundamental modes which have high n-factors at the roughness position is found, before linear stability characteristics are recovered. Nonlinear interactions between the fundamental modes are intensified with increasing mean flow distortion and initiate low-frequency modes in the subharmonic range, which become resonantly amplified in the far wake. By further increasing the roughness height to the upper limit of the medium height range, the fundamental and the subsequent low-frequency interaction modes reach a nonlinear amplitude level in the near wake and, thereby, initiate the laminar flow breakdown before the mean flow can stabilize.
The instability and transition mechanisms associated with critical and super-critical roughness elements are addressed in the second part of the thesis. With the change from a sub-critical to a critical configuration the perturbations maintain a modal, Tollmien-Schlichting (TS) character in centerline region of the low-aspect ratio element, but in the outer spanwise domain an inviscid Kelvin-Helmholtz (KH) type instability is found. With increasing roughness Reynolds number the KH-type instability extends towards the roughness centerline. Finally, for large super-critical elements transition is already triggered in the shear layers upstream and around the element and the boundary layer reattaches in a turbulent state in the roughness near field.