Interaction of a laminar boundary layer with roughness elements in the close vicinity of an airfoil leading edge
Final Report Abstract
Roughness-induced boundary layer transition is one of the key factors contributing to performance losses of airfoils. A detailed knowledge of the associated transition mechanisms is thus essential for a new design of highly efficient airfoils and - in recent times - especially wind turbine blades. In this context the present project is devoted to the stability and transition of an airfoil boundary layer which is disturbed by an isolated, cylindrical roughness of low aspect (height-to-diameter) ratio. The experimental investigations were 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 upstream of the roughness, which allow for phase-locked perturbation tracking with hot-wire anemometry and Particle Image Velocimetry (PIV). The present project has two main emphases. The first focus is set on medium height, subcritical roughness elements. Downstream of these elements the transition process is governed by wave-type instability modes, which grow and interact in their streamwise evolution, before the laminar flow breakdown is initiated far downstream. In contrast, for large, supercritical roughness elements, which are focused on in the second part, transition occurs in the near roughness vicinity and a modal perturbation growth is bypassed. In the medium height range the mean flow is destabilized in the roughness near field and a formation of vortical structures is seen. However, in the downstream development the mean flow stabilizes and the boundary layer stability characteristics are only affected in the near wake of the roughness. The present study takes advantage from a joint investigation of the interaction of the roughness with ‘natural’ meaning uncontrolled as well as controlled perturbations. At first, the near-wake instability is studied based on the uncontrolled perturbation spectrum at the roughness position to obtain a characteristic perturbation evolution in the roughness wake. These investigations are complemented by an excitation of controlled single-mode perturbations. Here, the influence of the perturbation amplitude and frequency is considered in addition to the effect of boundary layer instability resulting from a variation of the roughness height and the pressure gradient. It is shown that especially the roughness height and the perturbation amplitude at the roughness position have a significant effect on the transition mechanism in the roughness wake. Subsequently, controlled multi-mode perturbations and low-amplitude perturbation bands mimic a more ’natural’, quasi-continuous perturbation spectrum at the roughness position and allow for a variety of nonlinear mode interactions. Three stages of nonlinearity, which depend mainly on the mean flow distortion (roughness height), are identified consistently with the findings in the ’natural’ cases: At the lower limit of the medium height range, a weak nonlinear growth of modal instabilities is found in the near wake, before linear stability characteristics are recovered with the mean flow stabilization. The nonlinear stage in the near wake is dominated by fundamental modes which have high n-factors based on linear theory at the roughness position. 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 roughness near field and, thereby, initiate the laminar flow breakdown before the mean flow can stabilize. The mechanisms associated with instability and transition downstream of high - meaning critical and super-critical - roughness elements are addressed by combined hot-wire and PIV measurements. With the change from a subcritical to a critical configuration the perturbations maintain a modal, Tollmien- Schlichting (TS) character in the near wake centerline region of the low-aspect ratio element. However, the nature of the instability in the outer spanwise domain changes and can be linked to an inviscid Kelvin- Helmholtz (KH) type shear layer instability. With increasing roughness Reynolds number the KH-type instability domain extends towards the roughness centerline and as a result the spanwise domain of the TS wave-type instability is decreased. Finally, for large super-critical elements periodic shear layer perturbations in the near wake centerline region roll-up into a shedding of vortices, which is characteristic for isolated elements. However, as transition is already triggered in the shear layers upstream and around the element, the vortices in the centerline region quickly break down and the boundary layer reattaches in a turbulent state in the roughness near field. In conclusion, the present experimental project provides a detailed study of the instability mechanisms associated with medium height, subcritical roughness elements on the one hand and high, super-critical ones on the other hand with the focus on low to moderate perturbation levels at the roughness position. Throughout the entire project the experimental results have been extensively compared with theoretical predictions and previous findings in the literature. Moreover, all results have been published on conferences, in high-impact factor journals and in a Phd-thesis. Thereby, the present work is the basis for future investigations with an excitation of ’quasi-natural’ (in time and space) perturbation modes, which are similar to continuous spectrum perturbations observed typically in ’natural’-roughness disturbed boundary layers. Based on these and the present study, new mechanism-based transition prediction methods can be developed with the goal of an enhanced design of wind turbine blades and airfoils in general.
Publications
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2012. Interaction of a Laminar Boundary Layer with a Cylindrical Roughness Element near an Airfoil Leading Edge 42nd AIAA Fluid Dynamics Conference, New Orleans, USA, AIAA 2012-3077
Plogmann, B. and Würz, W. and Krämer, E.
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2012. Interaction of a Three-Dimensional Roughness Element with a TS- wave near an Airfoil Leading edge 16th International Conference on the Methods of Aerophysical Research, Kazan, Russia
Plogmann, B. and Würz, W. and Krämer, E.
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2013. The influence of a cylindrical roughness element on a perturbed laminar boundary layer. 43rd AIAA Fluid Dynamics Conference, San Diego, USA, AIAA 2013- 3109
Plogmann, B. and Würz, W.
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201.4 On the disturbance evolution downstream of a cylindrical roughness element. Journal of Fluid Mechanics, 758, 238–286
Plogmann, B. and Würz, W. and Krämer, E.
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2014. Interaction of a cylindrical roughness element and a two-dimensional TS-wave. In New Results in Numerical and Experimental Fluid Mechanics IX (ed. A. Dillmann, G. Heller, E. Krämer, H. P. Kreplin, W. Nitsche & U. Rist), pp. 163–172. Springer
Plogmann, B. and Würz, W. and Krämer, E.
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2014. Nonlinear mode interactions in the wake of a medium height roughness element 8th IUTAM Symposium on Laminar Turbulent Transition, Rio de Janeiro, Brazil
Plogmann, B. and Würz, W. and Krämer, E.
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2014. Stability of a boundary layer flow in the wake of a medium height roughness element. 44th AIAA Fluid Dynamics Conference, Atlanta, USA, AIAA 2014-2886
Plogmann, B. and Würz, W. and Krämer, E.
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2015. Experimental investigations on roughness initiated instability and transition in airfoil boundary layers Phd thesis, Institute of Aerodynamics and Gas Dynamics, University of Stuttgart
Plogmann, B.
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Transitional flow in the wake of a moderate to large height cylindrical roughness element Exp Fluids (2015) 56:212
Plogmann, B. and Würz, W. and Krämer, E.
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Nonlinear disturbance evolution downstream of a medium height roughness element. In New Results in Numerical and Experimental Fluid Mechanics X (ed. A. Dillmann et. al.). Cham: Springer 2016
Plogmann, B. and Würz, W. and Krämer, E.
