Control of crossflow-induced laminar-turbulent transition in a 3-d boundary-layer flow using plasma actuators
Final Report Abstract
Using direct numerical simulations the applicability of dielectric barrier discharge (DBD) plasma actuators to delay the laminar-to-turbulent transition caused by crossflow vortices (CFVs) has been explored. Delaying the transition on aerodynamic surfaces is one means to improve the efficiency of aerodynamic devices such as aircraft or wind turbines because the skin-friction drag is significantly lower for the laminar boundary-layer state. The base flow of the newly designed DLR-Göttingen swept flat-plate experiment has been used for the investigations. It is a model flow for a three-dimensional boundary-layer flow as it develops on the upper side of a swept wing. The main focus has been on the control of transition caused by traveling CFVs, but also investigations on the control of steady CFVs have been performed. Two different methods have been investigated in detail, namely upstream flow deformation (UFD) and direct attenuation of grown, nonlinear CFVs. Furthermore, conclusions on the applicability of the method of base-flow manipulation to control transition caused by traveling CFVs have been drawn. In the UFD technique (AP2) the plasma actuators are used to excite steady CFV control modes that are spaced narrower than the most amplified mode and less unstable with respect to secondary instability. The resulting narrow-spaced CFVs generate a useful mean-flow distortion and hinder the naturally growing CFV mode in growth, yielding delayed transition to turbulence. It has been shown that forcing in or against the direction of the crossflow (CF) is effective with respect to the excitation of the control modes. Successful delay of transition has been achieved both for flow scenarios dominated by steady and by traveling CFVs, yielding a significant reduction of the skin-friction drag. Further investigations on the unsteady UFD method have shown the potential of narrowspaced traveling CFVs to control the CF induced transition. The stabilizing effect achieved by traveling control modes is somewhat weaker than for the classical steady UFD method. However, the energy requirements would be significantly lower in practice, because this approach makes full use of the inherent unsteadiness of the plasma-induced body force. Based on the findings of the investigations on the steady UFD method it can be concluded that also the previously investigated method of base-flow manipulation (AP1) is suitable to control transition caused by traveling CFVs. For both methods the effect on the stability properties of the flow is comparable and (locally) high-amplitude CFVs are excited by the forcing. Compared to the UFD method the plasma actuators are positioned farther downstream for the direct-attenuation approach (AP3). The applicability of the method to control steady nonlinear CFVs has been demonstrated previously. The current investigations have been focused on the control of traveling CFVs. The localized forcing against the direction of the CF is then aimed at decreasing the amplitude of the traveling CFVs that move over the (unsteadily driven) actuators by tackling the three-dimensional disturbance state. It has been shown that transition can be delayed. However, the efficiency is found to be significantly higher when employing the UFD technique, because the instability inherent to the base flow is then exploited.
Publications
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Crossflow transition control by upstream flow deformation using plasma actuators. J. Appl. Phys., 121:063303, 2017
P. C. Dörr and M. J. Kloker
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Effect of Upstream Flow Deformation Using Plasma Actuators on Crossflow Transition Induced by Unsteady Vortical Free-Stream Disturbances. AIAA-2017-3114
P. C. Dörr, M. J. Kloker, and A. Hanifi
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Control of Traveling Crossflow Vortices Using Plasma Actuators. In W. E. Nagel, D. H. Kröner, and M. M. Resch, editors, High Performance Computing in Science and Engineering ’17, pages 1-13. Springer Berlin, 2018
P. C. Dörr, Z. Guo, J. M. F. Peter, and M. J. Kloker
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Numerical Investigations on Tollmien-Schlichting-Wave Attenuation Using Plasma Actuator Vortex Generators. AIAA J., 56(4):1305-1309, 2018
P. C. Dörr and M. J. Kloker
