Control of tollmien-schlichting waves on a natural laminar airfoil via dynamic surface modulation

Abstract

The study presented here demonstrates experimental mitigation of Tollmien-Schlichting (TS)waves on Natural Laminar Flow (NLF) wind tunnel models. Experiments were first conducted on an unswept NLF airfoil at a chord-based Reynolds number of 9:90 x 10^5 and then on a 30 degree swept-back NLF airfoil at a chord-based Reynolds number of 8:45 x 10^5. Control of TS waves was facilitated by dynamic surface modifications using piezoelectrically-driven oscillating surface (PDOS) actuators. On each model, the actuators were located at three streamwise locations on the airfoil suction side. For TS wave control, a disturbance was introduced to the flow using the upstream actuator, which phase-locked the TS waves. The downstream actuators were used to mitigate the induced TS waves by introducing anti-phase disturbances with the proper amplitude. The disturbances included either single frequency or multi-frequency input waveforms. The TS waves were mitigated in both open- and closedloop control schemes for the unswept model, and in open-loop for the swept-back model. In open-loop control, the results demonstrate that dynamic surface modification can be used to mitigate the TS waves for varying frequency bandwidths even in the presence of a large adverse pressure gradient. In addition, a closed-loop scheme, using an iterative learning control algorithm, succeeded in reducing the induced disturbance amplitudes to 11% of their original values. The experiments demonstrated the ability not only to mitigate the waves but also to amplify them if transition to turbulence is desired. With actuators implemented on the swept-back configuration, experiments indicated that this flow control method feasibly suppressed the Tollmien-Schlichting waves in a similar fashion. In addition, for the unswept model without actuation, a separation bubble was identified near the third PDOS actuator. Preliminary results showed that using the upstream PDOS yielded mitigation of the separation bubble.