Andreas Pätzold

Active Cancellation of Tollmien-Schlichting Instabilities in Compressible Flows Using Closed-Loop Control

The present paper reports on active cancellation of natural Tollmien- Schlichting (TS) instabilities on an unswept wing in compressible flows. The research concentrates on closed-loop active wave control (AWC) experiments at Mach numbers ranging from 0.2 up to 0.4. These high velocities result in thin boundary layers and therefore in TS frequencies up to 10 kHz. Therefore, the resolution of the applied sensors as well as the amplitude and frequency domain of the actuators are subject to challenging requirements. Additionally, the velocity of the convective TS waves demands a powerful, optimized control algorithm working in real time. The AWC principle applied here delays TS induced transition by stabilizing the linear disturbance waves initiating the laminar-turbulent transition process. This method is based on the wave superposition principle, i.e. the superposition of artificially generated anti-disturbances and the naturally occurring TS disturbances. The energy consumption with this method is considerably lower than the stabilization achieved by manipulating the local mean velocity profile (e.g. boundary layer suction).

On active control of laminar–turbulent transition on two-dimensional wings

This paper gives an overview of drag reduction on aerofoils by means of active control of Tollmien–Schlichting (TS) waves. Wind-tunnel experiments at Mach numbers of up to Mx=0.42 and model Reynolds numbers of up to Rec=2×106, as well as in-flight experiments on a wing glove at Mach numbers of M<0.1 and at a Reynolds number of Rec=2.4×106, are presented. Surface hot wires were used to detect the linearly growing TS waves in the transitional boundary layer. Different types of voice-coil- and piezo-driven membrane actuators, as well as active-wall actuators, located between the reference and error sensors, were demonstrated to be effective in introducing counter-waves into the boundary layer to cancel the travelling TS waves. A control algorithm based on the filtered-x least mean square (FxLMS) approach was employed for in-flight and high-speed wind-tunnel experiments. A model-predictive control algorithm was tested in low-speed experiments on an active-wall actuator system. For the in-flight experiments, a reduction of up to 12 dB (75% TS amplitude) was accomplished in the TS frequency range between 200 and 600 Hz. A significant reduction of up to 20 dB (90% TS amplitude) in the flow disturbance amplitude was achieved in high-speed wind-tunnel experiments in the fundamental TS frequency range between 3 and 8 kHz. A downstream shift of the laminar–turbulent transition of up to seven TS wavelengths is presented. The cascaded sensor–actuator arrangement given by Sturzebecher & Nitsche in 2003 for low-speed wind-tunnel experiments was able to shift the transition Δx=240 mm (18% x/c) downstream by a TS amplitude reduction of 96 per cent (30 dB). By using an active-wall actuator, which is much shorter than the cascaded system, a transition delay of seven TS wavelengths (16 dB TS amplitude reduction) was reached.

In-flight experiments on active TS-wave control on a 2D-laminar wing glove

In-flight measurements to delay laminar-turbulent transition by means of active Tollmien-Schlichting (TS) wave cancellation were carried out on a 2Dlaminar wing glove for a sailplane. The sensor-actuator system attached to the wing glove consisted of an array of surface hot-wire reference sensors to detect oncoming TS-waves upstream of a membrane actuator and surface hot-wire error sensors downstream of the actuator. The method applied was based on the dampening of naturally occurring instabilities through superimposition of a counter wave, which was calculated by a fast digital signal processor (DSP), using a closed loop feed-forward control algorithm. The flight experiments validated this system under varying atmospheric conditions successfully. Further attention was directed to the dampening of instabilities in the span-wise direction.

In-flight experiments for delaying laminar—turbulent transition on a laminar wing glove

Abstract This article describes in-flight measurements to delay laminar—turbulent transition by means of active Tollmien—Schlichting (TS) wave cancellation. The damping of unstable TS waves in the boundary layer leads to downstream shifting of the laminar—turbulent transition and therefore to the reduction of skin friction. In-flight experiments were carried out using a laminar wing glove for a sailplane. A sensor—actuator system attached to the wing glove consisted of an array of surface hot-wire reference sensors to detect oncoming TS-waves upstream of a membrane actuator and surface hot-wire error sensors downstream of the actuator. The method applied to delay laminar—turbulent transition is based on damping of naturally occurring instabilities through superimposition of the counter wave, which is calculated by a fast digital signal processor using a closed-loop feed-forward control algorithm. The experiments were carried out at flight velocities in the region of 20 m/s, which corresponds to a chord Reynolds number of about 2 million. The results show a damping of ∼ 50 per cent reduction of the local amplitudes of the instabilities. It is anticipated that using an actuator with a high resonance frequency and a minimal reaction time will improve damping significantly in future experiments.

Development of a Sensor-Actuator-System for Active Control of Boundary Layer Instabilities in Compressible Flows

This paper describes the development of an active control system for delaying laminar-turbulent transition in compressible flows. Convective boundary layer instabilities are detected during their linear amplification stage by a reference sensor and reduced downstream through destructive interference of artificial counterwaves.

Sensors and Actuators for Laminar Flow Flight Experiments

The present chapter is focussed on different measurement techniques for in-flight experiments. Laminar flow investigation for low and high flight velocities were performed. Several types of surface sensors were used for detection of laminar-turbulent transition. Methods for acquiring information about boundary conditions like angle of attack, flight velocity and pressure distribution are an essential part of this work, too. The method of delaying laminar-turbulent transition by active boundary layer control was transferred from wind tunnel experiments to a glider in real flight. For this purpose, a sensor actuator system was developed and, together with a realtime controller, applied to a glider.

Learning from Dolphin Skin – Active Transition Delay by Distributed Surface Actuation

The goal of this project was the development of an active laminarisation method in order to reduce skin friction drag. Laminar-turbulent boundary layer transition on unswept two-dimensional wings is mainly caused by Tollmien-Schlichting (TS-) waves. Based on an actively driven compliant wall as part of the wing’s surface, a method for attenuation of these convective instabilities was developed. Different arrangements of piezo-membrane actuators were investigated with an array of highly sensitive surface flow sensors and appropriate control strategies. Spanwise differentiated and streamwise cascaded actuation were used as well as inclined wall displacement. The onset of transition could be shifted downstream by 100mm or six average TS-wave lengths. Additionally, the investigation of the boundary layer flow downstream of the active wall area and an efficiency estimation are presented in this contribution.

Investigation on Actuator Arrays for Active Wave Control on a 2D Airfoil

This paper describes different actuators for delaying laminar-turbulent transition by active boundary layer control. Since transition on an unswept 2D wing is initiated by Tollmien-Schlichting instabilities, friction drag can be reduced by attenuation of these waves. The performed experiments are based on earlier investigation with single spanwise as well as streamwise cascaded actuation. As a consequent step and essential part of this work actuator arrays consisting of slots and oscillating surface membranes for spatially distributed actuation were developed. A single spanwise sensor actuator system was applied to a glider, because flow instabilities of free atmosphere can not be simulated in wind tunnel experiments. In this way the method of active TS wave attenuation could successfully be verified under actual flight conditions.