Inken Peltzer

New Results in Numerical and Experimental Fluid Mechanics VIII

This volume contains the contributions to the 17th Symposium of STAB (German Aerospace Aerodynamics Association). STAB includes German scientists and engineers from universities, research establishments and industry doing research and project work in numerical and experimental fluid mechanics and aerodynamics, mainly for aerospace but also for other applications. Many of the contributions collected in this book present results from national and European Community sponsored projects. This volume gives a broad overview of the ongoing work in this field in Germany and spans a wide range of topics: airplane aerodynamics, multidisciplinary optimization and new configurations, hypersonic flows and aerothermodynamics, flow control (drag reduction and laminar flow control), rotorcraft aerodynamics, aeroelasticity and structural dynamics, numerical simulation, experimental simulation and test techniques, aeroacoustics as well as the new fields of biomedical flows, convective flows, aerodynamics and acoustics of high-speed trains.

Large scale separation flow control experiments within the German Flow Control Network

This is an overview on flow control experiments for flow separation control conducted in the DNW-NWB atmospheric low-speed wind tunnel performed within the German Flow Control Network. Emphasis is given on the experimental setup using the DLR F15 wall-to-wall two-dimensional high-lift model. Examples of successful flow control for enhancement of lift are given for leading edge boundary layer control and flap separation control, both by means of pulsed jet actuation.

Experimental AFC Approaches on a Highly Loaded Compressor Cascade

This paper describes the impact of active separation control by means of pulsed blowing in a highly loaded compressor cascade. Experimental investigations with AFC were undertaken in order to increase the performance of the stator cascade. Two different concepts of actuation were tried. At first, pulsed blowing out of the casing was used to reduce the secondary flow structures. Secondly, the flow was excited with actuators mounted on the blade’s suction side, suppressing the pressure-induced flow separation. In a final step, both actuator concepts were combined with selected excitation amplitudes and frequencies. These demonstrations show that the gain achieved in both actuator concepts can be combined, using certain excitation parameters and no interaction with negative effects occur.

Active Flow Control on an Industry-Relevant Civil Aircraft Half Model

The article presented describes an approach to active flow control by means of pulsed blowing from the flap shoulder in order to delay turbulent flow separation in low-speed flows. The experiments were carried out on an industrial low-scale high-lift wind tunnel model, a specific landing configuration model employed in the industrial aircraft design process. The results verified the concept of pulsed blowing as a suitable tool for separation control on a complex model at a Mach Number relevant for take-off and landing (Ma = 0.2) and a reasonable Reynolds Number (Re = 1.6 ·106). Lift was increased significantly over a broad range of angles of attack with only moderate energy input necessary.

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.

Piezo-Polymer-Composite Unimorph Actuators for Active Cancellation of Flow Instabilities Across Airfoils

This article presents a smart device for active cancellation of flow instabilities. An array of two piezo unimorph actuators fabricated in piezo-polymer-composite technology is combined with a thin silicone membrane to mimic a movable wall with a closed surface. By locally displacing the thin membrane, a surface wave is generated that interferes with naturally occurring flow instabilities within the boundary layer of an airfoil. Using flow sensors and an intelligent control enables a destructive interference and therefore, an attenuation of natural flow instabilities. This leads to a delay of transition. The boundary layer remains laminar which means drag is reduced. Within the next pages, the setup of the device with actuators, membrane, sensors, and control is introduced. The main focus of this article is on actuator design, modeling, and implementation for wind tunnel experiments. Results of actuator characterization are presented. The non-linear behavior of the piezoactuator (harmonic distortions and impact of high electric fields) is investigated in detail. This study concludes with the results obtained in wind tunnel experiments which prove the functionality of the presented approach. A maximal attenuation of natural occurring flow instabilities of 80% is achieved.

A piezo-actuated closed loop MEMS system for active delay of transition

This paper presents the design, fabrication and application of a highly integrated closed loop MEMS system for an active control of aerodynamic flow instabilities across airfoils. A special piezo-polymercomposite (PPC) technology was used for the fabrication of powerful piezo-microactuators that were integrated with hotwire flow sensors and a digital control system. This work shows first wind tunnel experiments that prove the principal suitability of the developed device for dampening disturbances in the boundary layer. The local amplitudes of natural disturbances, so called Tollmien-Schlichting (TS) waves, have been reduced by 42%.

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.

Active Flow Control on a Highly Loaded Compressor Cascade with Non-steady Boundary Conditions

This paper discusses the effect of a periodical disturbance in the wake of a highly loaded axial compressor on the pressure distribution and the secondary flow characteristics of the turbo machinery stator blades. A large scale axial compressor cascade, consisting of six two dimensional passages, has been used for this investigation. The test rig is equipped with an active flow control system to enhance the operating range of the compressor. Results that have been achieved by means of side wall actuation are also presented.