Ralf Petz

Adaptive Closed-Loop Separation Control on a High-Lift Configuration Using Extremum Seeking

We present experimental results on adaptive closed-loop separation control on a 2-D generic high-lift configuration. Because model-based closed-loop flow control suffers from the lack of sufficient simple physical models for this configuration, a non-model-based control strategy, namely, the gradient-based extremum-seeking scheme, is used here. The controller exploits spanwise distributed pressure measurements and adjusts pulsed jets near the leading edge of the single-slotted flap. The jets are used for flow excitation to suppress separation over the flap at high angles of attack, high deflection angles, or to reattach an already separated flow. Starting from a single-input/single-output design, the extremum-seeking scheme is extended to both a single-input/single-output slope-seeking approach and a multi-input/multi -output approach. Multi-input/multi -output control accounts for spanwise-distributed, small-scale separation phenomena and shows the best performance. Additionally, this case even improves lift gain compared to preliminary open-loop studies. A lift increase is not only observed for angles of attack for which the unactuated flow obviously separates, but as well for smaller angles, which were assumed before to lead to an unseparated flow. Hence, closed-loop results demonstrate the capability of slope-seeking control to adjust the control signal automatically in an energy-efficient sense such that separation is minimized even in the presence of disturbances.

Active Separation Control on the Flap of a Two-Dimensional Generic High-Lift Configuration

The paper describes experimental results of controlling flow separation by periodic excitation on the flap of a generic high-lift configuration. The single slotted flap of the two-dimensional test model is equipped with a robust and reliable actuator system that fits inside the flap. The flow is excited using a pulsed wall jet that emanates from the upper surface near the flaps leading edge through a small spanwise-oriented slot By preventing the flow from separating or by reattaching the separated flow, lift and drag are substantially improved, resulting in a lift-to-drag ratio enhancement of 20-25 %. Because of the actuator assembly with spanwise individually addressable segments, the separated flow can be forced to attach only to certain parts of the flap. Local spanwise excitation is thus used to generate a rolling moment without the need to deflect an aileron.

Adaptive flow control using slope seeking

Besides controller synthesis based on a high dimensional discretization or low dimensional description of the Navier-Stokes equation or based on black-box models identified in wind tunnel experiments, model-free methods such as extremum seeking control can be used advantageously to control fluid flows. This contribution gives a survey on several successful applications of extremum seeking control showing its versatility and ease of application. Emphasis is put on the slope seeking variant of extremum seeking control. SISO and MISO examples are considered. Extremum seeking control is applied to minimize the drag of a bluff body, to increase the lift of a generic high-lift wing, and to reduce the noise emitted by a turbo-machine

Separation Control by Periodic Excitation and its Application to a High Lift Configuration

In a joint experimental and numerical study, the effect of periodic excitation on a twocomponent high lift configuration is investigated. At Reynolds numbers between 160, 000 and 2 × 10 the flow is influenced by periodic blowing and suction through a slot near the flap leading edge. The effects the excitation frequency and intensity are investigated using PIV measurements and numerical simulations based on the Unsteady Reynoldsaveraged Navier-Stokes equations (URANS) for different Reynolds numbers. Comparison of measured aerodynamic forces and flow visualisation to the numerical simulations allows a detailed analysis of the dominant structures in the flow field and the effect of flow control on these. The mean aerodynamic lift can be significantly enhanced by active flow control whilst the mean detachment on the flap is delayed.

Control of Separation on the Flap of a Three-Element High-Lift Configuration

This paper describes a joint experimental and numerical investigation of the control of the flow over the flap of a three-element high-lift configuration by means of periodic excitation. At Reynolds numbers between 0.3 ◊ 10 6 and 1 ◊ 10 6 the flow is influenced by periodic blowing or periodic blowing/suction through slots near the flap leading edge. The delay of flow separation by periodic vertical excitation could be identified in the experiments as well as numerical simulations based on the Unsteady Reynolds-averaged Navier-Stokes equations (URANS). As a result, the mean aerodynamic lift of this practically relevant wing configuration could be significantly enhanced. By investigating dierent excitation frequencies and intensities optimum control parameters could be found. The behaviour of the aerodynamic forces with varying flap deflection angle are measured on a finite swept wing. Scientific visualisation of the numerical simulations of an infinite swept wing allows a detailed analysis of the structures in this complex flow field and the eect of flow control on these.

AeroMEMS Wall Hot-Wire Anemometer on Polyimide Substrate Featuring Top Side or Bottom Side Bondpads

Design, manufacturing, calibration, and basic characterization of a microelectromechanical systems (MEMS) wall hot wire sensor on a flexible polyimide substrate are presented. A configuration exhibiting bond pads on the top side of the foil, as well as an improved setup featuring a through-foil metallization and bottom side bond pads were established. Both sensor designs make use of a highly sensitive nickel thin-film resistor spanning a reactive ion etched cavity in a polyimide substrate. The polyimide base material enables the sensor to be adapted to curved aerodynamic surfaces, e.g., airfoils and turbine blades. A mismatch of curvature of aerodynamic surface and silicon sensor surface, as observed with previously presented MEMS hot-wire anemometers is avoided. The combination of polyimides low thermal conductivity and a cavity featuring FEM-optimized dimensions accounts for a very low-power consumption (<25 mW). Fluctuations in wall shear stress up to 85 kHz can be resolved in constant-temperature mode. An average sensitivity of 0.166 V/(N/m2) is achieved in a wall shear stress range from 0 to 0.72 N/m2. The specifically designed through-foil metallization process allows for electrical contacts to be positioned on the backside of the substrate, thus effectively minimizing aerodynamic disturbances.

Active Control of Flow Separation on a Swept Constant Chord Half Model in a High-Lift Configuration

The purpose of this manuscript is to address the impact of active separation control by means of pulsed blowing in a three-dimensional and complex flow environment. Experimental investigations are undertaken in order to enhance the performance of a three-element high-lift configuration by preventing the flow separation on the single-slotted flap. The configuration includes some less investigated aspects as the wing has a constant sweep angle of 30 and a finite wing span. An eort is made to implement an actuator system inside the small flap in order to excite the flow locally with the desired frequency and amplitude. The results show that, despite the strong three-dimensionality due to sweep and finite wing span, periodic excitation is able to delay separation on the flap or to reattach an already separated flow. Lift is improved by 10% to 12% over a wide range of angle of attack and flap settings. The influence on drag by periodic excitation seems to be ambiguous because in some cases the higher induced drag due to the attached flow compensates the drag benefit resulting from the elimination of the recirculation region. However, preliminary tests show that high amplitude forcing in the wing tip area is able to reduce the drag by as much as 10%.

Designing Actuators for Active Separation Control Experiments on High-Lift Configurations

Designing actuators for experimental investigations that deal with the active control of separation by periodic excitation is of immense importance. The conclusions drawn from such experiments heavily rely on the actuator performance. Actuation frequency and amplitude as well as actuator location and jet direction play an important part that is still not fully understood in some aspects. Once the experiments are successfully performed the everlasting question of power consumption versus aerodynamic benefit arises. In order to estimate an overall figure of merit not only the aerodynamic benefits have to be taken into account but also actuator weight and volume as well as initial and maintenance costs and system complexity. Although many experiments are performed in this field of research scaling actuator performance from wind tunnel experiments to full-size applications is only possible for those who use zero-net-mass-flux actuators which require only electrical input. Once compressed air in combination with valves is used to excite the flow it is difficult to estimate the actuator performance due to longer ducting system and different tube diameters with additional pressure losses. The paper gives an overview of three different actuator designs that aim at enhancing the aerodynamic performance of high-lift configurations by suppressing flow separation on a single slotted flap. All experiments were performed in the Collaborative Research Centre 557 Control of complex shear flows set up at the Berlin University of Technology.

Family of Micromachined Wall Hot-Wire Sensors on Polyimide Foil

Micromachined wall hot-wire sensors composed of a highly sensitive, nickel, thin-film resistor spanning an air-filled cavity in a mechanically flexible substrate are presented. Cavity design and sensor materials are optimized to reduce thermal losses, thus enabling measurement of high-frequency fluctuations in fluid flows. Successfully realized sensors featuring wire widths of 2 and 5 μm, wire lengths from 400 to 2000 μm, and various cavity dimensions were characterized in wind-tunnel experiments. Static sensor calibration, cutoff frequency determination using a sine sweep, and recording of angular characteristics were conducted at wall shear stresses of up to 1 N/m 2 , obtained at 20 m/s freestream velocity in an open wind tunnel on a flat plate with fully developed turbulent flow. An overheat ratio of 1.8 was used for hours without thermal failure of the sensors, and a maximum cutoff frequency in still air of 73 kHz was obtained. The highest average sensitivity of 0.196 V/(N/m 2 ) was recorded for a sensor of 5-μm wire width and a length-to-width ratio of 400 in a wall shear stress range from 0 to 1 N/m 2 with a power consumption of less than 30 mW.

Increasing Lift by Means of Active Flow Control on the Flap of a Generic High-Lift Configuration

This paper demonstrates the use of active separation control on a high-lift configuration in order to enhance the aerodynamic performance in terms of lift and drag. The aim is to delay boundary layer separation on the flap’s upper surface by periodic excitation using a pulsed wall jet The experimental and numerical results show a massive improvement of almost all aerodynamic coefficients over a wide range of angles of attack and flap deflection angles. By actuating with correct excitation parameters, the jet formed by the single slotted flap can be reattached or kept attached, depending on the conditions, to the surface. Lift is increased by up to 12% while drag is reduced by the same amount. As a result, the lift to drag ratio defining the aerodynamic quality is improved by up to 25%. A numerical calculation on the basis of unsteady Reynolds-averaged Navier Stokes equations is also presented to determine the influence of different excitation parameters.