Wolfgang Nitsche

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.

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.

Active cancellation of Tollmien–Schlichting instabilities on a wing using multi-channel sensor actuator systems

Abstract This paper describes investigations on active attenuation of naturally occurring Tollmien–Schlichting (TS) instabilities on an unswept wing. TS disturbances are canceled in their linear stage by superimposition with artificially generated counter waves in order to shift the laminar–turbulent transition downstream. In this method, the need for energy input is considerably lower than the stabilization by manipulation of the local mean velocity profile (e.g. boundary layer suction). An optimum TS wave cancellation is achieved by a fast digital signal processor performing an adaptive control algorithm. The oncoming traveling TS disturbances are detected by a reference sensor upstream of an actuator. The reference signal is directly calculated into an actuator signal by means of a linear transfer function. The transfer function is continuously adapted to minimize the error signal measured downstream of the actuator. Previous investigations with a single control system have shown the successful TS wave damping by an adaptive control algorithm. These investigations led to an improvement in the set up of sensors and actuators used in the field of TS wave control. As a follow up of these experiments, recent investigations focus on the application of multi-channel control systems. This approach is essential to attenuate three-dimensionally dominated boundary layer instabilities by spanwise arranged sensor–actuator systems. Furthermore, the employment of multiple sensor–actuator systems allows a streamwise repeated damping to extend the delay of laminar–turbulent transition considerably.

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.

Extensions of adaptive slope-seeking for active flow control

To speed up gradient estimation in a slope-seeking controller two different modifications are proposed in this study. In a first approach, the gradient estimation is based on a locally identified black-box model. A further improvement is obtained by applying an extended Kalman filter to estimate the local gradient of an input—output map. Moreover, a simple method is outlined to adapt the search radius in the classical extremum- and slope-seeking approach to reduce the perturbations near the optimal state. To show the versatility of the slope-seeking controller for flow control applications two different wind tunnel experiments are considered, namely with a two-dimensional bluff body and a generic three-dimensional car model (Ahmed body).

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

Active Separation Control with Pulsed Jets in a Critically Loaded Compressor Cascade

DOI: 10.2514/1.J050931 In this contribution the impact of active flow control by means of pulsed blowing in a critically loaded compressor cascadewillbedescribed.Becauseofthehighloadingoftheblades,afullythree-dimensionalandcomplex flowfieldis developing. Experimental investigations with active flow control were undertaken to increase the turning and the pressure rise of the stator cascade, by suppressing separation phenomena in the passage flowfield. Two different concepts of actuators were used. First, pulsed blowing out of the endwalls was used to reduce the secondary flow structures. Second, the flow was excited with actuators flush-mounted on the blade’s suction surface. The best performance was reached by using both actuator concepts in combination. A multivariable closed-loop control approach was used to control both separation phenomena simultaneously. Heavy disturbances could be compensated and a stabilization of the cascade operation point was achieved.

Active Drag Control for a Generic Car Model

Experimental and numerical investigations were carried out aiming at the reduction of the total aerodynamic drag of a generic car model by means of active separation control. For two different configurations separate control approaches were tested, taking into account the differences in the wake topology of the models. The targeted excitation of the respective dominant structures in the wake region leads to their effective attenuation. The experiments as well as the numerical simulations showed that a weakening of a spanwise vortex in the separated flow over the slant is strongly coupled with the occurrence of stronger streamwise vortices along the slant edges and vice versa.

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.

Infrared based visualization of wall shear stress distributions with a high temporal and spatial resolution

These images are the result of a new infrared based measurement technique which allows the visualization of wall shear stress distributions with a high spatial and temporal resolution*. The experiments were conducted for the flow around a wall mounted cylinder with a height to diameter ratio of H/D = 2 for a Reynolds-Number of 40000 Re = D . The technique is not only able to capture the characteristic flow features displayed in figure 1, but can also visualize the unsteady processes in the wake of the cylinder with a good temporal resolution (Fig. 2)**.