Wolfgang Neise

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

Experimental verification of a radial mode analysis technique using wall‐flush mounted sensors

Sound fields in the inlet and outlet ducts of axial fans, compressors, and aircraft engines propagate as higher‐order acoustical modes in a wide‐frequency range. Decomposition of the sound field into azimuthal and radial modes permits direct conclusions on the sound generation processes and can lead to the identification of aerodynamic source areas. An established experimental method for assessing the sound field is to circumferentially traverse a radial microphone rake over 360 degrees, to measure the sound pressure in the duct at multiple circumferential and radial positions. On the inlet side of a turbo‐machine this procedure cannot be employed because the wake of the rake would disturb the inlet flow conditions of the machine and alter its acoustic characteristics. Computer simulations have shown that the radial mode structure of the sound field can also be determined by measuring the sound pressure only at the inner duct wall, but at different axial locations. In this paper, experiments are described...

Technology approach to aero engine noise reduction

Transportation noise is one of the most pressing environmental problems of modern societies. Aircraft noise is second only to road traffic noise in drawing complaints from the public about noise pollution. Therefore intensive research efforts are necessary on the national levels as well as the European level to reduce the noise load around airports. The most effective and economical way to reach this goal is noise reduction at the source. The aero engines of todays transport aircraft are the dominant noise sources for most flight conditions, although airframe noise does play an important role for landing aircraft. In this paper noise reduction studies for aero engines are described in which DLR are involved. The topics discussed are low noise fan design, active noise control using wall-flush loudspeakers as secondary sources, ANC using active stators as secondary sources, ANC using flow induced secondary sources at the rotor tips, reduction of low-pressure turbine noise, and flight tests for validation of add-on noise reduction devices.

Extensions of extremum-seeking control to improve the aerodynamic performance of axial turbomachines

Different extremum-seeking control methods are applied to a high pressure axial fan in order to reduce flow separation and increase the pressure ratio via injection of a pulsed air stream. To improve the control performance the plain SISO-extremum controller is extended by a slope-seeker. To speed up control, a Kalman filter is applied that estimates the local gradient of the static input-output map of the process significantly faster than in the classical concept of extremum-seeking control. Due to that, control can be accelerated up to ten times. The usable range of the aerodynamic characteristic diagram can be increased and flow separation can be hold to smaller flow coefficients.

Active Control to Improve the Aerodynamic Performance and Reduce the Tip Clearance Noise of Axial Turbomachines

The tip clearance flow of axial turbomachines is important for their aerodynamic and acoustic performance. The rotating instability phenomena and the tip clearance noise are observed on axial turbomachines with larger tip clearance s. In this paper it is shown that it is possible to reduce the tip clearance noise and improve the aerodynamic performance of the fan by actively controlling the tip clearance flow. To achieve this, air is injected into the gap either through slits in the casing wall or throu gh the rotor blades and out of the blade tips. The experiments were made with a high -pressure axial fan of D = 452.4 mm impeller d iameter connected to an anechoically terminated outlet duct . The fan has 24 rotor blades and 17 stator vanes . Two tip clearanc e gap s � = 0.7 % and 5.6% of the blade cord length are used. Three configurations for air injection through the casing wall are investigated: 24 and 17 slit nozzles, and a uniform circumferential slit. With steady air injection through the opt imum configura tion it is possible to achieve significant improvements of the aerodynamic perfor mance and of the radiated noise level. Air injection out of the tip of the impeller blades is also effective in improving the aer odynamic and acoustic performance of the fan. The r otating instability components and the tip clearance noise can be suppressed with both inje ction methods. In case of the smaller tip clearance gap, the range of stable fan operation is enlarged.

Active control of the blade passage frequency noise level of an axial fan with aeroacoustic sound sources

This paper presents results of a research project on active control of the blade passage frequency tone of an axial fan. The secondary sound field is generated by aeroacoustic sources, which are produced by actively controlling the flow around the impeller blade tips. Both amplitude and phase can be controlled in such a way that a destructive superposition with the primary sound field is possible. The flow distortions can be achieved by using different actuators; results using steady and unsteady jets of compressed air and piezo-electric actuators are presented.

Active noise control in axial turbomachines by flow induced secondary sources.

In conventional active noise control experiments, loudspeakers are used to generate the secondary antiphase sound field to be superimposed with the sound waves radiated from the primary source. In the present study, aerodynamic sound sources are used instead for that purpose. This is achieved by disturbing the flow field around the blade tips in such a way that additional periodic forces are set up which in turn form the secondary sound sources. To disturb the flow, either air is blown into the blade tip region through the casing wall, or piezo-electric actuators are installed in the fan casing wall to influence the flow conditions near the blade tips. The resulting aerodynamic sound sources are adjustable in both amplitude and phase. In this way active flow control is used to reduce the tonal noise components of the axial fan. Experimental results are presented for steady and unsteady air jets injected into the main flow at various axial positions relative to the rotor blades. It is shown that the method is successful for plane wave as well as for higher-order mode sound fields. The sound pressure level at the blade passing frequency was reduced by up to 20.5 dB.

Active Blade Tone Control in Axial Turbomachines by Flow Induced Secondary Sources in the Blade Tip Regime

To reduce the tonal noise of axial fans loudspeakers are typically used to generate the required anti-phase sound field. The space requirement and the weight of the loudspeakers inhibit practical application of this method. In the present study the secondary sound field is generated by injecting high speed air jets into the rotor blade tip regime. The air jets set up additional periodic forces on the blade tips which in turn form the required secondary acoustic sources. The jets are driven by a compressed air supply through small nozzles mounted flush with the inner casing wall. It has been shown, that this approach is very effective in controlling higher order mode sound fields at blade passage frequency and its harmonics. The main goal of the present work is to introduce fast feedback control to this application.

ACTIVE FLOW CONTROL TO IMPROVE THE AERODYNAMIC AND ACOUSTIC PERFORMANCE OF AXIAL TURBOMACHINES

The tip clearance flow of axial turbomachines is important for their aerodynamic a nd acoustic performance. The rotating instability phenomena and the tip clearance noise are observed on axial turbomachines with significant tip clearance. Previous investigations show that it is possible to reduce the tip clearance noise and improve the aerodynamic performance of the fan by mounting a turbulence generator into the tip clearance gap. In this paper it is shown that these improvements can be obtained without any modification of the tip clearance gap itself by actively controlling the tip clearance flow. To achieve this, air is injected into the gap through slit nozzles mounted flush with the inner casing wall. With steady air injection it is possible to obtain ‐ with a small injected mass flow ‐ a remarkable reduction of the noise level along with an improved aerodynamic performance. With larger injected mass flows, significant improvements of the ae rodynamic performance a re obtained at t he expense of a steep increase of the noise level. Unsteady air injection synchronized with the impeller rotation yields a significant improvement of the aerodynamic performance acc ompanied by a substantial i ncrease of the noise level. Rotating instability and tip clearance noise c an be reduced in both cases. Flow investigations with a simplified stationary 2D blade cascade show that steady air injection leads to a diminished b lade tip vortex and with it t o an improved aerodynamic performance.

Adaptive Control in an Axial Turbofan: Model-Free Implementation with Short Response Time

An adaptive control strategy is implemented in a single-input/single-output experiment to improve the aerodynamic performance of an axial turbofan. Pulsed blowing into the blade tip region is used to prevent flow separation. A slope-seeking control approach is shown to be able to extend the operating region of the engine without an explicit process model. By this, flow separation can be mitigated for smaller flow coefficients. The proposed closed-loop control strategy is capable of both driving the system back to stable operating conditions automatically and stabilizing operation in the presence of large-amplitude disturbances. Moreover, it is possible to accelerate the closed-loop control performance by one order of magnitude, compared with classical extremum-seeking control. By this, an extended Kalman filter is applied that enables a rapid determination of gradients of the corresponding characteristic diagram.