Lars Neuhaus

A New Modular Fan Rig Noise Test and Radial Mode Detection Capability

A new major large-scale fan rig test facility, UFFA (Universal Fan Facility for Acoustic), has been designed with the objective to allow test bed changes for engine representative OGVs and bypass duct annulus and liners, for reduced build times, and higher fidelity investigation of aft fan noise technologies. An important enhancement consisted in the implementation of three Radial Mode Detection (RMD) devices in the bypass duct and further downstream in the nozzle equivalent plane. High effort was spend on the realisation of a wall-flush mounted sensor array, which has the advantage not to disturb the flow and the acoustic field. However, the separation of different radial mode contributions is realised only implicitly by the analysis of the axial wave number spectrum, which is particularly challenging if sensors are installed only at the outer duct wall. More robust from the numerical point of view is the established technique to directly measure the radial structure of the sound field with sensor rakes. It is one of the main objectives of this paper to verify whether both techniques deliver the same experimental results also at the high targeted frequencies up to kR=75. As the examination of recently obtained data revealed, the sensor rake measurements were influenced by aerodynamic perturbations originating from the fan rotor wakes. The radial mode analysis could be significantly improved by incorporation of appropriate aerodynamic eigenfunctions. Further investigated was the sensitivity of mode detection with sensor rakes against manufacturing and installation tolerances. I. Introduction A major new large scale fan rig test facility, UFFA (Universal Fan Facility for Acoustic), has been designed, manufactured and commissioned by AneCom AeroTest GmbH, Wildau, Germany. The new modular fan rig test facility builds on previous Rolls-Royce large scale fan rig design experience 1 but extends the experimental capability to allow test bed changes for engine representative OGVs and bypass duct annulus and liners, for reduced build times, and higher fidelity investigation of aft fan noise technologies. In order to meet the challenging noise targets in a timely manner for future aircraft, the industry requires engine and nacelle representative high Technology Readiness Level validation test vehicles to deliver the technology ready for implementation in to the full scale product. An important enhancement consisted in the implementation of three Radial Mode Detection (RMD) devices in the bypass duct, i.e. radial sensor rakes directly at OGV exit and radial sensor rakes respectively a wall-flush mounted sensor array further downstream in the nozzle equivalent plane of the UFFA test facility. The sensor arrays were developed by DLR within the frame of the EU FP 6 project VITAL 2 , based on predicted sound field characteristics provided by Rolls-Royce. At the maximum targeted frequency of kR=75 more than 2500 modes of azimuthal mode orders up to m=85 and radial mode orders up to n=10 should be resolved. Major objectives of future RMD measurements are (1) the accurate assessment of fan rig design changes, (2) the provision of high quality data

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.

Broadband Sound Power Determination in Flow Ducts

This paper presents a new experimental technique, which enables not only the calculation of in-duct transmitted sound power for dominant tones, but also for broadband noise over a wide frequency range. The new method also distinguishes between sound power transmitted in and against the direction of flow. It can be performed using wall-flush installed pressure sensors as well as radial sensor rakes in the flow. The method is mathematically derived based on the analytical solution of the wave equation describing sound propagation in ducts with superimposed static mean flow. The analysis technique requires cross-correlation measurements, e.g. between azimuthally traversable sensor arrays and at least one fixed reference sensor. It assumes an invariant mean broadband noise sound field. The broadband sound power determination method was experimentally verified in a low speed DLR laboratory fan experiment against the ISO 5136 standard for in-duct sound power determination. The experimental setup consists of a single-stage compressor in a flow duct with a diameter of 500 mm. An azimuthally traversable duct section located 2m downstream of the rotor can be equipped alternatively with a sampling tube for measurements after ISO 5136 or microphone rakes for in-duct mode detection. For mode detection, 8 microphones were installed in the moving duct section, which was traversed over 36 azimuthal positions. A wall-flush mounted microphone was used as a reference sensor. Experimental results are presented for three operating conditions and two different compressor tip clearances.

Experiments on an Axial Fan Stage: Time-Resolved Analysis of Rotating Instability Modes

Rotating Instability (RI) occurs at off-design conditions in axial compressors, predominantly in rotor configurations with large tip clearances. Characteristic spectral signatures with side-by-side peaks below the blade passing frequency are typically referred to RI located in the clearance region next to the leading edge (LE). Each peak can be assigned to a dominant circumferential mode. RI is the source of the clearance noise and an indicator for critical operating conditions. Earlier studies at an annular cascade pointed out that RI modes of different circumferential orders occur stochastically distributed in time and independently from each other, which is contradictory to existing explanations of the RI. Purpose of the present study is to verify the generality with regard to axial rotor configurations.Experiments were conducted on a laboratory axial fan stage mainly using unsteady pressure measurements in a sensor ring near the rotor LE. A mode decomposition based on cross spectral matrices was used to analyze the spectral and modal RI patterns upstream of the rotor. Additionally, a time-resolved analysis based on a spatial Discrete-Fourier-Transform was applied to clarify the temporal characteristics of the RI modes and their potential interrelations. The results and a comparison with the previous findings on the annular cascade corroborate a new hypothesis about the basic RI mechanism. This hypothesis implies that instability waves of different wavelengths are generated stochastically in a shear layer resulting from a backflow in the tip clearance region.Copyright

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 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.

EXPERIMENTAL ASSESSMENT OF NOISE GENERATION AND TRANSMISSION IN A HIGH-PRESSURE TRANSONIC TURBINE STAGE

The noise originating from the core of an aero-engine is usually difficult to quantify and the knowledge about its Generation and propagation is less advanced than that for other engine components. In order to overcome the difficulties associated with dynamic measurements in the crowded core region, dedicated experiments have been set up in order to investigate the processes associated with the generation of noise in the combustor, its propagation through the turbine and the interaction of these two components, which may produce additional - so-called indirect combustion-noise. In the current work, a transonic turbine stage installed at the Laboratorio di Fluidodinamica delle Macchine of the Politechnico di Milano was exposed to acoustic, entropic, and vortical disturbances. The incoming and outgoing sound fields were analyzed in detail by two large arrays of microphones. The mean flow field and the disturbances were carefully mapped by several aerodynamic and thermal probes. The results include transmission and reflection characteristics of the turbine stage, latter one was found to be much lower than usually assumed. The modal decomposition of the acoustic field in the upstream and downstream section show beside the expected rotor-stator interaction modes additional modes. At the frequency of entropy or respectively vorticity excitation, a significant increase of the overall sound power level was observed.

Active Flow Control to Improve the Aerodynamic Performance of Axial Turbomachines

Axial turbomachines have a radial gap between the casing and the rotor blades. The static pressure difference between the suction and the pressure side of the impeller blades produces a secondary flow over the tip of the rotor blades. This tip clearance flow is important for the aerodynamic performance of the fan. Fan pressure and efficiency drop, and the usable range of the performance characteristics is diminished as the rotor flow is stalled at low flow rates. Previous investigations have shown that one method for increasing the aerodynamic performance is to control the flow in the tip clearance gap via air injection into the gap. The goal of this paper is to compare the different effects of steady and unsteady air injection on the aerodynamic performance curves and to implement various closed-loop extremum-seeking control algorithms. The main purpose of these active flow control methods is to stabilize the flow at operating points, where it is stalled otherwise. To compare the effect of steady and unsteady air injection, the aerodynamic performance curves (fan pressure rise and efficiency) were measured for different sets of frequencies with the air injection rate held constant. To control the air injection rate automatically and to find optimal actuation parameters, a SISO-extremum-seeking control algorithm was applied. For the improvement of the control performance, the controller was extended by a slope-seeker. Moreover, an extended Kalman filter was used to speed up the control via a faster slope detection to accelerate the estimation of the local gradient of the static input-output map of the process. This new approach led to an almost fivefold increase in closed-loop control speed.Copyright

Active control of the aerodynamic and acoustic performance of axial turbomachines.

The tip clearance flow of axial turbomachines is important for their aerodynamic and acoustic pe rformance. The rotating instability phenomena and the tip clearance noise are observed on axial turbomach ines with significant tip clearance. Previous investig ations show that it is possible to reduce the tip clea rance 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 co ntrolling 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 r eduction of the noise level along with an improved aerodynamic performance. With larger injected mass flows, significant improvements of the aerodynamic performance are obtained at the expense of a steep increase of the noise level. Unsteady air injection synchronized with the i mpeller rotation yields a significant improvement of the aerodynamic performance accompanied by a substantial increase of the noise level. Rotating instabi lity and tip clearance noise can be reduced in both cases.