Christoph Brandstetter

Aeroelastic Investigation of a Transonic Research Compressor

The trend in modern compressor design is towards higher stage loading and less structural damping, resulting in increased flutter risk. The understanding of the underlying aeroelastic effects, especially at highly loaded BLISK rotors, is small. This paper reports on the analysis of flutter phenomena in a modern transonic compressor.The geometry examined here is the one-and-a-half stage transonic research compressor operated by Technische Universitat Darmstadt. High blade deflections recorded during throttling measurements point to an aerodynamic excitation. Therefore, numerical investigations are carried out using the CFD-Code TRACE developed at the German Aerospace Center (DLR). Simulations are compared to measured compressor speed lines to validate the steady state results. The open source Finite Element code CalculiX is used to simulate the rotor blade eigenmodes and -frequencies. The results are then used in time-linearized calculations to determine the onset of flutter. These calculations confirm that there is an aerodynamic excitation of the first torsional eigenmode and blade flutter is at risk.A sensitivity study is carried out to further investigate the aerodynamic conditions under which structural vibrations become unstable and to identify influencing factors.Copyright

Mechanism of Nonsynchronous Blade Vibration in a Transonic Compressor Rig

This paper presents a numerical study on blade vibration for the transonic compressor rig at the Technische Universit€at Darmstadt (TUD), Darmstadt, Germany. The vibration was experimentally observed for the second eigenmode of the rotor blades at nonsynchronous frequencies and is simulated for two rotational speeds using a time-linearized approach. The numerical simulation results are in close agreement with the experiment in both cases. The vibration phenomenon shows similarities to flutter. Numerical simulations and comparison with the experimental observations showed that vibrations occur near the compressor stability limit due to interaction of the blade movement with a pressure fluctuation pattern originating from the tip clearance flow. The tip clearance flow pattern travels in the backward direction, seen from the rotating frame of reference, and causes a forward traveling structural vibration pattern with the same phase difference between blades. When decreasing the rotor tip gap size, the mechanism causing the vibration is alleviated.

Unsteady wall pressure measurement in a one-and-a-half stage axial transonic compressor during stall inception

This paper presents the experimental investigation of the stall inception of a state-of-the-art transonic compressor that resembles a typical front stage of a commercial jet engine. The compressor was designed and manufactured in cooperation with Rolls-Royce Deutschland and assembled in the Darmstadt Transonic Compressor test facility at Technische Universität Darmstadt, Germany. Utilising the 1.5-stage design it is possible to investigate stall inception for different rotor inflow conditions. The aerodynamic features prior and during stall inception are investigated by means of unsteady pressure probes in the casing above the rotor. All stall inception events have a spike-type stall character. However, in some cases the spikes are preceded by modal-type activities. In addition to the unsteady wall pressure, speed lines are presented. The combination of the data shows a clear effect of measurement position and averaging process on the shapes of the speed lines but did not allow to predict the type of stall inception.

Realistic Inlet Distortion Patterns Interacting with a Transonic Compressor Stage

The formation and the interaction of inlet distortions is a safety risk in the operation of an aircraft engine. The numerical simulation of an aircraft, including the engine nacelle and the turbo-machine inside, is not possible during the design process as it is too time-consuming. To gain insight into the effects, and the impact on the engine, in particular, experiments are necessary. Due to the complexity of generating and measuring distortion patterns screens are usually used. The screens generate a total pressure drop that is constant in space and time. In this paper the interaction of a transonic compressor stage with two complex, but more realistic distortion patterns is investigated. A delta wing represents a longitudinal vortex, which is representative of e.g. a ground vortex. A stalled engine inlet is modelled by a bevelled beam that generates a massive separation bubble, which is ingested into the rotor. The interaction of the distortion and the compressor is measured at different speeds and operating points. The influence of the delta wing seems small and is difficult to measure due to the small size of the distorted area. In contrast, the beam causes a global alteration of the flow. It changes the behaviour of the rotor around the whole circumference and along the whole span.

PIV measurements of the transient flow structure in the tip region of a transonic compressor near stability limit

The flow structures in an axial compressor that lead to shortlength scale stall inception are investigated using optical measurements in a high-speed one and a half stage compressor. During transient throttling procedures, velocity was measured in a tangential plane at 92% channel height, intersecting the tip leakage vortex. The results show large scale disturbances of the secondary flow structure, which results from unsteady breakdown of the tip leakage vortex. It was possible to resolve spill forward several revolutions before the occurrence of rotating stall. This effect leads to local flow separations on the blade suction side and the development of radial vortex structures. The vortices are transported to the adjacent blade and cause further separations. Both effects are described in literature but were measured directly for the first time in a transonic compressor in this investigation. They are visualized for several time steps during transient throttling maneuvers and compared to blade vibration amplitudes. During the final phase before rotating stall occurs, asynchronous blade vibrations correlate with axial velocity in the region around the blade leading edge. INTRODUCTION The stability limit of a transonic compressor stage is typically determined by flow effects in the rotor blade tip region, where tip leakage flow leads to a strong secondary flow structure, which forms the tip leakage vortex that travels through the blade passage [Adamcyzk (1993)]. At transonic blade speeds, this vortex intersects with the passage shock of the subsequent rotor blade and tends to increase in size [Hoeger (1998)]. Intense blockage and inhomogeneous blade loading are caused by highly throttled conditions. Numerical and experimental investigations have shown the significant influence of the tip leakage vortex behavior on the aerodynamic stability limit of the compressor stage [Suder (1996)]. A rotor is called tip-critical when stall initiation occurs close to the casing wall. Effects, such as breakdown of the tip leakage vortex [Schlechtriem and Lötzerich (1997), Furukawa et. al. (1999)], and spill of fluid around the blade leading edge, are described in literature [Vo (2001), Bergner (2007)], but were not measured directly in a transonic rotor stage. Numerical and experimental investigations of subsonic rotors showed that periodic separations of vortices from the blade leading edge occur shortly before the inception of rotating stall. The vortices may be the initiator of a spike [Camp and Day (1998)], i.e. a short-length scale disturbance with high circumferential velocity (>70% rotor speed depending on the setup), developing into a rotating stall cell. The shape of the structures is described as a tornado-like vortex, with one end attached to the rotor blade and the other bound to the casing and traveling towards the adjacent blade [Inoue (2000), Pullan (2015)]. The operability limit may also be determined by mechanical vibrations occurring at highly throttled conditions. Especially at part-speed conditions, the unsteady forces produced by rotating stall cells can lead to large vibration amplitudes [Dodds (2015)]. For the rotor under investigation, nonsynchronous vibrations have been measured across the speed range. A numerical study, using time-linearised computational fluid dynamics (CFD), has recently linked these vibrations to the convective transport of tip leakage fluid in the tip region [Möller (2016)]. Measures that influence the structure of the tip flow directly, such as casing treatments, affect the stability limit independently of the aerodynamic or aeroelastic failure mechanism, even though the exact working mechanism is still under investigation [Holzinger (2016)]. The development of such stabilizing methods requires exact understanding of the phenomena at the stability limit that lead to the breakdown of stable flow conditions. The literature mentioned indicates that the transient phenomena are located close to the casing wall above 90% span [e.g. Inoue (2000)] and may lead to a convective coupling