Recent German Research on Periodical Unsteady Flow in Turbomachinery

Abstract

The main source of flow unsteadiness in turbomachinery is the aerodynamical interaction of the rotor and stator blade rows. The blades and vanes, moving relatively to each other, interact because of the viscous wakes and the potential effects of the blades. In addition, the wakes and potential effects superimpose with other flow patterns, for instance the tip clearance vortices and other secondary flow phenomena. Furthermore in transonic compressors the interaction of wakes and shocks plays an important role. As a result, the real flow field is highly periodically unsteady and very complex, especially in multistage turbomachinery. Although this fact has received increasing attention within recent years, blade row interactions effects are not yet typically addressed in current design systems of turbomachinery. Actually, there is a requirement of the ability of modern design methods to predict unsteady flow features. With increasing aerodynamic loading of the blades and higher Mach numbers the consideration of rotor-stator-interactions gains in importance. It is therefore one of the challenges of the present and future design of compressors and turbines to include beneficial unsteady effects to improve the engine parameters. This requires a detailed physical understanding of the unsteady flow field and the resulting effects on the performance and flow stability. In 2000 the joint research program “Periodical Unsteady Flow in Turbomachinery” was initiated. Partners of this project are five research groups from four german universities: Technische Universitat Berlin (Prof. Hourmouziadis), Universitat der Bundeswehr Munchen (Prof. Fottner, Prof. Pfitzner), Universitat Karlsruhe: Institut fur Thermische Stromungsmaschinen (Prof. Wittig, Dr. Dullenkopf), Institut fur Hydromechanik (Prof. Rodi), Technische Universitat Dresden (Prof. Vogeler), and a research group from the: German Aerospace Centre (Deutsches Zentrum fur Luft- und Raumfahrt), (Prof. Weyer, Prof. Monig). This 5-year program was funded by the German Research Foundation (Deutsche Forschungsgemeinschaft) and coordinated by Professor Hourmouziadis (Technische Universitat Berlin). The aim of this joint project is to contribute to an improved physical understanding of the periodical unsteady flow phenomena and to provide more reliable prediction methods of these complex flow conditions in turbomachinery. Selected aspects of flow unsteadiness in turbomachines were investigated with complementing experimental and numerical investigations. Different flow conditions of different complexity were investigated in detail. After a 3-year period of the project, first results of the research group are published in a special issue of the Journal of Flow, Turbulence and Combustion (Flow Turbulence Combust 69, 2002). After the end of the joint project, in the present paper selected results of each research group, which addresses different aspects of periodical unsteady flow in turbomachinery, are discussed. However, it is not the intention of the present paper to give a general survey on this field of research. The following topics are selected to provide insight into the work of the joint research group: (1) Experimental Investigation of Rotor-Stator-Interactions in an Axial Compressor, (2) Influence Of Periodically Unsteady Flow On The Boundary Layer Development Of A Highly Loaded Linear Turbine- And Compressor Cascade, (3) Flow Conditions on a Flat Plate under Oscillating Inlet Conditions, (4) Simultaneous Measurements of Flow and Heat Transfer in a Periodically Unsteady Flow, (5) Turbulence- and Transition Modelling for Unsteady RANS simulations, and (6) Direct Numerical Simulations of Transitional Flow in Turbine-Related Geometries.