Oszillierender senkrechter Verdichtungsstoss in einer ebenen Düse
This investigation centers on the effects of oscillating normal shocks in unsteady flow. Systematic measurements in a turbine model in an annular cascade preceded the detailed investigations. This setup was used to detect the phenomena which cause shock induced flutter. The phenomenon "oscillating shock" was then isolated and investigated in a 2-D nozzle. The objectives of the investigation were the Installation of a system for the excitation of periodic oscillations of a normal shock in a nozzle. Measurement of the behaviour of the normal shock and the unsteady pressures in the frequency range existing in turbomachines (0 to 200 Hz excitation frequency). Investigation of the effects of the boundary layer on the response of an unsteady pressure transducer under the influence of an oscillating shock. Creation of a computation program for nonlinearized unsteady flows in nozzles containing an oscillating normal shock; emphasis is put on sharp shock capturing. Introduction of viscosity models in the computational program for estimating the influence of the boundary layer on the behaviour of the shock and the unsteady pressures. For the measurement, different current methods were used. The determination of the unsteady shock position was made by both a laser-2-focus-velocimeter and a line scan camera. The unsteady pressures were measured with unsteady pressure transducers at several positions in the side wall of the nozzle. Close to the shock, laser holography was used to obtain information about the zone of interaction between the shock and the boundary layer. The computation method developed is based on the Euler equations in conservative form. For viscous computations, the viscosity terms are included as perturbing terms inside the system of equations. With the aid of the flux-vector-splitting-method, the domains of physical influence are correctly taken into account. Thus, the shock is sharply captured (within only two mesh points). The main results of this work are The boundary layer over an unsteady pressure transducer has a quasi-steady behaviour with respect to the phase lag, but not with respect to the pressure amplitude. This means that the pressure fluctuations are in phase with the shock movement. The pressure amplitude depends on the frequency and on the boundary layer thickness. The wall region influenced by the shock is increased due to the boundary layer. The pressure increase or decrease takes place in a larger region on the wall. Thus, the pressure transducer sees the arriving shock before it has actually reached the position of the pressure transducer. The computational program developed was successfully validated with other available computational methods for steady state flows. The computed unsteady phenomena match well the results of the measurements. The computational method is tested and can now be upgraded for the computation of unsteady flows through oscillating cascades.
Laboratoire de thermique appliquée et de turbomachines
Record created on 2007-04-25, modified on 2016-08-08