Development of an experimental test section for forcing unsteady flow in a linear compressor cascade using circular rods

Upstream propagating waves impinging on a cascade of compressor blades were examined in an effort to better understand the influence of downstream components on high cycle fatigue in turbine engines. An array of cylinders was used to simulate the unsteady field generated by a rotor downstream of a set of stators. The unsteady flow upstream of a single cylinder and an array of cylinders, with and without an upstream cascade, was examined experimentally and computationally. Computational results indicate that the cylinders would only shed coherently when placed downstream of a set of blades. Coherent shedding is created when each of the cylinders in the array shed a vortex at the same instant in time. The computational results were verified experimentally and the required flow conditions for coherent vortex shedding were examined. Coherent vortex shedding was maximized by placing the cylinders in the centerline of the blade passages. The unsteady velocity was measured over a cascade blade with the cylinders located in an array downstream of the blades. Unsteady velocities measured along the blade indicate that the downstream cylinders create upstream propagating velocity fluctuations that are maximum at the trailing edge. The increasing amplitude of the unsteady velocities towards the trailing edge of the blade was seen both experimentally and computationally. Additionally, the computational results show that the unsteady fluctuations in the pressure along the blade surface also increases towards the trailing edge of the blade. The magnitude of the upstream propagating velocity fluctuations was increased with increasing freestream velocity. Unsteady velocities generated by individual cylinders were superposed to recreate the unsteady flowfield of the cylinder array and compared favorably with the cylinder array results towards the trailing edge of the blade.