Loss Coefficient Estimation in a Controlled Diffusion Cascade Using Large Eddy Simulation

Abstract

A Controlled Diffusion (CD) cascade is studied numerically using the Large Eddy Simulation (LES) technique. The Reynolds number of the flow is approximately 700,000 based on chord length, and a wide range of inlet flow angles are studied. The aim of the study is to assess the capability of LES to be used in an industrial context. A ‘thin-slice’ representation of the cascade is used as the reference grid, and the effects of the imposition of a narrow span are studied by performing comparison simulations with a domain that has a span five times larger than the thin-slice grid. Whilst the instantaneous flow-fields of the thin-slice and wide-domain simulations are qualitatively similar, the thin-slice simulations suffer from flow confinement problems caused by the imposition of the narrow span. The non-unity Axial Velocity Density Ratio of the flow enforces the use of inviscid wall spanwise boundaries, which have a severe parasitic influence on the development of the flow in the thin-slice simulations. The resultant data obtained from the thin-slice simulations are therefore compromised and the computed loss estimation is considered unreliable. Whilst the inviscid spanwise walls also affect the flow near the boundaries in the wide domain simulations, there is sufficient region of span from which reliable flow data and loss estimations can be obtained. The results presented in this paper show that it is necessary to perform Large Eddy Simulations on domains that are free from constraints which are commonly imposed in numerical studies performed in a design process, which results in a computational cost that may be too high to incorporate parametric LES studies into the design cycle of turbomachinery components for a number of years.