Towards long length scale unsteady modelling in turbomachines

Abstract The modelling issue of long length scale unsteadiness in turbomachines is discussed in this paper and a new three-dimensional CFD method with a hierarchy of body force models is proposed. A program has been developed to deal with the long length scale problem using efficient coarse meshes and large time steps, together with body force models. The distributed viscous body force can be calculated either using a very simple viscous wall shear stress model or extracted from a ‘stand-alone’ three-dimensional single passage steady calculation using a fine mesh. By eliminating the need to compute fine viscous scales, the proposed method is several orders of magnitude more efficient than ‘standard’ fine mesh N-S unsteady calculations while maintaining respectable resolution down to the scale of blade-to-blade variation, including wake/potential interactions between blade rows. Two sample cases with high-frequency low-amplitude and low-frequency high-amplitude, respectively, are used to illustrate the accuracy of the model.