Comparing Frequency-Based Flow Solutions to Traditional Unsteady Fluid Dynamics Analysis in Turbomachinery

Unsteady simulations are applied to resolve the time-dependent flow in turbomachinery. Consequently, the interaction between stationary and rotating parts requires an appropriate interface to transfer the flow quantities of the domains. Traditionally a so called domain scaling is used in combination with a sliding mesh approach. This method requires the pitch of the simulated blades to be equal in order to allow the use of periodic boundary conditions while cutting down the number of represented blade passages. Thus computational time can be saved. On the other hand, the flow solution is altered due to the scaling of the geometry. In order to overcome this problem, the non-linear harmonic (NLH) approach has been introduced. It allows for non identical blade pitches of the rows, since the flow solution is based on a Fourier-decomposition. Hence, the oscillating influence due to the perturbations of adjacent blade rows is approximated. Within the present study an unsteady Navier-Stokes-simulation is compared to a NLH computation in order to explain differences of the approaches. Therefore, an unsteady full-annulus simulation with about 40 million gridpoints as well a NLHs simulation with only one simulated blade passage per row with 1.38 million gridpoints was performed. Additionally, a NLHs simulation with a finer mesh resolution of 7 million gridpoints is included.