Numerical Analysis of Flow and Acoustic Fields of a Ducted Axial Fan

In this thesis, the turbulent low Mach number flow and acoustic field of an axial fan is predicted and analyzed by Large Eddy Simulation (LES). The prediction of the acoustic field is based on a two step approach. First, the flow field is determined for various tip clearances and for different operating and inflow conditions, subsequently the acoustic field is computed by using source terms determined from the flow field solution.

High sensitivity of the tip-leakage flow is investigated for varying tip-gap sizes and for the design and off-design operating conditions. The impact of tip-gap size on the vortical structures in the tip-gap region is investigated and various tip-leakage vortices are identified. Moreover, the effect of the inflow condition on the flow field is investigated. Computations are performed with disturbed inflow conditions using a synthetic turbulence generation method.

Spatio-temporal and modal analyses of the tip-vortex system of the studied axial fan are performed based on highly resolved LES by proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD). This study is focused on the POD and DMD analysis of the unsteady flow phenomena inside the tip-gap region at the off-design operating condition compared to the design condition. The findings are used to explore the noise generation mechanism of the blade-wake interaction.

Finally, a highly resolved acoustic field is computed based on the acoustic source terms which are determined by the flow field. It shows that the strongest acoustic sources occur in the regions with highest turbulent kinetic energy, i.e., in the tip vortex, blade wake, and hub region. The results successfully prove that increasing the tip-gap size results in an acoustic field with higher frequencies and broadband noise level. Further acoustic analysis is focused on the impact of the disturbed inflow condition on the acoustic field.