Numerical investigation of two-dimensional diffusers at low Reynolds numbers

Diffusers are important aerodynamic devices in many engineering applications, but the major part of research in this area is focused on devices operating at high Reynolds numbers. However, in numerous applications, including medical, diffusers may be employed in low Reynolds number flows. A specific feature of this regime is that the flow can undergo a laminar to turbulent transition within the diffuser and this can strongly influence the performance and design of the device. This thesis analyses the effect of inlet conditions on the case of two-dimensional diffusers operating at low Reynolds numbers with a laminar and transitional boundary layer discharging in a stationary atmosphere. Numerical simulations were performed with the joint implementation of the open-source mesh software Gmsh and OpenFOAM. In particular, 9 different cases are presented varying the inlet turbulence intensity (0.05, 3, and 10 percent) and the inlet velocity profile, characterised by different displacement thicknesses. For each case, a varying number of unsteady CFD simulations were performed using the k − ω Transitional SST RANS model that considers the possible laminarisation of the boundary layer. The pressure recovery coefficient at the outlet is analysed in detail, showing the relevant reduction of pressure recovery in the case of a laminar inlet velocity profile compared with the high Reynolds numbers case in the literature. Furthermore, the use of the modified effectiveness is remarked as an important factor in the analysis of velocity profiles with a high blockage factor. Velocity profiles at the outlet are compared, showing a relevant difference between a laminar and turbulent velocity profile. Finally, a setup for a future experimental study of two-dimensional diffusers is presented.