Numerical Investigation of Subsonic Flow through an Aggressive Flat Bottom Diffuser

Date The final copy of this thesis has been examined by the signatories, and we Find that both the content and the form meet acceptable presentation standards Of scholarly work in the above mentioned discipline. Airflow through an aggressive, constant pressure gradient, flat-bottomed 2D diffuser is simulated with the compressible version of the stabilized, implicit finite element code PHASTA. The freestream Mach number of fluid entering the diffuser is held at a value of with a PI feedback loop. For a quasi 1D flow, the expansion ratio of produces a Mach number of by the end of the diffuser or Aerodynamics Interface Plane (AIP). However, the compact geometry and high targeted pressure gradient of result in massive asymmetric separation off of the curved ceiling. To improve this situation, wall suction is applied to the ceiling, floor, and corners of the duct as a flow control surrogate while the geometry is iterated to better achieve the targeted pressure gradient. After iterating geometry, the separation dynamics are studied in greater detail with both Unsteady Reynolds Averaged Navier Stokes (URANS) and Delayed Detached-Eddy Simulations (DDES). The duct naturally develops a strong vortical structure downstream of the AIP which can be limited to the upper half of the duct with corner suction. However, the structure of the secondary flow with just corner suction differs substantially between RANS and DDES. Experimental results are not yet available for comparison. Tangential blowing is also studied, but results are only available for flow control on the floor. RANS simulations indicate that floor blower is moderately more effective at maintaining steady, attached flow at the AIP than the floor suction used in other simulations. Figure 7: 2D profile of the diffuser examined in this work. The location of suction on the upper surface of the diffuser is shown in green and suction on the lower surface is shown in blue. Figure 8: of the First Node Off of the Wall. The picture is taken from a time averaged Series 12 simulation with no flow control activated. The first element height is uniformly. Figure 10: Construction of Provided Geometry. The first 41% of the diffuser is approximated by two ellipses (solid black), followed by a set of splines (light green) and then an arc to maintain Figure 20: Ramp curvature and CFD pressure distribution on an intermediate geometry after some iteration. The altered curvature of the new geometry is shown …