Numerical Investigation of Subsonic Flow through an Aggressive Flat Bottom Diffuser

Mati, Nicholas Alexander

Graduate Thesis Or Dissertation

Numerical Investigation of Subsonic Flow through an Aggressive Flat Bottom Diffuser Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/bz60cw60m

Abstract

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 M = 0.7 with a PI feedback loop. For a quasi 1D flow, the expansion ratio of A_AIP / A_Inlet = 2.33 produces a Mach number of 0.25 by the end of the diffuser or Aerodynamics Interface Plane (AIP). However, the compact geometry and high targeted pressure gradient of dp/dx=162 kPa/m 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 then 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.

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