Experimental investigation of a three-dimensional separated diffuser [electronic resource]
Keywords: Mechanical Engineering

Boundary layer separation is important in many engineering applications. Flows in aerodynamic devices such as diffusers, aircraft engines' compressors, and airfoils face significant adverse pressure gradients and as such are at risk of separating. Boundary layer separation creates a separation bubble between the primary flow and the bounding surface. The bubble is highly unsteady and the interaction with the primary flow may cause sub-optimal performance or even catastrophic loss of functionality. This work investigates critical aspects of practical separated flows. The test case is an asymmetric three-dimensional stalled diffuser, with unambiguously specified boundary conditions, that is fed by a fully-developed low-aspect ratio rectangular duct. Particle Image Velocimetry (PIV) is used to measure mean flow, turbulence, and higher-order statistics in the diffuser for multiple Reynolds numbers and spanwise planes, and at high spatial resolution and with low uncertainty. The mean flow development, Reynolds stress development, and turbulence scaling in the reverse flow region and in the separated shear layer are investigated. The turbulence structure and integral length scales are examined using two-point correlations and structure parameters. Additionally, the effect of three-dimensionality and the Reynolds number effect on the mean flow and turbulence are discussed. The boundary layer separates off the smoothly contoured bottom diverging wall of the diffuser due to the strong adverse pressure gradient in the streamwise direction. The lower-momentum corner regions at the junctions of the diffuser walls experiences additional spanwise adverse pressure gradient. At the diffuser throat, flow in the corner regions is affected by vestiges of the secondary flow in the inlet duct. Hence, the flow in corner regions experiences more significant flow reversal and the separation points in these planes are upstream of the separation points of the interior planes. This results in a reverse flow region that is three-dimensional. Additionally, the PDFs of the streamwise velocities at the separation and reattachment points on the bottom wall are positively skewed, indicating that the events resulting in forward motion at these locations are more energetic and less frequent than the events resulting in backward motion. Reynolds number has only a mild effect on the mean flow. High levels of the Reynolds stresses develop in the separated shear layer due to the adverse pressure gradient, as an inflection point is generated in the mean velocity profile and then displaced away from the wall as the boundary layer separates. The additional strain rate components and the secondary flow in the inlet duct modify the characteristics of the Reynolds stresses in the corner regions of the diffuser. There is no sharp change in the Reynolds stress behavior across the separation line because there are significant levels of turbulent transport of kinetic energy into the reverse flow region. Two-point correlation profiles show that the largest eddies in the separated shear layer penetrate deep into the reverse flow region and that the integral length scales peak just before reattachment. The turbulence in this flow is energetic, locally isotropic in some portion of the flow, strongly inhomogeneous, and is a strong function of Reynolds number. The experiment described here and the experimental data provided in this thesis will serve well as a benchmark test case for validating CFD models.