Graduate Seminar in Fluid Mechanics
4:10 pm Presentation
“Distinct Modes of Unsteadiness in a Separating Turbulent Boundary Layer”
Presented by WEN WU (Profs. Rajat Mittal & Charles Meneveau)
Flow separation is ubiquitous in external as well as internal aerodynamics: wings and fuselages at high angles-of-attack, flow past external objects, shock-wave/boundary layer interactions, diffusers, corners, and junctions are just a few examples of this kind of flow. The separated flow is typically unsteady across a broad range of frequencies. In addition to high-frequency unsteadiness associated with turbulent fluctuations, the flow also displays a low frequency “breathing” or “flapping” mode and a higher frequency “shedding” mode. These unsteady models lead to many issues in applications, such as degradation of system performance and aerodynamic noise. They also bring additional difficulties for the prediction and control of these flows due to the complexity of intermittency. Better and more sophisticated tools are required for analysis of these unsteady modes. We generate a realistic separation bubble via adverse pressure gradient induced by a prescribed suction profile and perform direct numerical simulation of a turbulent boundary layer (Re=500) separated by this APG. The resulting flow exhibits two distinct unsteady modes frequencies separated by a factor of three. The higher-frequency motion scales well with the characteristics of a canonical plane mixing layer, corresponding to the generation of quasi two-dimensional spanwise vortices by KH instability. The lower frequency mode is more difficult to characterize but the use of dynamic mode decomposition (DMD) indicates that the low-frequency mode is associated with elongated streamwise vortices, the scale of which agrees well with the Görtler vortices generated due to curvature effects in the flow.
4:35 pm Presentation
“Effect of Different Axial Casing Groove Geometries on the Performance and Efficiency of a Compressors”
Presented by SUBHRA SHANKHA KOLEY (Adviser: Prof. Katz)
The impact of varying the geometry of axial casing grooves were investigated by carrying out performance tests and flow measurements. It has been shown in earlier studies that skewed semi-circular grooves installed near the blade leading edge (LE) have multiple effects on the flow structure, including ingestion of the tip leakage vortex (TLV), suppression of backflow vortices, and periodic variations of flow angle. To determine which of these phenomena is a key contributor, the present study examines the impact of several grooves, all with the same inlet geometry, but with outlets aimed at different directions. The “U” grooves that have circumferential exits aimed against the direction of blade rotation, achieve the highest stall margin improvement of well above 60%, but cause a 2.0% efficiency loss near the best efficiency point (BEP). The “S” grooves, which have exits aimed with the blade rotation, achieve a relatively moderate stall margin improvement of 36%, but they do not reduce the BEP efficiency. Other grooves, which are aligned with and against the flow direction at the exit from upstream inlet guide vanes, achieve lower improvements. These trends suggest that causing high periodic variations in flow angle around the blade leading edge is particularly effective in extending the stall margin, but also reduces the peak efficiency. In contrast, maintaining low flow angles near the LE achieves more moderate improvement in stall margin, without the maximum efficiency loss. Hence, of the geometries tested, the S grooves appear to have the best overall impact on the machine performance. In order to further elucidate the flow we are conducting flow visualization experiments using cavitation which will be followed by PIV measurements to get an quantitative estimation of the flow.