Graduate Seminar in Fluid Mechanics

February 16, 2018 @ 4:00 pm – 5:00 pm
132 Gilman Hall

4:00 pm Presentation

 Large Eddy Simulation Including Population Dynamics Model for Polydisperse Droplet Evolution in Turbulence”

 Presented by ADITYA KANDASWAMY AIYER (Adviser: Prof. Meneveau)

Previous studies have shown that dispersion patterns of oil droplets in the ocean following an oil spill depend critically on droplet diameter. Hence predicting the evolution of the droplet size distribution is of critical importance for predicting macroscopic features of oil dispersion in the ocean. We adopt a population dynamics model of polydisperse droplet distributions for use in Large Eddy Simulation. We generalize a breakup model from Reynolds averaging approaches to LES in which the breakup is modeled as due to bombardment of droplets by turbulent eddies of various sizes. The breakage rate is expressed as an integral of a collision frequency times a breakage efficiency over all eddy sizes. An empirical fit to the integral is proposed in order to avoid having to recalculate the integral at every LES grid point and time step. The fit is tested by comparison with various stirred tank experiments. As a flow application for LES we consider a turbulent jet emanating from a source where oil droplets are released. The advected velocity and concentration fields of the droplets are described using an Eulerian approach. We apply this LES model to study the change of the oil droplet size distribution due to breakup, caused by interaction of turbulence generated by the jet with the oil droplets.

4:25 pm Presentation

On the Interactions of a Rotor Blade Tip flow with Axial Casing Grooves in an Axial Compressor Near Best Efficiency Point

 Presented by HUANG CHEN (Adviser: Prof. Katz)

Previous studies have shown that axial casing grooves (ACGs) are effective in delaying the onset of stall, but degrade the performance of axial turbomachines around the best efficiency point. Our recent experimental study (GT2017-65099) in the JHU refractive index-matched liquid facility have examined the effects of ACGs on delaying stall of a one and half stage compressor. The semicircular ACGs based on Müller et al. (GT2011-45364) reduce the stall flow rate by 40% with a slight decrease in pressure rise at higher flow rates.  Efficiency measurements conducted as part of the present study show that the ACGs cause a 2.4% peak efficiency loss. They are followed by detailed characterizations of the impact of the ACGs on the flow structure and turbulence in the tip region at high flow rates away from stall. Comparisons with the flow structure without casing grooves and at low flow rate are aimed at exploring relevant flow features that might be associated with the reduced efficiency. The Stereo-PIV measurements in several meridional and radial planes show that the periodic inflow into the groove peaks when the rotor blade pressure side (PS) overlaps with the downstream end of the groove, but diminishes when this end faces the blade suction side (SS). The entrainment of the PS boundary layer and its vorticity during the inflow phase generates counter-rotating radial vortices at the entrance to the groove, and a “discontinuity” in the appearance of the tip leakage vortex (TLV). While being exposed to the blade SS, the backward tip leakage flow causes flow separation and formation of a counter-rotating vortex at the downstream corner of the groove, which migrates towards the passage with increasing flow rate. Interactions of this corner vortex with the TLV cause fragmentation of the latter, creating a broad area with secondary flows and elevated turbulence level. Consequently, the vorticity shed from the blade tip remains scattered from the groove corner to the blade tip long after the blade clears this groove. Compared to a smooth endwall, the groove also increases the flow angle near the blade tip leading edge and varies it periodically. Accordingly, the magnitude of circulation shed from the blade tip and leakage flow increase near the leading edge. The insight from these observations might guide the development of ACGs that take advantage of the effective stall suppression by the ACGs but alleviate the adverse effects at high flowrates.

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