GRADUATE SEMINAR IN FLUID MECHANICS
4:15-4:40 p.m. Presentation
“Simultaneous PLIF and PIV Measurements on Refractive Index Matched Immiscible Buoyant Oil Jet Fragmentation in Water”
Presented by XINZHI XUE (Adviser: Prof. Katz)
Subsurface oil well blowout generate immiscible turbulent buoyant oil jets, which breaks up into a cloud of oil droplets. Understanding of the fragmentation is essential for evaluating the spreading of the oil jet and its interaction with the surrounding water in the near field, and for determining the droplet size distribution needed for modeling the subsequent transport of the oil. There is limited experimental data on the near field behavior of the opaque oil jet because of the inability to perform phase distribution measurements there. Injecting silicone oil into sugar water, which have the same refractive index, as surrogates for crude oil and water, respectively, enables us to observe the breakup process. The dynamic similarity is maintained by keeping nearly the same interfacial tension as well as density and viscosity ratios. The mixing process is visualized by simultaneous applications of planar laser induced fluorescence(PLIF) by premixing the oil with dye, and particle image velocimetry (PIV). The PLIF images are used for measuring the droplet sizes as well. Results show that with increasing Reynolds number, the jet spreading angle evaluated from the PLIF images increases, and its centerline velocity decreases at a faster rate. Beyond about 7 nozzle diameters, the turbulence peaks at the center of the jet, and its magnitudes scales with centerline velocity. As expected, the oil ligament fragmentation occurs primarily in regions of high strain-rate fluctuations in the near field shear layer and at the end of the potential core. This latter moves closer to the nozzle, and the resulting characteristic droplet sizes decrease with increasing Reynolds number. In both cases, the fragmentation process generates compound droplets, each containing multiple layers of oil and water.
4:40-5:00 p.m. Presentation
“Common Features of the Turbulence Structures in the Tip Region of Axial Turbomachines”
Presented by YUANCHAO LI (Adviser: Prof. Katz)
The flow in turbomachines is inherently complex and turbulent. Modern design processes rely heavily on CFD, especially RANS simulations to elucidate the flow, raising the questions about the applicability of popular turbulence models to turbomachines. Several axial turbomachines, including two waterjet pumps and an aviation compressor, have been studied experimentally in the JHU refractive index-matched facility in the past few years. This talk summarizes some common features in turbulence structure observed in these machines, such as the anisotropic distributions of Reynolds stresses and mechanisms (e.g., production, transport) causing it. Among all the machines, elevated turbulent kinetic energy (TKE) is observed to be associated with the characteristic vortical structures, e.g., the tip leakage vortex (TLV) and the shear layer connecting TLV to the blade suction side tip corner. High TKE is also evident near the blade tip corners and in some cases, in the layer separated from the endwall boundary where the leakage flow meets the opposite-directional main passage flow. The dominant terms of turbulence production rates show similar distributions in these machines, such as high shear production in the shear layer and opposite effects by flow contraction/stretching in the production of <u′z2> and <u′r2> near the TLV center. Moreover, eddy-viscosities estimated using individual stress and strain rate components reveal extreme spatial variability and inconsistency, suggesting that the popular eddy-viscosity based models are not applicable for these machines. However, the common turbulence features can serve as useful references for numerical simulations in which local production and anisotropy should be taken into account.