Graduate Seminar in Fluid Mechanics
4:00 pm Presentation
“Drag Modulation in Particle-Laden Viscoelastic Turbulence”
Presented by AMIR ESTEGHAMATIAN (Adviser: Prof. Zaki)
Polymer additives are widely used in order to reduce drag in turbulent flows. In many applications, the flow is laden with suspended finite-size particles that affect the state of the turbulence and polymer conformation, simultaneously. Previously we showed that in a dilute concentration of particles (5% solid volume fraction) in turbulent channel flow, polymers can introduce interesting changes in the particle migration behavior and intermittency, but remain an effective way to reduce drag. In this talk, we examine the competing effects of particles and viscoelasticity in a semi-dilute concentration (20% solid volume fraction) of neutrally buoyant particles. Despite that the Reynolds stresses are nearly eradicated, a drag increase is observed at higher elasticity beyond a certain Weissenberg number. It will be shown that the onset of viscoelastic drag increase in the particle-laden flows occurs due to increased polymer stresses. By comparing the deformation and expansion of polymers in both single-phase and particle-laden conditions, we will explain the role of particles in increasing the polymer stresses.
4:25 pm Presentation
“Measurements of Flow Structure and Turbulence in the Nearfield of an Oil Jet in Water”
Presented by XINZHI XUE (Adviser: Prof. Katz)
Experimental observations on the flow structures occurring in the near field of crude oil jet breakup in water is a challenge owing to optical obstruction. Therefore, the available experimental data on the structure of the immiscible liquid jet is restricted to either the periphery or the far-field of this jet. To probe into the near field, refractive index-matched silicone oil and sugar water are utilized as a surrogate liquid pair. Their density ratio, viscosity ratio, and interfacial tension are closely matched with those of crude oil and seawater. Simultaneous planar induced fluorescence (PLIF) and particle image velocimetry (PIV) measurements are conducted by labeling the oil and seeding both phases with particles. Compound droplets containing multiple water droplets, some with smaller oil droplets, form regularly at Re>1358, the origin of some of the encapsulated water droplets can be traced back to the entrained water ligaments during the initial roll-up of K-H vortices. The profiles of the mean velocity, as well as normal and shear Reynolds stresses for the immiscible oil jet are compared with those of the single-phase water jet at the same Reynolds number. The spreading rate of the near field of the oil jet is lower than that of the single-phase flow, but increases with the Reynolds number, presumably owing to the reduction in droplet sizes. The turbulence is dampened in the oil phase due to its higher viscosity, creating quiescent islands within the oil, consistent with the PLIF observations that while the oil droplets are deformed by the jet’s shear field, the interior water droplets are nearly spherical.