Velocimetry of Confined Cylinder Wakes in non-Newtonian Fluid Flows

Sammanfattning

This thesis investigates non-Newtonian fluid flow around a cylinder confined in asquare channel at a blockage ratio of 0.2 and Reynolds number (Re) between 10and 1000. The study is the first to explore viscoelastic (VE) and elastoviscoplastic(EVP) flows at velocity- and length-scales large enough to demonstrate inertialphenomena and the findings are compared to pre-existing Newtonian referencessuch as the von Kármán vortex street and successive transitions to turbulence inthe wake. Two shear-thinning solutions of Carbopol 940 (0.15 % wt./vol.) and one ofPolyacrylamide (0.1 % wt./vol.) are prepared as model fluids and characterisedusing Steady Shear and Small Amplitude Oscillatory Shear rheometry. PlanarParticle Image Velocimetry is then used to capture time-resolved flow fielddata at the midspan of the cylinder between non-dimensionalised streamwisecoordinates x/D, of -5 and 25. The test section measures 50 mm in height andthe resulting spatial resolution of the vector field is 1.16 mm−1. Vortex shedding in the flow of EVP fluids is observed for the first time, witha unique symmetric vortex shedding preceding the appearance of the traditionalasymmetric vortex street. With an increase in fluid yield stress from 0.07 Pa to2.32 Pa the vortex shedding is completely suppressed due to a dampening effecton the inertial forces. However, the relaxation of elastic stresses in the higheryield stress fluid causes the formation of a negative wake at the combinationof high Weissenberg number W i, and low Re, which has only previously beenobserved for rising gas bubbles in viscoelastic solutions. Decreasing the power-law index (increasing the amount of shear-thinning) and increasing the elasticity,the flow of the VE fluid is destabilized compared to the lower yield stress, leadingto a Strouhal number (St) in excess of 0.25 in the laminar vortex sheddingregime. As the shear layers transition to turbulence there is a clear decrease in Stleading to the proposition that the elevated St may be a product of elastoinertialturbulence. Apart from the flow statistics, the thesis also explores results related to theflow-type parameter, Proper Orthogonal Decomposition and the Q-criterion ina concerted effort to identify relevant flow structures. Further probing of the non-dimensional space as well as exploration of non-Newtonian spanwise instabilitiesand numerical validation efforts are but a few of the natural continuations madepossible by this work.