Flow separation

About: Flow separation is a research topic. Over the lifetime, 16708 publications have been published within this topic receiving 386926 citations.

Generalized reynolds number for non-newtonian fluids

Abstract: An extended version of the generalized Reynolds number was derived to characterize the duct flow of non-Newtonian gelled fluids of the Herschel-Bulkley-Extended (HBE) Type. This number allows also estimating the transition from laminar to turbulent flow conditions. An experimental investigation was conducted with a capillary rheometer for several non-Newtonian gelled fluids to evaluate the introduced HBE-generalized Reynolds number Regen HBE. A good correlation between the experimental results and the theory could be found for laminar flow conditions. For one of the examined gelled fuels, the necessary high Reynolds numbers could be realized so that the transition from the laminar to the turbulent flow regime could be measured. Because of its general description, the HBE-generalized Reynolds number can also be applied to Newtonian liquids as well as to non-Newtonian fluids of the Herschel-Bulkley (HB), Ostwald-de-Waele (power-law, PL), and Bingham type.

Unsteady laminar flow of gas near an infinite flat plate

Abstract: Boundary-layer equations for the unsteady flow near an effectively infinite flat plate set into motion in its own plane are subjected to von Mises's transformation. Solutions are obtained for the flows in which gravity is neglected, the Prandtl number σ is arbitrary, and the plate has a constant temperature and a velocity that is either uniform or, with dissipation neglected, non-uniform. Explicit solutions are obtained for the case in which the viscosity μ r varies directly as the absolute temperature T r . Solutions are also obtained for the diffusion of a plane vortex sheet in a gas, and for the boundary layer near a uniformly accelerated plate of constant temperature when gravity is not neglected. For the non-uniform motion of a heat-insulated plate, dissipation not being negligible, a solution is obtained when σ is 1 and μ r ∝ T r . The relative importance of free convection due to gravity and forced convection due to viscosity is discussed, and a solution is obtained, with μ r ∝ T r , for the free convection current set up near a plate that is at rest in a gas at a temperature different from that of the plate, dissipation being neglected.

Polymer-induced drag reduction: Effects of the variations in elasticity and inertia in turbulent viscoelastic channel flow

Abstract: In this work we systematically investigate the effects of the flow elasticity and inertia in polymer-induced drag reduction through (pseudo)spectral simulations of a turbulent channel flow of a dilute polymer solution. Viscoelastic effects are modeled by the finite-extensibility nonlinear elastic dumbbell model with the Peterlin approximation. The present work updates the low Weissenberg results (Weτ0⩽50) reported in earlier works by Sureshkumar et al. [Phys. Fluids 9, 743 (1997)] and Dimitropoulos et al. [J. Non-Newtonian Fluid Mech. 79, 433 (1998)] for a zero shear rate friction Reynolds number, Reτ0=125, by allowing for a lower value for the numerical diffusivity. In addition, we examine two effects on drag reduction: (A) high elasticity, by varying Weτ0 from 62.5 to 125 for a constant Reτ0=125, (B) friction Reynolds number, Reτ0=180, 395, and 590, for a constant Weτ0=50. In the high elasticity region, the mean Reynolds, Remean, continues to increase with increasing Weτ0, albeit at a smaller rate. Thus...

Formation of Turbulent Vortex Breakdown: Intermittency, Criticality, and Global Instability

Abstract: This study provides quantitative insight into the formation of vortex breakdown and the onset of global instability in a turbulent swirling jet. A water jet is guided through a rotating honeycomb that imparts the rotational motion, passed through a contraction, and discharged into a large water tank. The flow states evolving at increasing swirl are mapped out via time-resolved particle image velocimetry. The experimental results scale properly with the swirl number based on the axial momentum flux when the commonly used boundary-layer approximations are omitted. The instantaneous velocity field reveals that vortex breakdown occurs intermittently at a wide range of swirl numbers before it appears in the mean flow. At this intermittent state, the evolving breakdown bubble oscillates heavily between two streamwise locations where the vortex core is subcritical. Upon further increasing the swirl, the breakdown oscillations decay and a region of reversed flow appears in the mean flowfield. The formation of this socalled axisymmetric breakdown state is accompanied by a supercritical-to-subcritical transition of the inflowing vortex core. The reversed flow region is found to grow linearly with increasing swirl until the flow undergoes a supercritical Hopf bifurcation to a global single-helical mode, and vortex breakdown adopts a spiral shape. The global mode shape is extracted from the particle image velocimetry snapshots by means of proper orthogonal decomposition and Fourier analysis. The present experiment reveals that, at gradually increasing swirl, the jet first transitions to an axisymmetric breakdown state that remains globally stable until a critical swirl number is exceeded. This sequence of flow states agrees well with the transient formation of vortex breakdown observed in laminar flows.