Flow separation

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

Transient growth analysis of flow through a sudden expansion in a circular pipe

Abstract: Results are presented from a numerical study of transient growth experienced by infinitesimal perturbations to flow in an axisymmetric pipe with a sudden 1-2 diametral expansion. First, the downstream reattachment point of the steady laminar flow is accurately determined as a function of Reynolds number and it is established that the flow is linearly stable at least up to Re=1400. A direct method is used to calculate the optimal transient energy growth for specified time horizon tau, Re up to 1200, and low-order azimuthal wavenumber m. The critical Re for the onset of growth with different m is determined. At each Re the maximum growth is found in azimuthal mode m=1 and this maximum is found to increase exponentially with Re. The time evolution of optimal perturbations is presented and shown to correspond to sinuous oscillations of the shear layer. Suboptimal perturbations are presented and discussed. Finally, direct numerical simulation in which the inflow is perturbed by Gaussian white noise confirms the presence of the structures determined by the transient growth analysis. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3313931]

Steady non-Newtonian flow past a circular cylinder: a numerical study

Abstract: The steady and incompressible flow of non-Newtonian fluids past a circular cylinder is investigated for power law indices n between 0.2 and 1.4, blockage ratios of 0.037, 0.082 and 0.164, and the Reynolds numbers Re of 1, 20 and 40, using a stream function/vorticity formulation. The governing field equations have been solved by using a second-order accurate finite difference method to determine the drag coefficient, wake length, separation angle and flow patterns, and to investigate their dependence on power law index, blockage ratio and Reynolds number. The results reported here provide fundamental knowledge on the dependence of engineering flow parameters on blockage ratio and power law index, and further show that the effects on stream line and iso-vorticity patterns which result from an increase in the blockage ratio are similar to those which result from a decrease in the power law index.

Three‐dimensional nature of flow near a sill

Abstract: Velocity and density sections across Knight Inlet, British Columbia, demonstrate that lateral recirculations are a first-order feature of the flow in the lee of the sill. The flow over this sill has been used as a typical example of two-dimensional hydraulics, with a lower layer that thins and accelerates as it moves downstream below an almost stagnant layer that widens downstream, creating a distinctive wedge shape. However, we find that rather than being stagnant, the velocity in this wedge-shaped layer is actually quite large, consisting of a swiftly recirculating dipole vortex during flood tide and a monopole vortex during ebb tide, though we may have missed the matching half of a dipole with our sampling during ebb. The recirculations during flood tide carry an amount of water equal to 25% of the tidal flux, while the monopole during ebb tide carries 20% of the tidal flux. These recirculations bias along-channel estimates of volume flux, especially in the middle wedge-shaped layer, and demonstrate that accurate volume fluxes in the lee wave are only possible if three-dimensional surveys are made. Our three-dimensional survey shows that there is a net isopycnal convergence of water in the middle layer at a rate adequate to close the volume budget of the flow without recourse to diapycnal fluxes. We also calculate the strength of the vorticity in the recirculations observed during flood tide and attribute their formation to boundary layer separation.

Theoretical modelling of cavitation in piston ring lubrication

Abstract: Mathematical models of piston ring dynamics and lubrication are sensitive to the boundary conditions adopted to describe the cavitation occurring in the diverging outlet region of the lubricant film between the piston ring and cylinder wall In this paper, such sensitivity is investigated by applying different models of gaseous cavitation, flow separation and fluid film reformation to the analysis of a single compression ring from a diesel engine Significant differences are predicted in hydrodynamic pressure profiles, lubricant film boundaries, lubricant film thickness, oil flow and frictionSuch indications of substantial differences in piston ring operating characteristics associated with the distinct cavitation boundary conditions considered highlights the need for further research in this field However, the lack of detailed experimental data to validate the predictive models suggests that future progress must be based upon combined theoretical and experimental approaches to the problem It i