We study the transition to turbulence of channel flow of finite-size particle suspensions at low volume fraction, i.e., φ ≈0.001. The critical Reynolds number above which turbulence is sustained reduces to Re ≈ 167...We study the transition to turbulence of channel flow of finite-size particle suspensions at low volume fraction, i.e., φ ≈0.001. The critical Reynolds number above which turbulence is sustained reduces to Re ≈ 1675, in the presence of few particles, independently of the initial condition, a value lower than that of the corresponding single-phase flow, i.e., Re ≈1775. In the dilute suspension, the initial arrangement of the particles is important to trigger the transition at a fixed Reynolds number and particle volume fraction. As in single phase flows, streamwise elongated disturbances are initially induced in the flow. If particles can induce oblique disturbances with high enough energy within a certain time, the streaks breakdown, flow experiences the transition to turbulence and the particle trajectories become chaotic, Otherwise, the streaks decay in time and the particles immigrate towards the channel core in a laminar flow.展开更多
Hydrodynamic instability and the mechanisms for turbulence onset and self-sustaining are two research questions that have puzzled the scientists for over a century, yet renewed interest and significant progress occurr...Hydrodynamic instability and the mechanisms for turbulence onset and self-sustaining are two research questions that have puzzled the scientists for over a century, yet renewed interest and significant progress occurred in the last decade or so. We know only few exact analytical solutions of the Navier-Stokes equations, the equations governing the flow of simple fluids like water and air, and most of these are never observed in applications or laboratory experiments since these configurations are prone to instabilities that quickly bring the flow to the turbulent regime.展开更多
An existing phase-fieldmodel of two immiscible fluids with a single soluble surfactant present is discussed in detail.We analyze the well-posedness of the model and provide strong evidence that it is mathematically il...An existing phase-fieldmodel of two immiscible fluids with a single soluble surfactant present is discussed in detail.We analyze the well-posedness of the model and provide strong evidence that it is mathematically ill-posed for a large set of physically relevant parameters.As a consequence,critical modifications to the model are suggested that substantially increase the domain of validity.Carefully designed numerical simulations offer informative demonstrations as to the sharpness of our theoretical results and the qualities of the physical model.A fully coupled hydrodynamic test-case demonstrates the potential to capture also non-trivial effects on the overall flow.展开更多
A 3D lattice Boltzmann(LB)model with twenty-seven discrete velocities is presented and used for the simulation of three-dimensional porous media flows.Its accuracy in combination with the half-way bounce back boundary...A 3D lattice Boltzmann(LB)model with twenty-seven discrete velocities is presented and used for the simulation of three-dimensional porous media flows.Its accuracy in combination with the half-way bounce back boundary condition is assessed.Characteristic properties of the gas diffusion layers that are used in polymer electrolyte fuel cells can be determined with this model.Simulation in samples that have been obtained via X-ray tomographic microscopy,allows to estimate the values of permeability and relative effective diffusivity.Furthermore,the computational LB results are compared with the results of other numerical tools,as well as with experimental values.展开更多
基金supported by the European Research Council Grant No.ERC-2013-CoG-616186,TRITOSthe Swedish Research Council(VR)
文摘We study the transition to turbulence of channel flow of finite-size particle suspensions at low volume fraction, i.e., φ ≈0.001. The critical Reynolds number above which turbulence is sustained reduces to Re ≈ 1675, in the presence of few particles, independently of the initial condition, a value lower than that of the corresponding single-phase flow, i.e., Re ≈1775. In the dilute suspension, the initial arrangement of the particles is important to trigger the transition at a fixed Reynolds number and particle volume fraction. As in single phase flows, streamwise elongated disturbances are initially induced in the flow. If particles can induce oblique disturbances with high enough energy within a certain time, the streaks breakdown, flow experiences the transition to turbulence and the particle trajectories become chaotic, Otherwise, the streaks decay in time and the particles immigrate towards the channel core in a laminar flow.
文摘Hydrodynamic instability and the mechanisms for turbulence onset and self-sustaining are two research questions that have puzzled the scientists for over a century, yet renewed interest and significant progress occurred in the last decade or so. We know only few exact analytical solutions of the Navier-Stokes equations, the equations governing the flow of simple fluids like water and air, and most of these are never observed in applications or laboratory experiments since these configurations are prone to instabilities that quickly bring the flow to the turbulent regime.
文摘An existing phase-fieldmodel of two immiscible fluids with a single soluble surfactant present is discussed in detail.We analyze the well-posedness of the model and provide strong evidence that it is mathematically ill-posed for a large set of physically relevant parameters.As a consequence,critical modifications to the model are suggested that substantially increase the domain of validity.Carefully designed numerical simulations offer informative demonstrations as to the sharpness of our theoretical results and the qualities of the physical model.A fully coupled hydrodynamic test-case demonstrates the potential to capture also non-trivial effects on the overall flow.
基金Financial support from BFE via projects 103078 and 153708 is kindly acknowledgedpartial support from KTH Mechanics.
文摘A 3D lattice Boltzmann(LB)model with twenty-seven discrete velocities is presented and used for the simulation of three-dimensional porous media flows.Its accuracy in combination with the half-way bounce back boundary condition is assessed.Characteristic properties of the gas diffusion layers that are used in polymer electrolyte fuel cells can be determined with this model.Simulation in samples that have been obtained via X-ray tomographic microscopy,allows to estimate the values of permeability and relative effective diffusivity.Furthermore,the computational LB results are compared with the results of other numerical tools,as well as with experimental values.
