Effect of cylinder proximity to the wall on channel flow heat transfer enhancement

Heat-transfer enhancement in a uniformly heated slot mini-channel due to vortices shed from an adiabatic circular cylinder is numerically investigated. The effects of gap spacing between the cylinder and bottom wall on wall heat transfer and pressure drop are systemically studied. Numerical simulations are performed at Re=100$\mathit{Re}=100$, 0.1?Pr?10$0.1?\mathrm{Pr}?10$ and a blockage ratio of D/H=1/3$D/H=1/3$. Results within the thermally developing flow region show heat transfer augmentation compared to the plane channel. It was found that when the obstacle is placed in the middle of the duct, maximum heat transfer enhancement from channel walls is achieved. Displacement of circular cylinder towards the bottom wall leads to the suppression of the vortex shedding, the establishment of a steady flow and a reduction of both wall heat transfer and pressure drop. Performance analysis indicates that the proposed heat transfer enhancement mechanism is beneficial for low-Prandtl-number fluids.     

Relative periodic orbits in plane Poiseuille flow

A branch of relative periodic orbits is found in plane Poiseuille flow in a periodic domain at Reynolds numbers ranging from Re=3000$\mathit{Re}=3000$ to Re=5000$\mathit{Re}=5000$. These solutions consist in sinuous quasi-streamwise streaks periodically forced by quasi-streamwise vortices in a self-sustained process. The streaks and the vortices are located in the bulk of the flow. Only the amplitude, but not the shape, of the averaged velocity components does change as the Reynolds number is increased from 3000 to 5000. We conjecture that these solutions could therefore be related to large- and very large-scale structures observed in the bulk of fully developed turbulent channel flows.     

An Eulerian–Eulerian model for gravity currents driven by inertial particles

In this work we introduce an Eulerian–Eulerian formulation for gravity currents driven by inertial particles. The model is based on the equilibrium Eulerian approach and on an asymptotic expansion of the two-phase flow equations. The final model consists of conservation equations for the continuum phase (carrier fluid), an algebraic equation for the disperse phase (particles) velocity that accounts for settling and inertial effects, and a transport equation for the disperse phase volume fraction. We present highly resolved two-dimensional (2D) simulations of the flow for a Reynolds number of Re=3450$\mathit{Re}=3450$ (this particular choice corresponds to a value of Grashof number of Gr=Re²/8=1.5×10⁶$\mathit{Gr}={\mathit{Re}}^2/8=1.5\times {10}^6$) in order to address the effect of particle inertia on flow features. The simulations capture physical aspects of two-phase flows, such as particle preferential concentration and particle migration down turbulence gradients (turbophoresis), which modify substantially the structure and dynamics of the flow. We observe the migration of particles from the core of Kelvin–Helmholtz vortices shed from the front of the current as well as their accumulation in the current head. This redistribution of particles in the current affects the propagation speed of the front, bottom shear stress distribution, deposition rate and sedimentation. This knowledge is helpful for the interpretation of the geologic record.