Chris D. Dritselis

Numerical study of educed coherent structures in the near-wall region of a particle-laden channel flow

The interaction of small heavy solid particles with turbulence near the wall of a vertical downward channel flow is investigated by using direct numerical simulation (DNS) and Lagrangian particle tracking. The interest is focused on the effect of the particles on the near-wall coherent structures obtained by conditional sampling of DNS results of a particle-laden turbulent channel flow. The coherent structures are detected from instantaneous flow fields by using the vortex definition of Jeong and Hussain [J. Fluid Mech. 285, 69 (1995)]. The Reynolds number of the particle-free flow is Reτ≈180 based on the friction velocity and the wall half distance. The particle response time is 200 wall units and the average mass and volume fractions φm=0.5 and φv=6.8×10−5, respectively. The particle diameter is smaller than the Kolmogorov length scale and the grid spacing, the latter being small enough to adequately resolve the smaller fluid flow scales. The feedback effect of the particles on the carrier phase is take...

Numerical investigation of momentum exchange between particles and coherent structures in low Re turbulent channel flow

The interaction between particles and coherent structures is studied by using discrete particle simulation combined with direct numerical simulation of gaseous flow in a vertical channel. A conditional sampling scheme is used to examine the modifications of the near-wall quasistreamwise vortices by the momentum exchange between the phases. The particle effect on the fluid flow is modeled by a point-force approximation. The particle diameters are smaller than both the smallest flow length scales and the computational grid spacing. Results are obtained for particle ensembles with four response times ranging from 10 to 200 wall units in numerical simulations with and without gravitational settling in the streamwise direction and interparticle collisions. It is found that the size of the quasistreamwise vortices is increased up to 25% in the presence of particles. The increase is larger for the smallest inertia particles studied, which is partly due to their locally nonuniform spatial distribution. The underl...

Transient Laminar MHD Natural Convection Cooling in a Vertical Cylinder

A numerical study is presented of transient laminar natural convection cooling of an electrically conductive fluid, placed in a vertical cylinder in the presence of an axial magnetic field. The cylindrical wall is suddenly cooled to a uniform temperature, thus setting the fluid to motion. The cooling process starts with the development of momentum and thermal boundary layers along the cylindrical cold wall, followed by the intrusion of the cooled fluid into the bulk, and finally, by fluid stratification. A range of Hartmann, Rayleigh, and Prandtl numbers are studied for which the flow remains laminar in all stages. It is found that the increase of the magnetic field reduces the heat transfer rate and decelerates the cooling process. This can be attributed to the damping of the fluid motion by the magnetic field, which results in the domination of conduction over convection heat transfer. The increase of the Rayleigh number enhances heat transfer, but the cooling process lasts longer due to the higher temperature of the hot fluid. The flow deceleration and the reduction of heat transfer are less intense for fluids with low Prandtl number.

Magnetic field effect on the cooling of a low-Pr fluid in a vertical cylinder

Results of direct numerical simulations are presented for the transient and turbulent natural convection cooling of an initially isothermal quiescent liquid metal placed in a vertical cylinder in the presence of a vertical magnetic field. The electrically conductive low-Prandtl number fluid is put to motion when the cylindrical wall is suddenly cooled to a uniform lower temperature. For this particular cooling process, the flow is characterized by three sequential almost discrete stages: (a) development of momentum and thermal boundary layers along the cylindrical cold wall, (b) intrusion of the cooled fluid into the main fluid body, and (c) flow and thermal stratification. The selected Rayleigh numbers in the present study are high enough so that turbulent convection is established. The numerical results show that the magnetic field has no observable effect at the initial stage of the vertical boundary layer development and conduction heat transfer is favored during the intrusion stage. An interesting ef...

LES of particle-laden turbulent channel flow with transverse roughness elements on one wall

The effect of wall roughness on the transport of solid particles in a turbulent channel flow is numerically investigated by means of large eddy simulation coupled with a Lagrangian particle‐tracking scheme. Two‐dimensional transverse square elements separated by a rectangular cavity are placed on the lower wall of the channel. Results were obtained for several values of the cavity width to the roughness height ratio. It is shown that the deposition of particles, as well as the particle accumulation near the walls, and their tendency to preferentially concentrate in flow regions of low streamwise fluid velocity are significantly affected by the roughness elements.

Low-Prandtl number MHD cooling in a vertical cylindrical container

Results of direct numerical simulations are presented for the strongly transient and turbulent natural convection cooling of an initially isothermal quiescent liquid metal placed in a cylinder under the effect of an external vertical uniform and constant magnetic field. The electrically conductive low Prandtl number fluid is put to motion when the vertical sidewall is suddenly cooled to a uniform lower temperature. See Sarris et al. [1] for further details.