Massive heat transfer enhancement of Rayleigh-Benard turbulence over rough surfaces and under horizontal vibration

We carried out direct numerical simulations of turbulent Rayleigh-Benard convection(RBC)with accounting for both the roughness and the external vibration over the Rayleigh number range 10^(7)≤Ra≤10^(11) and the vibration frequency range 0<ω<1400.The triangular rough elements are uniformly distributed over the top and bottom surfaces,and the vibration is applied in the horizontal direction.It is shown that under the combined action of roughness and horizontal vibration,with increasing the vibration frequency ω,the heat transfer is initially decreased a little and then greatly enhanced after ω exceeds the critical value.The physical reason for massive heat-transfer-enhancement is that high frequency vibration destabilizes thermal boundary layers(BL)over rough surfaces,triggers abundant emissions of thermal plumes,and strengthens the motion of large-scale circulation(LSC),which consequently thins the thickness of thermal BL and heightens the convective transport.In addition,it is shown that vibration-induced heat-transfer-enhancement can obviously affect the scaling behavior between the heat flux and the Rayleigh number,and the scaling exponent increases with increasing ω,whereas the influence of vibration on the scaling behavior between the intensity of LSC and Ra is very weak.

Effect of Stokes number on energy modulation of the fluid in turbulent particle-laden channel flows

The effect of Stokes number on the kinetic energy(KE)budget in particle-laden turbulent channel flows is examined by conducting two-way coupled direct numerical simulations using the Eulerian-Lagrangian approach.The friction Reynolds number of the single phase channel flow is Re_(τ)=180,the particle mass loading and volume fraction areφ_(m)=0.2,φ_(v)≈10−4,and the Stokes numbers range from St^(+)=14–92.The statistics show that due to the presence of solid particles,the mean velocity is reduced in the vicinity of the wall but enhanced in the outer region,and the off-streamwise intensity of fluctuated velocity and the Reynolds stress are reduced in the whole channel.The analysis on the budgets of turbulent kinetic energy(TKE)finds that the presence of particles induces a significant reduction on both the production and dissipation rates.With increasing Stokes number St^(+),both the production and dissipation rates exhibit non-monotonical trends,i.e.,both initially decrease for St^(+)<40 and then transit to growth after St^(+)>40.This suggests that the particle-induced suppression on TKE production and dissipation is the strongest nearly at St^(+)=40.It is also found that particles act as an additional sink/source term in the budgets of both mean-flow kinetic energy(MKE)and TKE.In addition,we investigate the influence of St^(+)on the“zero point”which indicates the balance of exchanging energy between the particle and fluid phases.It is shown that with increasing St^(+),the“zero point”moves toward the wall,suggesting that the position of perfect following between particle and fluid is closer to the wall with larger St^(+).The present results reveal the Stokes number effects on the spatial transport mechanisms of MKE,TKE in turbulent channel flows laden with inertial particles.

Tuning turbulent convection through rough element arrangement

The tuning of turbulent Rayleigh-Bénard(RB)convection in a box is realized numerically by designed rough element arrangement.Considering the nonlinear dynamics of the thermal turbulence system,five models with rough elements of different widths and the same height are proposed to tune the fluid flow heat-transport capacity.Numerical simulations are performed using spectral element method for Rayleigh number in the range 10^(6)≤Ra≤10^(9) and a fixed Prandtl number Pr=0.7.It is found that heat transport is enhanced for large roughness widths as the interaction between the large-scale circulation and secondary flows inside the cavity regions between the rough elements promotes the eruptions of thermal plumes,but is suppressed for small ones as more heat are trapped inside the cavities.In all the rough models studied,different scaling exponents for the heat transport are identified and the influences of roughness arrangement on flow structure are studied.