Significance of coherent structures in augmented heat flux in roughness-aided Rayleigh-Bénard convection

In the present work, we investigate the effect of conical roughness on heat flux and coherent structures for Ra=10⁸ in a 3D cubic cell filled with air. Two roughness set-ups are explored, and unlike the uniform case, height of the conical elements varies as 50-100% of the maximum roughness height for the irregular case. In addition, we also study their half-variants (HU and HR), where the rough surface is considered only on the bottom plate. While heat flux increases significantly in the rough cases, roughness weakly impacts the flow strength. The heat flux increases due to emission of a large number of intense thermal plumes, which is confirmed by an increase in plume volume fraction V_pl. Larger temperature variance in the rough cases confirms the frequent emission of thermal plumes. A shift in mean temperature in the bulk region is observed for HU and HR cases due to a surge in hot plumes emitting from the rough bottom surface. LSC remains in the diagonal plane in the smooth case, whereas it is found aligned along the Cartesian planes in the rough cases. The strength and orientation of the LSC are quantified by Reynolds number based on angular velocity. In the plane of LSC, we also obtain greater local heat flux in rough cases, which indicates augmented plume morphology. Second-order structure function of vertical velocity and temperature reveals energetic flow structures in the rough cases, whereas power spectra of temperature fluctuations reveal the existence of BO59 scaling in the bulk region.