Turbulent thermal convection over rough plates with varying roughness geometries

We present a systematic investigation of the effects of roughness geometry on turbulent Rayleigh-Benard convection (RBC) over rough plates with pyramid-shaped and periodically distributed roughness elements. Using a parameter lambda defined as the height of a roughness element over its base width, the heat transport, the flow dynamics and the local temperatures are measured for the Rayleigh number range 7.50 <= 10(7) <= Ra <= 1.31 <= 10(11) and Prandtl numbers Pr from 3.57 to 23.34 at four values of lambda (0.5, 1.0, 1.9 and 4.0). It is found that the heat transport scaling, i.e. Nu similar to Ra-alpha where Nu is the Nusselt number, may be classified into three regimes in turbulent RBC over rough plates. In Regime I, the system is in a dynamically smooth state. The heat transport scaling is the same as that in a smooth cell. In Regimes II and III, the heat transport is enhanced. When lambda is increased from 0.5 to 4.0, alpha increases from 0.36 to 0.59 in Regime II and it increases from 0.30 to 0.50 in Regime III. The experiment thus clearly demonstrates that the heat transport scaling in turbulent RBC can be manipulated using lambda in the heat transport enhanced regime. Previous studies suggest that the transition to heat transport enhanced regime, i.e. from Regime I to Regime II, occurs when the thermal boundary layer (BL) thickness becomes smaller than the roughness height. Direct measurements of the viscous BL in the present study suggest that the transition from Regime II to Regime III is likely a result of the viscous BL thickness becoming smaller than the roughness height. The scaling exponent of the Reynolds number Re with respect to Ra changes from 0.471 to 0.551 when lambda is increased from 0.5 to 4.0, suggesting a change of the dynamics of the large-scale circulation. Interestingly, the transition from Regime II to Regime III in terms of the heat transport scaling is not reflected in the Re scaling with Ra. It is also found that increasing lambda increases the clustering of thermal plumes which effectively increases the plume lifetime. This leads to a great increase in the probability of observing large temperature fluctuations in the bulk flow, which corresponds to the formation of more coherent plumes or plume clusters that are ultimately responsible for the enhanced heat transport.