Xiaojue Zhu

Roughness-Facilitated Local 1/2 Scaling Does Not Imply the Onset of the Ultimate Regime of Thermal Convection

In thermal convection, roughness is often used as a means to enhance heat transport, expressed in Nusselt number. Yet there is no consensus on whether the Nusselt vs Rayleigh number scaling exponent (Nu∼Ra^{β}) increases or remains unchanged. Here we numerically investigate turbulent Rayleigh-Bénard convection over rough plates in two dimensions, up to Ra≈10^{12}. Varying the height and wavelength of the roughness elements with over 200 combinations, we reveal the existence of two universal regimes. In the first regime, the local effective scaling exponent can reach up to 1/2. However, this cannot be explained as the attainment of the so-called ultimate regime as suggested in previous studies, because a further increase in Ra leads to the second regime, in which the scaling saturates back to a value close to the smooth wall case. Counterintuitively, the transition from the first to the second regime corresponds to the competition between bulk and boundary layer flow: from the bulk-dominated regime back to the classical boundary-layer-controlled regime. Our study demonstrates that the local 1/2 scaling does not necessarily signal the onset of ultimate turbulence.

Direct numerical simulation of Taylor–Couette flow with grooved walls: torque scaling and flow structure

We present direct numerical simulations of Taylor–Couette flow with grooved walls at a fixed radius ratio

Universal nanodroplet branches from confining the Ouzo effect

{\it\eta}=r_{i}/r_{o}=0.714

Disentangling the origins of torque enhancement through wall roughness in Taylor-Couette turbulence.

η=ri/ro=0.714 with inner cylinder Reynolds number up to

Wall roughness induces asymptotic ultimate turbulence

Re_{i}=3.76\times 10^{4}

Mass and Moment of Inertia Govern the Transition in the Dynamics and Wakes of Freely Rising and Falling Cylinders

Rei=3.76×104, corresponding to Taylor number up to

Diffusive interaction of multiple surface nanobubbles: shrinkage, growth, and coarsening

Ta=2.15\times 10^{9}

Transition to the ultimate regime in two-dimensional Rayleigh-Benard convection

Ta=2.15×109. The grooves are axisymmetric V-shaped obstacles attached to the wall with a tip angle of 90°. Results are compared to the smooth wall case in order to investigate the effects of grooves on Taylor–Couette flow. We focus on the effective scaling laws for the torque, flow structures, and boundary layers. It is found that, when the groove height is smaller than the boundary layer thickness, the torque is the same as that of the smooth wall cases. With increasing

AFiD-GPU: a versatile Navier-Stokes Solver for Wall-Bounded Turbulent Flows on GPU Clusters

Ta

Turbulent thermal superstructures in Rayleigh-Bénard convection

Ta, the boundary layer thickness becomes smaller than the groove height. Plumes are ejected from the tips of the grooves and secondary circulations between the latter are formed. This is associated with a sharp increase of the torque, and thus the effective scaling law for the torque versus