50 research outputs found
Mass and moment of inertia govern the transition in the dynamics and wakes of freely rising and falling cylinders
In this Letter, we study the motion and wake-patterns of freely rising and
falling cylinders in quiescent fluid. We show that the amplitude of oscillation
and the overall system-dynamics are intricately linked to two parameters: the
particle's mass-density relative to the fluid and
its relative moment-of-inertia . This supersedes the
current understanding that a critical mass density ( 0.54) alone
triggers the sudden onset of vigorous vibrations. Using over 144 combinations
of and , we comprehensively map out the parameter space covering
very heavy () to very buoyant () particles. The entire
data collapses into two scaling regimes demarcated by a transitional Strouhal
number, . separates a mass-dominated regime from a
regime dominated by the particle's moment of inertia. A shift from one regime
to the other also marks a gradual transition in the wake-shedding pattern: from
the classical ~(2-Single) vortex mode to a ~(2-Pairs) vortex mode.
Thus, auto-rotation can have a significant influence on the trajectories and
wakes of freely rising isotropic bodies.Comment: Phys. Rev. Lett. (accepted): 5 pages and supplemental materia
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 with inner cylinder Reynolds
number up to , corresponding to Taylor number up to
. The grooves are axisymmetric V-shaped obstacles attached
to the wall with a tip angle of . 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 , 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 to a
sharp increase of the torque and thus the effective scaling law for the torque
vs. becomes much steeper. Further increasing does not result in an
additional slope increase. Instead, the effective scaling law saturates to the
"ultimate" regime effective exponents seen for smooth walls. It is found that
even though after saturation the slope is the same as for the smooth wall case,
the absolute value of torque is increased, and the more the larger size of the
grooves.Comment: Accepted by JFM, 27 pages, 23 figure
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 () increases or remains unchanged. Here we numerically
investigate turbulent Rayleigh-B\'enard convection over rough plates in two
dimensions, up to . 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 leads to the second regime, in
which the scaling saturates back to a value close to the smooth 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 clearly demonstrates that the local scaling does not signal the
onset of asymptotic ultimate thermal convection.Comment: Submitted, 11 pages, 5figur
Controlling heat transport and flow structures in thermal turbulence using ratchet surfaces
In this combined experimental and numerical study on thermally driven
turbulence in a rectangular cell, the global heat transport and the coherent
flow structures are controlled with an asymmetric ratchet-like roughness on the
top and bottom plates. We show that, by means of symmetry breaking due to the
presence of the ratchet structures on the conducting plates, the orientation of
the Large Scale Circulation Roll (LSCR) can be locked to a preferred direction
even when the cell is perfectly leveled out. By introducing a small tilt to the
system, we show that the LSCR orientation can be tuned and controlled. The two
different orientations of LSCR give two quite different heat transport
efficiencies, indicating that heat transport is sensitive to the LSCR direction
over the asymmetric roughness structure. Through a quantitative analysis of the
dynamics of thermal plume emissions and the orientation of the LSCR over the
asymmetric structure, we provide a physical explanation for these findings. The
current work has important implications for passive and active flow control in
engineering, bio-fluid dynamics, and geophysical flows.Comment: 5 pages, 5 figures, Physical Review Letters (in Press
Flow organization and heat transfer in turbulent wall sheared thermal convection
We perform direct numerical simulations of wall sheared Rayleigh-B\'enard
(RB) convection for Rayleigh numbers up to , Prandtl number unity, and
wall shear Reynolds numbers up to . Using the Monin-Obukhov length
we identify three different flow states, a buoyancy dominated regime
(; with the thermal
boundary layer thickness), a transitional regime (; with the height of the domain), and a shear dominated
regime (). In the buoyancy dominated regime the flow
dynamics are similar to that of turbulent thermal convection. The transitional
regime is characterized by rolls that are increasingly elongated with
increasing shear. The flow in the shear dominated regime consists of very
large-scale meandering rolls, similar to the ones found in conventional Couette
flow. As a consequence of these different flow regimes, for fixed and with
increasing shear, the heat transfer first decreases, due to the breakup of the
thermal rolls, and then increases at the beginning of the shear dominated
regime. For the Nusselt number effectively scales as
, with while we find
in the buoyancy dominated regime. In the transitional regime the effective
scaling exponent is , but the temperature and velocity profiles
in this regime are not logarithmic yet, thus indicating transient dynamics and
not the ultimate regime of thermal convection
Transition to the ultimate regime in two-dimensional Rayleigh-B\'enard convection
The possible transition to the so-called ultimate regime, wherein both the
bulk and the boundary layers are turbulent, has been an outstanding issue in
thermal convection, since the seminal work by Kraichnan [Phys. Fluids 5, 1374
(1962)]. Yet, when this transition takes place and how the local flow induces
it is not fully understood. Here, by performing two-dimensional simulations of
Rayleigh-B\'enard turbulence covering six decades in Rayleigh number Ra up to
for Prandtl number Pr , for the first time in numerical
simulations we find the transition to the ultimate regime, namely at
. We reveal how the emission of thermal plumes enhances
the global heat transport, leading to a steeper increase of the Nusselt number
than the classical Malkus scaling [Proc.
R. Soc. London A 225, 196 (1954)]. Beyond the transition, the mean velocity
profiles are logarithmic throughout, indicating turbulent boundary layers. In
contrast, the temperature profiles are only locally logarithmic, namely within
the regions where plumes are emitted, and where the local Nusselt number has an
effective scaling , corresponding to the
effective scaling in the ultimate regime.Comment: 6 pages, 4figure
Threshold current density for diffusion-controlled stability of electrolytic surface nanobubbles
Understanding the stability mechanism of surface micro/nanobubbles adhered to gas-evolving electrodes is essential for improving the efficiency of water electrolysis, which is known to be hindered by the bubble coverage on electrodes. Using molecular simulations, the diffusion-controlled evolution of single electrolytic nanobubbles on wettability-patterned nanoelectrodes is investigated. These nanoelectrodes feature hydrophobic islands as preferential nucleation sites and allow the growth of nanobubbles in the pinning mode. In these simulations, a threshold current density distinguishing stable nanobubbles from unstable nanobubbles is found. When the current density remains below the threshold value, nucleated nanobubbles grow to their equilibrium states, maintaining their nanoscopic size. However, for the current density above the threshold value, nanobubbles undergo unlimited growth and can eventually detach due to buoyancy. Increasing the pinning length of nanobubbles increases the degree of nanobubble instability. By connecting the current density with the local gas oversaturation, an extension of the stability theory for surface nanobubbles [Lohse and Zhang, Phys. Rev. E91, 031003(R) (2015)] accurately predicts the nanobubble behavior found in molecular simulations, including equilibrium contact angles and the threshold current density. For larger systems that are not accessible to molecular simulations, continuum numerical simulations with the finite difference method combined with the immersed boundary method are performed, again demonstrating good agreement between numerics and theories.</p
Three-dimensional Turbulent Reconnection within Solar Flare Current Sheet
Solar flares can release coronal magnetic energy explosively and may impact
the safety of near-earth space environments. Their structures and properties on
macroscale have been interpreted successfully by the generally-accepted
two-dimension standard model invoking magnetic reconnection theory as the key
energy conversion mechanism. Nevertheless, some momentous dynamical features as
discovered by recent high-resolution observations remain elusive. Here, we
report a self-consistent high-resolution three-dimension magnetohydrodynamical
simulation of turbulent magnetic reconnection within a flare current sheet. It
is found that fragmented current patches of different scales are spontaneously
generated with a well-developed turbulence spectrum at the current sheet, as
well as at the flare loop-top region. The close coupling of tearing-mode and
Kelvin-Helmholtz instabilities plays a critical role in developing turbulent
reconnection and in forming dynamical structures with synthetic observables in
good agreement with realistic observations. The sophisticated modeling makes a
paradigm shift from the traditional to three-dimension turbulent reconnection
model unifying flare dynamical structures of different scales.Comment: 15 pages, 8 figure, accepted for publication in ApJ