1,399 research outputs found
Heat transport and flow structure in rotating Rayleigh-B\'enard convection
Here we summarize the results from our direct numerical simulations (DNS) and
experimental measurements on rotating Rayleigh-B\'enard (RB) convection. Our
experiments and simulations are performed in cylindrical samples with an aspect
ratio \Gamma varying from 1/2 to 2. Here \Gamma=D/L, where D and L are the
diameter and height of the sample, respectively. When the rotation rate is
increased, while a fixed temperature difference between the hot bottom and cold
top plate is maintained, a sharp increase in the heat transfer is observed
before the heat transfer drops drastically at stronger rotation rates. Here we
focus on the question of how the heat transfer enhancement with respect to the
non-rotating case depends on the Rayleigh number Ra, the Prandtl number Pr, and
the rotation rate, indicated by the Rossby number Ro. Special attention will be
given to the influence of the aspect ratio on the rotation rate that is
required to get heat transport enhancement. In addition, we will discuss the
relation between the heat transfer and the large scale flow structures that are
formed in the different regimes of rotating RB convection and how the different
regimes can be identified in experiments and simulations.Comment: 12 pages, 10 figure
A concurrent precursor inflow method for Large Eddy Simulations and applications to finite length wind farms
In order to enable simulations of developing wind turbine array boundary
layers with highly realistic inflow conditions a concurrent precursor method
for Large Eddy Simulations is proposed. In this method we consider two domains
simultaneously, i.e. in one domain a turbulent Atmospheric Boundary Layer (ABL)
without wind turbines is simulated in order to generate the turbulent inflow
conditions for a second domain in which the wind turbines are placed. The
benefit of this approach is that a) it avoids the need for large databases in
which the turbulent inflow conditions are stored and the correspondingly slow
I/O operations and b) we are sure that the simulations are not negatively
affected by statically swept fixed inflow fields or synthetic fields lacking
the proper ABL coherent structures. Sample applications are presented, in
which, in agreement with field data a strong decrease of the power output of
downstream wind-turbines with respect to the first row of wind-turbines is
observed for perfectly aligned inflow.Comment: 13 pages, 5 figure
Radial boundary layer structure and Nusselt number in Rayleigh-Benard convection
Results from direct numerical simulations for three dimensional
Rayleigh-Benard convection in a cylindrical cell of aspect ratio 1/2 and Pr=0.7
are presented. They span five decades of Ra from to . Good numerical resolution with grid spacing Kolmogorov
scale turns out to be crucial to accurately calculate the Nusselt number, which
is in good agreement with the experimental data by Niemela et al., Nature, 404,
837 (2000). In underresolved simulations the hot (cold) plumes travel further
from the bottom (top) plate than in the fully resolved case, because the
thermal dissipation close to the sidewall (where the grid cells are largest) is
insufficient. We compared the fully resolved thermal boundary layer profile
with the Prandtl-Blasius profile. We find that the boundary layer profile is
closer to the Prandtl Blasius profile at the cylinder axis than close to the
sidewall, due to rising plumes in that region.Comment: 10 pages, 6 figure
Sidewall effects in Rayleigh-B\'enard convection
We investigate the influence of the temperature boundary conditions at the
sidewall on the heat transport in Rayleigh-B\'enard (RB) convection using
direct numerical simulations. For relatively low Rayleigh numbers Ra the heat
transport is higher when the sidewall is isothermal, kept at a temperature
(where is the temperature difference between the
horizontal plates and the temperature of the cold plate), than when the
sidewall is adiabatic. The reason is that in the former case part of the heat
current avoids the thermal resistance of the fluid layer by escaping through
the sidewall that acts as a short-circuit. For higher Ra the bulk becomes more
isothermal and this reduces the heat current through the sidewall. Therefore
the heat flux in a cell with an isothermal sidewall converges to the value
obtained with an adiabatic sidewall for high enough Ra ().
However, when the sidewall temperature deviates from the heat
transport at the bottom and top plates is different from the value obtained
using an adiabatic sidewall. In this case the difference does not decrease with
increasing Ra thus indicating that the ambient temperature of the experimental
apparatus can influence the heat transfer. A similar behavior is observed when
only a very small sidewall region close to the horizontal plates is kept
isothermal, while the rest of the sidewall is adiabatic. The reason is that in
the region closest to the horizontal plates the temperature difference between
the fluid and the sidewall is highest. This suggests that one should be careful
with the placement of thermal shields outside the fluid sample to minimize
spurious heat currents.Comment: 27 pages, 16 figure
Generalized coupled wake boundary layer model: applications and comparisons with field and LES data for two wind-farms
We describe a generalization of the Coupled Wake Boundary Layer (CWBL) model
for wind-farms that can be used to evaluate the performance of wind-farms under
arbitrary wind inflow directions whereas the original CWBL model (Stevens et
al., J. Renewable and Sustainable Energy 7, 023115 (2015)) focused on aligned
or staggered wind-farms. The generalized CWBL approach combines an analytical
Jensen wake model with a "top-down" boundary layer model coupled through an
iterative determination of the wake expansion coefficient and an effective wake
coverage area for which the velocity at hub-height obtained using both models
converges in the "deep-array" portion (fully developed region) of the
wind-farm. The approach accounts for the effect of the wind direction by
enforcing the coupling for each wind direction. Here we present detailed
comparisons of model predictions with LES results and field measurements for
the Horns Rev and Nysted wind-farms operating over a wide range of wind inflow
directions. Our results demonstrate that two-way coupling between the Jensen
wake model and a "top-down" model enables the generalized CWBL model to predict
the "deep-array" performance of a wind-farm better than the Jensen wake model
alone. The results also show that the new generalization allows us to study a
much larger class of wind-farms than the original CWBL model, which increases
the utility of the approach for wind-farm designers.Comment: 17 pages, 11 figure
Modeling space-time correlations of velocity fluctuations in wind farms
An analytical model for the streamwise velocity space-time correlations in
turbulent flows is derived and applied to the special case of velocity
fluctuations in large wind farms. The model is based on the Kraichnan-Tennekes
random sweeping hypothesis, capturing the decorrelation in time while including
a mean wind velocity in the streamwise direction. In the resulting model, the
streamwise velocity space-time correlation is expressed as a convolution of the
pure space correlation with an analytical temporal decorrelation kernel. Hence,
the spatio-temporal structure of velocity fluctuations in wind farms can be
derived from the spatial correlations only. We then explore the applicability
of the model to predict spatio-temporal correlations in turbulent flows in wind
farms. Comparisons of the model with data from a large eddy simulation of flow
in a large, spatially periodic wind farm are performed, where needed model
parameters such as spatial and temporal integral scales and spatial
correlations are determined from the large eddy simulation. Good agreement is
obtained between the model and large eddy simulation data showing that spatial
data may be used to model the full temporal structure of fluctuations in wind
farms.Comment: Submitted to Wind Energ
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
Optimal Prandtl number for heat transfer in rotating Rayleigh-Benard convection
Numerical data for the heat transfer as a function of the Prandtl (Pr) and
Rossby (Ro) numbers in turbulent rotating Rayleigh-Benard convection are
presented for Rayleigh number Ra = 10^8. When Ro is fixed the heat transfer
enhancement with respect to the non-rotating value shows a maximum as function
of Pr. This maximum is due to the reduced efficiency of Ekman pumping when Pr
becomes too small or too large. When Pr becomes small, i.e. for large thermal
diffusivity, the heat that is carried by the vertical vortices spreads out in
the middle of the cell, and Ekman pumping thus becomes less efficient. For
higher Pr the thermal boundary layers (BLs) are thinner than the kinetic BLs
and therefore the Ekman vortices do not reach the thermal BL. This means that
the fluid that is sucked into the vertical vortices is colder than for lower Pr
which limits the efficiency of the upwards heat transfer.Comment: 5 pages, 6 figure
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