525 research outputs found
Convection in an ideal gas at high Rayleigh numbers
Numerical simulations of convection in a layer filled with ideal gas are
presented. The control parameters are chosen such that there is a significant
variation of density of the gas in going from the bottom to the top of the
layer. The relations between the Rayleigh, Peclet and Nusselt numbers depend on
the density stratification. It is proposed to use a data reduction which
accounts for the variable density by introducing into the scaling laws an
effective density. The relevant density is the geometric mean of the maximum
and minimum densities in the layer. A good fit to the data is then obtained
with power laws with the same exponent as for fluids in the Boussinesq limit.
Two relations connect the top and bottom boundary layers: The kinetic energy
densities computed from free fall velocities are equal at the top and bottom,
and the products of free fall velocities and maximum horizontal velocities are
equal for both boundaries
Elliptical instability of compressible flow in ellipsoids
Elliptical instability is due to a parametric resonance of two inertial modes
in a fluid velocity field with elliptical streamlines. This flow is a simple
model of the motion in a tidally deformed, rotating body. Elliptical
instability typically leads to three-dimensional turbulence. The associated
turbulent dissipation together with the dissipation of the large scale mode may
be important for the synchronization process in stellar and planetary binary
systems. In order to determine the influence of the compressibility on the
stability limits of tidal flows in stars or planets, we calculate the growth
rates of perturbations in flows with elliptical streamlines within ellipsoidal
boundaries of small ellipticity. In addition, the influence of the orbiting
frequency of the tidal perturber and the viscosity of the fluid are
taken into account
Transition to finger convection in double-diffusive convection
Finger convection is observed experimentally in an electrodeposition cell in
which a destabilizing gradient of copper ions is maintained against a
stabilizing temperature gradient. This double-diffusive system shows finger
convection even if the total density stratification is unstable. Finger
convection is replaced by an ordinary convection roll if convection is fast
enough to prevent sufficient heat diffusion between neighboring fingers, or if
the thermal buoyancy force is less than 1/30 of the compositional buoyancy
force. At the transition, the ion transport is larger than without an opposing
temperature gradient
Transitions in turbulent rotating Rayleigh-B\'enard convection
Numerical simulations of rotating Rayleigh-B\'enard convection are presented
for both no slip and free slip boundaries. The goal is to find a criterion
distinguishing convective flows dominated by the Coriolis force from those
nearly unaffected by rotation. If one uses heat transport as an indicator of
which regime the flow is in, one finds that the transition between the flow
regimes always occurs at the same value of a certain combination of Reynolds,
Prandtl and Ekman numbers for both boundary conditions. If on the other hand
one uses the helicity of the velocity field to identify flows nearly
independent of rotation, one finds the transition at a different location in
parameter space
High Rayleigh number convection with double diffusive fingers
An electrodeposition cell is used to sustain a destabilizing concentration
difference of copper ions in aqueous solution between the top and bottom
boundaries of the cell. The resulting convecting motion is analogous to
Rayleigh-B\'enard convection at high Prandtl numbers. In addition, a
stabilizing temperature gradient is imposed across the cell. Even for thermal
buoyancy two orders of magnitude smaller than chemical buoyancy, the presence
of the weak stabilizing gradient has a profound effect on the convection
pattern. Double diffusive fingers appear in all cases. The size of these
fingers and the flow velocities are independent of the height of the cell, but
they depend on the ion concentration difference between top and bottom
boundaries as well as on the imposed temperature gradient. The scaling of the
mass transport is compatible with previous results on double diffusive
convection
Modelling multiphase chemistry in deliquescent aerosols and clouds using CAPRAM3.0i
Modelling studies were performed with the multiphase mechanism RACM- MIM2ext/CAPRAM 3.0i to investigate the tropospheric multiphase chemistry in deli- quesced particles and non-precipitating clouds using the SPACCIM model framework. Simulations using a non-permanent cloud scenario were carried out for two different environmental conditions focusing on the multiphase chemistry of oxidants and other linked chemical subsystems. Model results were analysed by time-resolved reaction flux analyses allowing advanced interpretations. The model shows significant effects of multiphase chemical interactions on the tropospheric budget of gas-phase oxidants and organic com- pounds. In-cloud gas-phase OH radical concentration reductions of about 90 % and 75 % were modelled for urban and remote conditions, respectively. The reduced in-cloud gas- phase oxidation budget increases the tropospheric residence time of organic trace gases by up to about 30 %. Aqueous-phase oxidations of methylglyoxal and 1,4-butenedial were identified as important OH radical sinks under polluted conditions. The model revealed that the organic C3 and C4 chemistry contributes with about 38 %/48 % and 8 %/9 % consid- erably to the urban and remote cloud / aqueous particle OH sinks. Furthermore, the simulations clearly implicate the potential role of deliquescent particles to operate as a reactive chemical medium due to an efficient TMI/HOx,y chemical processing including e.g. an effective in-situ formation of OH radicals. Considerable chemical differences be- tween deliquescent particles and cloud droplets, e.g. a circa 2 times more efficient daytime iron processing in the urban deliquescent particles, were identified. The in-cloud oxidation of methylglyoxal and its oxidation products is identified as efficient sink for NO3 radicals in the aqueous phase
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