22,204 research outputs found
Large eddy simulations of weakly heated stratocumulus top boudary layer
No full textPerforming Large Eddy Simulations (LES) of marine stratocumulus in the weakly heated boundary layer is an opportunity to evaluate the relative importance of radiative cooling and of a wind shear in cloud top region on cloud structure. It is shown that cooling due to longwave radiation influences the convective circulation in the atmospheric boundary layer and counteracts dilution caused by the wind shear
Tertiary patterns in inclined layer convection
No full textConvection in an inclined layer generates various types of spatio-temporal patterns due to interaction of buoyancy and shear. At small angles of incline, the secondary instability of the uniform base state occurs in the form of buoyancy dominated longitudinal rolls. Above a critical angle of incline marking a co-dimension 2 point, shear driven transverse roll instabilities take over as the secondary instabilities. Computing the location of the co-dimension 2 point for varying thermal driving and inclination angle and determining all secondary bifurcations together with the resulting tertiary states allows to characterize the nonlinear phase diagram of inclined layer convection system. The semi-analytically computed phase diagram quantitatively matches experimental observations by Daniels et al. Close to the co-dimension 2 point, a subcritical secondary bifurcation leading to bistability is identified. In the bistable region, heteroclinic cycles generate bursting behavior
Downstream Evolution of Perturbations in a Zero Pressure Gradient Turbulent Boundary Layer
No full textThis abstract examines the evolution of perturbations generated by various trips in a turbulent boundary layer. Measurements taken using hot wire anemometry show that the evolution towards the natural state is strongly dependent on the formation mechanisms of the boundary layer, this being different for different wall normal distribution of the trips' blockage. It is observed that standard boundary layer properties are recovered, after an adaptation region, with higher momentum thickness than the natural case. Two-point measurements with time resolved velocity in the inner region are studied to explore the different formation mechanisms
'Synthetic' Large Scale Motions Organize Small Scale Motions in the Turbulent Boundary Layer
No full textThe relationship between large- and small-scale motions in a non-equilibrium turbulent boundary layer was studied experimentally. A zero-pressure-gradient flat plate turbulent boundary layer was perturbed by a short array of two-dimensional roughness elements, both statically and under dynamic actuation. The dynamic forcing generated a `synthetic' very-large-scale motion (VLSM) within the flow which was observed by phase-locked flow measurements. The phase-relationship between both synthetic and natural VLSMs and the small scale motions within the boundary layer was studied by cross-correlation and cospectral techniques, to reveal an organizing effect of the artificial VLSM on smaller scales
Relation of skewness factor and convection velocity in turbulent boundary layer
No full textThe paper is devoted to prove the relation between skewness factor and convection velocity in turbulent boundary layer. It appears that skewness factor can be used as an indicator of convection velocity of coherent structures, which is not always equal to the average flow velocity. The analysis has been performed based upon velocity profiles measured with hot-wire technique in turbulent boundary layer with pressure gradient corresponding to turbomachinery conditions. The results show that the cross product term of skewness factor decomposed by spectral filtering, which is also alternative measure of amplitude modulation, describes the convection velocity in zero pressure gradient turbulent boundary layer
Investigation of Lagrangian coherent structures in a wake-induced boundary layer transition
No full textThe evolution of coherent structures in a flat plate boundary layer transition induced by the cylinder wake is investigated using the particle image velocimetry (PIV) technique. The finite-time Lyapunov exponent (FTLE), which characterizes the amount of stretching about the flow trajectory, is used to extract the Lagrangian coherent structures. It is revealed that secondary vortex is induced by the cylinder wake vortices in the near wall region,which would evolve into hairpin vortex as it convects downstream. The subsequent evolvement of the hairpin vortex, characterized by the regeneration of offspring hairpin vortex upstream of it, leads to the appearance of the hairpin packet and the boundary layer finally reaches a turbulent state
Cyclonic vortex in a rotating layer with isolated heat source
No full textFormation of cyclonic vortex from isolated heat source in a rotating fluid layer was studied experimentally and numerically. It was shown that structure of the vortex strongly depends on main governing parameters such as Grasshof number and Rossby number. PIV measurements were carried out for fluids with different values of Prandtl number (from 40 to 250). Numerical simulations were done using CFD software FlowVision
Turbulent dispersion in cloud-topped boundary layers
No full textCompared to dry boundary layers, dispersion in cloud-topped boundary layers has received less attention. In this LES based numerical study we investigate the dispersion of a passive tracer in the form of Lagrangian particles for four kinds of atmospheric boundary layers: 1) a dry convective boundary layer (for reference), 2) a "smoke" cloud boundary layer in which the turbulence is driven by radiative cooling, 3) a stratocumulus topped boundary layer and 4) a shallow cumulus topped boundary layer. We show that the dispersion characteristics of the smoke cloud boundary layer as well as the stratocumulus situation can be well understood by borrowing concepts from previous studies of dispersion in the dry convective boundary layer. A general result is that the presence of clouds enhances mixing and dispersion – a notion that is not always reflected well in traditional parameterization models, in which clouds usually suppress dispersion by diminishing solar irradiance. The dispersion characteristics of a cumulus cloud layer turn out to be markedly different from the other three cases and the results can not be explained by only considering the well-known top-hat velocity distribution. To understand the surprising characteristics in the shallow cumulus layer, this case has been examined in more detail by 1) determining the velocity distribution conditioned on the distance to the nearest cloud and 2) accounting for the wavelike behaviour associated with the stratified dry environment.Infrastructures, Systems and ServicesTechnology, Policy and Managemen
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Inner plasma structure of the low-latitude reconnection layer
No full textWe report a clear transition through a reconnection layer at the low-latitude magnetopause which shows a complete traversal across all reconnected field lines during northwestward interplanetary magnetic field (IMF) conditions. The associated plasma populations confirm details of the electron and ion mixing and the time history and acceleration through the current layer. This case has low magnetic shear with a strong guide field and the reconnection layer contains a single density depletion layer on the magnetosheath side which we suggest results from nearly field-aligned magnetosheath flows. Within the reconnection boundary layer, there are two plasma boundaries, close to the inferred separatrices on the magnetosphere and magnetosheath sides (Ssp and Ssh) and two boundaries associated with the Alfvén waves (or Rotational Discontinuities, RDsp and RDsh). The data are consistent with these being launched from the reconnection site and the plasma distributions are well ordered and suggestive of the time elapsed since reconnection of the field lines observed. In each sub-layer between the boundaries the plasma distribution is different and is centered around the current sheet, responsible for magnetosheath acceleration. We show evidence for a velocity dispersion effect in the electron anisotropy that is consistent with the time elapsed since reconnection. In addition, new evidence is presented for the occurrence of partial reflection of magnetosheath electrons at the magnetopause current layer
Scale interaction in a mixing layer: The role of the large-scale gradients
No full textThe interaction between scales is investigated in a turbulent mixing layer. The large-scale amplitude modulation of the small scales already observed in other works depends on the crosswise location. Large-scale positive fluctuations correlate with a stronger activity of the small scales on the low speed-side of the mixing layer, and a reduced activity on the high speed-side. However, from physical considerations we would expect the scales to interact in a qualitatively similar way within the flow and across different turbulent flows. Therefore, instead of the large-scale fluctuations, the large-scale gradients modulation of the small scales has been additionally investigated
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