scholarly journals Simultaneous temperature and velocity Lagrangian measurements in turbulent thermal convection

2016 ◽  
Vol 794 ◽  
pp. 655-675 ◽  
Author(s):  
O. Liot ◽  
F. Seychelles ◽  
F. Zonta ◽  
S. Chibbaro ◽  
T. Coudarchet ◽  
...  
Keyword(s):  
Heat Flux ◽  
Thermal Convection ◽  
Finite Size ◽  
Frequency Spectra ◽  
Velocity Statistics ◽  
Lagrangian Velocity ◽  
Temperature Spectra ◽  
Temperature Signal ◽  
Lagrangian Tracking ◽  
Neutrally Buoyant

We report joint Lagrangian velocity and temperature measurements in turbulent thermal convection. Measurements are performed using an improved version (extended autonomy) of the neutrally buoyant instrumented particle (Shewet al.,Rev. Sci. Instrum., vol. 78, 2007, 065105) that was used by Gasteuilet al.(Phys. Rev. Lett., vol. 99, 2007, 234302) to performed experiments in a parallelepipedic Rayleigh–Bénard cell. The temperature signal is obtained from a radiofrequency transmitter. Simultaneously, we determine a particle’s position and velocity with one camera, which grants access to the Lagrangian heat flux. Due to the extended autonomy of the present particle, we obtain well-converged temperature and velocity statistics, as well as pseudo-Eulerian maps of velocity and heat flux. Present experimental results have also been compared with the results obtained by a corresponding campaign of direct numerical simulations and Lagrangian tracking of massless tracers. The comparison between experimental and numerical results shows the accuracy and reliability of our experimental measurements and points also out the finite-size effects of the particle. Finally, the analysis of Lagrangian velocity and temperature frequency spectra is shown and discussed. In particular, we observe that temperature spectra exhibit an anomalous$f^{-2.5}$frequency scaling, likely representing the ubiquitous passive and active scalar behaviour of temperature.

Heat flux enhancement by regular surface roughness in turbulent thermal convection

2014 ◽  
Vol 763 ◽  
pp. 109-135 ◽  
Author(s):  
Sebastian Wagner ◽  
Olga Shishkina
Keyword(s):  
Heat Flux ◽  
Surface Roughness ◽  
Thermal Convection ◽  
Large Scale ◽  
Secondary Flows ◽  
Scaling Exponent ◽  
Wall Roughness ◽  
Regular Surface ◽  
Large Scale Circulation ◽  
Flux Enhancement

AbstractDirect numerical simulations (DNS) of turbulent thermal convection in a box-shaped domain with regular surface roughness at the heated bottom and cooled top surfaces are conducted for Prandtl number $\mathit{Pr}=0.786$ and Rayleigh numbers $\mathit{Ra}$ between $10^{6}$ and $10^{8}$. The surface roughness is introduced by four parallelepiped equidistantly distributed obstacles attached to the bottom plate, and four obstacles located symmetrically at the top plate. By varying $\mathit{Ra}$ and the height and width of the obstacles, we investigate the influence of the regular wall roughness on the turbulent heat transport, measured by the Nusselt number $\mathit{Nu}$. For fixed $\mathit{Ra}$, the change in the value of $\mathit{Nu}$ is determined not only by the covering area of the surface, i.e. the obstacle height, but also by the distance between the obstacles. The heat flux enhancement is found to be largest for wide cavities between the obstacles which can be ‘washed out’ by the flow. This is also manifested in an empirical relation, which is based on the DNS data. We further discuss theoretical limiting cases for very wide and very narrow obstacles and combine them into a simple model for the heat flux enhancement due to the wall roughness, without introducing any free parameters. This model predicts well the general trends and the order of magnitude of the heat flux enhancement obtained in the DNS. In the $\mathit{Nu}$ versus $\mathit{Ra}$ scaling, the obstacles work in two ways: for smaller $\mathit{Ra}$ an increase of the scaling exponent compared to the smooth case is found, which is connected to the heat flux entering the cavities from below. For larger $\mathit{Ra}$ the scaling exponent saturates to the one for smooth plates, which can be understood as a full washing-out of the cavities. The latter is also investigated by considering the strength of the mean secondary flow in the cavities and its relation to the wind (i.e. the large-scale circulation), that develops in the core part of the domain. Generally, an increase in the roughness height leads to stronger flows both in the cavities and in the bulk region, while an increase in the width of the obstacles strengthens only the large-scale circulation of the fluid and weakens the secondary flows. An increase of the Rayleigh number always leads to stronger flows, both in the cavities and in the bulk.

scholarly journals Lagrangian velocity statistics of directed launch strategies in a Gulf of Mexico model

2004 ◽  
Vol 11 (1) ◽  
pp. 35-46 ◽  
Author(s):  
M. Toner ◽  
A. C. Poje
Keyword(s):  
Velocity Field ◽  
Numerical Model ◽  
Gulf Of Mexico ◽  
Spatial Dependence ◽  
Density Functions ◽  
Particle Trajectories ◽  
Field Reconstruction ◽  
Velocity Statistics ◽  
Lagrangian Velocity

Abstract. The spatial dependence of Lagrangian displacement and velocity statistics is studied in the context of a data assimilating numerical model of the Gulf Mexico. In the active eddy region of the Western Gulf, a combination of Eulerian and Lagrangian measures are used to locate strongly hyperbolic regions of the flow. The statistics of the velocity field sampled by sets of drifters launched specifically in these hyperbolic regions are compared to those produced by randomly chosen launch sites. The results show that particle trajectories initialized in hyperbolic regions preferentially sample a broader range of Eulerian velocities than do members of ensembles of randomly launched drifters. The velocity density functions produced by the directed launches compare well with Eulerian velocity pdfs. Implications for the development of launch strategies to improve Eulerian velocity field reconstruction from drifter data are discussed.

scholarly journals Numerical Study of Rotating Thermal Convection on a Hemisphere

Fluids ◽  
2020 ◽  
Vol 5 (4) ◽  
pp. 185
Author(s):  
Patrick Fischer ◽  
Charles-Henri Bruneau ◽  
Hamid Kellay
Keyword(s):  
Heat Flux ◽  
Shear Layer ◽  
Convective Heat ◽  
Thermal Convection ◽  
Soap Bubble ◽  
Convective Heat Flux ◽  
Benard Convection ◽  
Velocity Equation ◽  
Rayleigh Benard Convection ◽  
Rayleigh Benard

Numerical simulations of rotating two-dimensional turbulent thermal convection on a hemisphere are presented in this paper. Previous experiments on a half soap bubble located on a heated plate have been used for studying thermal convection as well as the effects of rotation on a curved surface. Here, two different methods have been used to produce the rotation of the hemisphere: the classical rotation term added to the velocity equation, and a non-zero azimuthal velocity boundary condition. This latter method is more adapted to the soap bubble experiments. These two methods of forcing the rotation of the hemisphere induce different fluid dynamics. While the first method is classically used for describing rotating Rayleigh–Bénard convection experiments, the second method seems to be more adapted for describing rotating flows where a shear layer may be dominant. This is particularly the case where the fluid is not contained in a closed container and the rotation is imposed on only one side of it. Four different diagnostics have been used to compare the two methods: the Nusselt number, the effective computation of the convective heat flux, the velocity and temperature fluctuations root mean square (RMS) generation of vertically aligned vortex tubes (to evaluate the boundary layers) and the energy/enstrophy/temperature spectra/fluxes. We observe that the dynamics of the convective heat flux is strongly inhibited by high rotations for the two different forcing methods. Also, and contrary to classical three-dimensional rotating Rayleigh–Bénard convection experiments, almost no significant improvement of the convective heat flux has been observed when adding a rotation term in the velocity equation. However, moderate rotations induced by non-zero velocity boundary conditions induce a significant enhancement of the convective heat flux. This enhancement is closely related to the presence of a shear layer and to the thermal boundary layer just above the equator.

scholarly journals Do finite-size neutrally buoyant particles cluster?

2013 ◽  
Vol T155 ◽  
pp. 014056
Author(s):  
L Fiabane ◽  
R Volk ◽  
J-F Pinton ◽  
R Monchaux ◽  
A Cartellier ◽  
...  
Keyword(s):  
Finite Size ◽  
Neutrally Buoyant

Modulation of isotropic turbulence by particles of Taylor length-scale size

2010 ◽  
Vol 650 ◽  
pp. 5-55 ◽  
Author(s):  
FRANCESCO LUCCI ◽  
ANTONINO FERRANTE ◽  
SAID ELGHOBASHI
Keyword(s):  
Isotropic Turbulence ◽  
Volume Fraction ◽  
Density Ratio ◽  
Particle Diameter ◽  
Finite Size ◽  
Rate Of Change ◽  
Spherical Particles ◽  
Length Scale ◽  
Frequency Spectra ◽  
Scale Size

This study investigates the two-way coupling effects of finite-size solid spherical particles on decaying isotropic turbulence using direct numerical simulation with an immersed boundary method. We fully resolve all the relevant scales of turbulence around freely moving particles of the Taylor length-scale size, 1.2≤d/λ≤2.6. The particle diameter and Stokes number in terms of Kolmogorov length- and time scales are 16≤d/η≤35 and 38≤τp/τk≤178, respectively, at the time the particles are released in the flow. The particles mass fraction range is 0.026≤φm≤1.0, corresponding to a volume fraction of 0.01≤φv≤0.1 and density ratio of 2.56≤ρp/ρf≤10. The maximum number of dispersed particles is 6400 for φv=0.1. The typical particle Reynolds number is of O(10). The effects of the particles on the temporal development of turbulence kinetic energy E(t), its dissipation rate (t), its two-way coupling rate of change Ψp(t) and frequency spectra E(ω) are discussed.In contrast to particles with d < η, the effect of the particles in this study, with d > η, is that E(t) is always smaller than that of the single-phase flow. In addition, Ψp(t) is always positive for particles with d > η, whereas it can be positive or negative for particles with d < η.

scholarly journals Frequency spectra of turbulent thermal convection with uniform rotation

2014 ◽  
Vol 90 (4) ◽  
Author(s):  
Hirdesh K. Pharasi ◽  
Krishna Kumar ◽  
Jayanta K. Bhattacharjee
Keyword(s):  
Thermal Convection ◽  
Uniform Rotation ◽  
Frequency Spectra
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