arXiv subjects

O. Shildkrot

Publications and source records attributed to O. Shildkrot.

7 recordsLinked to original sources

arXiv2026

Turbophoresis of inertial particles in inhomogeneous turbulence generated by oscillating grids

Turbophoresis in inhomogeneous turbulent flows leads to the formation of large-scale nonuniform particle number density distributions of inertial particles. This effect is associated with an effective drift velocity directed toward regions of lower turbulence intensity. It depends on the Stokes and Reynolds numbers, as well as on the gradient of the turbulence intensity. In the present study, turbophoretic transport is experimentally investigated in air turbulent flows generated by one-grid and two-grid oscillating systems. The flow velocity field is measured using Particle Image Velocimetry, and the particle spatial distribution is obtained by applying image processing techniques. To isolate the effect of particle accumulation due to turbophoresis from that associated with mean fluid flow, the measured particle number density of inertial particles is normalized by the corresponding distribution obtained for noninertial tracer particles under identical flow conditions. The measurements show preferential accumulation of inertial particles in regions of minimum mean-square turbulent velocity, consistent with the expected behavior of turbophoretic transport.

physics.flu-dyn↗

arXiv2026

Experimental study of turbulent thermal diffusion of inertial particles in a convective turbulence forced by oscillating grids

We investigate the phenomenon of turbulent thermal diffusion of inertial solid particles in laboratory experiments with convective turbulence forced by one or two oscillating grids in the air. Turbulent thermal diffusion causes a non-diffusive contribution to turbulent flux of particles described in terms of an effective drift velocity directed opposite to the gradient of the mean fluid temperature. For inertial particles, this effective drift velocity depends on the Stokes and Reynolds numbers. In the experiments, fluid velocity and spatial distribution of inertial particles are measured using a Particle Image Velocimetry (PIV) system, and the temperature field is measured in many locations by a temperature probe equipped with 12 thermocouples. Measurements of temperature and particle number density spatial distributions have demonstrated the formation of large-scale clusters of inertial particles in the vicinity of the mean temperature minimum due to turbulent thermal diffusion. In the experiments, the effective drift velocity caused by turbulent thermal diffusion that results in the formation of large-scale clusters of inertial particles (having the diameter $10 \mu m$) is in 1.5 -- 2.5 times larger than that for noninertial particles (having the diameter $0.7 \mu m$) depending on the level of turbulence. This is in agreement with the theoretical predictions.

physics.flu-dyn↗

arXiv2025

Experimental investigation of turbulence and turbulent thermal diffusion in strongly inhomogeneous and anisotropic forced convection

We investigate properties of turbulence and turbulent transport of non-inertial particles described in terms of turbulent thermal diffusion in strongly inhomogeneous and anisotropic convection forced by two similar turbulence generators with oscillating membrane and a steady grid in the air flow (with the Rayleigh number about $10^8$). Velocity field and spatial distribution of particles are measured using Particle Image Velocimetry system. The temperature distribution is measured in many locations using a temperature probe equipped with 12 E - thermocouples. In the forced convection, the gradients of the mean temperature field and the particle number density in the horizontal direction in the core flow are much stronger than in the vertical direction. The mean fluid velocity structure show transition between a single-roll pattern for isothermal turbulence to double-roll patterns with increase of the temperature difference between the bottom and upper walls of the chamber. For larger temperature differences, the mean fluid velocity structure returns to a single-roll pattern. In the turbulent regions with large mean temperature gradients, the dominant effect of the large-scale particle clustering is turbulent thermal diffusion, resulting in that the maximum of the mean particle number density is located in the regions with minimum of the mean temperature and vise versa. Deviations from this feature is observed in the regions with strong mean fluid velocities where the mean temperature gradients are small.

physics.flu-dyn↗

arXiv2023

Experimental study of turbulent thermal diffusion of particles in an inhomogeneous forced convective turbulence

We investigate experimentally phenomenon of turbulent thermal diffusion of micron-size solid particles in an inhomogeneous convective turbulence forced by one vertically-oriented oscillating grid in an air flow. This effect causes formation of large-scale inhomogeneities in particle spatial distributions in a temperature-stratified turbulence. We perform detailed comparisons of the experimental results with those obtained in our previous experiments with an inhomogeneous and anisotropic stably stratified turbulence produced by a one oscillating grid in the air flow. Since the buoyancy increases the turbulent kinetic energy for convective turbulence and decreases it for stably stratified turbulence, the measured turbulent velocities for convective turbulence are larger than those for stably stratified turbulence. This tendency is also seen in the measured vertical integral turbulent length scales. Measurements of temperature and particle number density spatial distributions show that particles are accumulated in the vicinity of the minimum of the mean temperature due to phenomenon of turbulent thermal diffusion. This effect is observed in both, convective and stably stratified turbulence, where we find the effective turbulent thermal diffusion coefficient for micron-size particles. The obtained experimental results are in agreement with theoretical predictions.

physics.flu-dyn↗

arXiv2022

Experimental study of turbulent transport of nanoparticles in convective turbulence

We perform experimental study of turbulent transport of nanoparticles in convective turbulence with the Rayleigh number $\sim 10^8$ in the air flow. We measure temperature field in many locations by a temperature probe equipped with 11 E-thermocouples. Nanoparticles of the size $\sim 70$ nm in diameter are produced by Advanced Electrospray Aerosol Generator. To determine the number density of nanoparticles, we use Condensation Particle Counter. We demonstrate that the joint action of turbulent effects (which are important in the core flow) and molecular effects (which are essential near the boundaries of the chamber) results in an effective accumulation of nanoparticles at the cold wall of the chamber. The turbulent effects are characterised by turbulent diffusion and turbulent thermal diffusion of nanoparticles, while the molecular effects are described by the Brownian diffusion and thermophoresis, as well as the adhesion of nanoparticles at the cold wall of the chamber. In different experiments in convective turbulence in a chamber with the temperature difference $ΔT$ between the bottom and top walls varying between $ΔT= 29$ K to $ΔT= 61$ K, we find that the mean number density of nanoparticles decreases exponentially in time. For instance, the characteristic decay time of the mean number density of nanoparticles varies from 12.8 min for $ΔT= 61$ K to 24 min for $ΔT= 29$ K. For better understanding of experimental results, we perform one-dimensional mean-field numerical simulations of the evolution of the mean number density of nanoparticles for conditions pertinent to the laboratory experiments. The obtained numerical results are in a good agreement with the experimental results.

physics.flu-dyn↗

arXiv2022

Experimental study of turbulent thermal diffusion of particles in inhomogeneous and anisotropic turbulence

We study experimentally turbulent thermal diffusion of small particles in inhomogeneous and anisotropic stably stratified turbulence produced by one oscillating grid in the air flow. The velocity fields have been measured using a Particle Image Velocimetry (PIV). We have determined various turbulence characteristics: the mean and turbulent velocities, two-point correlation functions of the velocity field and an integral scale of turbulence from the measured velocity fields. The temperature field have been measured with a temperature probe equipped with 12 E thermocouples. Spatial distributions of micron size particles have been determined by a PIV system using the effect of the Mie light scattering by particles in the flow. The experiments have demonstrated that particles are accumulated at the minimum of mean fluid temperature due to phenomenon of turbulent thermal diffusion. Using measured spatial distributions of particles and temperature fields, we have determined the effective turbulent thermal diffusion coefficient of particles in inhomogeneous temperature stratified turbulence. This experimental study has clearly detected phenomenon of turbulent thermal diffusion in inhomogeneous turbulence.

physics.flu-dyn↗

arXiv2020

Detection of standing internal gravity waves in experiments with convection over a wavy heated wall

Convection over a wavy heated bottom wall in the air flow has been studied in experiments with the Rayleigh number $\sim 10^8$. It is shown that the mean temperature gradient in the flow core inside a large-scale circulation is directed upward, that corresponds to the stably stratified flow. In the experiments with a wavy heated bottom wall, we detect large-scale standing internal gravity waves excited in the regions with the stably stratified flow. The wavelength and the period of these waves are much larger than the turbulent spatial and time scales, respectively. In particular, the frequencies of the observed large-scale waves vary from 0.006 Hz to 0.07 Hz, while the turbulent time in the integral scale is about 0.5 s. The measured spectra of these waves contains several localized maxima, that implies an existence of waveguide resonators for the large-scale standing internal gravity waves. For comparisons, experiments with convection over a smooth plane bottom wall at the same mean temperature difference between bottom and upper walls have been also conducted. In these experiments various locations with a stably stratified flow are also found and the large-scale standing internal gravity waves are observed in these regions.

physics.flu-dyn↗