Numerical simulation of particle flow in a sand trap

2022

We hypothesize that large eddies play a major role in the entrainment of aeolian sand and dust particles. To test this, wind-tunnel experiments are carried out to measure the entrainment rate of various particle sizes under different flow conditions. Wind tunnel flows are usually neutrally stratified with no large eddies which typically develop in convective atmospheric boundary layers. Here, a novel technique is applied by deploying a piece of randomly fluttering cloth to generate large eddies similar to convective eddies in atmospheric boundary layers, which we call quasi-convective turbulence. The characteristics of quasi-convective turbulence are analyzed with respect to neutral turbulence in the Monin-Obukhov similarity framework, and the probability distributions of surface shear stress are examined. We show that for given mean flow speed and in comparison with neutral flow conditions, quasi-convective turbulence increases the surface shear stress and alters its probability distribution, and hence substantially enhances the entrainment of sand and dust particles. Our hypothesis is thus confirmed by the wind-tunnel experiments. We also explain why large eddies are important to aeolian entrainment and transport.

Earth Surface Processes and Landforms, 2003

A simple low-cost sand catcher has been developed for use in wind-tunnel experiments. It is 500 mm high with a 300 mm by 20 mm inlet opening, and is designed to measure total mass transport directly. The length is 280 mm and its walls slowly expand at an angle of 5·6°. The top view is similar to that of the widely used BSNE sampler. Each sidewall is vented with three ventilation screens. Two sets of experiments were conducted on dune sand and at different wind velocities to investigate to what degree the catcher samples isokinetically, efficiently and non-selectively. The results indicate that the catcher measures sufficiently isokinetically (inlet wind velocity over outside wind velocity >0·9). Catch efficiency was around 0·75 and the degree of non-selectivity was >0·80 for 95 per cent of the particle sizes of the test material. It was also observed that the influence of wind velocity on the tested sampler characteristics was of minor importance so that no specific corrections need to be made for different wind velocities. Copyright © 2003 John Wiley & Sons, Ltd.

Atmospheric Chemistry and Physics

We hypothesize that large eddies play a major role in the entrainment of aeolian particles. To test this, wind-tunnel experiments are carried out to measure the particle entrainment rate for various sizes and flow conditions. Wind-tunnel flows are usually neutrally stratified with no large eddies, which are typically seen in convective atmospheric boundary layers. Here, a novel technique is applied, by deploying a piece of randomly fluttering cloth, to generate large eddies similar to convective eddies, here referred to as quasi-convective turbulence. The characteristics of quasi-convective turbulence are analyzed with respect to neutral turbulence in the Monin-Obukhov similarity framework, and the probability distributions of surface shear stress are examined. We show that for a given mean flow speed and in comparison with neutral flow conditions, quasi-convective turbulence increases the surface shear stress and alters its probability distribution and hence substantially enhances the entrainment of sand and dust particles. Our hypothesis is thus confirmed by the wind-tunnel experiments. We also explain why large eddies are important to aeolian entrainment and transport.

The European Physical Journal E, 2007

The airborne transport of particles on a granular surface by the saltation mechanism is studied through numerical simulation of particles dragged by turbulent air flow. We calculate the saturated flux qs and show that its dependence on the wind strength u * is consistent with several empirical relations obtained from experimental measurements. We propose and explain a new relation for fluxes close to the threshold velocity ut, namely, qs = a(u * − ut) α with α ≈ 2. We also obtain the distortion of the velocity profile of the wind due to the drag of the particles and find a novel dynamical scaling relation. We also obtain a new expression for the dependence of the height of the saltation layer as function of the strength of the wind.

Aquatic Sciences, 1996

The hydrodynamical, fluid and particle parameters which control flushing rates, flow ceils, and accumulation rates of particulate matter in cylindrical (MultiPIT) sediment traps were quantified in a flume simulation using a seeding technique for 25-45 ~m particles. Particle collection was found to be a trap-and particle-specific filtering process encompassing advective and gravitational entry of particles over a reduced trap aperture area, and gravitational-turbulent removal of particles at the bottom of the internal flow cell. Trapping efficiency increased up to 10-fold with increasing horizontal flow velocity (1-30 cm • s -1). For given flow velocity, the trap overand undercollected particles relative to their weight, i.e. (theoretical) Stokes settling velocity. The trapping efficiency increased with increasing trap Reynolds number ReT, changed by the approaching velocity in our experiments. Opposite findings from earlier experiments using the flume seeding technique and changing Rex by altering the trap diameter (Butman, 1986) are discussed. Semi-empirical equations are derived for the accumulation process of light, heavy and intermediate particles. From these, measured trap fluxes can be converted into in-situ verticle particle flux except for light particles.

Proceedings of ICAR5/GCTE …, 2002

Aeolian transport of fine to medium sized sand particles is usually occurring as saltation, the jumping movement of grains over the surface. Saltating grains receive momentum from the near-surface wind, which causes particle lift, entrainment, acceleration, and when the ...

Journal of Geophysical Research, 2004

1] We carried out a wind tunnel experiment on aeolian transport of sand. Fluorescencedyed grains of sand were embedded in the sand bed in a wind tunnel, and their dispersion was recorded by a video camera. Dispersion of colored sand both downwind and in the crosswind direction are examined. The concentration of colored sand decreases as an exponential function of the downwind distance y, Ae Àky , whereas the cross-sectional distribution is approximated by a Gaussian distribution with the standard deviation proportional to ffiffi ffi y p . The characteristic length scale is 1000 times greater in the downwind than in the crosswind direction. A simple stochastic model is proposed to explain these findings. We also found that our experimental results conform to the assumptions of the Werner model, which reproduces aeolian dune dynamics well. We derived conditions on the parameters of the Werner model from the experimental results and suggest the applicability of the Werner model to real problems such as sandy coasts, deserts, and regions whose surface is contaminated.

The Review of scientific instruments, 2013

A dedicated 4-m-high vertical wind tunnel has been designed and constructed at the University of Geneva in collaboration with the Groupe de compétence en mécanique des fluides et procédés énergétiques. With its diverging test section, the tunnel is designed to study the aero-dynamical behavior of non-spherical particles with terminal velocities between 5 and 27 ms(-1). A particle tracking velocimetry (PTV) code is developed to calculate drag coefficient of particles in standard conditions based on the real projected area of the particles. Results of our wind tunnel and PTV code are validated by comparing drag coefficient of smooth spherical particles and cylindrical particles to existing literature. Experiments are repeatable with average relative standard deviation of 1.7%. Our preliminary experiments on the effect of particle to fluid density ratio on drag coefficient of cylindrical particles show that the drag coefficient of freely suspended particles in air is lower than those m...

Geomorphology, 2014

Although moderate attention has been paid to the response of the aeolian mass transport rate to wind gusts, it is still unclear how the particle size and volumetric concentration affect this relation. Very little is known about the response time of the particle speed, and specifically, how the sensor scale and elevation affect measurements of this variable. The present study addresses this knowledge gap through a series of wind tunnel experiments in which a gusty wind was generated by programming the fan motor to adjust to a randomly selected rpm every 10 s. Beds consisting of either medium or coarse sand were investigated through synchronous, co-located measurements of the local wind speed and particle speed/count rate obtained via a customized laser Doppler anemometry (LDA) system. The vertically integrated sand transport rate (Q) and the wind speed in the freestream were quantified using a passive sand trap and pitot tube, respectively. The results of the experiments indicate that the response of the aeolian transport system to wind gusts is generally faster in terms of the particle speed than the mass transport rate, while the degree of correlation is found to vary with the sensor elevation, as well as with the particle size and volumetric concentration. In essence, the coupling within the transport system is demonstrated to be strongly scale dependent.

Wind Tunnels and Experimental Fluid Dynamics Research, 2011