Large Eddy Simulation Study of Turbulent Flow During Transport of Aeolian Material




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In order to understand aeolian transportation, it is important to understand the origin of the aeolian source materials. The wind-driven hopping motion of sand called saltation is the dominating form of aeolian transport. When saltating particles splash on the sediment bed, they release fine dust. Aerodynamic surface stress imposed by the mixing processes of the inertia-dominated surface layer drives saltation. Due to the high spatial and temporal variability of the imposed stress, saltation itself becomes intermittent. Saltation is initiated when the surface shear velocity exceeds the fluid threshold of the sand particles. During saltation, splashing sand grains release fine dust known as aerosols. Saltation sustains itself with the continuation of the splashing of sand grains. This process stops when the shear velocity falls below the impact threshold value. So sand mobilization starts with the exceedance of the fluid threshold and continues until the shear value falls below the impact threshold. Even when the time-averaged shear velocity is below the fluid threshold, saltation can still be started due to turbulent fluctuations. This also accounts for transport intermittency which is difficult to capture via transport models. We utilized the dynamic properties of large-eddy-simulations to record a time series of the shear velocity imposed by the flow. This time series was later conditionally sampled to filter out the frequencies where saltation was active. These time series were used to recover probability density functions from which a compensated shear value was calculated. This compensated subthreshold shear velocity exhibited a linear relationship with the actual time-averaged shear velocity. A new transport model based on the compensated subthreshold shear velocity was proposed that performed well against field data measurements. Atmospheric surface layer flows are known to exhibit alternative patches of long sinuous structures of momentum excess and deficit. While the former is associated with negative vertical velocities, the latter is associated with positive vertical velocities. The high stress beneath these high momentum structures (HMRs) initiate saltation but the downwash from aloft traps the dust in the saltation layer itself and inhibits dust suspension. The low momentum structures (LMRs) are capable of dust suspension but they barely start sand mobilization. This paradox poses a challenge to explain the entrainment mechanisms. The existence of the paradox was established by analyzing the data from field measurements. These HMRS and LMRs were investigated with the help of quadrant analysis and classified into four categories: sweeps, ejections, inner and outer interactions. These four turbulent production mechanisms were investigated along with the hysteresis phenomena to propose three possible dust entrainment pathways. Numerically these dust entrainment modes were accounted for by conditional sampling techniques. The numerical results substantiated the proposed modes of dust entrainment. The density normalized conditions of the LES simulation make the results applicable to both terrestrial and Martian conditions. The credibility of the simulation results are established with a grid insensitivity test conducted with three different mesh densities.



Engineering, Mechanical