The apparent roughness of a sand surface blown by wind from an analytical model of saltation

We present an analytical model of aeolian sand transport. The model quantifies the momentum transfer from the wind to the transported sand by providing expressions for the thickness of the saltation layer and the apparent surface roughness. These expressions are derived from basic physical principles and a small number of assumptions. The model further predicts the sand transport rate (mass flux) and the impact threshold (the smallest value of the wind shear velocity at which saltation can be sustained). We show that, in contrast to previous studies, the present model's predictions are in very good agreement with a range of experiments, as well as with numerical simulations of aeolian saltation. Because of its physical basis, we anticipate that our model will find application in studies of aeolian sand transport on both Earth and Mars

Jump at the onset of saltation

We reveal a discontinuous transition in the saturated flux for aeolian saltation by simulating explicitly particle motion in turbulent flow. The discontinuity is followed by a coexistence interval with two metastable solutions. The modification of the wind profile due to momentum exchange exhibits a second maximum at high shear strength. The saturated flux depends on the strength of the wind as $q_s=q_0+A(u_*-u_t)(u_*^2+u_t^2)$

Turbulent Friction in Rough Pipes and the Energy Spectrum of the Phenomenological Theory

The classical experiments on turbulent friction in rough pipes were performed by J. Nikuradse in the 1930's. Seventy years later, they continue to defy theory. Here we model Nikuradse's experiments using the phenomenological theory of Kolmog\'orov, a theory that is widely thought to be applicable only to highly idealized flows. Our results include both the empirical scalings of Blasius and Strickler, and are otherwise in minute qualitative agreement with the experiments; they suggest that the phenomenological theory may be relevant to other flows of practical interest; and they unveil the existence of close ties between two milestones of experimental and theoretical turbulence.Comment: Accepted for publication in PRL; 4 pages, 4 figures; revised versio

Optimised mixing and flow resistance during shear flow over a rib roughened boundary

A series of numerical investigations has been performed to study the effect of lower boundary roughness on turbulent flow in a two-dimensional channel. The roughness spacing to height ratio, w/k, has been investigated over the range 0.12 to 402 by varying the horizontal rib spacing. The square roughness elements each have a cross-sectional area of (0.05 H)2, where H is the full channel height. The Reynolds number, Reτ is fixed based on the value of the imposed pressure gradient, dp/dx, and is in the range 6.3 × 103 − 4.5 × 104. A Reynolds Averaged Navier–Stokes (RANS) based turbulence modelling approach is adopted using a commercial CFD code, ANSYS-CFX 14.0. Measurements of eddy viscosity and friction factor have been made over this range to establish the optimum spacings to produce maximum turbulence enhancement, mixing and resistance to flow. These occur when w/k is approximately 7. It is found that this value is only weakly dependent on Reynolds number, and the decay rate of turbulence enhancement as a function of w/k ratio beyond this optimum spacing is slow. The implications for heat transfer design optimisation and particle transport are considered

Rough boundary treatment method for the shear-stress transport k - ω model

[[abstract]]Most of existing rough boundary treatment methods for the shear-stress transport model require much finer grids than smooth boundary treatment methods. Knopp, Eisfeld, and Calvo (2009, A new extension for k–ω turbulence models to account for wall roughness. International Journal of Heat and Fluid Flow, 30(1), 54–65.) developed a rough boundary treatment method that allows the same grid resolution as smooth boundary treatment methods, but its effect of grid resolution on the computed velocity is strong. This study aims to improve the method of Knopp et al. (2009, A new extension for k–ω turbulence models to account for wall roughness. International Journal of Heat and Fluid Flow, 30(1), 54–65.) and to reduce the effect of grid resolution. This work newly incorporates the effect of grid resolution on boundary values of , the inverse time scale. The method generates the logarithmic velocity profile of open channel flows over rough beds with a velocity shift relative to that over smooth beds. The computed velocity shift, depending on the dimensionless roughness, generally agrees with the measured ones. The effect of the grid resolution on velocity shift that is computed using the present method is only 31% of that using the method of Knopp et al. (2009, A new extension for k–ω turbulence models to account for wall roughness. International Journal of Heat and Fluid Flow, 30(1), 54–65.) in the transitionally rough regime. The effect of grid resolution in the transitionally rough regime is stronger than in both the hydraulically smooth regime and the fully rough regime. The present method is applied to simulate open channel flow over a bed with suddenly changing roughness. The computed velocities are consistent with the measured ones. The method reveals a sharp response of the shear velocity to the sudden change in roughness.[[notice]]補正完