We investigate the relaxation process toward the equilibrium regime of saltation transport in the context of nonuniform conditions. Relaxation phenomena can be described in terms of a characteristic ...length scale that measures the distance for the particle flux to adapt to a spatial change in flow or boundary conditions. We conducted wind tunnel experiments to document the influence of the upwind mass flux on the relaxation process. For zero upwind mass flux conditions, the relaxation process is monotone and the relaxation length is independent of the wind strength. In contrast, for nonzero upwind flux conditions (obtained by releasing particles in the flow from a finite height), the relaxation process is nonmonotone and is well captured by damped harmonic oscillations. Importantly, the relaxation length increases with increasing air flow velocity but is almost insensitive to the magnitude of the upwind flux. Our experimental outcomes clearly indicate that the relaxation of far from equilibrium transport regimes strongly deviates from a simple exponential behavior.
Key Points
The saturation length is independent of the wind strength in the case of zero upwind mass flux conditions
In the case of finite upwind flux condition, the relaxation process is nonmonotone and is well captured by a damped harmonic oscillation
The relaxation length determined in the latter case scales linearly with the friction velocity contrasting with the former case
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The work presented here focuses on the analysis of a turbulent boundary layer saturated with saltating particles. Experiments were carried out in a wind tunnel 15m long and 0.6m wide at the ...University of Aarhus in Denmark with sand grains 242 μm in size for wind speeds ranging from the threshold speed to twice its value. The saltating particles were analysed using particle image velocimetry (PIV) and particle-tracking velocimetry (PTV), and vertical profiles of particle concentration and velocity were extracted. The particle concentration was found to decrease exponentially with the height above the bed, and the characteristic decay height was independent of the wind speed. In contrast with the logarithmic profile of the wind speed, the grain velocity was found to vary linearly with the height. In addition, the measurements indicated that the grain velocity profile depended only slightly on the wind speed. These results are shown to be closely related to the features of the splash function that characterizes the impact of the saltating particles on a sandbed. A numerical simulation is developed that explicitly incorporates low-velocity moments of the splash function in a calculation of the boundary conditions that apply at the bed. The overall features of the experimental measurements are reproduced by simulation.
We report on wind tunnel measurements on saltating particles in a turbulent boundary layer and provide evidence that over an erodible bed the particle velocity in the saltation layer and the ...saltation length are almost invariant with the wind strength, whereas over a nonerodible bed these quantities vary significantly with the air friction speed. It results that the particle transport rate over an erodible bed does not exhibit a cubic dependence with the air friction speed, as predicted by Bagnold, but a quadratic one. This contrasts with saltation over a nonerodible bed where the cubic Bagnold scaling holds. Our findings emphasize the crucial role of the boundary conditions at the bed and may have important practical consequences for aeolian sand transport in a natural environment.
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We report experiments on windblown sand that highlight a transition from saltation to collisional regime above a critical dimensionless mass flux or Shields number. The transition is first seen ...through the mass flow rate Q, which deviates from a linear trend with the Shields number and seems to follow a quadratic law. Other physical evidences confirm the change of the transport properties. In particular, the particle velocity and the height of the transport layer increases with increasing Shields number in the collisional regime while the latter are invariant with the wind strength in the saltation regime. Discrete numerical simulations support the experimental findings and ascertain that mid-air collisions are responsible for the change of transport regime.
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Although vapor exchanged across hyper‐arid surfaces without free liquid affects the water budget of sand seas, its mechanism is poorly documented for want of accurate instruments with fine spatial ...resolution. To rectify this, we report bulk density profiles and spatiotemporal variations of vapor mass fraction just below the surface of a mobile dune, acquired with a multi‐sensor capacitance probe sensitive to tiny water films adsorbed on sand grains. We also record wind speed and direction, ambient temperature and relative humidity, net radiation flux, and subsurface temperature profiles over 2 days. The data validate a non‐linear model of vapor mass fraction. Unlike heat, which conducts through grains, vapor percolates across the interstitial pore space by advection and diffusion. On time scales longer than evaporation, adsorbed films equilibrate with their surroundings and hinder molecular diffusion. Their non‐linear coupling with subsurface temperature generates inflections in vapor profiles without counterpart in simpler diffusive systems. Pore advection arises as wind induces subtle pressure variations over the topography. During periods of aeolian transport, flowing sand dehydrates the surface intermittently, triggering evanescent vapor waves of amplitude decaying exponentially downward on a characteristic length implying an adsorption rate governed by a kinetic‐limited activated process. Finally, the probe yields diffusive and advective exchanges with the atmospheric boundary layer. During the day, their combined flux is smaller than expected, yet nearly proportional to the difference between vapor mass fraction at the surface and aloft. Under stabler stratification at night, or during aeolian sand transport, this relation no longer holds.
Plain Language Summary
Deserts inhale and exhale water vapor through their surface. Although this process affects the water balance over vast sand seas, it is poorly understood for want of sensitive instruments. We discover how it operates using a new probe that detects tiny amounts of moisture on sand grains. Our analysis reveals that vapor infiltration is considerably slower in dry sand, and that wind flowing over a dune creates weak internal air currents contributing to the transport of moisture. Their strength depends on dune location, wind speed and direction. When wind is strong enough to let dry sand meander over a dune, the resulting rapid variation in surface moisture sends evanescent waves of humidity downward. An analysis of these waves implies that water evaporation from individual sand grains behaves like a slow chemical reaction. The exchange of moisture with the atmosphere is not always driven by the difference between humidity at the dune surface and in the ambient, as current models assume, and it is weaker than they predict. In future, the new probe can be used as “ground truth” to calibrate satellite observations over deserts, explore extra‐terrestrial environments holding scant water, and detect moisture contamination in pharmaceutical products.
Key Points
We detect tiny spatiotemporal moisture variations below a hyper‐arid mobile dune with a new capacitance instrument
We predict moisture profiles, internal waves and wind‐driven pore advection. Grain evaporation is a kinetic‐limited, activated process
The moisture surface flux is weaker than expected, and not always proportional to the mass fraction difference between surface and ambient
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
We present a 1 year study of 50 dunes in a small field of barchan dunes in Mauritania. We documented the morphological evolution of the dunes and their migration rates and measured at 10 min ...intervals the interdune sand transport, the wind strength, and its direction for the same interval of time. The dune heights H0 range between 2 and 5 m, and their celerity c is found to be well approximated by the standard migration law: c = Q0/H0, with Q0 ≈ 50 m3/m yr. From both the interdune sand flux and the migration rate of the dunes we were able to estimate the spatially averaged sand flux at the dune crest as well as the bulk sand flux associated with the mass of sand transported by the dune. We found that the sand flux at the crest was about 3 times greater than the interdune mass transport rate, whereas the bulk sand flux was surprisingly of the same order as the interdune flux. Moreover, we analyzed carefully the interdune sand transport data, which can be well described by the Sorensen law. The cumulative mass of sand transported during moderate wind events within 1 year was much greater than that transported during strong wind events.
We investigate experimentally the impact process of sand particles onto a cohesive granular packing made of similar particles. We use a sand-oil mixture with varying liquid content to tune the ...cohesive strength of the packing. The outcome of the impact is analyzed in terms of the production of ejected particles from the packing. We quantify this production as a function of the impact velocity of the particles for increasing cohesion strength. We identified three different regimes depending on the cohesion number Co, defined as the ratio of the interparticle cohesive force to the particle weight. For small cohesion (i.e., Co ⪅ 1 ), the ejection process is not modified by the cohesion. For intermediate cohesion (i.e., 1 ⪅ Co ⪅ 20 ), the ejection process becomes less efficient: the number of ejected particles per impact for a given impact velocity is decreased but the critical impact velocity to trigger the ejection process remains unchanged. Finally, for strong cohesion (i.e., Co ⪆ 20 ), we observed a progressive increase of the critical impact velocity. These experimental results confirm spectacularly the outcomes of recent numerical simulations on the collision process of a particle onto a cohesive packing and open avenues to model the aeolian transport of moist sand. Published by the American Physical Society 2024
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We investigate numerically the impact process of a particle of diameter d and velocity V_{i} onto a cohesive granular packing made of similar particles via two-dimensional discrete element method ...simulations. The cohesion is ensured by liquid bridges between neighboring particles and described by short range attraction force based on capillary modeling. The outcome of the impact is analyzed through the production of ejected particles from the packing, referred to as the splash process. We quantify this production as a function of the impact velocity for various capillary strength Γ and liquid content Ω. The numerical data indicate that the splash process is modified when the dimensionless cohesion number Co=6Γ/ρ_{p}gd^{2} (where ρ_{p} is the particle density, d its diameter, and g the gravitational acceleration) exceeds a critical value of the order of the unity. Above this value, we highlight that the ejection process is triggered above a threshold impact Froude number, Fr=V_{i}/sqrtgd, which depends both on Γ and Ω and scales as Γ^{β}Ω^{δ}, where the values of the exponents are found close to 1/2 and 1/6, respectively, and can be derived from rational physical arguments. Importantly, we show that, above the threshold, the number of splashed particles follows a linear law with the impact Froude number as in the cohesionless case.
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9.
Aeolian Erosion Thresholds for Cohesive Sand Besnard, J.‐B.; Dupont, P.; Moctar, A. Ould ...
Journal of geophysical research. Earth surface,
November 2022, Volume:
127, Issue:
11
Journal Article
Peer reviewed
Open access
Moisture is known to affect sand cohesion and therefore transport threshold and rate of aeolian sand transport. Aeolian sand transport has been widely documented for dry sand beds, but much more ...sparsely in the context of moist sand beds. One major challenge in the collection of reliable field or laboratory data for moist sand is to have an accurate control of the moisture level within the sand bed, which is prone to strongly vary over time and space due to evaporation. To suppress the variability of moisture content due to evaporation, we devised a new approach based on the use of a non‐volatile liquid (namely silicon oil instead of water) which ensures a proper control of the liquid content. We thus conducted wind‐tunnel experiments employing this approach and observed that the aerodynamic threshold friction velocity increases linearly with liquid content, while the dynamic threshold is found to be unchanged in the range of cohesion strength investigated in this study. These outcomes indicate that the difference between the static and dynamic threshold increases with increasing cohesion. This may have strong implications on aeolian transport of moist sand, and in particular on hysteretic behaviors.
Plain Language Summary
Moisture has a significant influence on the initiation of motion of sand by wind and also on the resulting sand transport rate. While the moisture level is usually very low in sandy desert areas and thus irrelevant there, the aeolian sand transport on sandy beaches or in the context of coastal dune morphodynamics is expected to be crucially dependent on the moisture content within the sand. The literature regarding the impact of the moisture on wind blown sand is sparse, and available data exhibit considerable discrepancies regarding the magnitude of the moisture effects. These discrepancies highlight that it is experimentally difficult to control the moisture levels within the sand because of evaporation. To circumvent this difficulty, we carried wind‐tunnel experiments with sand‐oil mixtures instead of sand‐water mixtures. Oil plays the same role as water in generating cohesion but has the significant advantage to have a very low evaporation rate in the standard conditions of temperature and pressure. With these sand‐oil mixtures, we were able to assess with an unprecedented accuracy the magnitude of the cohesion effects on the aerodynamic erosion threshold and also on the threshold characterizing the cessation of transport, known as the dynamic threshold.
Key Points
We devised a new approach to assess the aerodynamic and dynamic transport thresholds for cohesive sand beds in wind‐tunnel experiments
The aerodynamic threshold is found to increase linearly with the amount of liquid content within the sand bed
Unlike the aerodynamic threshold, the dynamic threshold remains unchanged in the range of cohesive strength investigated so far
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
We report wind-tunnel measurements on aeolian sand transport aiming at characterizing the distribution of the length and height of trajectories of the saltating particles. We employ a simple ...horizontal sand trap device to assess the distribution of saltation length while the distribution of saltation height is inferred from the measurements of the particle lift-off velocity by means of particle velocimetry tracking techniques. Our measurements reveal that the saltation length and height present a continuum distribution which decreases monotonously and exhibits a long tail that can be well described by a lognormal law. Interestingly, these distributions are found almost invariant with the flow strength. As a consequence, the mean saltation length (lmacr) and height (hmacr) are independent of the flow strength confirming previous indirect measurements. The influence of the flow strength is only seen through the tail of the saltation length distribution: the higher the Shields number, the flatter the distribution tail. Finally, experiments carried out with sand of different sizes show that the mean saltation length and height are not related to the sand grain size through a simple manner but depend instead linearly with the height zfzf of the Bagnold focus point: l approximately 6zf and h approximately 0.6zf. This last result emphasizes that the focus height is an important characteristic length scale of the saltation transport.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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