Aerodynamic entrainment threshold: effects of boundary layer flow conditions

Abstract

Experimental data are presented demonstrating the influence of boundary layer flow conditions on aerodynamic entrainment of grains in the absence of intersaltation collisions. New methods are proposed for (I) the unambiguous determination of aerodynamic threshold for any grain population and (2) approximation of the probability density function (PDF) distributions of threshold shear velocity for aerodynamic entrainment.In wind tunnel experiments, the orderly spatial development of flow conditions within a developing boundary layer over the roughened surface of a flat plate constrains the aerodynamic threshold condition in terms of both mean and fluctuating values. Initial grain dislodgements and subsequent erosion from narrow strips of loose, finely fractionated ballotini were recorded photographically as wind speed was increased. Boundary layer parameters, including average threshold shear velocity ( U.,), were calculated using the momentum integral method.Direct observations show that sporadic oscillation of grains preceded dislodgement. At slightly higher velocities most grains rolled over their neighbours before entering saltation. Initial entrainment in spatially semi-organized flurries of 50 or more grains was followed by quiescent periods at airflow velocities close to threshold. These observations provide strong circumstantial evidence linking both the nature and spatial pattern of initial grain motions to sweep events during the fluid bursting process.For each grain fraction, values of U., were found to span an unexpectedly wide range and to decrease downwind from the leading edge of the plate as turbulence intensity increased. A probabilistic entrainment model is applied to the aerodynamic threshold condition so as to incorporate the effects of changing turbulent flow regimes over the plate. Analysis of strip erosion curves gives both an objective definition of the threshold condition and usable approximations of the PDF for U,, required by the model and for future stochastic treatment of the threshold condition.

Aeolian saltation on Mars at low wind speeds

“…As previously mentioned, wind tunnel experiments with slowly increasing u * typically show some grains in irregular creep or participating in occasional low bounces at wind energies below where most other grains are set into motion, so that initiation of particle motion is gradational for u * around the value of u *tf [ Bagnold , , , pp. 32–33, 92–93; Willetts et al ., ; Williams et al ., ; Carneiro et al ., ; Swann et al ., ] (also cf. Lavelle and Mofjeld [] and Kirchner et al .…”

Height profile of the mean velocity of an aeolian saltating cloud: Wind tunnel measurements by Particle Image Velocimetry

“…In this method, various movement parameters are measured in a wind tunnel or in the field (Nalpanis, 1985;White, 1985;White and Mounla, 1991;Rasmussen and Mikkelsen, 1991;Willetts et al, 1991;Williams et al, 1994, Zou et al, 2001. Reliable measurement technology is the key to studying the velocity of a saltating sand cloud by experiment.…”

Sediment Mobilisation by the Wind

“…While the relationships shown in Figure 18.11 have been found to be satisfactory for loose, dry, flat and homogeneous surfaces (Williams, Butterfield and Clark, 1994), the critical thresholds of motion and grain transport fluxes on natural sediment beds are also influenced by variations in factors such as sediment mixtures, surface crusting, surface slope, moisture and vegetation. The relationships between these parameters are frequently complex and are not yet fully understood.…”