Experimental study of aeolian sand ripples in a wind tunnel

Abstract

The topographic parameters and propagation velocity of aeolian sand ripples reflect complex erosion, transport, and deposition processes of sand on the land surface. In this study, three Nikon cameras located in the windward (0-1 m), middle (4.5-5.5 m), and downwind (9-10 m) zones of a 10 m long sand bed are used to continuously record changes in sand ripples. Based on the data extracted from these images, this study reaches the following conclusions. (1) The initial formation and full development times of sand ripples over a flatbed decrease with wind velocity. (2) The wavelengths of full development sand ripples are approximately twice the wavelengths of initially formed sand ripples. Both wavelengths increase linearly with friction velocity. During the developing stage of sand ripples, the wavelength increases linearly with time. (3) The propagation velocity of full development sand ripples is approximately 0.6 times that of the initially formed sand ripples. The propagation velocity of both initial and full development of sand ripples increase as power functions with respect to friction velocity. During the developing stage of sand ripples, the propagation velocity decreases with time following a power law. These results provide new information for understanding the formation and evolution of aeolian sand ripples and help improve numerical simulations.

Aeolian Ripple Migration and Associated Creep Transport Rates

“…The differences are magnified in wind tunnels with small working cross section areas and in wind tunnel experiments that use extreme environmental conditions, e.g., hurricane wind speeds [101]. McKenna Neuman and Maljaars [102] also surmised that these differences between saltation dynamics in wind tunnel and field experiments may have scaled with wind tunnel dimensions, but because this facet of wind tunnel modeling has received little attention [38], substantive differences will continue to occur in reported findings. These scaling differences in boundary layer-surface interactions may provide the key to understanding some of the inconsistencies of saltation models [77] and ripple dynamics reported in field and wind tunnels.…”

“…The ripple migration rates reported are anomalously slow relative to the other wind tunnel data. This has been treated as a result of an order of magnitude error in the ordinate label for Figure 1 [38], and we made a similar assumption based on comparing the data in the figure with the range of migration rates reported in the text. Ling et al [66] did not report the measurement height for wind speed, but for the shear velocity conversions, we specified the elevation as 0.5 m based on information from Cheng (personal communication, April 2019).…”

“…Cheng et al [38] performed a set of wind tunnel experiments with 0.15 mm sands that included measuring equilibrium ripple migration rates at four different shear velocities. Relevant to this study, they found that equilibrium ripple length increased linearly with shear velocity and the ripple migration rate increased exponentially with shear velocity.…”

“…Many papers have addressed the nature of ripples and the dynamics of ripple formation [8,[26][27][28][29][30]. Aspects of ripple geometry, especially key relationships between length, height, sand grain size, and migration rates, have also received considerable attention [31][32][33][34][35][36][37][38], as have sedimentary structures caused by aeolian ripple migration [39][40][41]. Finally, there has been substantial interest in ripple formation and migration on Mars [12,20,[42][43][44], although much of the interest in Martian bedforms involves the afore-mentioned larger aeolian forms of megaripples, transverse aeolian ridges, and dunes.…”

A Broad Continuum of Aeolian Impact Ripple Morphologies on Mars is Enabled by Low Wind Dynamic Pressures

“…The ubiquity of aeolian ripples, their importance in the rock record for interpreting past environments, and their intriguing, repeating geometrical form have motivated many investigations into factors controlling ripple development, shape, and size (e.g., Anderson, 1990;Andreotti et al, 2006;Bagnold, 1941, pp. 144-166;Cheng et al, 2018;Durán et al, 2011Durán et al, , 2014Ellwood et al, 1975;Manukyan & Prigozhin, 2009;Pelletier, 2009;Rasmussen et al, 2015;Schmerler et al, 2016;Sharp, 1963;Walker, 1981). Nevertheless, the physics of aeolian ripples has proven to be more complex than their simple forms might initially suggest, and uncertainties remain about how environmental factors combine to control ripple wavelength, size, and morphology (Andreotti et al, 2006;Durán et al, 2011;McKenna Neuman & Bédard, 2016).…”