Initiation and evolution of current ripples on a flat sand bed under turbulent water flow

Abstract

We investigate the formation and dynamics of sand ripples under a turbulent water flow. Our experiments were conducted in an open flume with spherical glass beads between 100 and 500 microm in diameter. The flow Reynolds number is of the order of 10,000 and the particle Reynolds number of the order of 1 to 10. We study the development of ripples by measuring their wavelength and amplitude in course of time and investigate the influence of the grain size and the flow properties. In particular, we demonstrate two different regimes according to the grain size. For fine grains, a slow coarsening process (i.e., a logarithmic increase of the wavelength and amplitude) takes place, while for coarser grains, this process occurs at a much faster rate (i.e., with a linear growth) and stops after a finite time. In the later case, a stable pattern is eventually observed. Besides, we carefully analyze the wavelength of ripples in the first stages of the instability as a function of the grain size and the shear velocity of the flow, and compare our results with other available experimental data and with theoretical predictions based on linear stability analyses.

Bedforms in a turbulent stream: ripples, chevrons and antidunes

“…Although dunes and ripples were previously seen as two distinct modes of the same linear instability (Richards 1980, Sumer & Bakioglu 1984, McLean 1990, it is now proposed that dunes result from the coarsening of ripples, through the non-linear increase of their wavelength (Raudkivi & Witte 1990, Raudkivi 2006, Fourrière et al 2010. Such a coarsening has been reported in numerous experimental studies (Mantz 1978, Gyr & Schmid 1989, Baas 1994, Coleman & Melville 1994a, Baas 1999, Robert & Uhlman 2001, Coleman et al 2003, Venditti et al 2005b, Langlois & Valance 2007, Rauen et al 2008.…”

Sand Ripples and Dunes

“…Initial ripple wavelengths are typically in the range 100-800d both in water (Baas 1994(Baas , 1999Betat et al 1999;Coleman & Melville 1996;Fourrière et al 2010;Langlois & Valance 2007) and in viscous fluids (Charru & Mouilleron-Arnould 2002, Kuru et al 1995. together with the most amplified mode computed from the three hydrodynamical models.…”

Radar Albedo Feature

“…On Mars, centimeter-scale trough cross-bedding in Burns formation sandstones composed of >50 wt% evaporitic component and weathered mafic silicates suggests that these ripples formed subaqueously in flowing viscous brines (Lamb et al 2012). (3.1) Wave-formed ripples (oscillation ripples): They are formed by oscillatory water flow (Langlois and Valance 2007) (Fig. 1b).…”