Effect of depth on flow over a fixed dune

Abstract

Laser Doppler velocimeter (LDV) measurements were carried out to study the effect of depth on the flow over a train of fixed two-dimensional dunes. Conventionally averaged velocity and turbulence parameters reveal large peaks in the streamwise and vertical components of turbulent intensities and shear stress, along the shear layer emanating from the dune crest. A secondary peak in the streamwise turbulence profiles some distance beyond the shear layer indicates maintenance of turbulence generated on the previous dune and convection of the flow history from one dune to the next. Analyses based on triple products and quadrant decomposition of velocity fluctuations reveals the central role of the shear layer in dictating the flow properties over the entire depth. The depth influences the flow in the near-bed region and the length of the separation zone is longer at a shallower depth. The streamwise mean profiles collapse onto a single curve in the outer region beyond the shear layer, indicating a degree of similarity and independence from the near-bed flow. The profiles of the vertical component of turbulence reveal a systematic dependence on flow depth, with lower turbulence intensity at larger depths. The quantitative effect of flow depth is evident in the measurements at all levels, including triple products and quadrant decomposition.

Large-eddy simulation of three-dimensional dunes in a steady, unidirectional flow. Part 1. Turbulence statistics

“…For this reason, most dune shapes studied experimentally and numerically are quite similar. We base our geometry on the 2D one used by Balachandar et al (2007). A sinusoidal wave in the spanwise direction with an amplitude A, and a wavelength λ is superimposed on the streamwise position of all points in the domain.…”

“…At a higher deceleration, the shear layer over the lobe becomes broader and is lifted toward the top surface by the upward moving of the mean flow. An internal boundary layer is developed after the reattachment on the stoss side of dunes (Kostaschuk & Church 1993;Best et al 2001;Nelson et al 1993;Kadota & Nezu 1999;Bennett & Best 1995;Venditti & Bennett 2000;Balachandar et al 2007), which results in a near-wall peak of the normal stresses in Figure 19(a,c). The internal boundary layer has higher streamwise stressess in the saddle plane, since the local Reynolds number is higher; however, the spanwise stresses (Figure 19(c)) near the bed have a larger peak in the lobe plane due to the spanwise motion of the internal boundary layer.…”

“…Although flow over dunes has been extensively studied in field experiments (Matthes 1947;Jackson 1976;Flemming 1978;Gabel 1993;Kostaschuk & Church 1993;Bennett & Best 1995;Babakaiff & Hickin 1996;Kostaschuk & Villard 1996;Roden 1998;Carling et al 2000;Kostaschuk 2000;Best et al 2001), fixed 2D dunes have been the focus of the studies in laboratory experiments (Müller & Gyr 1986;Nelson et al 1993;Kadota & Nezu 1999;Schmeeckle et al 1999;Venditti & Bennett 2000;Hyun et al 2003;Balachandar et al 2007;Balachandar & Patel 2008), numerical simulations (Yoon & Patel 1996;Schmeeckle et al 1999;Yue et al 2005;Polatel et al 2006;Yue et al 2006;Stoesser et al 2008;Grigoriadis et al 2009;Omidyeganeh & Piomelli 2011), as well as theoretical studies (McLean & Smith 1986;Nelson & Smith 1989;McLean et al 1999). A comprehensive review of literature on dunes is given by Best (2005).…”

Bed Forms and Flow Mechanisms Associated with Dunes

“…Such studies also provide a basis for a better understanding of the effects of dune three-dimensionality. The laser-Doppler (LDV) technique has been used in several laboratory fixed sediment bed form investigations (e.g., Balachandar et al, 2007;Balachandar and Patel 2008;Bennett and Best 1995;Coleman et al 2006;Kadota and Nezu 1999;Lyn 1993;McLean et al 1994 and1996;Nelson et al 1993;Nelson et al 1995;Van Mierlo and de Ruiter 1988;Venditti and Bennett 2000). These studies have suggested that near-bed turbulence over much of the stoss side of a dune deviates markedly from either classical boundary-layer or wake turbulence.…”

“…Extensive research regarding fluid flow over dunes has been undertaken in the past forty years (Engelund and Fredsøe 1982;Lyn 1993;McLean et al 1994;Nelson et al 1993;Raudkivi 1966). To summarize, five zones ( Figure 3) have been recognized within the dune-flow interaction region (Balachandar et al, 2007;Balachandar et al 2008;Bennett and Best, 1995;Best 2003). A typical dune and the flow over the dune are indicated in Figure 4 (Balachandar et al, 2007).…”