Flow resistance in natural, turbulent channel flows: The need for a fluvial fluid mechanics

Abstract

In fluvial environments, feedbacks among flow, bed forms, sediment, and macrophytes result in a complex fluid dynamics. The assumptions underpinning standard tools in hydraulics are commonly violated and alternative approaches must be formulated. I argue that we should question the assumption that classical notions in fluid mechanics provide the foundations for the techniques of the future. Recent work on turbulent dissipation, interscale modulation of the dynamics, intermittency, and the role of complex forcings is discussed. An agenda for future work is proposed that involves improving our characterization of complex forcings and developing better understanding of the behavior of the velocity gradient tensor in complex, fluvial environments. This leads to the formulation of modeling tools relevant to fluvial fluid mechanics, rather than a reliance on methods developed elsewhere. One avenue by which such methods might be developed is suggested based on the stretched spiral vortex as a baseline topology. This would result in a nonequilibrium model for turbulence that has greater potential to capture the dynamics in which we are interested. Although these ideas are raised in the context of a future fluvial fluid mechanics, they are applicable to any situation where turbulent flows are forced in complicated ways.

Large eddy simulation of the velocity-intermittency structure for flow over a field of symmetric dunes

“…While the velocity-intermittency quadrant technique has been shown to be a useful way to characterise the flow field from single-point measurements, the implications of the success of this method for developing improved closure schemes for environmental flows forced in a complex fashion should also be noted [74]. Kolmogorov noted the potential importance of flow macrostructure on turbulence structure [44] (see his equations 3 and 4).…”

A joint velocity-intermittency analysis reveals similarity in the vertical structure of atmospheric and hydrospheric canopy turbulence

“…However, it also arises for canopy flows because of the complex nature of the forcing of the flow, which has clear implications for momentum and scalar exchange [21]. Intermittent turbulence excited by multiple scales is also an area of contemporary and fundamental concern [22,23] and this leads to an absence of equilibrium between turbulence production and dissipation that gives new scaling laws for the cascade dynamics in turbulence [24]. The implications for the role played by vortical structures in these dynamics remains a subject of inquiry (see the recent collection of papers on this theme in Fluid Dynamics…”

Fifty years ofWater Resources Research: Legacy and perspectives for the science of hydrology

“…Church and Ferguson [], Savenije [], Bertuzzo et al . [], and Keylock [] focus on river morphology, river estuaries, and fluvial fluid mechanics, while Runkel [] and Hipsey et al . [] direct their attention to stream hydrodynamics and aquatic ecosystems, respectively.…”

“…del Jesus et al [2015] discuss point rainfall statistics derived from satellite rainfall measurements. Church and Ferguson [2015], Savenije [2015], Bertuzzo et al [2015], and Keylock [2015] focus on river morphology, river estuaries, and fluvial fluid mechanics, while Runkel [2015] and Hipsey et al [2015] direct their attention to stream hydrodynamics and aquatic ecosystems, respectively. Harvey and Gooseff [2015] concentrate on rivers, looking at how small-scale physical drivers link to larger-scale fluvial and geomorphic processes and ecological consequences.…”