Turbulence structure in stably stratified open-channel flow

Komori, Satoru; Ueda, Hiromasa; Ogino, Fumimaru; Mizushina, Tokurō

doi:10.1017/s0022112083000944

Cited by 155 publications

(106 citation statements)

References 17 publications

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“…Komori et al 2 used steam to heat the surface of water in an inclined open channel at relatively low Reynolds number. Similar to the later results of Armenio and Sarkar, 1 it was found by Komori et al 2 that Ri g governs the effect of buoyancy on the local turbulence.…”

Section: Introductionmentioning

confidence: 99%

Large eddy simulation of stably stratified open channel flow

2005

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Large eddy simulation has been used to study flow in an open channel with stable stratification imposed at the free surface by a constant heat flux and an adiabatic bottom wall. This leads to a stable pycnocline overlying a well-mixed turbulent region near the bottom wall. The results are contrasted with studies in which the bottom heat flux is nonzero, a difference analogous to that between oceanic and atmospheric boundary layers. Increasing the friction Richardson number, a measure of the relative importance of the imposed surface stratification with respect to wall-generated turbulence, leads to a stronger, thicker pycnocline which eventually limits the impact of wall-generated turbulence on the free surface. Increasing stratification also leads to an increase in the pressure-driven mean streamwise velocity and a concomitant decrease in the skin friction coefficient, which is, however, smaller than in the previous channel flow studies where the bottom buoyancy flux was nonzero. It is found that the turbulence in any given region of the flow can be classified into three regimes ͑unstratified, buoyancy-affected, and buoyancy-dominated͒ based on the magnitude of the Ozmidov length scale relative to a vertical length characterizing the large scales of turbulence and to the Kolmogorov scale. Since stratification does not strongly influence the near-wall turbulent production in the present configuration, the behavior of the buoyancy flux, turbulent Prandtl number, and mixing efficiency is qualitatively different from that seen in stratified shear layers and in channel flow with fixed temperature walls, and, furthermore, collapse of quantities as a function of gradient Richardson number is not observed. The vertical Froude number is a better measure of stratified turbulence in the upper portion of the channel where buoyancy, by providing a potential energy barrier, primarily affects the transport of turbulent patches generated at the bottom wall. The characteristics of free-surface turbulence including the kinetic energy budget and pressure-strain correlations are examined and found to depend strongly on the surface stratification.

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Section: Introductionmentioning

confidence: 99%

Large eddy simulation of stably stratified open channel flow

2005

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“…and Anderson 1948; Mellor and Yamada 1974;Launder 1975;Nunes Vaz and Simpson 1994;Gerz and Schumann 1996;Burchard et al 1998;Armenio and Sarkar 2002;Taylor et al 2005), laboratory facilities (e.g., Itsweire et al 1986;Rohr et al 1988;Komori et al 1983;Komori and Nagata 1996), and field surveys (e.g. Osborn and Cox 1972;Osborn 1980;Anwar 1983;West and Shiono 1988;West and Oduyemi 1989;Peters 1997;van der Ham 1999;Etemad Shahidi and Imberger 2006).…”

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confidence: 99%

“…Furthermore, we investigate countergradient transport and its role in coastal and estuarine flows. Komori et al (1983) were the first to identify countergradient fluxes in the outer layer of a thermally stratified open-channel flow with weak shear. In turbulent flows, the signs of the ensemble mean vertical turbulent transports of salinity and SPM are usually opposite to those of the mean gradients of salinity and SPM.…”

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confidence: 99%

“…This term applies to countergradient fluxes when they persist for longer than half the buoyancy period. Such fluxes have been measured in thermally and salinitystratified homogeneous shear flows (Komori et al 1983;Schumann 1987;Gerz and Schumann 1996), in unsheared stratified flows (Komori and Nagata 1996), in linearly saltstratified grid-generated turbulence (Itsweire et al 1986;Rohr et al 1988), in stratified flows with a gradient in turbulent properties or slightly inhomogeneous turbulent conditions (Schumann 1987;Komori and Nagata 1996), and in a tidal flow with strong SPM stratification (Dyer et al 2004). Schumann (1987) investigated the mechanisms driving countergradient transport in uniform stably stratified flows with second-order model equations.…”

mentioning

confidence: 99%

“…This more or less coincided with their observations of countergradient fluxes. Komori et al (1983) attributed countergradient heat fluxes to intermittent buoyancy-driven turbulent motions, which become wavelike as a result of strong stratification. Based on large eddy simulation (LES) data and laboratory measurements, Gerz and Schumann (1996) proposed a conceptual model for the turbulent motions that determine the countergradient transport of momentum and heat in homogeneous stratified shear flows.…”

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confidence: 99%

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An explanation for salinity- and SPM-induced vertical countergradient buoyancy fluxes

Nijs

Pietrzak

2011

Ocean Dynamics

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Measurements of turbulent fluctuations of velocity, salinity, and suspended particulate matter (SPM) are presented. The data show persistent countergradient buoyancy fluxes. These countergradient fluxes are controlled by the ratio of vertical turbulent kinetic energy (VKE) and available potential energy (APE) terms in the buoyancy flux equation. The onset of countergradient fluxes is found to approximately coincide with larger APE than VKE. It is shown here that the ratio of VKE to APE can be written as the square of a vertical Froude number. This number signifies the onset of the dynamical significance of buoyancy in the transport of mass. That is when motions driven by buoyancy begin to actively determine the vertical turbulent transport of mass. Spectral and quadrant analyses show that the occurrence of countergradient fluxes coincides with a change in the relative importance of turbulent energetic structures and buoyancydriven motions in the transport of mass. Furthermore, these analyses show that with increasing salinity-induced Richardson number (Ri), countergradient contributions expand to the larger scales of motions and the relative importance of outward and inward interactions increases. At the smaller scales, at moderate Ri, the countergradient buoyancy fluxes are physically associated with an asymmetry in transport of fluid parcels by energetic turbulent motions. At the large scales, at large Ri, the countergradient buoyancy fluxes are physically associated with convective motions induced by buoyancy of incompletely dispersed fluid parcels which have been transported by energetic motions in the past. Moreover, these convective motions induce restratification and enhanced settling of SPM. The latter is generally the result of salinity-induced convective motions, but SPM-induced buoyancy is also found to play a role.

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Turbulence structure and scalar transfer in stratified free–surface flows

Nagaosa¹,

Saito²

1997

AIChE Journal

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Turbulence structure in stably stratified open-channel flow

Cited by 155 publications

References 17 publications

Large eddy simulation of stably stratified open channel flow

Large eddy simulation of stably stratified open channel flow

An explanation for salinity- and SPM-induced vertical countergradient buoyancy fluxes

Turbulence structure and scalar transfer in stratified free–surface flows

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