Turbulence structure and scalar transfer in stratified free–surface flows

Nagaosa, Ryuichi; Saito, Takayuki

doi:10.1002/aic.690431003

Cited by 39 publications

(40 citation statements)

References 29 publications

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“…Although the mathematical representation of the boundary conditions for this experiment is likely to be complicated, it has been argued that they are best approximated by a fixed temperature difference across the channel. 3 Nagaosa and Saito 4 reach similar conclusions from a direct nu-merical simulation ͑DNS͒ of open channel flow with fixed ⌬T across the channel. They considered a friction Reynolds number of 150, a Prandtl number of 1, and friction Richardson numbers of 0, 10, and 20.…”

Section: Introductionsupporting

confidence: 62%

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: Introductionsupporting

confidence: 62%

Large eddy simulation of stably stratified open channel flow

2005

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“…Townsend (1948) was one of the first to report global intermittency in a laboratory experiment. Its existence was also well documented in the atmospheric boundary layer (ABL) literature (e.g., Mahrt, 1989;Sun et al, , 2004Nakamura and Mahrt, 2005). Global intermittent turbulence (a.k.a.…”

Section: Introductionmentioning

confidence: 96%

“…Unfortunately, no such results were documented in those studies. We would like to emphasize that the field experimental studies (Sun et al, , 2004Nakamura and Mahrt, 2005;Cullen et al, 2007;Coulter and Doran, 2002) almost always elaborate on temporal intermittency. Thus, there is a need for documenting simulated temporal intermittency results in order to make the DNS studies relevant to the ABL research.…”

Section: Published By Copernicus Publications On Behalf Of the Europementioning

confidence: 99%

“…This type of non-stationary time series has been widely observed in geographically and climatologically diverse regions around the world. For example, they were observed in field experiments in Kansas, USA (Nakamura and Mahrt, 2005), Cabauw, the Netherlands (Holtslag et al, 2013), and Summit, Greenland (Cullen et al, 2007) (see Fig. 1a,b,c).…”

Section: Introductionmentioning

confidence: 99%

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Direct numerical simulation of intermittent turbulence under stably stratified conditions

Basu

2015

Nonlin. Processes Geophys.

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Abstract. In this paper, we simulate intermittent turbulence (also known as bursting events) in stably stratified openchannel flows using direct numerical simulation. Clear signatures of this intriguing phenomenon are observed for a range of stabilities. However, the spatiotemporal characteristics of intermittency are found to be strongly stability dependent. In general, the bursting events are much more frequent near the bottom wall than in the upper-channel region. A steady coexistence of laminar and turbulent flows is detected at various horizontal planes in very stable cases. This spatially intermittent pattern is found to propagate downstream and strongly correlate with the temporal evolution of intermittency. Lastly, a long standing hypothesis by Blackadar (1979), i.e., the strong connection between local stability and intermittent turbulence, is corroborated by this modeling study.

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“…9 The assumption of using a nondeformable free surface has been motivated in several studies. [10][11][12][13] Of particular interest is the paper by Tsai et al 13 on a numerical study of a stressdriven turbulent shear flow with a nondeformable free surface. They concluded that despite the idealization, the flow exhibits many features observed in laboratory and field experiments.…”

Section: Introductionmentioning

confidence: 99%

Turbulent oscillating channel flow subjected to a free-surface stress

2010

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The channel flow subjected to a wind stress at the free surface and an oscillating pressure gradient is investigated using large-eddy simulations. The orientation of the surface stress is parallel with the oscillating pressure gradient and a purely pulsating mean flow develops. The Reynolds number is typically Reω=106 and the Keulegan–Carpenter number—the ratio between the oscillation period and advection time scale—is KC=80. Results compare favorably to the data from direct numerical simulations obtained over a single period. A slowly pulsating mean flow occurs with the turbulent flow essentially being statistically steady. Logarithmic boundary layers are present at both the bottom wall and the free surface. Turbulent streaks are observed in the bottom and free-surface layer. The viscous sublayer below the free surface is, however, much thinner. This observation is verified by simulations we performed for a purely wind-driven channel flow. For the oscillating flow, additional low-speed splats (localized regions of upwelling) occur at the free surface when the mean velocity and stress are in the same direction.

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Turbulence structure and scalar transfer in stratified free–surface flows

Cited by 39 publications

References 29 publications

Large eddy simulation of stably stratified open channel flow

Large eddy simulation of stably stratified open channel flow

Direct numerical simulation of intermittent turbulence under stably stratified conditions

Turbulent oscillating channel flow subjected to a free-surface stress

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