Internal Waves and Turbulence in the Upper Central Equatorial Pacific: Lagrangian and Eulerian Observations

2002 ◽  
Vol 32 (9) ◽  
pp. 2619-2639 ◽  
Author(s):  
R-C. Lien ◽  
E. A. D'Asaro ◽  
M. J. McPhaden
Keyword(s):  
Kinetic Energy ◽  
Internal Waves ◽  
Dissipation Rate ◽  
Equatorial Pacific ◽  
Turbulence Kinetic Energy ◽  
Shear Instability ◽  
Inertial Subrange ◽  
Central Equatorial Pacific ◽  
Turbulence Mixing ◽  
Microstructure Measurements

Abstract In the shear stratified flow below the surface mixed layer in the central equatorial Pacific, energetic near-N (buoyancy frequency) internal waves and turbulence mixing were observed by the combination of a Lagrangian neutrally buoyant float and Eulerian mooring sensors. The turbulence kinetic energy dissipation rate ε and the thermal variance diffusion rate χ were inferred from Lagrangian frequency spectral levels of vertical acceleration and thermal change rate, respectively, in the turbulence inertial subrange. Variables exhibiting a nighttime enhancement include the vertical velocity variance (dominated by near-N waves), ε, and χ. Observed high levels of turbulence mixing in this low-Ri (Richardson number) layer, the so-called deep-cycle layer, are consistent with previous microstructure measurements. The Lagrangian float encountered a shear instability event. Near-N waves grew exponentially with a 1-h timescale followed by enhanced turbulence kinetic energy and strong dissipation rate. The event supports the scenario that in the deep-cycle layer shear instability may induce growing internal waves that break into turbulence. Superimposed on few large shear-instability events were background westward-propagating near-N waves. The floats' ability to monitor turbulence mixing and internal waves was demonstrated by comparison with previous microstructure measurements and with Eulerian measurements.

scholarly journals What controls the deep cycle? Proxies for equatorial turbulence.

2021 ◽  
Author(s):  
W. D. Smyth ◽  
S. J. Warner ◽  
J. N. Moum ◽  
H. Pham ◽  
S. Sarkar
Keyword(s):  
Seasonal Variation ◽  
Energy Dissipation ◽  
Kinetic Energy ◽  
Dissipation Rate ◽  
Energy Dissipation Rate ◽  
Equatorial Pacific ◽  
Predictive Capacity ◽  
Microstructure Measurements ◽  
Mean Current

AbstractFactors thought to influence deep cycle turbulence in the equatorial Pacific are examined statistically for their predictive capacity using a 13-year moored record that includes microstructure measurements of the turbulent kinetic energy dissipation rate. Wind stress and mean current shear are found to be most predictive of the dissipation rate. Those variables, together with the solar buoyancy flux and the diurnal mixed layer thickness, are combined to make a pair of useful parameterizations. The uncertainty in these predictions is typically 50% greater than the uncertainty in present-day in situ measurements. To illustrate the use of these parameterizations, the record of deep cycle turbulence, measured directly since 2005, is extended back to 1990 based on historical mooring data. The extended record is used to refine our understanding of the seasonal variation of deep cycle turbulence.

Influence of inlet boundary conditions in computations of turbulent jet flames

2018 ◽  
Vol 28 (6) ◽  
pp. 1433-1456 ◽  
Author(s):  
Michał T. Lewandowski ◽  
Paweł Płuszka ◽  
Jacek Pozorski
Keyword(s):  
Kinetic Energy ◽  
Boundary Conditions ◽  
Dissipation Rate ◽  
Turbulence Kinetic Energy ◽  
Turbulent Jet ◽  
Jet Flows ◽  
Jet Flames ◽  
Mean Velocity ◽  
Content Type ◽  
Inlet Conditions

Purpose This paper aims to assess the sensitivity of numerical simulation results of turbulent reactive flow to the formulation of inlet boundary conditions. The analysis concerns the profiles of the mean velocity the turbulence kinetic energy k and its dissipation rate ϵ. It is intended to provide guidance to the determination of inlet conditions when only global flow data are available. This situation can be met both in simple laboratory experiments and in industrial full-scale applications, when measurements are either incomplete or infeasible, resulting in lack of detailed inlet data. Design/methodology/approach Two turbulence–chemistry interaction models were studied: eddy dissipation concept and partially stirred reactor. Three different velocity profiles and related turbulence statistics were applied to present feasible scenarios and their consequences. Simulations with the most appropriate inlet data were accompanied with profiles of turbulent quantities obtained with a proposed method. This method was contrasted to other approaches popular in the literature: the pre-inlet pipe and the separate cold flow simulations of a burner. The methodology was validated on two laboratory-scale jet flames: Delft Jet-in-Hot-Coflow and Sandia CHN B. The simulations were carried out with open source code OpenFOAM. Findings The proposed relations for turbulence kinetic energy and its dissipation rate at the inlet are found to provide results comparable to those obtained with the use of experimental data as inlet boundary conditions. Moreover, from a certain location downstream the jet, weakly dependent on the Reynolds number, the influence of inlet conditions on flow statistics was found to be negligible. Originality/value This work reveals the consequences of the use of rather crude assumptions made for inlet boundary conditions. Proposed formulas for the profiles for k and epsilon are attractive alternatives to other approaches aiming to determine the inlet boundary conditions for turbulent jet flows.

Momentum Flux in Plane, Parallel Jets

2004 ◽  
Vol 126 (4) ◽  
pp. 665-670 ◽  
Author(s):  
Robert E. Spall ◽  
Elgin A. Anderson ◽  
Jeffrey Allen
Keyword(s):  
Kinetic Energy ◽  
Dissipation Rate ◽  
Momentum Flux ◽  
Stokes Equations ◽  
Turbulence Kinetic Energy ◽  
Navier Stokes ◽  
Navier Stokes Equations ◽  
Integral Constant ◽  
Plane Parallel ◽  
Merge Point

The evolution of the streamwise momentum flux for two turbulent, plane, parallel jets discharging through slots in a direction normal to a wall was studied both numerically and experimentally. The numerical results, obtained by solving the Reynolds-averaged Navier-Stokes equations employing a standard k−ε turbulence model, predicted to within experimental error measured integrals of the momentum flux downstream of the merge point for jet spacing S/d=5. Integration of the streamwise component of the Reynolds-averaged Navier-Stokes equations over a control volume results in an integral constant that was evaluated numerically for jet spacings S/d=3, 5, 7, 9, and 11, and for different levels of turbulence kinetic energy and dissipation rate at the jet inlet boundaries. Results revealed that the integral constant is decreased as the jet spacing increases, and is also decreased as jet entrainment rates are increased due to higher levels of inlet turbulence kinetic energy, or alternatively, decreased levels of dissipation rate. Streamwise distance to the merge point was also found to decrease for increased levels of turbulence kinetic energy or decreased levels of dissipation rate at the jet inlet.

scholarly journals Turbulence Kinetic Energy budget during the afternoon transition – Part 2: A simple TKE model

2015 ◽  
Vol 15 (21) ◽  
pp. 29807-29869 ◽  
Author(s):  
E. Nilsson ◽  
M. Lothon ◽  
F. Lohou ◽  
E. Pardyjak ◽  
O. Hartogensis ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Simple Model ◽  
Kinetic Energy ◽  
Dissipation Rate ◽  
Turbulence Kinetic Energy ◽  
Time Varying ◽  
Near Surface ◽  
Surface Buoyancy ◽  
Surface Buoyancy Flux ◽  
Shear Production

Abstract. A simple model for turbulence kinetic energy (TKE) and the TKE budget is presented for sheared convective atmospheric conditions based on observations from the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) field campaign. It is based on an idealized mixed-layer approximation and a simplified near-surface TKE budget. In this model, the TKE is dependent on four budget terms (turbulent dissipation rate, buoyancy production, shear production and vertical transport of TKE) and only requires measurements of three input available (near-surface buoyancy flux, boundary layer depth and wind speed at one height in the surface layer). This simple model is shown to reproduce some of the observed variations between the different studied days in terms of near-surface TKE and its decay during the afternoon transition reasonably well. It is subsequently used to systematically study the effects of buoyancy and shear on TKE evolution using idealized constant and time-varying winds during the afternoon transition. From this, we conclude that many different TKE decay rates are possible under time-varying winds and that generalizing the decay with simple scaling laws for near-surface TKE of the form tα may be questionable. The model's errors result from the exclusion of processes such as elevated shear production and horizontal advection. The model also produces an overly rapid decay of shear production with height. However, the most influential budget terms governing near-surface TKE in the observed sheared convective boundary layers are included, while only second order factors are neglected. Comparison between modeled and averaged observed estimates of dissipation rate illustrate that the overall behavior of the model is often quite reasonable. Therefore, we use the model to discuss the low turbulence conditions that form first in the upper parts of the boundary layer during the afternoon transition and are only apparent later near the surface. This occurs as a consequence of the continuous decrease of near-surface buoyancy flux during the afternoon transition. This region of weak afternoon turbulence is hypothesized to be a "pre-residual layer", which is important in determining the onset conditions for the weak sporadic turbulence that occur in the residual layer once near-surface stratification has become stable.

A Macroscopic Turbulence Model for Flow in a Porous Medium

1999 ◽  
Vol 121 (2) ◽  
pp. 427-433 ◽  
Author(s):  
A. Nakayama ◽  
F. Kuwahara
Keyword(s):  
Porous Medium ◽  
Kinetic Energy ◽  
Turbulent Flow ◽  
Turbulence Model ◽  
Dissipation Rate ◽  
Large Scale ◽  
Saturated Porous Medium ◽  
Turbulence Kinetic Energy ◽  
Spatially Periodic ◽  
Model Equations

A complete set of macroscopic two-equation turbulence model equations has been established for analyzing turbulent flow and heat transfer within porous media. The volume-averaged transport equations for the mass, momentum, energy, turbulence kinetic energy and its dissipation rate were derived by spatially averaging the Reynolds-averaged set of the governing equations. The additional terms representing production and dissipation of turbulence kinetic energy are modeled introducing two unknown model constants, which are determined from a numerical experiment using a spatially periodic array. In order to investigate the validity of the present macroscopic turbulence model, a macroscopically unidirectional turbulent flow through an infinite array of square rods is considered from both micro- and macroscopic-views. It has been found that the stream-wise variations of the turbulence kinetic energy and its dissipation rate predicted by the present macroscopic turbulence model agree well with those obtained from a large scale microscopic computation over an entire field of saturated porous medium.

scholarly journals Observations of Turbulence Mixing and Vorticity in a Littoral Surface Boundary Layer

2008 ◽  
Vol 38 (3) ◽  
pp. 648-669 ◽  
Author(s):  
R-C. Lien ◽  
B. Sanford ◽  
W-T. Tsai
Keyword(s):  
Boundary Layer ◽  
Kinetic Energy ◽  
Surface Waves ◽  
Turbulence Kinetic Energy ◽  
Surface Boundary ◽  
Vertical Profiles ◽  
Langmuir Circulation ◽  
Surface Boundary Layer ◽  
Turbulence Mixing ◽  
Shear Turbulence

Abstract Measurements of small-scale vorticity, turbulence velocity, and dissipation rates of turbulence kinetic energy ɛ were taken in a littoral fetch-limited surface wave boundary layer. Drifters deployed on the surface formed convergence streaks with ∼1-m horizontal spacing within a few minutes. In the interior, however, no organized pattern of velocity, vorticity, or turbulence mixing intensity was found at a similar horizontal spatial scale. The turbulent Langmuir number La was 0.6–1.3, much larger than the 0.3 of the typical open ocean, suggesting comparable importance of wind-driven turbulence and Langmuir circulation. Observed ɛ are explained by the wind-driven shear turbulence. The production rate of turbulence kinetic energy associated with the vortex force is about 10−7 W kg−1, slightly smaller than that generated by the wind-driven turbulence. The rms values of the streakwise component of vorticity σζ|| and the vertical component of vorticity σζz have a similar magnitude of ∼0.02 s−1. Vertical profiles of ɛ, σζ||, and σζz showed a monotonic decrease from the surface. Traditionally, surface convergence streaks are regarded as signatures of Langmuir circulation. Two large-eddy simulations with and without Stokes drift were performed. Both simulations produced surface convergence streaks and vertical profiles of ɛ, vorticity, and velocity consistent with observations. The observations and model results suggest that the presence of surface convergence streaks does not necessarily imply the existence of Langmuir circulation. In a littoral surface boundary layer where surface waves are young, fetch-limited, and weak, and La = O(1), the turbulence mixing in the surface mixed layer is primarily due to the wind-driven shear turbulence, and convergence streaks exist with or without surface waves.

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