Investigation of turbulent momentum flux in the typhoon boundary layer

Abstract Relationships among the horizontal pressure gradient, the Coriolis force, and the vertical momentum transport by turbulent fluxes are investigated using data collected from the 1999 Cooperative Atmosphere–Surface Exchange Study (CASES-99). Wind toward higher pressure (WTHP) adjacent to the ground occurred about 50% of the time. For wind speed at 5 m above the ground stronger than 5 m s−1, WTHP occurred about 20% of the time. Focusing on these moderate to strong wind cases only, relationships among horizontal pressure gradients, Coriolis force, and vertical turbulent transport in the momentum balance are investigated. The magnitude of the downward turbulent momentum flux consistently increases with height under moderate to strong winds, which results in the vertical convergence of the momentum flux and thus provides a momentum source and allows WTHP. In the along-wind direction, the horizontal pressure gradient is observed to be well correlated with the quadratic wind speed, which is demonstrated to be an approximate balance between the horizontal pressure gradient and the vertical convergence of the turbulent momentum flux. That is, antitriptic balance occurs in the along-wind direction when the wind is toward higher pressure. In the crosswind direction, the pressure gradient varies approximately linearly with wind speed and opposes the Coriolis force, suggesting the importance of the Coriolis force and approximate geotriptic balance of the airflow. A simple one-dimensional planetary boundary layer eddy diffusivity model demonstrates the possibility of wind directed toward higher pressure for a baroclinic boundary layer and the contribution of the vertical turbulent momentum flux to this phenomenon.

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Turbulent flux events in a nearly neutral atmospheric boundary layer

Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences ◽

10.1098/rsta.2006.1949 ◽

2007 ◽

Vol 365 (1852) ◽

pp. 841-858 ◽

Cited By ~ 26

Author(s):

Roddam Narasimha ◽

S. Rudra Kumar ◽

A Prabhu ◽

S.V Kailas

Keyword(s):

Boundary Layer ◽

Atmospheric Boundary Layer ◽

Momentum Flux ◽

Laboratory Data ◽

Time Interval ◽

Mean Velocity ◽

Wall Unit ◽

Turbulent Momentum ◽

High Reynolds ◽

The Time Domain

We propose here a novel method of analysing turbulent momentum flux signals. The data for the analysis come from a nearly neutral atmospheric boundary layer and are taken at a height of 4 m above ground corresponding to 1.1×10 5 wall units, within the log layer for the mean velocity. The method of analysis involves examining the instantaneous flux profiles that exceed a given threshold, for which an optimum value is found to be 1 s.d. of the flux signal. It is found feasible to identify normalized flux variation signatures separately for positive and negative ‘flux events’—the sign being determined by that of the flux itself. Using these signatures, the flux signal is transformed to one of events characterized by the time of occurrence, duration and intensity. It is also found that both the average duration and the average time-interval between successive events are of order 1 s, about four orders of magnitude higher than a wall unit in time. This episodic description of the turbulence flux in the time domain enables us to identify separately productive, counter-productive and idle periods (accounting, respectively, for 36, 15 and 49% of the time), taking as criterion the generation of the momentum flux. A ‘burstiness’ index of 0.72 is found for the data. Comparison with laboratory data indicates higher (/lower) ejection (/sweep) quadrant occupancy but lower (/higher) contributions to flux from the ejection (/sweep) quadrant at the high Reynolds numbers of the atmospheric boundary layer. Possible connections with the concept of active and passive motion in a turbulent boundary layer are briefly discussed.

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Wind-Driven Mixing below the Oceanic Mixed Layer

Journal of Physical Oceanography ◽

10.1175/jpo-d-10-05020.1 ◽

2011 ◽

Vol 41 (8) ◽

pp. 1556-1575 ◽

Cited By ~ 28

Author(s):

Alan L. M. Grant ◽

Stephen E. Belcher

Keyword(s):

Boundary Layer ◽

Shear Layer ◽

Mixed Layer ◽

Momentum Flux ◽

Layer Structure ◽

Mixed Layer Depth ◽

Layer Depth ◽

Turbulent Momentum ◽

Bulk Model ◽

Shear Production

Abstract This study describes the turbulent processes in the upper ocean boundary layer forced by a constant surface stress in the absence of the Coriolis force using large-eddy simulation. The boundary layer that develops has a two-layer structure, a well-mixed layer above a stratified shear layer. The depth of the mixed layer is approximately constant, whereas the depth of the shear layer increases with time. The turbulent momentum flux varies approximately linearly from the surface to the base of the shear layer. There is a maximum in the production of turbulence through shear at the base of the mixed layer. The magnitude of the shear production increases with time. The increase is mainly a result of the increase in the turbulent momentum flux at the base of the mixed layer due to the increase in the depth of the boundary layer. The length scale for the shear turbulence is the boundary layer depth. A simple scaling is proposed for the magnitude of the shear production that depends on the surface forcing and the average mixed layer current. The scaling can be interpreted in terms of the divergence of a mean kinetic energy flux. A simple bulk model of the boundary layer is developed to obtain equations describing the variation of the mixed layer and boundary layer depths with time. The model shows that the rate at which the boundary layer deepens does not depend on the stratification of the thermocline. The bulk model shows that the variation in the mixed layer depth is small as long as the surface buoyancy flux is small.

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The vertical structure of turbulent momentum flux in the lower part of the atmospheric boundary layer

Meteorologische Zeitschrift ◽

10.1127/0941-2948/2007/0205 ◽

2007 ◽

Vol 16 (4) ◽

pp. 367-373 ◽

Cited By ~ 13

Author(s):

Rostislav Kouznetsov ◽

Valerii F. Kramar ◽

Margarita A. Kallistratova

Keyword(s):

Boundary Layer ◽

Atmospheric Boundary Layer ◽

Momentum Flux ◽

Vertical Structure ◽

Turbulent Momentum

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Polymer-laden homogeneous shear-driven turbulent flow: a model for polymer drag reduction

Journal of Fluid Mechanics ◽

10.1017/s0022112010001394 ◽

2010 ◽

Vol 657 ◽

pp. 189-226 ◽

Cited By ~ 11

Author(s):

ASHISH ROBERT ◽

T. VAITHIANATHAN ◽

LANCE R. COLLINS ◽

JAMES G. BRASSEUR

Keyword(s):

Boundary Layer ◽

Turbulent Boundary Layer ◽

Potential Energy ◽

Momentum Flux ◽

Weissenberg Number ◽

Velocity Fluctuations ◽

Homogeneous Shear ◽

Polymer Molecules ◽

Elastic Potential Energy ◽

Turbulent Momentum

Drag reduction (DR) under a turbulent boundary layer implies the suppression of turbulent momentum flux to the wall, a large-eddy phenomenon. Our hypothesis is that the essential mechanisms by which dilute concentrations of long-chain polymer molecules reduce momentum flux involve only the interactions among turbulent velocity fluctuations, polymer molecules and mean shear. Experiments indicate that these interactions dominate in a polymer-active ‘elastic layer’ outside the viscous sublayer and below a Newtonian inertial layer in a polymer-laden turbulent boundary layer. We investigate our hypothesis by modelling the suppression of momentum flux with direct numerical simulation (DNS) of homogeneous turbulent shear flow (HTSF) and the finite extensible nonlinear elastic with Peterlin approximation (FENE-P) model for polymer stress. The polymer conformation tensor equation was solved using a new hyperbolic algorithm with no artificial diffusion. We report here on the equilibrium state with fixed mean shear rate S, where progressive increases in non-dimensional polymer relaxation time WeS (shear Weissenberg number) or concentration parameter 1 − β produced progressive reductions in Reynolds shear stress, turbulence kinetic energy and turbulence dissipation rate, concurrent with increasing polymer stress and elastic potential energy. The changes in statistical variables underlying polymer DR with 1 − β, WeS, %DR and polymer-induced changes to spectra are similar to experiments in channel and pipe flows and show that the experimentally measured increase in normalized streamwise velocity variance is an indirect consequence of DR that is true only at lower DR. Comparison of polymer stretch and elastic potential energy budgets with channel flow DNS showed qualitative correspondence when distance from the wall was correlated to WeS. As WeS increased, the homogeneous shear flow displayed low-DR, high-DR and maximum-DR (MDR) regimes, similar to experiments, with each regime displaying distinctly different polymer–turbulence physics. The suppression of turbulent momentum flux arises from the suppression of vertical velocity fluctuations primarily by polymer-induced suppression of slow pressure–strain rate correlations. In the high-Weissenberg-number MDR-like limit, the polymer nearly completely blocks Newtonian inter-component energy transfer to vertical velocity fluctuations and turbulence is maintained by the polymer contribution to pressure–strain rate. Our analysis from HTSF with the FENE-P representation of polymer stress and its comparisons with experimental and DNS studies of wall-bounded polymer–turbulence supports our central hypothesis that the essential mechanisms underlying polymer DR lie directly in the suppression of momentum flux by polymer–turbulence interactions in the presence of mean shear and indirectly in the presence of the wall as the shear-generating mechanism.

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An Eddy Injection Method for Large-Eddy Simulations of Tornado-Like Vortices

Monthly Weather Review ◽

10.1175/mwr-d-16-0339.1 ◽

2017 ◽

Vol 145 (5) ◽

pp. 1937-1961 ◽

Cited By ~ 8

Author(s):

George H. Bryan ◽

Nathan A. Dahl ◽

David S. Nolan ◽

Richard Rotunno

Keyword(s):

Boundary Layer ◽

Momentum Flux ◽

Inner Core ◽

Initial Conditions ◽

Vertical Component ◽

Large Eddy Simulations ◽

Turbulent Fluctuations ◽

Turbulent Eddies ◽

Large Eddy ◽

Turbulent Momentum

Abstract The structure and intensity of tornado-like vortices are examined using large-eddy simulations (LES) in an idealized framework. The analysis focuses on whether the simulated boundary layer contains resolved turbulent eddies, and whether most of the vertical component of turbulent momentum flux is resolved rather than parameterized. Initial conditions are first generated numerically using a “precursor simulation” with an axisymmetric model. A three-dimensional “baseline” LES is then integrated using these initial conditions plus random perturbations. With this baseline approach, the inner core of the simulated vortex clearly contains resolved turbulent eddies (as expected); however, the boundary layer inflow has very weak resolved turbulent eddies, and the subgrid model accounts for most of the vertical turbulent momentum flux (contrary to the design of these simulations). To overcome this problem, a second precursor simulation is conducted in which resolved turbulent fluctuations develop within a smaller, doubly periodic LES domain. Perturbation flow fields from this precursor LES are then “injected” into the large-domain LES at a specified radius. With this approach, the boundary layer inflow clearly contains resolved turbulent fluctuations, often organized as quasi-2D rolls, which persist into the inner core of the simulation; thus, the simulated tornado-like vortex and its inflowing boundary layer can be characterized as LES. When turbulence is injected, the inner-core vortex structure is always substantially different, the boundary layer inflow is typically deeper, and in most cases the maximum wind speeds are reduced compared to the baseline simulation.

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Investigation of Air-Sea Turbulent Momentum Flux over the Aegean Sea with a Wind-Wave Coupling Model

Atmosphere ◽

10.3390/atmos12091208 ◽

2021 ◽

Vol 12 (9) ◽

pp. 1208

Author(s):

Panagiotis Portalakis ◽

Maria Tombrou ◽

John Kalogiros ◽

Aggeliki Dandou ◽

Qing Wang

Keyword(s):

Boundary Layer ◽

Momentum Flux ◽

Wave Spectrum ◽

Aegean Sea ◽

Coupling Model ◽

Stable Stratification ◽

Flux Divergence ◽

Near Surface ◽

Turbulent Momentum ◽

Surface Similarity

Near surface turbulent momentum flux estimates are performed over the Aegean Sea, using two different approaches regarding the drag coefficient formulation, a wave boundary layer model (referred here as KCM) and the most commonly used Coupled Ocean–Atmosphere Response Experiment (COARE) algorithm. The KCM model incorporates modifications in the energy-containing wave spectrum to account for the wave conditions of the Aegean Sea, and surface similarity to account for the stratification effects. Airborne turbulence data during an Etesian outbreak over Aegean Sea, Greece are processed to evaluate the simulations. KCM estimates found up to 10% higher than COARE ones, indicating that the wave-induced momentum flux may be insufficiently parameterized in COARE. Turbulent fluxes measured at about 150 m, and reduced to their surface values accounting for the vertical flux divergence, are consistently lower than the estimates. Under unstable atmospheric stratification and low to moderate wind conditions, the residuals between estimates and measurements are less than 40%. On the other hand, under stable stratification and strong winds, the majority of the residuals are more than 40%. This discrepancy is associated with the relatively high measurement level, shallow boundary layer, and the presence of a low level jet.

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Turbulence Characteristics Across a Range of Idealized Urban Canopy Geometries

Boundary-Layer Meteorology ◽

10.1007/s10546-021-00658-6 ◽

2021 ◽

Author(s):

Lewis P. Blunn ◽

Omduth Coceal ◽

Negin Nazarian ◽

Janet F. Barlow ◽

Robert S. Plant ◽

...

Keyword(s):

Momentum Flux ◽

Mixing Layer ◽

Urban Canopy ◽

Turbulence Closure ◽

Length Scale ◽

Good Representation ◽

Mixing Length ◽

Turbulence Characteristics ◽

First Order ◽

Turbulent Momentum

AbstractGood representation of turbulence in urban canopy models is necessary for accurate prediction of momentum and scalar distribution in and above urban canopies. To develop and improve turbulence closure schemes for one-dimensional multi-layer urban canopy models, turbulence characteristics are investigated here by analyzing existing large-eddy simulation and direct numerical simulation data. A range of geometries and flow regimes are analyzed that span packing densities of 0.0625 to 0.44, different building array configurations (cubes and cuboids, aligned and staggered arrays, and variable building height), and different incident wind directions ($$0^\circ $$ 0 ∘ and $$45^\circ $$ 45 ∘ with regards to the building face). Momentum mixing-length profiles share similar characteristics across the range of geometries, making a first-order momentum mixing-length turbulence closure a promising approach. In vegetation canopies turbulence is dominated by mixing-layer eddies of a scale determined by the canopy-top shear length scale. No relationship was found between the depth-averaged momentum mixing length within the canopy and the canopy-top shear length scale in the present study. By careful specification of the intrinsic averaging operator in the canopy, an often-overlooked term that accounts for changes in plan area density with height is included in a first-order momentum mixing-length turbulence closure model. For an array of variable-height buildings, its omission leads to velocity overestimation of up to $$17\%$$ 17 % . Additionally, we observe that the von Kármán coefficient varies between 0.20 and 0.51 across simulations, which is the first time such a range of values has been documented. When driving flow is oblique to the building faces, the ratio of dispersive to turbulent momentum flux is larger than unity in the lower half of the canopy, and wake production becomes significant compared to shear production of turbulent momentum flux. It is probable that dispersive momentum fluxes are more significant than previously thought in real urban settings, where the wind direction is almost always oblique.

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