Horizontal Momentum Research Articles

Simulations of Smoke Flow Fields in a Wind Tunnel Under the Effect of an Air Curtain for Smoke Confinement

Computational Fluid Dynamics (CFD) simulation results, obtained with Fire Dynamics Simulator (FDS 6.0.1), are presented in order to analyze the performance of an air curtain in blocking fire-induced smoke in a tunnel configuration. The flow and temperature fields are discussed for different air curtain jet velocities and for a range of smoke inlet temperatures. The key objective is the determination of the effectiveness of a vertical air curtain in blocking the fire-induced smoke spreading downstream of the air curtain, as function of the momentum of the air curtain. The results are presented in non-dimensional form, in terms of a ‘momentum ratio’ R, defined as \( R = \frac{{\rho_{j} A_{j} V_{j}^{2} }}{{\rho_{s} A_{s} V_{s}^{2} }} \). This is the ratio of the vertically downward air curtain momentum to the horizontal smoke layer momentum at the position of the air curtain. This allows interpretation of the results, obtained at reduced-scale, in full-scale configurations. The smoke blocking is quantified by means of sealing effectiveness E, defined as one minus the ratio of the average temperature increase in the region downstream of the air curtain to the average temperature increase in the same region without activated air curtain. For small values of R, the sealing effectiveness E increases as the momentum ratio R increases. A maximum sealing effectiveness, E ≈ 60%, is attained for R = 8–10. Higher values of R lead to less effective sealing because the downward impinging air flow pushes smoke into the downward region. For very high values of R the effectiveness increases again, due to dilution of the smoke pushed in the downward region.

Stratospheric gravity wave momentum flux from radio occultations

AbstractTriples of GPS radio occultation (RO) temperature data are used to derive horizontal and vertical gravity wave (GW) parameters in the stratosphere between 20 km and 40 km from which the vertical flux of horizontal momentum is determined. Compared to previous studies using RO data, better limiting values for the sampling distance (Δd≤250 km) and the time interval (Δt≤15 min) are used. For several latitude bands the mean momentum fluxes (MFs) derived in this study are considerably larger than MF from other satellite missions based on horizontal wavelengths calculated between two adjacent temperature profiles along the satellite track. Error sources for the estimation of MF from RO data and the geometrical setup for the applied method are investigated. Another crucial issue discussed in this paper is the influence of different background separation methods to the final MF. For GW analysis a measured temperature profile is divided into a fluctuation and a background and it is assumed that the fluctuation is caused by GWs only. For the background separation, i.e., the detrending of large‐scale processes from the measured temperature profile, several methods exist. In this study we compare different detrending approaches and for the first time an attempt is made to detrend RO data with ERA‐Interim data from the European Centre for Medium‐Range Weather Forecasts. We demonstrate that the horizontal detrending based on RO data and ERA‐Interim gives more consistent results compared with a vertical detrending.

Synthetic thermosphere winds based on CHAMP neutral and plasma density measurements

AbstractMeridional winds in the thermosphere are key to understanding latitudinal coupling and thermosphere‐ionosphere coupling, and yet global measurements of this wind component are scarce. In this work, neutral and electron densities measured by the Challenging Minisatellite Payload (CHAMP) satellite at solar low and geomagnetically quiet conditions are converted to pressure gradient and ion drag forces, which are then used to solve the horizontal momentum equation to estimate low latitude to midlatitude zonal and meridional “synthetic” winds. We validate the method by showing that neutral and electron densities output from National Center for Atmospheric Research (NCAR) Thermosphere Ionosphere Mesosphere Electrodynamics‐General Circulation Model (TIME‐GCM) can be used to derive solutions to the momentum equations that replicate reasonably well (over 85% of the variance) the winds self‐consistently calculated within the TIME‐GCM. CHAMP cross‐track winds are found to share over 65% of the variance with the synthetic zonal winds, providing further reassurance that this wind product should provide credible results. Comparisons with the Horizontal Wind Model 14 (HWM14) show that the empirical model largely underestimates wind speeds and does not reproduce much of the observed variability. Additionally, in this work we reveal the longitude, latitude, local time, and seasonal variability in the winds; show evidence of ionosphere‐thermosphere (IT) coupling, with enhanced postsunset eastward winds due to depleted ion drag; demonstrate superrotation speeds of ∼27 m/s at the equator; discuss vertical wave coupling due the diurnal eastward propagating tide with zonal wave number 3 and the semidiurnal eastward propagating tide with zonal wave number 2.

Gravity currents propagating into ambients with arbitrary shear and density stratification: vorticity‐based modelling

We develop a vorticity‐based approach for modelling quasi‐steady, supercritical gravity currents propagating into a finite‐height channel with arbitrary density and velocity stratification. The model enforces the conservation of mass, horizontal and vertical momentum. In contrast to previous approaches, it does not rely on empirical, energy‐based closure assumptions. Instead, the effective energy loss of the flow can be calculated a posteriori. The present model results in the formulation of a second‐order, nonlinear ordinary differential equation (ODE) that can be solved in a straightforward fashion to determine the gravity‐current velocity, along with the downstream ambient velocity and density profiles. Comparisons between model predictions and direct numerical simulations (DNS) show excellent agreement. Furthermore, they indicate that, for high Reynolds numbers, the gravity‐current height adjusts itself so as to maximize the loss of energy.

Investigation of gravity wave activity based on operational radiosonde data from 13 years (1997-2009): Climatology and possible induced variability

Atmospheric gravity waves (GWs) are important for the dynamics of the atmosphere. The analysis of 13 years of routine radiosonde data from Prague (50.01°N, 14.27°E) with temporal highly resolved temperature, pressure and wind measurements is presented in order to derive a climatology of gravity wave activity in the lower stratosphere. An annual cycle with a maximum during winter and a minimum during summer is identified. Gravity wave activity is twice as high during winter as during summer. Winter periods are investigated by wavelet analysis. They show similar periods in vertical flux of horizontal momentum and pressure variance time series. These features may be attributed to planetary waves. When analyzing individual years, maxima of gravity wave activity and vertical flux of horizontal momentum often appears together with minima in surface pressure. We speculate therefore that at least parts of the interannual variations of gravity wave activity may due to cyclones.

Particle stresses in dilute, polydisperse, two-way coupled turbulent flows.

Direct numerical simulations are performed of turbulent planar Couette flow which are seeded with two-way coupled particles at low volume concentration. Based on an understanding of the development of particle stress (horizontal momentum carried vertically on average by the particle phase) in monodisperse systems at various particle Stokes numbers, several bidisperse and continuously polydisperse systems are simulated which are chosen to understand how flows containing blends of particle Stokes numbers can be effectively modeled in the dilute regime. Under noninteracting conditions, the particle stresses from particles with different inertia and different feedback stresses are shown to be linearly additive, providing a convenient method for effectively representing dispersed phase stress in polydisperse systems. While this is true, it is demonstrated that a single effective particle size is in general not sufficient at representing the entire mixture.

A Particle Tracking Model for Sedimentation from Buoyant Jets

AbstractSediment-laden turbulent buoyant jets are commonly found in natural and engineered environments. Examples include volcanic eruptions, deep sea hydrothermal vents, discharge of partially-treated wastewater, and dredging operations. A horizontal sediment buoyant jet is characterized by a horizontal momentum jet, rising plume, and a horizontal surface gravity current. The settling of particles from a sediment buoyant jet depends on the complex interaction of particles with turbulent fluctuations and the mean flow—in particular particle re-entrainment due to the external irrotational flow induced by the jet. A three-dimensional (3D) stochastic particle tracking model is proposed to predict the sedimentation from an arbitrarily inclined sediment-laden buoyant jet in a stagnant ambient. The simple model predicts the entire 3D jet flow field via a coupling of semianalytical models for the mean flow as well as turbulent fluctuations. The mean flow for the turbulent buoyant jet and the surface gravity curr...

Characteristics of Vertical Wind Fluctuations in the Lower Troposphere at Syowa Station in the Antarctic Revealed by the PANSY Radar

Using wind data over three years from July 2012-June 2015 from the PANSY radar, an MST radar, newly installed at Syowa Station (39.59°E, 69.0°S), statistical characteristics of vertical winds and vertical momentum fluxes in the Antarctic lower troposphere are examined. Frequency spectra covering a wide frequency range from (30 d)−1 to (8 min)−1 are divided into three frequency regions obeying power laws with different scaling exponents. The transition frequencies are different between horizontal and vertical wind spectra. Vertical fluxes of horizontal momentum were estimated for two wave period ranges of 8 min–2 h and 2 h–1 d which have almost equal logarithmic scales. The momentum fluxes are larger for longer period components. There are evidences showing that the vertical wind disturbances in the lower troposphere are due to gravity waves forced by topography aligned in the north-south direction. First, the strong disturbances are observed when horizontal winds are strong near the surface. Second, zonal winds tend to almost zero around the top of the disturbances. Third, frequency spectra are large at a wide range of frequency below a critical level, as is consistent with the phase modulation of mountain waves by unsteady mean flow.

Performance of One-Way and Two-Way Nesting Techniques Using the Shelf Circulation Modelling System for the Eastern Canadian Shelf

The performance of one-way and two-way nesting techniques is assessed in this study using model results produced by a regional ocean circulation modelling system for the eastern Canadian shelf. The assessment is made in terms of dynamical consistency between the parent model (PM) and child model (CM), representation of general circulation features, and reduction of numerical noise generated during the interaction of the PM and CM. It is demonstrated that the feedback from the CM to the PM in numerical experiments using two-way nesting ensures that the large-scale circulation produced by the PM and CM be dynamically consistent over the region where the two model domains overlap. In comparison with one-way nesting, two-way nesting leads to a better representation of coastal currents over the Gulf of St. Lawrence and the Scotian Shelf and improves the large-scale circulation in the results produced by the PM. This study also examines an alternative two-way nesting technique based on the semi-prognostic method in which differences between the PM and CM densities are used to adjust the horizontal momentum balance in the PM. Model results demonstrate the advantage of the semi-prognostic method in eliminating numerical noise during the feedback from the CM to PM while ensuring dynamical consistency between the two model components.

A note on the vertical distribution of momentum transport in water waves

Abstract In this paper, the classical problem of horizontal waveinduced momentum transport is analyzed once again. A new analytical approach has been employed to reveal the vertical variation of this transport in the Eulerian description. In mathematical terms, this variation is shown to have (after “smoothing out” the surface corrugation) the character of a generalized function (distribution) and is described by a classical function in the water depths and by an additional Dirac-delta-function component on the averaged free surface. In terms of physics, the considered variation consists of two entities: (i) a continuous distribution of the mean momentum transport flux density (tensorial radiation pressure) over the entire water column, and (ii) an additional momentum transport flux concentrated on the mean free surface level (tensorial radiation surface pressure). Simple analytical formulae describing this variation have been derived. This allowed a conventional expression to be derived, describing the depth-integrated excess of horizontal momentum flux due to the presence of waves (the so-called “radiation stress”), confirming to some extent the correctness of the whole analysis carried out. The results obtained may be important to the ocean dynamics, especially in view of their possible application in the field of hydrodynamics of wave-dominated coastal zones.

On the Mass-Flux Representation of Vertical Transport in Moist Convection

Abstract This study investigates to what extent the convective fluxes formulated within the mass-flux framework can represent the total vertical transport of heat and moisture in the cloud layer and whether the same approach can be extended to represent the vertical momentum transport using large-eddy simulations (LESs) of six well-documented cloud cases, including both deep and shallow convection. Two methods are used to decompose the LES-resolved vertical fluxes: decompositions based on the coherent convective features using the mass-flux top-hat profile and by two-dimensional fast Fourier transform (2D-FFT) in terms of wavenumbers. The analyses show that the convective fluxes computed using the mass-flux formula can account for most of the total fluxes of conservative thermodynamic variables in the cloud layer of both deep and shallow convection for an appropriately defined convective updraft fraction, a result consistent with the mass-flux dynamic view of moist convection and previous studies. However, the mass-flux approach fails to represent the vertical momentum transport in the cloud layer of both deep and shallow convection. The 2D-FFT and other analyses suggest that such a failure results from a number of reasons: 1) the complicated momentum distribution in the cloud layer cannot be well described by the simple top-hat profile; 2) shear-driven small-scale eddies are more efficient momentum carriers than coherent convective plumes; 3) the phase relationship between vertical velocity and horizontal momentum components is substantially different from that between vertical velocity and conservative thermodynamic variables; and 4) the structure of horizontal momentum can change substantially from case to case even in the same climate regime.

Energy dissipation and transfer processes during the breaking of modulated wave trains

In this work numerical simulation of the breaking induced by the modulational instability are presented. The study is focused on the energy and momentum exchanged taking place at the air-water interface during the breaking process. Simulations are performed by a two-fluids Navier-Stokes solver combined with the VOF technique, which provides a detailed description of the flow in both phases. Results obtained by two different initial conditions are presented in terms of free surface shapes, energy in air and water, vertical fluxes of horizontal momentum and vorticity.

Development of efficient GPU parallelization of WRF Yonsei University planetary boundary layer scheme

Abstract. The planetary boundary layer (PBL) is the lowest part of the atmosphere and where its character is directly affected by its contact with the underlying planetary surface. The PBL is responsible for vertical sub-grid-scale fluxes due to eddy transport in the whole atmospheric column. It determines the flux profiles within the well-mixed boundary layer and the more stable layer above. It thus provides an evolutionary model of atmospheric temperature, moisture (including clouds), and horizontal momentum in the entire atmospheric column. For such purposes, several PBL models have been proposed and employed in the weather research and forecasting (WRF) model of which the Yonsei University (YSU) scheme is one. To expedite weather research and prediction, we have put tremendous effort into developing an accelerated implementation of the entire WRF model using graphics processing unit (GPU) massive parallel computing architecture whilst maintaining its accuracy as compared to its central processing unit (CPU)-based implementation. This paper presents our efficient GPU-based design on a WRF YSU PBL scheme. Using one NVIDIA Tesla K40 GPU, the GPU-based YSU PBL scheme achieves a speedup of 193× with respect to its CPU counterpart running on one CPU core, whereas the speedup for one CPU socket (4 cores) with respect to 1 CPU core is only 3.5×. We can even boost the speedup to 360× with respect to 1 CPU core as two K40 GPUs are applied.

Multi-year observations of gravity wave momentum fluxes at low and middle latitudes inferred by all-sky meteor radar

Abstract. We have applied a modified composite day analysis to the Hocking (2005) technique to study gravity wave (GW) momentum fluxes in the mesosphere and lower thermosphere (MLT). Wind measurements from almost continuous meteor radar observations during June 2004–December 2008 over São João do Cariri (Cariri; 7° S, 36° W), April 1999–November 2008 over Cachoeira Paulista (CP; 23° S, 45° W), and February 2005–December 2009 over Santa Maria (SM; 30° S, 54° W) were used to estimate the GW momentum fluxes and variances in the MLT region. Our analysis can provide monthly mean altitude profiles of vertical fluxes of horizontal momentum for short-period (less than 2–3 h) GWs. The averages for each month throughout the entire data series have shown different behavior for the momentum fluxes depending on latitude and component. The meridional component has almost the same behavior at the three sites, being positive (northward), for most part of the year. On the other hand, the zonal component shows different behavior at each location: it is positive for almost half the year at Cariri and SM but predominantly negative over CP. Annual variation in the GW momentum fluxes is present at all sites in the zonal component and also in SM at 89 km in the meridional component. The seasonal analysis has also shown a 4-month oscillation at 92.5 km over SM in the zonal component and over CP at the same altitudes but for the meridional component.

Wind-blown pool fire, Part II: Comparison of measured flame geometry with semi-empirical correlations

Experimental measurements of flame drag, flame tilt and flame length, which were obtained for 2m diameter Jet A fires in crosswinds of 3–10m/s and described in Part I of this paper, were compared to values predicted using published semi-empirical correlations. Results were shown to be influenced in part by differences in the method used to measure flame geometry and in the boundary conditions between different experiments. Flame tilt values were predicted most successfully by correlations involving a combination of the Froude number U2/gD and the non-dimensionalized heat release rate Q⁎. Values estimated using similar correlations for flame drag and flame length were less successful, indicating gaps in the relevant physics modeled by those correlations. Fuel vapor density played an important role in determining the extent of flame drag. At high wind speeds, the horizontal momentum of the wind dominated over buoyancy effects in producing flame drag, physical effects not necessarily captured in the existing correlations. Flame length appeared to be governed by factors such as fuel vapor density and heat of combustion of the fuel, as well as wind speed and fuel burning rate. The restriction in air entrainment under conditions of large flame drag greatly increased flame length, particularly at high wind speeds, and should also be considered in future correlations.

Laboratory testing of a displacement ventilation diffuser for underfloor air distribution systems

Underfloor air distribution (UFAD) systems use the underfloor plenum beneath a raised floor to provide conditioned air through floor-mounted diffusers, which typically discharge cool air with both horizontal and vertical momentum components. These systems usually create a vertical temperature stratification when in cooling mode and this has an impact on energy, indoor air quality and thermal comfort. The purpose of this study was to characterize the stratification performance of a previously unstudied type of floor diffuser that discharges air horizontally, with almost no vertical velocity component, and that aims to combine the benefits of both UFAD and displacement ventilation (DV) strategies.We performed 19 full scale laboratory experiments in which we varied the number of diffusers and the internal loads over a range of values typically found in office spaces. We quantified the amount of thermal stratification by measuring the dimensionless temperature at ankle height and found a degree of stratification that is typical of DV systems – higher than is typical in UFAD systems. We developed a model based on these results that can be used to simulate these systems in whole building energy simulation tools, such as EnergyPlus, and simplified UFAD design tools.

Prognostic Residual Mean Flow in an Ocean General Circulation Model and its Relation to Prognostic Eulerian Mean Flow

AbstractRecent work has shown that taking the thickness-weighted average (TWA) of the Boussinesq equations in buoyancy coordinates results in exact equations governing the prognostic residual mean flow where eddy–mean flow interactions appear in the horizontal momentum equations as the divergence of the Eliassen–Palm flux tensor (EPFT). It has been proposed that, given the mathematical tractability of the TWA equations, the physical interpretation of the EPFT, and its relation to potential vorticity fluxes, the TWA is an appropriate framework for modeling ocean circulation with parameterized eddies. The authors test the feasibility of this proposition and investigate the connections between the TWA framework and the conventional framework used in models, where Eulerian mean flow prognostic variables are solved for. Using the TWA framework as a starting point, this study explores the well-known connections between vertical transfer of horizontal momentum by eddy form drag and eddy overturning by the bolus velocity, used by Greatbatch and Lamb and Gent and McWilliams to parameterize eddies. After implementing the TWA framework in an ocean general circulation model, the analysis is verified by comparing the flows in an idealized Southern Ocean configuration simulated using the TWA and conventional frameworks with the same mesoscale eddy parameterization.

Filament Frontogenesis by Boundary Layer Turbulence

AbstractA submesoscale filament of dense water in the oceanic surface layer can undergo frontogenesis with a secondary circulation that has a surface horizontal convergence and downwelling in its center. This occurs either because of the mesoscale straining deformation or because of the surface boundary layer turbulence that causes vertical eddy momentum flux divergence or, more briefly, vertical momentum mixing. In the latter case the circulation approximately has a linear horizontal momentum balance among the baroclinic pressure gradient, Coriolis force, and vertical momentum mixing, that is, a turbulent thermal wind. The frontogenetic evolution induced by the turbulent mixing sharpens the transverse gradient of the longitudinal velocity (i.e., it increases the vertical vorticity) through convergent advection by the secondary circulation. In an approximate model based on the turbulent thermal wind, the central vorticity approaches a finite-time singularity, and in a more general hydrostatic model, the central vorticity and horizontal convergence are amplified by shrinking the transverse scale to near the model’s resolution limit within a short advective period on the order of a day.

On the integral quantities of a low amplitude solitary wave shoaling on a mild slope

A weakly viscous 2D numerical simulation is carried out to investigate the momentum, momentum flux parameter, energy, circulation and angular momentum of a low amplitude solitary wave propagating over uniform depth and then shoaling on a mild slope. The integral measures are studied in detail by analysing separately the inviscid and viscous contributions to their evolution. In the shoaling region, the horizontal momentum is significantly reduced by the horizontal dynamic pressure component caused by the weight of the wave over the shoaling region. The potential/kinetic energy and the circulation are modified by the action of the shear stress on the rigid boundary. For angular momentum, the inviscid contributions of the moment of the pressure and the weight are dominant. It is the resultant torque which acts on the wave which results in the high vertical gradient in the horizontal velocity and in the distinctive overturning shape of a wave close to breaking.

Eulerian Volume Transport Induced by Spatially Damped Internal Equatorial Kelvin Waves

AbstractThe Eulerian volume transport in internal equatorial Kelvin waves subject to viscous attenuation is investigated theoretically by integrating the horizontal momentum equations in the vertical. In terms of small perturbations, the time-averaged horizontal transports are determined to second order in wave steepness. The total Lagrangian volume transport in this problem consists of a Stokes transport plus an Eulerian transport. It is known that the Stokes transport, that is, the vertically integrated Stokes drift, in inviscid internal equatorial Kelvin waves vanishes identically in the rigid-lid approximation for arbitrary vertical variation of the Brunt–Väisälä frequency. The present study considers spatial wave damping due to viscosity. The Stokes transport still becomes zero, but now the radiation stresses due to decaying waves become source terms for the Eulerian mean transport. Calculations of the wave-induced Eulerian transport yield a jetlike symmetric mean flow along the equator from west to east for each baroclinic component, with compensating westward flows on both sides. The flow system scales as the internal equatorial Rossby radius in the north–south direction. The total eastward part of the Eulerian volume flux centered about the equator is estimated to about 0.2 Sv (1 Sv ≡ 106 m3 s−1) for the first baroclinic mode.