Upslope Internal-Wave Stokes Drift, and Compensating Downslope Eulerian Mean Currents, Observed above a Lakebed

2016 ◽  
Vol 46 (6) ◽  
pp. 1947-1961 ◽  
Author(s):  
Stephen M. Henderson
Keyword(s):  
Boundary Layer ◽  
Internal Wave ◽  
Internal Waves ◽  
Outer Boundary ◽  
Stokes Drift ◽  
Surface Velocity ◽  
Intermediate Water ◽  
Mean Velocity ◽  
Near Surface ◽  
Velocity Fluctuations

AbstractIn a small lake, where flows were dominated by internal waves with 10–32-h period, slow but persistent mean transport of water over many wave periods was examined. Acoustic Doppler profilers (ADPs) and a vertical string of temperature loggers were deployed where the lower thermocline intersected the sloping lakebed. Near (<1 m above) the bed, internal waves, coherent with a lakewide seiche, propagated upslope at ~0.023 m s−1. Near-bed wave-induced water velocity fluctuations had a standard deviation of <0.02 m s−1. Near the surface, velocity fluctuations had similar magnitude, but lateral wave propagation was unclear. Averaged over many wave periods, the near-bed Eulerian velocity flowed downslope at ~0.01 m s−1, and was roughly cancelled by an upslope internal-wave Stokes drift (estimated by assuming that weakly nonlinear waves propagated without change of form). To examine net transport, while relaxing approximations used to estimate the Stokes drift, the observed temperature range (9°–25°C) was divided into 0.5°C increments, and the depth-integrated, wave-averaged flux of water in each temperature class was calculated. The coldest (near-bed) water was slowly transported onshore, opposite the Eulerian mean velocity. Onshore flux of warm near-surface water was comparable to an Eulerian-mean flux, indicating minimal near-surface Stokes drift. Intermediate water, from the middle of the water column and the outer boundary layer, was transported offshore by an offshore Stokes drift. The downslope near-bed Eulerian mean velocity, together with intensification of mean stratification within 0.4 m of the bed, may enhance boundary layer mixing.

Tracking of velocity variations at depth in the presence of surface velocity fluctuations

Geophysics ◽  
2013 ◽  
Vol 78 (1) ◽  
pp. U1-U8 ◽  
Author(s):  
Benoit de Cacqueray ◽  
Philippe Roux ◽  
Michel Campillo ◽  
Stefan Catheline
Keyword(s):  
Surface Waves ◽  
Body Waves ◽  
Surface Velocity ◽  
Small Scale ◽  
Beam Forming ◽  
Near Surface ◽  
Velocity Fluctuations ◽  
Wave Separation ◽  
Double Beam ◽  
Velocity Variations

We tested a small-scale experiment that is dedicated to the study of the wave separation algorithm and to the velocity variations monitoring problem itself. It handles the case in which velocity variations at depth are hidden by near-surface velocity fluctuations. Using an acquisition system that combines an array of sources and an array of receivers, coupled with controlled velocity variations, we tested the ability of beam-forming techniques to track velocity variations separately for body waves and surface waves. After wave separation through double beam forming, the arrival time variations of the different waves were measured through the phase difference between the extracted wavelets. Finally, a method was tested to estimate near-surface velocity variations using surface waves or shallow reflection and compute a correction to isolate target velocity variations at depth.

scholarly journals Mean Velocity Decomposition and Vertical Eddy Diffusivity of the Pacific Ocean from Surface GDP Drifters and 1000-m Argo Floats

2016 ◽  
Vol 46 (6) ◽  
pp. 1751-1768 ◽  
Author(s):  
Stephen M. Chiswell
Keyword(s):  
Eddy Diffusivity ◽  
Surface Velocity ◽  
Mean Velocity ◽  
Argo Data ◽  
Near Surface ◽  
Argo Floats ◽  
Velocity Decomposition ◽  
The Mean ◽  
Vertical Eddy Diffusivity ◽  
Vertical Eddy

AbstractWith the relatively recent development of Global Drifter Program (GDP) drifters that measure the near-surface ocean velocity and Argo floats that can be used to derive both the intermediate-ocean (1000 m) velocity and the mean dynamic height of the surface relative to 1000 dbar, there now exists the opportunity to directly observe the mean velocity decomposition of the ocean. This study computes the mean Ekman velocity by subtracting the mean referenced velocity derived from Argo data from the mean surface velocity derived from GDP data. This Ekman velocity is slightly stronger than previous observations and shows a spatial structure consistent with a vertical eddy diffusivity that is linearly dependent on wind stress. To do this analysis, the author has to deal with the fact that GDP drifters often lose their drogues, and a product of this research is validation of the wind-slip correction applied to GDP drifters that have lost their drogues.

The infrared measurement cascade: Connecting large scale meteorologically induced surface temperature perturbations to local spatial velocity structures

2020 ◽  
Author(s):  
Benjamin Schumacher ◽  
Marwan Katurji ◽  
Jiawei Zhang
Keyword(s):  
Boundary Layer ◽  
Velocity Field ◽  
Brightness Temperature ◽  
High Speed ◽  
Surface Brightness ◽  
Large Scale ◽  
Mean Velocity ◽  
Near Surface ◽  
Multi Scale ◽  
Infrared Cameras

&lt;p&gt;The evolution of micrometeorological measurements has been recently manifested by developments in methodological and analytical techniques using spatial surface brightness temperature captured by infrared cameras (Schumacher et al. 2019, Katurji and Zawar-Reza 2016). The Thermal Image Velocimetry (TIV) method can now produce accurate 2D advection-velocities using high speed (&gt;20Hz) infrared imagery (Inagaki 2013, Schumacher 2019). However, to further develop TIV methods and achieve a novel micrometeorological measurement technique, all scales of motion within the boundary layer need to be captured.&lt;/p&gt;&lt;p&gt;Spatial observations of multi-frequency and multi-scale temperature perturbations are a result from the turbulent interaction of the overlying atmosphere and the surface. However, these surface signatures are connected to the larger scales of the atmospheric boundary layer (McNaughton 2002, Tr&amp;#228;umner 2015). When longer periods (a few hours to a few days) of spatial surface brightness temperatures are observed, the larger scale information needs to be accounted for to build a comprehensive understanding of surface-atmospheric spatial turbulent interactions. Additionally, the time-frequency decomposition of brightness temperature perturbations shows longer periods of 4-15 minutes superimposed over shorter periods of ~ 4&amp;#8211;30 seconds. This suggests that that boundary layer dynamic scales (of longer periods) can influence brightness temperature perturbations on the local turbulent scale. An accurate TIV algorithm needs to account for all scales of motion when analysing the time-space variability of locally observed spatial brightness temperature patterns.&lt;/p&gt;&lt;p&gt;To analyse these propositions temporally high resolved geostationary satellite infrared data from the Himawari 8 satellite was compared to near-surface and high speed (20 Hz) measured air and brightness temperature using thermocouple measurements and infrared cameras. The satellite provides a temporal resolution of 10-minutes and a horizontal resolution of 2 by 2 km per pixel and therefore captures the atmospheric meso &amp;#947; and micro &amp;#945; scale which signals are usually active for ~10 minutes to &lt; 12 hours.&amp;#160;Moreover, the Himawari 8 brightness temperature was used to create the near-surface mean velocity field using TIV. Afterwards, the velocity field was compared to the in-situ measured wind velocity over several days during January 2019.&lt;/p&gt;&lt;p&gt;The results show that the atmospheric forcing from the micro &amp;#945; scale to lower atmospheric scales has a major impact on the near-surface temperature over several minutes. A significant (p-value: 0.02) positive covariance between the Himawari 8 measurement and the local measured temperature 1.5 cm above the ground on a 10 minute average, specifically concerning cooling and heating patterns, has been found.&lt;/p&gt;&lt;p&gt;Further analysis demonstrates that the retrieved near-surface 2-D velocity field calculated from the Himawari 8 brightness temperature perturbations is correctly representing the mean velocity. This finding allows the classification of meso-scale atmospheric forcing and its direct connection to local scale turbulent 2-D velocity measurements. This extends the TIV algorithm by a multi-scale component which allows to address inter-scale boundary layer analysis from a new point of view.&amp;#160;In respect to the current findings a new experiment will focus on the repeated induced local velocity patterns from large scale forcing which will be measured through the surface brightness temperature.&lt;/p&gt;

Turbulent mixing in a sloping benthic boundary layer energized by internal waves

2000 ◽  
Vol 418 ◽  
pp. 59-76 ◽  
Author(s):  
G. N. IVEY ◽  
K. B. WINTERS ◽  
I. P. D. DE SILVA
Keyword(s):  
Boundary Layer ◽  
Internal Waves ◽  
Buoyancy Frequency ◽  
Benthic Boundary Layer ◽  
Incoming Wave ◽  
Velocity Fluctuations ◽  
Reflected Waves ◽  
Wave Forcing ◽  
Orthogonal Components ◽  
Vertical Density

A laboratory study was carried out to directly measure the turbulence properties in a benthic boundary layer (BBL) above a uniformly sloping bottom where the BBL is energized by internal waves. The ambient fluid was continuously stratified and the steadily forced incoming wave field consisted of a confined beam, restricting the turbulent activity to a finite region along the bottom slope. Measurements of dissipation showed some variation over the wave phase, but cycle-averaged values indicated that the dissipation was nearly constant with height within the BBL. Dissipation levels were up to three orders of magnitude larger than background laminar values and the thickness of the BBL could be defined in terms of the observed dissipation variation with height. Assuming that most of the incoming wave energy was dissipated within the BBL, predicted levels of dissipation were in good agreement with the observations.Measurements were also made of density and two orthogonal components of the velocity fluctuations at discrete heights above the bottom. Cospectral estimates of density and velocity fluctuations showed that the major contributions to both the vertical density flux and the momentum flux resulted from frequencies near the wave forcing frequency, rather than super-buoyancy frequencies, suggesting a strong nonlinear interaction between the incident and reflected waves close to the bottom. Within the turbulent BBL, time-averaged density fluxes were significant and negative near the wave frequencies but negligible at frequencies greater than the buoyancy frequency N. While dissipation rates were high compared to background laminar values, they were low compared to the value of εtr ≈ 15vN2, the transition value often used to assess the capacity of a stratified flow to produce mixing. Existing models relating mixing to dissipation rate rely on the existence of a positive-definite density flux at frequencies greater than N as a signature of fluid mixing and therefore cannot apply to these experiments. We therefore introduce a simple model, based on the concept of diascalar fluxes, to interpret the mixing in the stratified fluid in the BBL and suggest that this may have wider application than to the particular configuration studied here.

Evaluation of boundary-layer and urban-canopy parameterizations for simulating wind in Miami during Hurricane Irma (2017)

2021 ◽  
Author(s):  
Eric A. Hendricks ◽  
Jason C. Knievel ◽  
David S. Nolan
Keyword(s):  
Boundary Layer ◽  
Urban Environment ◽  
Weather Prediction ◽  
Urban Canopy ◽  
Surface Velocity ◽  
Urban Region ◽  
Near Surface ◽  
Model Simulations ◽  
Closure Scheme ◽  
Hurricane Irma

AbstractThe simulated winds within the urban canopy of landfalling tropical cyclones are sensitive to the representation of the planetary-boundary and urban-canopy layers in numerical weather prediction models. In order to assess the sub-grid-scale parameterizations of these layers, mesoscale model simulations were executed and evaluated against near-surface observations as the outer wind field of Hurricane Irma (2017) interacted with the built-up region from downtown Miami northward to West Palm Beach. Four model simulations were examined, comprised of two different planetary boundary layer (PBL) parameterizations (a local closure scheme with turbulent kinetic energy prediction and a nonlocal closure scheme) and two different urban canopy models (UCMs) [a zeroth order bulk scheme and a multilayer Building Effect Parameterization (BEP) that mimics the three-dimensionality of buildings]. Overall, the simulated urban canopy winds were weakly sensitive to the PBL scheme and strongly sensitive to the UCM. The bulk simulations compared most favorably to an analyzed wind swath in the urban environment, while the BEP simulations had larger negative biases in the same region. There is uncertainty in magnitude of the urban environment biases due to the lack of many urban sheltered measurements in the wind swath analysis. Biases in the rural environment were similar among the bulk and BEP simulations. An improved comparison with the analyzed wind swath in the urban region was obtained by reducing the drag coefficient in BEP in one of the PBL schemes. The usefulness of BEP was demonstrated in its ability to predict realistic heterogeneous near-surface velocity patterns in urban regions.

Full-coverage film cooling. Part 1. Three-dimensional measurements of turbulence structure

1980 ◽  
Vol 101 (1) ◽  
pp. 129-158 ◽  
Author(s):  
S. Yavuzkurt ◽  
R. J. Moffat ◽  
W. M. Kays
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Film Cooling ◽  
Outer Boundary ◽  
Turbulence Structure ◽  
Reynolds Stresses ◽  
Mean Velocity ◽  
Test Plate ◽  
Apparent Paradox ◽  
Full Coverage

Hydrodynamic measurements were made with a triaxial hot wire in the full-coverage region and the recovery region following an array of injection holes inclined downstream, at 30° to the surface. The data were taken under isothermal conditions at ambient temperature and pressure for two blowing ratios: M = 0·9 and M = 0·4. (The ratio M = ρjetUjet/ρ∞U∞, where U is the mean velocity and ρ is the density. Subscripts jet and ∞ stand for injectant and free stream, respectively.) Profiles of the three mean-velocity components and the six Reynolds stresses were obtained at several spanwise positions at each of five locations down the test plate.In the full-coverage region, high levels of turbulence kinetic energy (TKE) were found for low blowing and low TKE levels for high blowing. This observation is especially significant when coupled with the fact that the heat transfer coefficient is high for high blowing, and low for low blowing. This apparent paradox can be resolved by the hypothesis that entrainment of the mainstream fluid must be more important than turbulent mixing in determining the heat transfer behaviour at high blowing ratios (close to unity).In the recovery region, the flow can be described in terms of a two-layer model: an outer boundary layer and a two-dimensional inner boundary layer. The inner layer governs the heat transfer.

On the Tendency to Self-Preservation in Axisymmetric Ducted Jets

1964 ◽  
Vol 86 (4) ◽  
pp. 765-771 ◽  
Author(s):  
R. Curtet ◽  
F. P. Ricou
Keyword(s):  
Boundary Layer ◽  
Equations Of Motion ◽  
Form Factors ◽  
Velocity Profiles ◽  
Stream Velocity ◽  
Mean Velocity ◽  
Velocity Fluctuations ◽  
Air Jet ◽  
The Mean ◽  
Secondary Stream

If it is assumed that the mean-velocity profiles of a ducted jet are similar in form sufficiently for downstream of the orifice it is possible, as shown in earlier papers [1, 2, 3], to integrate the equations of motion using the boundary-layer approximation and assuming a constant-energy secondary stream. It is necessary to know when and how this limiting profile is reached, and whether a similar tendency to self-preservation of the components of the velocity fluctuations is observed before the jet reaches the duct-wall boundary layer. Measurements have been made in an axisymmetric ducted air jet of the mean and fluctuating velocities, jet width, secondary-stream velocity, ductwall static pressure, and the boundary layer thickness. Results are compared with values predicted by the approximate jet theory. The authors define form factors calculated from measured profiles of mean velocities, of radial and longitudinal components of the velocity fluctuations, and of the shear stress. The variation of these form factors indicates a definite tendency to similarity for the mean velocity profiles; however, departures from similarity persist for the velocity fluctuations to the limit of measurements, about three duct diameters (40 nozzle diameters).

scholarly journals Effects of Wave Breaking on the Near-Surface Profiles of Velocity and Turbulent Kinetic Energy

2004 ◽  
Vol 34 (2) ◽  
pp. 490-504 ◽  
Author(s):  
Arne Melsom ◽  
Øyvind SÆtra
Keyword(s):  
Surface Stress ◽  
Wave Breaking ◽  
Traditional Approach ◽  
Surface Region ◽  
Breaking Waves ◽  
Stokes Drift ◽  
Surface Velocity ◽  
Momentum Exchange ◽  
Near Surface ◽  
The Mean

Abstract A theoretical model for the near-surface velocity profile in the presence of breaking waves is presented. Momentum is accumulated by growing waves and is released upon wave breaking. In effect, such a transition is a process involving a time-dependent surface stress acting on the mean current. In this paper, conventional theory for the Stokes drift is expanded to fourth-order accuracy in wave steepness. It is shown that the higher-order terms lead to an enhancement of the surface Stokes drift and a slight retardation of the Stokes volume flux. Furthermore, the results from the wave theory are used to obtain a bulk parameterization of momentum exchange during the process of wave breaking. The mean currents are then obtained by application of a variation of the “level 2.5” turbulence closure theory of Mellor and Yamada. When compared with the traditional approach of a constant surface stress, the mean Eulerian current exhibits a weak enhancement in the near-surface region, compensated by a negative shift deeper in the water column. However, it is found that the results of Craig and Banner and the results of Craig are not significantly affected by the present theory. Hence, this study helps to explain why the Craig and Banner model agrees well with observations when a realistic, time-varying surface stress acts on the drift currents.

scholarly journals Internal Wave Reflection on Shelf Slopes with Depth-Varying Stratification

2013 ◽  
Vol 43 (2) ◽  
pp. 248-258 ◽  
Author(s):  
Rob A. Hall ◽  
John M. Huthnance ◽  
Richard G. Williams
Keyword(s):  
Internal Wave ◽  
Internal Waves ◽  
Energy Flux ◽  
Internal Tide ◽  
Shelf Break ◽  
Maximum Slope ◽  
Near Surface ◽  
Shelf Slope ◽  
Linear Slope ◽  
Horizontal Wavelength

Abstract Reflection of internal waves from sloping topography is simple to predict for uniform stratification and linear slope gradients. However, depth-varying stratification presents the complication that regions of the slope may be subcritical and other regions supercritical. Here, a numerical model is used to simulate a mode-1, M2 internal tide approaching a shelf slope with both uniform and depth-varying stratifications. The fractions of incident internal wave energy reflected back offshore and transmitted onto the shelf are diagnosed by calculating the energy flux at the base of slope (with and without topography) and at the shelf break. For the stratifications/topographies considered in this study, the fraction of energy reflected for a given slope criticality is similar for both uniform and depth-varying stratifications. This suggests the fraction reflected is dependent only on maximum slope criticality and independent of the depth of the pycnocline. The majority of the reflected energy flux is in mode 1, with only minor contributions from higher modes due to topographic scattering. The fraction of energy transmitted is dependent on the depth-structure of the stratification and cannot be predicted from maximum slope criticality. If near-surface stratification is weak, transmitted internal waves may not reach the shelf break because of decreased horizontal wavelength and group velocity.

scholarly journals ESCOAMENTO NA ESTEIRA DE TURBINAS EÓLICAS: ANÁLISE ESPECTRAL DA TURBULÊNCIA MEDIANTE TESTES EM TÚNEL DE VENTO

2016 ◽  
Vol 38 ◽  
pp. 46
Author(s):  
Adrián Roberto Wittwer ◽  
Rodrigo Dorado ◽  
Gervásio Annes Degrazia ◽  
Acir Mércio Loredo-Souza ◽  
Bardo Ernst Josef Bodmann
Keyword(s):  
Boundary Layer ◽  
Wind Tunnel ◽  
Wind Farm ◽  
Wake Flow ◽  
Mean Velocity ◽  
Velocity Fluctuations ◽  
Velocity Deficit ◽  
Scale Models ◽  
Spectral Evaluation ◽  
Turbine Wake

The interaction between the incident wind and wind turbines in a wind farm causes mean velocity deficit and increased levels of turbulence in the wake. The turbulent flow is characterized by the superposition of the wind turbine wakes. In this work, the technique of turbulence spectral evaluation to reduce scale models in a boundary layer wind tunnel is presented, and different measurements of velocity fluctuations are analyzed. The results allow evaluating the spectrum configuration at different frequency ranges and the differences of the spectral behavior between the incident wind and the turbine wake flow.  

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