The Impact of Surface Contamination on the Flow Beneath Wind-Generated Waves

We report on an experimental study conducted to investigate flow characteristics in the near-surface layer beneath clean and surfactant-contaminated water surfaces in the presence of wind. The two-dimensional velocity field beneath the water surface was measured using particle image velocimetry. The water surface temperature measurements were made simultaneously using infrared imagery. The results show the existence of the viscous sublayer beneath both clean and contaminated water surfaces. Within the viscous sublayer in contaminated water, the mean streamwise velocity is 25–30% larger and the mean streamwise velocity gradients are more than a factor of two larger compared to that beneath clean water surfaces.

Estimating and Comparing Response Times in Traditional and Connected Environments

Transportation Research Record Journal of the Transportation Research Board ◽

10.1177/0361198119837964 ◽

2019 ◽

Vol 2673 (4) ◽

pp. 674-684 ◽

Cited By ~ 8

Author(s):

Anshuman Sharma ◽

Zuduo Zheng ◽

Jiwon Kim ◽

Ashish Bhaskar ◽

Md. Mazharul Haque

Keyword(s):

Decision Making ◽

Traffic Safety ◽

Driving Simulator ◽

Response Times ◽

Flow Characteristics ◽

Trajectory Data ◽

Stimulus Response ◽

Wide Range ◽

The Mean ◽

Response time (RT) is a critical human factor that influences traffic flow characteristics and traffic safety, and is governed by drivers’ decision-making behavior. Unlike the traditional environment (TE), the connected environment (CE) provides information assistance to drivers. This in-vehicle informed environment can influence drivers’ decision-making and thereby their RTs. Therefore, to ascertain the impact of CE on RT, this study develops RT estimation methodologies for TE (RTEM-TE) and CE (RTEM-CE), using vehicle trajectory data. Because of the intra-lingual inconsistency among traffic engineers, modelers, and psychologists in the usage of the term RT, this study also provides a ubiquitous definition of RT that can be used in a wide range of applications. Both RTEM-TE and RTEM-CE are built on the fundamental stimulus–response relationship, and they utilize the wavelet-based energy distribution of time series of speeds to detect the stimulus–response points. These methodologies are rigorously examined for their efficiency and accuracy using noise-free and noisy synthetic data, and driving simulator data. Analysis results demonstrate the excellent performance of both the methodologies. Moreover, the analysis shows that the mean RT in CE is longer than the mean RT in TE.

Parameterizations of Sea-Spray Impact on the Air–Sea Momentum and Heat Fluxes

Monthly Weather Review ◽

10.1175/mwr-d-11-00007.1 ◽

2011 ◽

Vol 139 (12) ◽

pp. 3781-3797 ◽

Cited By ~ 54

Author(s):

J.-W. Bao ◽

C. W. Fairall ◽

S. A. Michelson ◽

L. Bianco

Keyword(s):

Boundary Layer ◽

Wind Speed ◽

Weather Prediction ◽

Heat Fluxes ◽

Sea Spray ◽

Surface Boundary ◽

Near Surface ◽

The Mean ◽

Momentum And Heat Fluxes ◽

Abstract This paper focuses on parameterizing the effect of sea spray at hurricane-strength winds on the momentum and heat fluxes in weather prediction models using the Monin–Obukhov similarity theory (a common framework for the parameterizations of air–sea fluxes). In this scheme, the mass-density effect of sea spray is considered as an additional modification to the stratification of the near-surface profiles of wind, temperature, and moisture in the marine surface boundary layer (MSBL). The overall impact of sea-spray droplets on the mean profiles of wind, temperature, and moisture depends on the wind speed at the level of sea-spray generation. As the wind speed increases, the mean droplet size and the mass flux of sea-spray increase, rendering an increase of stability in the MSBL and the leveling-off of the surface drag. Sea spray also tends to increase the total air–sea sensible and latent heat fluxes at high winds. Results from sensitivity testing of the scheme in a numerical weather prediction model for an idealized case of hurricane intensification are presented along with a dynamical interpretation of the impact of the parameterized sea-spray physics on the structure of the hurricane boundary layer.

The impact of a deep-water plunging breaker on a wall with its bottom edge close to the mean water surface

10.1017/jfm.2018.109 ◽

2018 ◽

Vol 843 ◽

pp. 680-721 ◽

Cited By ~ 3

Author(s):

An Wang ◽

Christine M. Ikeda-Gilbert ◽

James H. Duncan ◽

Daniel P. Lathrop ◽

Mark J. Cooker ◽

...

Keyword(s):

Deep Water ◽

Water Surface ◽

Front Face ◽

Wave Crest ◽

Bottom Surface ◽

Mean Water Level ◽

The Mean ◽

The Impact ◽

Plunging Breaker ◽

Range Of Values

The impact of a deep-water plunging breaker on a finite height two-dimensional structure with a vertical front face is studied experimentally. The structure is located at a fixed horizontal position relative to a wave maker and the structure’s bottom surface is located at a range of vertical positions close to the undisturbed water surface. Measurements of the water surface profile history and the pressure distribution on the front surface of the structure are performed. As the vertical position,$z_{b}$(the$z$axis is positive up and$z=0$is the mean water level), of the structure’s bottom surface is varied from one experimental run to another, the water surface evolution during impact can be categorized into three classes of behaviour. In class I, with$z_{b}$in a range of values near$-0.1\unicode[STIX]{x1D706}_{0}$, where$\unicode[STIX]{x1D706}_{0}$is the nominal wavelength of the breaker, the behaviour of the water surface is similar to the flip-through phenomena first described in studies with shallow water and a structure mounted on the sea bed. In the present work, it is found that the water surface between the front face of the structure and the wave crest is well fitted by arcs of circles with a decreasing radius and downward moving centre as the impact proceeds. A spatially and temporally localized high-pressure region was found on the impact surface of the structure and existing theory is used to explore the physics of this phenomenon. In class II, with$z_{b}$in a range of values near the mean water level, the bottom of the structure exits and re-enters the water phase at least once during the impact process. These air–water transitions generate large-amplitude ripple packets that propagate to the wave crest and modify its behaviour significantly. At$z_{b}=0$, all sensors submerged during the impact record a nearly in-phase high-frequency pressure oscillation indicating possible air entrainment. In class III, with$z_{b}$in a range of values near$0.03\unicode[STIX]{x1D706}_{0}$, the bottom of the structure remains in air before the main crest hits the bottom corner of the structure. The subsequent free surface behaviour is strongly influenced by the instantaneous momentum of the local flow just before impact and the highest wall pressures of all experimental conditions are found.

Influence of Wall Heating on Turbulent Structure

ASME/JSME 2007 Thermal Engineering Heat Transfer Summer Conference, Volume 1 ◽

10.1115/ht2007-32438 ◽

2007 ◽

Author(s):

Shivani T. Gajusingh ◽

Kamran Siddiqui

Keyword(s):

Flow Behavior ◽

Streamwise Velocity ◽

Wall Region ◽

Turbulent Regime ◽

Square Channel ◽

Wall Heating ◽

Turbulent Characteristics ◽

The Mean ◽

The Impact ◽

Near Wall

An experimental study was conducted to investigate the impact of wall heating on the flow structure in the near-wall region inside a square channel. PIV was used to measure the two-dimensional velocity fields. The measurements were conducted for a range of mass flow rates that cover laminar and turbulent regimes. The results have shown that when a flow is unstably stratified via heating through a bottom wall, both the mean and turbulent characteristics are affected. The results have shown that the impact of wall heating on the flow behavior is significantly different for laminar and turbulent flow regimes. It was found that when a flow that is originally laminar is heated, the mean streamwise velocity in the near-wall region is significantly increased and turbulence is generated in the flow predominantly due to buoyancy. When the flow is in the turbulent regime, addition of heat reduces the magnitudes of mean streamwise velocity and turbulent properties. The reduction in the magnitudes of turbulent properties in this flow regime is due to the working of turbulence against the buoyancy forces.

Observations on turbulent-drag reduction in a dilute suspension of clay in sea-water

10.1017/s0022112076000116 ◽

1976 ◽

Vol 75 (1) ◽

pp. 29-47 ◽

Cited By ~ 94

Author(s):

Giselher Gust

Keyword(s):

Drag Reduction ◽

Clay Mineral ◽

Flow Structure ◽

Sea Water ◽

Streamwise Velocity ◽

Viscous Sublayer ◽

Mean Flow ◽

Turbulent Drag Reduction ◽

Turbulent Drag ◽

The Mean

Hot-wire anemometer measurements have been made in a dilute sea-water/claymineral suspension. For fully developed turbulent flows in an open channel with a smooth mud (from the North Sea) bottom, mean streamwise velocity profiles were measured for Reynolds numbers between 5400 and 27 800 (i.e. non-eroding and eroding flow rates) and compared with Newtonian flows under the same experimental conditions. For the clay-mineral suspensions, measurements of the kinematic viscosityv, Kármán's constantkand the mean streamwise velocity$\overline{u}$of the logarithmic layer seemed to verify a Newtonian flow structure. Although the distributions of concentration showed no substantial increase towards the wall, it was found that beneath this Newtonian core there existed a viscous sublayer whose thickness was enhanced by a factor of 2–5. The friction velocityu*determined by the gradient method in the viscous sublayer was reduced by as much as 40 %. The mean flow structure exhibited an additional new layer in the region 10 <y+< 30.The measurements indicate that turbulent-drag reduction occurs for the experimental clay-mineral suspension at non-eroding and also at eroding velocities. Agglomeration of suspended clay-mineral particles is suggested as possible explanation of this phenomenon.

Numerical and Experimental Study of Particle Rebounding Flow Characteristics in a Gas-Particle Flow Over Curved Bodies

Volume 1: Fora, Parts A, B, C, and D ◽

10.1115/fedsm2003-45741 ◽

2003 ◽

Author(s):

J. Y. Tu ◽

G. H. Yeoh ◽

Y. S. Morsi ◽

W. Yang

Keyword(s):

Stokes Equations ◽

Laser Doppler Anemometry ◽

Solid Wall ◽

Tube Bundle ◽

Flow Characteristics ◽

Cylindrical Body ◽

Particle Flow ◽

Particulate Flow ◽

The Mean ◽

The particle rebounding characteristics of a gas–particle flow over a cylindrical body and an in-line tube bundle arrangement is investigated. With the aim of adopting both numerical and experimental approaches, the mean particulate flow patterns, comprising both incident and rebound particles resulting from the impact of particles on solid walls, are examined. Experimentally, a two-dimensional Laser-Doppler Anemometry (LDA) technique is used to measure the instantaneous incident and rebound particle velocities in the immediate vicinity of body surfaces. Computationally, the Reynolds-Averaging Navier-Stokes equations are solved for the continuum gas phase; the results are used in conjunction with a Lagrangian trajectory model to predict the particle-rebound characteristics. For a single tube model, the computational observations, also confirmed through experiment, reveal a particle rebound zone where the mean particulate flow pattern is significantly modified due to the contribution of the rebound particles during particle-wall impact interaction. For the in-line tube bundle model, particles being rebounded from the first row of tubes at upstream migrated downstream and impinged the other tubes in an extremely complex and random disposition. Analysis of the effect of the above-mentioned parameters on the rebounding particle flow characteristics has provided a better understanding on the behaviour of particulate flow impinging on curved solid wall bodies.

Linking the variability of PM10 in Europe to the position of the extratropical jet

10.5194/egusphere-egu2020-3129 ◽

2020 ◽

Author(s):

Carlos Ordóñez ◽

Jose M. Garrido-Perez ◽

Ricardo García-Herrera ◽

David Barriopedro

Keyword(s):

Central Europe ◽

North Atlantic ◽

Lower Troposphere ◽

Time Lags ◽

Southern Europe ◽

Near Surface ◽

Mean Values ◽

Future Evolution ◽

The Mean ◽

We have investigated the impact of the polar jet on the winter PM10 (particulate matter with aerodynamic diameter &#8804; 10 &#956;m) concentrations in Europe during a 10-year period. For this purpose, we have computed the daily latitude and strength of the jet by using reanalysis wind fields in the lower troposphere over the eastern North Atlantic (0&#176;&#8211;15&#176; W). Then we have extracted daily average surface PM10 observations at ~440 sites from the European air quality database (AirBase).Four preferred jet positions have been identified over the 0&#176;&#8211;15&#176; W sector in winter: southern (south of 41&#176; N), central-southern (between 41&#176; N and 51&#176; N), central-northern (between 51&#176; N and 63&#176; N) and northern (north of 63&#176; N). They exert a stronger influence than the jet strength on the mean PM10 levels. Consequently, we have examined whether the full distribution of PM10 and the occurrence of PM10 extremes (exceedances of the local winter 95th percentiles) are also linked to the jet position.The northern position is associated with enhanced PM10 concentrations (on average ~9 &#956;g m&#8722;3 above the mean values) and threefold increases in the odds of PM10 extremes over northwestern / central Europe. Comparable increases have been found in southern Europe when the jet is in its central-northern position. In both cases, the rise in the PM10 concentrations is associated with blocking of the zonal flow over those regions and the impact on PM10 extremes is maximised for time lags of around 1&#8211;2 days. On the other hand, the mean sea level pressure (SLP) patterns of the central-southern jet position resemble a positive phase of the winter North Atlantic Oscillation (NAO), yielding large PM10 decreases (on average around &#8722;9 &#956;g m&#8722;3) in northwestern / central Europe. Similarly, the southern jet position results in low PM10 concentrations in southern Europe.These results demonstrate that winter near-surface PM10 concentrations in Europe are strongly sensitive to the jet latitude, with implications for future projections of air pollution. As there is no consensus on the future evolution of the North Atlantic jet in a warming climate, different responses among model simulations could be relevant to understand discrepancies in their climate change projections of PM10 and other pollutants.

The structure of the viscous sublayer and the adjacent wall region in a turbulent channel flow

10.1017/s0022112074001479 ◽

1974 ◽

Vol 65 (3) ◽

pp. 439-459 ◽

Cited By ~ 264

Author(s):

Helmut Eckelmann

Keyword(s):

Channel Flow ◽

Turbulent Channel Flow ◽

Streamwise Velocity ◽

Viscous Sublayer ◽

Wall Region ◽

Normal Velocity ◽

Mean Velocity ◽

Velocity Fluctuations ◽

Mean Values ◽

The Mean

Hot-film anemometer measurements have been carried out in a fully developed turbulent channel flow. An oil channel with a thick viscous sublayer was used, which permitted measurements very close to the wall. In the viscous sublayer between y+ ≃ 0·1 and y+ = 5, the streamwise velocity fluctuations decreased at a higher rate than the mean velocity; in the region y+ [lsim ] 0·1, these fluctuations vanished at the same rate as the mean velocity.The streamwise velocity fluctuations u observed in the viscous sublayer and the fluctuations (∂u/∂y)0 of the gradient at the wall were almost identical in form, but the fluctuations of the gradient at the wall were found to lag behind the velocity fluctuations with a lag time proportional to the distance from the wall. Probability density distributions of the streamwise velocity fluctuations were measured. Furthermore, measurements of the skewness and flatness factors made by Kreplin (1973) in the same flow channel are discussed. Measurements of the normal velocity fluctuations v at the wall and of the instantaneous Reynolds stress −ρuv were also made. Periods of quiescence in the − ρuv signal were observed in the viscous sublayer as well as very active periods where ratios of peak to mean values as high as 30:1 occurred.

The structure and dynamics of backflow in turbulent channels

10.1017/jfm.2019.774 ◽

2019 ◽

Vol 880 ◽

Cited By ~ 5

Author(s):

J. I. Cardesa ◽

J. P. Monty ◽

J. Soria ◽

M. S. Chong

Keyword(s):

Streamwise Velocity ◽

Viscous Sublayer ◽

Spanwise Direction ◽

Streamwise Direction ◽

Mean Velocity ◽

Number Range ◽

Structure And Dynamics ◽

The Mean ◽

Turbulent Plane ◽

Increasing Function

A statistical description of flow regions with negative streamwise velocity is provided based on simulations of turbulent plane channels in the Reynolds number range $547\leqslant Re_{\unicode[STIX]{x1D70F}}\leqslant 2003$. It is found that regions of backflow are attached and their density per surface area – in wall units – is an increasing function of $Re_{\unicode[STIX]{x1D70F}}$. Their size distribution along the three coordinates reveals that, even though in the mean they appear to be circular in the wall-parallel plane, they tend to become more elongated in the spanwise direction after reaching a certain height. Time-tracking of backflow regions in a $Re_{\unicode[STIX]{x1D70F}}=934$ simulation showed they convect downstream at the mean velocity corresponding to $y^{+}\approx 12$, they seldom interact with other backflow events, their statistical signature extends in the streamwise direction for at least $300$ wall units, and they result from a complex interaction between regions of high and low spanwise vorticity far beyond the viscous sublayer. This could explain why some statistical aspects of these near-wall events do not scale in viscous units; they are dependent on the $Re_{\unicode[STIX]{x1D70F}}$-dependent dynamics further away from the wall.

Scale and Attenuation of Water Bodies on Urban Heat Islands

Open House International ◽

10.1108/ohi-03-2017-b0022 ◽

2017 ◽

Vol 42 (3) ◽

pp. 108-111

Author(s):

Huanchun Huang ◽

Yingxia Yun ◽

Jiangang Xu ◽

Shizhen Wang ◽

Xin Zheng ◽

...

Keyword(s):

Water Body ◽

Water Surface ◽

Urban Heat Islands ◽

Water Bodies ◽

Urban Water ◽

Heat Islands ◽

Planning And Design ◽

Water Surfaces ◽

Urban Heat ◽

Urban water bodies play an important role in reducing summertime urban heat island (UHI) effects. Previous studies focused mainly on the impact of water bodies of large areas, and there is no analysis of the efficacy and scale effect of how small and medium-sized water bodies reduce the UHI effects. Hence, these studies could not provide theoretical support for the scientific planning and design of urban water bodies. This study aims to confirm, within different scale ranges, the efficacy of a water body in reducing the summertime UHI effects. We propose a scale sensitivity method to investigate the temporal and spatial relationship between urban water bodies and UHI. Based on the scale theory and geostatistical analysis method in landscape ecology, this study used the platforms of 3S, MATLAB, and SPSS to analyze the distance-decay law of water bodies in reducing summertime UHI effects, as well as the scale response at different water surface ratios. The results show that the influence of water surfaces on UHIs gradually decreases with increasing distance, and the temperature rises by 0.78 °C for every 100 m away from the water body. During daytime, there is a scaled sensitivity of how much water surfaces reduce the summertime UHI effects. The most sensitive radius from the water was found at the core water surface ratio of 200 m. A reduction of UHI intensity by 2.3 °C was observed for every 10% increase of the average core water surface ratio. This study provides a theoretical reference to the control of heat islands for the planning and design of urban water bodies.