The Experts below are selected from a list of 321 Experts worldwide ranked by ideXlab platform
Eugene S. Takle - One of the best experts on this subject based on the ideXlab platform.
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A numerical study of the influence of an air temperature-inversion layer and a seawater density-jump layer on the structure of interacting boundary layers
Boundary-Layer Meteorology, 1994Co-Authors: Le Ngoc Ly, Eugene S. TakleAbstract:The effects of an air-temperature inversion in the atmosphere and a seawater density jump in the ocean on the structure of the atmospheric and oceanic boundary layers are studied by use of a coupled model. The numerical model consists of a closed system of equations for velocities, Turbulent kinetic energy, Turbulent exchange coefficient, local Turbulent Length Scale, and stratification expressions for both air and sea boundary layers. The effects of the temperature inversion and the density jump are incorporated into the equations of Turbulent kinetic energy of the atmosphere and ocean by a parameterization. A series of numerical experiments was conducted to determine the effects of various strengths of the inversion layer and surface heat fluxes in the atmosphere and of the density-jump layer in the ocean on the structure of the interacting boundary layers. The numerical results show that the temperature inversion in the atmosphere and density jump in the ocean have strong influences on Turbulent structure [especially on the Turbulent exchange coefficient (TEC) and Turbulent kinetic energy (TKE)] and on air-sea interaction characteristics. Maxima of TKE and TEC strongly decrease with increasing strength of the inversion layer, and they disappear for strong inversions in the atmosphere. Certain strengths (density differences between the upper and the lower layers) of the density-jump layer in the ocean (Δρ_2 ⩾0.1 g/cm^3) produce double maxima in TEC-profiles and TKE-profiles in the ocean. The magnitudes of air-sea interaction characteristics such as geostrophic drag coefficient, and surface drift current increase with increasing strength of the density-jump layer in the ocean. The density-jump layer plays the role of a barrier that limits vertical mixing in the ocean. The numerical results agree well with available observed data and accepted quantitive understanding of the influences of a temperature inversion layer and a density-jump layer on the interacting atmospheric and oceanic boundary layers.
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A numerical study of the influence of an air temperature-inversion layer and a seawater density-jump layer on the structure of interacting boundary layers
Boundary-Layer Meteorology, 1994Co-Authors: Le Ngoc Ly, Eugene S. TakleAbstract:The effects of an air-temperature inversion in the atmosphere and a seawater density jump in the ocean on the structure of the atmospheric and oceanic boundary layers are studied by use of a coupled model. The numerical model consists of a closed system of equations for velocities, Turbulent kinetic energy, Turbulent exchange coefficient, local Turbulent Length Scale, and stratification expressions for both air and sea boundary layers. The effects of the temperature inversion and the density jump are incorporated into the equations of Turbulent kinetic energy of the atmosphere and ocean by a parameterization. A series of numerical experiments was conducted to determine the effects of various strengths of the inversion layer and surface heat fluxes in the atmosphere and of the density-jump layer in the ocean on the structure of the interacting boundary layers. The numerical results show that the temperature inversion in the atmosphere and density jump in the ocean have strong influences on Turbulent structure [especially on the Turbulent exchange coefficient (TEC) and Turbulent kinetic energy (TKE)] and on air-sea interaction characteristics. Maxima of TKE and TEC strongly decrease with increasing strength of the inversion layer, and they disappear for strong inversions in the atmosphere. Certain strengths (density differences between the upper and the lower layers) of the density-jump layer in the ocean (Δρ_2 ⩾0.1 g/cm^3) produce double maxima in TEC-profiles and TKE-profiles in the ocean. The magnitudes of air-sea interaction characteristics such as geostrophic drag coefficient, and surface drift current increase with increasing strength of the density-jump layer in the ocean. The density-jump layer plays the role of a barrier that limits vertical mixing in the ocean. The numerical results agree well with available observed data and accepted quantitive understanding of the influences of a temperature inversion layer and a density-jump layer on the interacting atmospheric and oceanic boundary layers.
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A numerical study of the influence of an air temperature-inversion layer and a seawater density-jump layer on the structure of interacting boundary layers
Boundary-Layer Meteorology, 1994Co-Authors: Le Ngoc Ly, Eugene S. TakleAbstract:The effects of an air-temperature inversion in the atmosphere and a seawater density jump in the ocean on the structure of the atmospheric and oceanic boundary layers are studied by use of a coupled model. The numerical model consists of a closed system of equations for velocities, Turbulent kinetic energy, Turbulent exchange coefficient, local Turbulent Length Scale, and stratification expressions for both air and sea boundary layers. The effects of the temperature inversion and the density jump are incorporated into the equations of Turbulent kinetic energy of the atmosphere and ocean by a parameterization. A series of numerical experiments was conducted to determine the effects of various strengths of the inversion layer and surface heat fluxes in the atmosphere and of the density-jump layer in the ocean on the structure of the interacting boundary layers.
Vitaliy Bychkov - One of the best experts on this subject based on the ideXlab platform.
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ON THE THEORY OF Turbulent FLAME VELOCITY
Combustion Science and Technology, 2007Co-Authors: Vitaliy Bychkov, Arkady Petchenko, V'yacheslav AkkermanAbstract:The renormalization ideas of self-similar dynamics of a strongly Turbulent flame front are applied to the case of a flame with realistically large thermal expansion of the burning matter. In that case a flame front is corrugated both by external turbulence and the intrinsic flame instability. The analytical formulas for the velocity of flame propagation are obtained. It is demonstrated that the flame instability is of principal importance when the integral Turbulent Length Scale is much larger than the cutoff waveLength of the instability. The developed theory is used to analyze recent experiments on Turbulent flames propagating in tubes.
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Effect of the Darrieus-Landau instability on Turbulent flame velocity.
Physical review. E Statistical nonlinear and soft matter physics, 2002Co-Authors: Maxim Zaytsev, Vitaliy BychkovAbstract:The propagation of Turbulent premixed flames influenced by the intrinsic hydrodynamic flame instability (the Darrieus-Landau instability) is considered in a two-dimensional case using the model nonlinear equation proposed recently by Bychkov [Phys. Rev. Lett. 84, 6122 (2000)]. The nonlinear equation takes into account both the influence of external turbulence and the intrinsic properties of a flame front, such as small but finite flame thickness and realistically large density variations across the flame front. Dependence of the flame velocity on the Turbulent Length Scale, Turbulent intensity, and density variations is investigated in the case of weak nonlinearity and weak external turbulence. It is shown that the Darrieus-Landau instability influences the flamelet velocity considerably. The obtained results are in agreement with experimental data on the Turbulent burning of moderate values of the Reynolds number.
Alberto Martilli - One of the best experts on this subject based on the ideXlab platform.
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A one-dimensional model of Turbulent flow through “urban” canopies (MLUCM v2.0): updates based on large-eddy simulation
Geoscientific Model Development, 2020Co-Authors: Negin Nazarian, E. Scott Krayenhoff, Alberto MartilliAbstract:Abstract. In mesoScale climate models, urban canopy flow is typically parameterized in terms of the horizontally averaged (1-D) flow and scalar transport, and these parameterizations can be informed by computational fluid dynamics (CFD) simulations of the urban climate at the microScale. Reynolds averaged Navier–Stokes simulation (RANS) models have previously been employed to derive vertical profiles of Turbulent Length Scale and drag coefficient for such parameterization. However, there is substantial evidence that RANS models fall short in accurately representing Turbulent flow fields in the urban roughness sublayer. When compared with more accurate flow modeling such as large-eddy simulations (LES), we observed that vertical profiles of Turbulent kinetic energy and associated Turbulent Length Scales obtained from RANS models are substantially smaller specifically in the urban canopy. Accordingly, using LES results, we revisited the urban canopy parameterizations employed in the one-dimensional model of Turbulent flow through urban areas and updated the parameterization of Turbulent Length Scale and drag coefficient. Additionally, we included the parameterization of the dispersive stress, previously neglected in the 1-D column model. For this objective, the PArallelized Large-Eddy Simulation Model (PALM) is used and a series of simulations in an idealized urban configuration with aligned and staggered arrays are considered. The plan area density ( λp ) is varied from 0.0625 to 0.44 to span a wide range of urban density (from sparsely developed to compact midrise neighborhoods, respectively). In order to ensure the accuracy of the simulation results, we rigorously evaluated the PALM results by comparing the vertical profiles of Turbulent kinetic energy and Reynolds stresses with wind tunnel measurements, as well as other available LES and direct numerical simulation (DNS) studies. After implementing the updated drag coefficients and Turbulent Length Scales in the 1-D model of urban canopy flow, we evaluated the results by (a) testing the 1-D model against the original LES results and demonstrating the differences in predictions between new (derived from LES) and old (derived from RANS) versions of the 1-D model, and (b) testing the 1-D model against LES results for a test case with realistic geometries. Results suggest a more accurate prediction of vertical Turbulent exchange in urban canopies, which can consequently lead to an improved prediction of urban heat and pollutant dispersion at the mesoScale.
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A One-Dimensional Model of Turbulent Flow Through ‘Urban’ Canopies: Updates Based on Large-Eddy Simulation
2019Co-Authors: Negin Nazarian, E. Scott Krayenhoff, Alberto MartilliAbstract:Abstract. In mesoScale climate models, urban canopy flow is typically parameterized in terms of the horizontally-averaged (1-D) flow and scalar transport, and these parameterizations can be informed by Computational Fluid Dynamics (CFD) simulations of the urban climate at the microScale. Reynolds Averaged Navier-Stokes Simulation (RANS) models have been previously employed to derive vertical profiles of Turbulent Length Scale and drag coefficient for such parameterization. However, there is substantial evidence that RANS models fall short in accurately representing Turbulent flow fields in the urban roughness sublayer. When compared with more accurate flow modeling such as Large Eddy Simulations (LES), we observed that vertical profiles of Turbulent kinetic energy and associated Turbulent Length Scales obtained from RANS models are substantially smaller specifically in the urban canopy. Accordingly, using LES results, we revisited the urban canopy parameterizations employed in the one-dimensional model of Turbulent flow through urban areas, and updated the parameterization of Turbulent Length Scale and drag coefficient. Additionally, we included the parameterization of the dispersive stress, previously neglected in the 1-D column model. For this objective, the PArallelized Large-Eddy Simulation Model (PALM) is used and a series of simulations in an idealized urban configuration with aligned and staggered arrays are considered. The plan area density is varied from 0.0625 to 0.44 to span a wide range of urban density (from sparsely developed to compact midrise neighborhoods, respectively). To ensure the accuracy of the simulation results, we rigorously evaluated the PALM results by comparing the vertical profiles of Turbulent kinetic energy and Reynolds stresses with wind tunnel measurements, as well as other available LES and DNS studies. After implementing the updated drag coefficients and Turbulent Length Scales in the 1-D model of urban canopy flow, we evaluated the results by a) testing the 1-D model against the original LES results, and demonstrating the differences in predictions between new (derived from LES) and old (derived from RANS) versions of the 1-D model, and b) testing the 1-D model against LES results for a test-case with realistic geometries. Results suggest a more accurate prediction of vertical Turbulent exchange in urban canopies, which can consequently lead to an improved prediction of urban heat and pollutant dispersion at the mesoScale.
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cfd simulation of airflow over a regular array of cubes part ii analysis of spatial average properties
Boundary-Layer Meteorology, 2007Co-Authors: Alberto Martilli, Jose Luis SantiagoAbstract:In the first part of this study, results of a computational fluid dynamics simulation over an array of cubes have been validated against a set of wind-tunnel measurements. In Part II, such numerical results are used to investigate spatially-averaged properties of the flow and passive tracer dispersion that are of interest for high resolution urban mesoScale modelling (e.g. non resolved obstacle approaches). The results show that vertical profiles of mean horizontal wind are linear within the canopy and logarithmic above. The drag coefficient, derived from the numerical results using the classical formula for the drag force, is height dependent (it decreases with height). However, a modification of the formula is proposed (accounting for subgrid velocity Scales) that makes the drag coefficient constant with height. Results also show that the dispersive fluxes are similar in magnitude to the Turbulent fluxes, and that they play a very important role within the canopy. Vertical profiles of Turbulent Length Scales (to be used in k–l closure schemes, where k is the Turbulent kinetic energy and l a Turbulent Length Scale) are also derived. Finally the distribution of the values around the mean over the reference volumes are analysed for wind and tracer concentrations.
Le Ngoc Ly - One of the best experts on this subject based on the ideXlab platform.
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A numerical study of the influence of an air temperature-inversion layer and a seawater density-jump layer on the structure of interacting boundary layers
Boundary-Layer Meteorology, 1994Co-Authors: Le Ngoc Ly, Eugene S. TakleAbstract:The effects of an air-temperature inversion in the atmosphere and a seawater density jump in the ocean on the structure of the atmospheric and oceanic boundary layers are studied by use of a coupled model. The numerical model consists of a closed system of equations for velocities, Turbulent kinetic energy, Turbulent exchange coefficient, local Turbulent Length Scale, and stratification expressions for both air and sea boundary layers. The effects of the temperature inversion and the density jump are incorporated into the equations of Turbulent kinetic energy of the atmosphere and ocean by a parameterization. A series of numerical experiments was conducted to determine the effects of various strengths of the inversion layer and surface heat fluxes in the atmosphere and of the density-jump layer in the ocean on the structure of the interacting boundary layers. The numerical results show that the temperature inversion in the atmosphere and density jump in the ocean have strong influences on Turbulent structure [especially on the Turbulent exchange coefficient (TEC) and Turbulent kinetic energy (TKE)] and on air-sea interaction characteristics. Maxima of TKE and TEC strongly decrease with increasing strength of the inversion layer, and they disappear for strong inversions in the atmosphere. Certain strengths (density differences between the upper and the lower layers) of the density-jump layer in the ocean (Δρ_2 ⩾0.1 g/cm^3) produce double maxima in TEC-profiles and TKE-profiles in the ocean. The magnitudes of air-sea interaction characteristics such as geostrophic drag coefficient, and surface drift current increase with increasing strength of the density-jump layer in the ocean. The density-jump layer plays the role of a barrier that limits vertical mixing in the ocean. The numerical results agree well with available observed data and accepted quantitive understanding of the influences of a temperature inversion layer and a density-jump layer on the interacting atmospheric and oceanic boundary layers.
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A numerical study of the influence of an air temperature-inversion layer and a seawater density-jump layer on the structure of interacting boundary layers
Boundary-Layer Meteorology, 1994Co-Authors: Le Ngoc Ly, Eugene S. TakleAbstract:The effects of an air-temperature inversion in the atmosphere and a seawater density jump in the ocean on the structure of the atmospheric and oceanic boundary layers are studied by use of a coupled model. The numerical model consists of a closed system of equations for velocities, Turbulent kinetic energy, Turbulent exchange coefficient, local Turbulent Length Scale, and stratification expressions for both air and sea boundary layers. The effects of the temperature inversion and the density jump are incorporated into the equations of Turbulent kinetic energy of the atmosphere and ocean by a parameterization. A series of numerical experiments was conducted to determine the effects of various strengths of the inversion layer and surface heat fluxes in the atmosphere and of the density-jump layer in the ocean on the structure of the interacting boundary layers. The numerical results show that the temperature inversion in the atmosphere and density jump in the ocean have strong influences on Turbulent structure [especially on the Turbulent exchange coefficient (TEC) and Turbulent kinetic energy (TKE)] and on air-sea interaction characteristics. Maxima of TKE and TEC strongly decrease with increasing strength of the inversion layer, and they disappear for strong inversions in the atmosphere. Certain strengths (density differences between the upper and the lower layers) of the density-jump layer in the ocean (Δρ_2 ⩾0.1 g/cm^3) produce double maxima in TEC-profiles and TKE-profiles in the ocean. The magnitudes of air-sea interaction characteristics such as geostrophic drag coefficient, and surface drift current increase with increasing strength of the density-jump layer in the ocean. The density-jump layer plays the role of a barrier that limits vertical mixing in the ocean. The numerical results agree well with available observed data and accepted quantitive understanding of the influences of a temperature inversion layer and a density-jump layer on the interacting atmospheric and oceanic boundary layers.
Hubert Chanson - One of the best experts on this subject based on the ideXlab platform.
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modelling upstream fish passage in standard box culverts interplay between turbulence fish kinematics and energetics
River Research and Applications, 2018Co-Authors: Hang Wang, Hubert ChansonAbstract:Box culverts are common hydraulic structures along rivers and streams, in rural and urban water systems. The expertise in fish-friendly culvert design is limited, sometimes leading to adverse impact on the catchment ecosystem or to uneconomical structures. Basic dimensional considerations highlight a number of key parameters relevant to any laboratory modelling of upstream fish passage, including the ratio of fish speed fluctuations to fluid velocity fluctuations, the ratios of fish dimensions to Turbulent Length Scale, and the fish species. Alternately, the equation of conservation of momentum may be applied to an individual fish, yielding some deterministic estimate of instantaneous thrust and power expended during fish swimming, including the associated energy consumption. The rate of work required by the fish to deliver thrust is proportional to the cube of the local fluid velocity, and the model results demonstrate the key role of slow-velocity regions in which fish will minimize their energy consumption when swimming upstream.
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INTEGRAL Turbulent Length AND TIME ScaleS IN HYDRAULIC JUMPS: AN EXPERIMENTAL INVESTIGATION AT LARGE REYNOLDS NUMBERS
2015Co-Authors: Hang Wang, Hubert ChansonAbstract:A hydraulic jump is a rapidly-varied open channel flow characterised by the sudden transition from a supercritical flow motion to a subcritical regime. The transition is associated with a rapid increase of water depth, a highly Turbulent flow with macro-Scale vortices, significant kinetic energy dissipation, a two-phase flow region and some strong turbulence interactions with the free surface leading to splashes and droplet projection. The phenomenon is not a truly random Turbulent process because of the existence of low-frequency, pseudo-periodic coherent structures and fluctuating motion in the jump roller. This study presents new measurements of Turbulent air-water flow properties in hydraulic jumps, including turbulence intensity, longitudinal and transverse integral Length and time Scales, for a range of Froude numbers (3.8 < Fr1 < 8.5) at large Reynolds numbers (3×10 4 < Re < 2×10 5 ). The results showed a combination of both fast and slow Turbulent components. The respective contributions of the fast and slow motions were quantified using a novel triple decomposition technique. The results highlighted the 'true' Turbulent characteristics linked to the fast, microscopic velocity turbulence of hydraulic jumps, while showing that slow-fluctuation turbulence intensity was a significant contribution to the total. The high-frequency advection Length Scale and integral Turbulent Length Scale exhibited some maxima in the lower shear flow next to the invert. The Turbulent Length Scales decreased along the roller as the fast turbulence was dissipated. Comparison between the longitudinal advection and integral Length Scales indicated that the advection and diffusion were not independent processes in the flow region immediately downstream of the jump toe. The impact of slow fluctuations was large in the free-surface region and relatively smaller in the lower shear flow.
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Turbulence and aeration in hydraulic jumps: free-surface fluctuation and integral Turbulent Scale measurements
Environmental Fluid Mechanics, 2012Co-Authors: Gangfu Zhang, Hang Wang, Hubert ChansonAbstract:In an open channel, a change from a supercritical to subcritical flow is a strong dissipative process called a hydraulic jump. Herein some new measurements of free-surface fluctuations of the impingement perimeter and integral Turbulent time and Length Scales in the roller are presented with a focus on turbulence in hydraulic jumps with a marked roller. The observations highlighted the fluctuating nature of the impingement perimeter in terms of both longitudinal and transverse locations. The results showed further the close link between the production and detachment of large eddies in jump shear layer, and the longitudinal fluctuations of the jump toe. They highlighted the importance of the impingement perimeter as the origin of the developing shear layer and a source of vorticity. The air–water flow measurements emphasised the intense flow aeration. The Turbulent velocity distributions presented a shape similar to a wall jet solution with a marked shear layer downstream of the impingement point. The integral Turbulent Length Scale distributions exhibited a monotonic increase with increasing vertical elevation within 0.2 < Lz/d1 < 0.8 in the shear layer, where Lz is the integral Turbulent Length Scale and d1 the inflow depth, while the integral Turbulent time Scales were about two orders of magnitude smaller than the period of impingement position longitudinal oscillations.
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Air entrainment and turbulence in hydraulic jumps: Free-surface fluctuations and integral Turbulent Scales
2012Co-Authors: Gangfu Zhang, Hang Wang, Hubert ChansonAbstract:An open channel flow can change from a supercritical to subcritical flow with a strong dissipative process: a hydraulic jump. Herein some new measurements of free-surface fluctuations next to the jump toe and integral Turbulent Scales in the roller are presented with a focus on Turbulent hydraulic jumps with a marked roller. The results highlighted the fluctuating nature of the impingement perimeter in terms of both longitudinal and transverse locations. The air-water flow measurements highlighted the intense flow aeration. The Turbulent velocity distributions presented a shape similar to a wall jet solution, and the integral Turbulent Length Scale distributions exhibited a monotonic increase with increasing vertical elevation within 0.2 < Lz/d1 < 0.8 in the shear layer, where Lz is the integral Turbulent Length Scale and d1 the inflow depth.
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turbulence measurements in the bubbly flow region of hydraulic jumps
Experimental Thermal and Fluid Science, 2008Co-Authors: Serhat Kucukali, Hubert ChansonAbstract:A hydraulic jump is characterized by a highly Turbulent flow with macro-Scale vortices, some kinetic energy dissipation and a bubbly two-phase flow structure. New air–water flow measurements were performed in a large-size facility using two types of phase-detection intrusive probes: i.e. single-tip and double-tip conductivity probes. These were complemented by some measurements of free-surface fluctuations using ultrasonic displacement meters. The void fraction measurements showed the presence of an advective diffusion shear layer in which the void fractions profiles matched closely an analytical solution of the advective diffusion equation for air bubbles. The free-surface fluctuations measurements showed large Turbulent fluctuations that reflected the dynamic, unsteady structure of the hydraulic jumps. The measurements of interfacial velocity and turbulence level distributions provided new information on the Turbulent velocity field in the highly-aerated shear region. The velocity profiles tended to follow a wall jet flow pattern. The air–water Turbulent integral time and Length Scales were deduced from some auto- and cross-correlation analyses based upon the method of Chanson [H. Chanson, Bubbly flow structure in hydraulic jump, Eur. J. Mech. B/Fluids 26 (3) (2007) 367–384], providing the Turbulent Scales of the eddy structures advecting the air bubbles in the developing shear layer. The Length Scale Lxz is an integral air–water turbulence Length Scale which characterized the transverse size of the large vortical structures advecting the air bubbles. The experimental data showed that the dimensionless integral Turbulent Length Scale Lxz/d1 was closely related to the inflow depth: i.e. Lxz/d1 = 0.2–0.8, with Lxz increasing towards the free-surface.
