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- Author or Editor: D. S. Henn x
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Abstract
Numerical calculations of two-dimensional convection between flat plates at high Rayleigh numbers are presented. They do not show the large cell aspect ratios observed by Rothermel and Agee in their numerical calculations at high Rayleigh numbers. The present results appear to be insensitive to grid resolution, the conventional test of numerical accuracy, large amplitude random perturbations and the numerical representation of the diffusion term. We therefore suggest that the mechanism observed by Rothermel and Agee, which produces increasingly larger aspect ratio cells with increasing Rayleigh number, is strongly influenced by the numerical method used to solve the equations.
Abstract
Numerical calculations of two-dimensional convection between flat plates at high Rayleigh numbers are presented. They do not show the large cell aspect ratios observed by Rothermel and Agee in their numerical calculations at high Rayleigh numbers. The present results appear to be insensitive to grid resolution, the conventional test of numerical accuracy, large amplitude random perturbations and the numerical representation of the diffusion term. We therefore suggest that the mechanism observed by Rothermel and Agee, which produces increasingly larger aspect ratio cells with increasing Rayleigh number, is strongly influenced by the numerical method used to solve the equations.
Abstract
A series of large-eddy simulators of free and sheared convective flow between moving flat plates is presented. Results for free convection are compared with laboratory data. The ratio of friction velocity to the convective velocity sale,u */w *,is identified as an important parameter in sheared convective flow determining the formation of longitudinal rolls. Rolls are found for u */w * ≥ 0.35, with aspect ratio decreasing as this parameter increases. It is shown that, in this regime, two-dimensional simulations with a proper choice of roll orientation and turbulence length-scale can produce correct velocity variances and roll aspect ratio.
Abstract
A series of large-eddy simulators of free and sheared convective flow between moving flat plates is presented. Results for free convection are compared with laboratory data. The ratio of friction velocity to the convective velocity sale,u */w *,is identified as an important parameter in sheared convective flow determining the formation of longitudinal rolls. Rolls are found for u */w * ≥ 0.35, with aspect ratio decreasing as this parameter increases. It is shown that, in this regime, two-dimensional simulations with a proper choice of roll orientation and turbulence length-scale can produce correct velocity variances and roll aspect ratio.
Abstract
The Gaussian puff model framework is extended to provide a description of velocity shear distortion effects. An efficient splitting-merging algorithm is presented so that a maximum puff size can be specified for a calculation. This localizes the Gaussian puffs so that they represent only a limited region of the flow and the accuracy of the representation is therefore controlled. The model is shown to perform well on the deformational flow of Smolarkiewicz, providing an accurate calculation of the highly distorted solution. The extended puff methodology allows practical applications of an efficient Lagrangian dispersion technique in complex flow fields.
Abstract
The Gaussian puff model framework is extended to provide a description of velocity shear distortion effects. An efficient splitting-merging algorithm is presented so that a maximum puff size can be specified for a calculation. This localizes the Gaussian puffs so that they represent only a limited region of the flow and the accuracy of the representation is therefore controlled. The model is shown to perform well on the deformational flow of Smolarkiewicz, providing an accurate calculation of the highly distorted solution. The extended puff methodology allows practical applications of an efficient Lagrangian dispersion technique in complex flow fields.
Abstract
The boundary-layer eddy structure under conditions similar to the cold-air outbreak of GALE IOP 2 is studied using numerical simulations. The simulations are run in two basic modes: a quasi-two-dimensional version that takes advantage of the observed “cloud-street” character of the flow, and a fully three-dimensional, unsteady simulation on a limited domain where periodic conditions are assumed to prevail. The two-dimensional simulation exhibits a cloud structure similar to that observed when the surface fluxes agree with the aircraft measurements. This requires very different values of effective surface roughness for temperature and humidity, which is unlikely to have been assumed in the absence of data. The three-dimensional simulation reveals that even when the eddy structure on this severely limited domain does not exhibit a dominant two-dimensional roll structure, the average turbulent statistics are quite consistent with those from the two-dimensional simulation. It is argued that a larger domain than can be readily used is needed to see a distinct cloud street pattern.
Abstract
The boundary-layer eddy structure under conditions similar to the cold-air outbreak of GALE IOP 2 is studied using numerical simulations. The simulations are run in two basic modes: a quasi-two-dimensional version that takes advantage of the observed “cloud-street” character of the flow, and a fully three-dimensional, unsteady simulation on a limited domain where periodic conditions are assumed to prevail. The two-dimensional simulation exhibits a cloud structure similar to that observed when the surface fluxes agree with the aircraft measurements. This requires very different values of effective surface roughness for temperature and humidity, which is unlikely to have been assumed in the absence of data. The three-dimensional simulation reveals that even when the eddy structure on this severely limited domain does not exhibit a dominant two-dimensional roll structure, the average turbulent statistics are quite consistent with those from the two-dimensional simulation. It is argued that a larger domain than can be readily used is needed to see a distinct cloud street pattern.
Abstract
An improved method for representing the small-scale structure of a turbulent scalar field using fractal recursion techniques is described. The model generalizes the fractal successive refinement method described by Sykes and Gabruk to include a more realistic description of the pseudodissipation field. that is, the square of the scalar gradient. Turbulent dissipation fields are known to be multifractal, so a multifractal generation technique has been incorporated into the fractal refinement model to yield a scalar field with isosurfaces but with a multifractal pseudodissipation field.
The model fields are compared with realizations from large-eddy simulations of turbulent scalar dispersion and shown to provide improved agreement with the small-scale structure. The simple combination of fractal and multifractal properties employed in the model also provides insight into the structure of the random scalar field. Finally, the generation technique is completely localized in physical space and is therefore applicable to inhomogeneous fields.
Abstract
An improved method for representing the small-scale structure of a turbulent scalar field using fractal recursion techniques is described. The model generalizes the fractal successive refinement method described by Sykes and Gabruk to include a more realistic description of the pseudodissipation field. that is, the square of the scalar gradient. Turbulent dissipation fields are known to be multifractal, so a multifractal generation technique has been incorporated into the fractal refinement model to yield a scalar field with isosurfaces but with a multifractal pseudodissipation field.
The model fields are compared with realizations from large-eddy simulations of turbulent scalar dispersion and shown to provide improved agreement with the small-scale structure. The simple combination of fractal and multifractal properties employed in the model also provides insight into the structure of the random scalar field. Finally, the generation technique is completely localized in physical space and is therefore applicable to inhomogeneous fields.
Abstract
Two-dimensional numerical computations of the developing boundary layer with a positive surface heat flux are presented. The model incorporates moisture phase-change effects; we are particularly interested in the cloud-street formation. The results show that cloud streets with large aspect ratios of about 10 can develop under certain conditions, and cloud top entrainment is identified as a crucial feature. Runs without phase change indicate roll merging to a certain extent, but do not product aspect ratios greater than 4.
Abstract
Two-dimensional numerical computations of the developing boundary layer with a positive surface heat flux are presented. The model incorporates moisture phase-change effects; we are particularly interested in the cloud-street formation. The results show that cloud streets with large aspect ratios of about 10 can develop under certain conditions, and cloud top entrainment is identified as a crucial feature. Runs without phase change indicate roll merging to a certain extent, but do not product aspect ratios greater than 4.
Abstract
Near-surface air temperature observations often have periods of missing data, and many applications using these datasets require filling in all missing periods. Multiple methods are available to fill missing data, but the comparative accuracy of these approaches has not been assessed. In this comparative study, five techniques were used to fill in missing temperature data: spatiotemporal correlations in the form of empirical orthogonal functions (EOFs), time series diurnal interpolation, and three variations of lapse rate–based filling. The method validation used sets of hourly surface temperature observations in complex terrain from five regions. The most accurate method for filling missing data depended on the number of available stations and the number of hours of missing data. Spatiotemporal correlations using EOF reconstruction were most accurate provided that at least 16 stations were available. Temporal interpolation was the most accurate method when only one or two stations were available or for 1-h gaps. Lapse rate–based filling was most accurate for intermediate numbers of stations. The accuracy of the lapse rate and EOF methods was found to be sensitive to the vertical separation of stations and the degree of correlation between them, which also explained some of the regional differences in performance. Horizontal distance was less significantly correlated with method performance. From these findings, guidelines are presented for choosing a filling method based on the duration of the missing data and the number of stations.
Abstract
Near-surface air temperature observations often have periods of missing data, and many applications using these datasets require filling in all missing periods. Multiple methods are available to fill missing data, but the comparative accuracy of these approaches has not been assessed. In this comparative study, five techniques were used to fill in missing temperature data: spatiotemporal correlations in the form of empirical orthogonal functions (EOFs), time series diurnal interpolation, and three variations of lapse rate–based filling. The method validation used sets of hourly surface temperature observations in complex terrain from five regions. The most accurate method for filling missing data depended on the number of available stations and the number of hours of missing data. Spatiotemporal correlations using EOF reconstruction were most accurate provided that at least 16 stations were available. Temporal interpolation was the most accurate method when only one or two stations were available or for 1-h gaps. Lapse rate–based filling was most accurate for intermediate numbers of stations. The accuracy of the lapse rate and EOF methods was found to be sensitive to the vertical separation of stations and the degree of correlation between them, which also explained some of the regional differences in performance. Horizontal distance was less significantly correlated with method performance. From these findings, guidelines are presented for choosing a filling method based on the duration of the missing data and the number of stations.
Abstract
Detailed statistics of the fluctuating concentration field produced by large-eddy simulations (LES) of the chemically reactive mixing of two species in a convectively driven mixed layer are presented. The effect of the turbulent mixing on the effective reaction rate between the species is analysed. The segregation between the species is shown to be significant for fast reactions, and therefore correct model predictions of the evolution of the species concentration requires an estimate of the segregation coefficient. Some simple modeling concepts for one-point second-order turbulence closure schemes are examined and compared with the LES results. The results are a promising indication that second-order closure schemes can be extended to provide a practical calculation of the turbulent mixing effects on fast chemical reactions.
Abstract
Detailed statistics of the fluctuating concentration field produced by large-eddy simulations (LES) of the chemically reactive mixing of two species in a convectively driven mixed layer are presented. The effect of the turbulent mixing on the effective reaction rate between the species is analysed. The segregation between the species is shown to be significant for fast reactions, and therefore correct model predictions of the evolution of the species concentration requires an estimate of the segregation coefficient. Some simple modeling concepts for one-point second-order turbulence closure schemes are examined and compared with the LES results. The results are a promising indication that second-order closure schemes can be extended to provide a practical calculation of the turbulent mixing effects on fast chemical reactions.
