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Thomas B. Gatski - One of the best experts on this subject based on the ideXlab platform.
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Accounting for Reynolds stress and Dissipation Rate anisotropies in inertial and noninertial frames
Physics of Fluids, 1998Co-Authors: T. Jongen, G. Mompean, Thomas B. GatskiAbstract:This paper presents a general procedure for including added effects, such as turbulent Dissipation Rate anisotropies, into algebraic stress formulations in both inertial and noninertial frames of reference. Explicit algebraic stress models, which assume an isotropic turbulent Dissipation Rate, have been developed previously and extended for application to noninertial frames. Independently, anisotropic Dissipation Rate models have also been developed. Recently, an algebraic, anisotropic Dissipation Rate model has been developed and used in conjunction with a full Reynolds stress closure. Unfortunately, in the theoretical formulations used previously for explicit algebraic models, the combination of the algebraic stress and algebraic Dissipation Rate models only appeared possible for inertial frames. The alternative procedure outlined here remedies this problem and allows for the construction of a composite model for both inertial and noninertial frames. This new composite model formulation is tested in homogeneous shear with and without rotation, and in strongly rotating channel and pipe flows using different types of anisotropic Dissipation Rate models.
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analysis and modelling of anisotropies in the Dissipation Rate of turbulence
Journal of Fluid Mechanics, 1997Co-Authors: Charles G. Speziale, Thomas B. GatskiAbstract:The modelling of anisotropies in the Dissipation Rate of turbulence is considered based on an analysis of the exact transport equation for the Dissipation Rate tensor. An algebraic model is systematically derived using integrity bases methods and tensor symmetry properties. The new model differs notably from all previously proposed models in that it depends nonlinearly on the mean velocity gradients. This gives rise to a transport equation for the scalar Dissipation Rate that is of the same general form as the commonly used model with one major exception: the coefficient of the production term is dependent on the invariants of both the rotational and irrotational strain Rates. The relationship between the new model and other recently proposed models is examined in detail. Some basic tests and applications of the model are also provided along with a discussion of the implications for turbulence modelling.
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Modeling the Dissipation Rate in rotating turbulent flows
Studies in Turbulence, 1992Co-Authors: Charles G. Speziale, Rishi Raj, Thomas B. GatskiAbstract:A variety of modifications to the modeled Dissipation Rate transport equation that have been proposed during the past two decades to account for rotational strains are examined. The models are subjected to two crucial test cases: the decay of isotropic turbulence in a rotating frame and homogeneous shear flow in a rotating frame. It is demonstRated that these modifications do not yield substantially improved predictions for these two test cases and in many instances give rise to unphysical behavior. An alternative proposal, based on the use of the tensor Dissipation Rate, is made for the development of improved models.
N. Soulopoulos - One of the best experts on this subject based on the ideXlab platform.
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Scalar Dissipation Rate statistics in turbulent swirling jets
Physics of Fluids, 2016Co-Authors: Viacheslav Stetsyuk, N. Soulopoulos, Yannis Hardalupas, A. M. K. P. TaylorAbstract:The scalar Dissipation Rate statistics were measured in an isothermal flow formed by discharging a central jet in an annular stream of swirling air flow. This is a typical geometry used in swirl-stabilised burners, where the central jet is the fuel. The flow Reynolds number was 29 000, based on the area-averaged velocity of 8.46 m/s at the exit and the diameter of 50.8 mm. The scalar Dissipation Rate and its statistics were computed from two-dimensional imaging of the mixture fraction fields obtained with planar laser induced fluorescence of acetone. Three swirl numbers, S, of 0.3, 0.58, and 1.07 of the annular swirling stream were considered. The influence of the swirl number on scalar mixing, unconditional, and conditional scalar Dissipation Rate statistics were quantified. A procedure, based on a Wiener filter approach, was used to de-noise the raw mixture fraction images. The filtering errors on the scalar Dissipation Rate measurements were up to 15%, depending on downstream positions from the burner exit. The maximum of instantaneous scalar Dissipation Rate was found to be up to 35 s−1, while the mean Dissipation Rate was 10 times smaller. The probability density functions of the logarithm of the scalar Dissipation Rate fluctuations were found to be slightly negatively skewed at low swirl numbers and almost symmetrical when the swirl number increased. The assumption of statistical independence between the scalar and its Dissipation Rate was valid for higher swirl numbers at locations with low scalar fluctuations and less valid for low swirl numbers. The deviations from the assumption of statistical independence were quantified. The conditional mean of the scalar Dissipation Rate, the standard deviation of the scalar Dissipation Rate fluctuations, the weighted probability of occurrence of the mean conditional scalar Dissipation Rate, and the conditional probability are reported.
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scalar Dissipation Rate measurements in a starting jet
arXiv: Fluid Dynamics, 2015Co-Authors: N. Soulopoulos, Y Hardalupas, A. M. K. P. TaylorAbstract:Measurements of the scalar Dissipation Rate are performed in an impulsively started gas jet, using planar laser induced fluorescence. The measurements are well resolved spatially. The deteriorating effect of experimental noise on this experiment is treated with a Wiener filter, which is shown to be applicable to this large-scale inhomogeneous flow. The accuracy of the scalar Dissipation Rate is within $20\%$, as determined from an explicit calculation of the filtering errors. The residual fields that remain after the filtering are analysed in detail and their statistical properties show that these resemble white noise to a good approximation. The level of corrections is minimal for the scalar field but it is of the order of $40\%$ for the scalar Dissipation Rate. An examination of the filtering operation using modeled spectra and the measured spatial resolution shows that the Wiener filter produces errors in the estimate of the scalar Dissipation Rate $\sim30\%$, for Taylor-scale Reynolds number up to 1000. The implications of this modelling are discussed with respect to common experimental situations and point out the relative merits of improving the spatial resolution as compared to improvements in the signal to noise ratio.
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Mixing and scalar Dissipation Rate statistics in a starting gas jet
Physics of Fluids, 2015Co-Authors: N. Soulopoulos, Yannis Hardalupas, Alexander TaylorAbstract:We quantify the temporal development of the mixing field of a starting jet by measuring the mixture fraction and the scalar Dissipation Rate and their statistics in an isothermal, impulsively started, gaseous jet. The scalar measurements are performed using planar laser induced fluorescence and, with appropriate processing of the resulting images, allow scalar Dissipation Rate measurements within 20%. The probability density functions of the mixture fraction, measured within a region of the order of 3 times the Batchelor length scale of the flow, are bimodal and skewed around a well-mixed radial location, which depends on the downstream distance and the time after the start of injection. The instantaneous distributions of the scalar Dissipation Rate reveal regions of high mixing at the jet periphery and at the developing vortex ring. The normalised probability density function (pdf) of the scalar Dissipation Rate at various flow positions and times after the start of injection has the same characteristic shape but differs from the usually suggested lognormal distribution at both low and high Dissipation values; the same, also, holds true for the pdf conditioned on different values of the mixture fraction. The mean of the scalar Dissipation Rate conditional on mixture fraction shows a variation across the mixture fraction range, which differs between flow locations and times after the start of injection; however, at later times and for larger downstream distances the conditional mean between flow locations has similar distributions. Implications of the measurements for the auto-ignition of gaseous jets are examined and demonstRate that near the nozzle exit or at earlier times conditions are un-favourable for auto-ignition.
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Scalar Dissipation Rate measurements in a starting jet
Experiments in Fluids, 2014Co-Authors: N. Soulopoulos, Yannis Hardalupas, Alexander TaylorAbstract:Measurements of the scalar Dissipation Rate are taken in an impulsively started gas jet, using planar laser-induced fluorescence. The measurements are well-resolved spatially. The deteriorating effect of experimental noise on this experiment is treated with a Wiener filter, which is shown to be applicable to this large-scale inhomogeneous flow. The accuracy of the scalar Dissipation Rate is within 20 %, as determined from an explicit calculation of the filtering errors. The residual fields that remain after the filtering are analysed in detail, and their statistical properties show that these resemble white noise to a good approximation. The level of corrections is minimal for the scalar field but it is of the order of 40 % for the scalar Dissipation Rate. An examination of the filtering operation using modelled spectra and the measured spatial resolution shows that the Wiener filter produces errors in the estimate of the scalar Dissipation Rate ∼30 %, for Taylor-scale Reynolds number up to 1,000. The implications of this modelling are discussed with respect to common experimental situations and point out the relative merits of improving the spatial resolution as compared to improvements in the signal-to-noise ratio.
Alexander Taylor - One of the best experts on this subject based on the ideXlab platform.
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Mixing and scalar Dissipation Rate statistics in a starting gas jet
Physics of Fluids, 2015Co-Authors: N. Soulopoulos, Yannis Hardalupas, Alexander TaylorAbstract:We quantify the temporal development of the mixing field of a starting jet by measuring the mixture fraction and the scalar Dissipation Rate and their statistics in an isothermal, impulsively started, gaseous jet. The scalar measurements are performed using planar laser induced fluorescence and, with appropriate processing of the resulting images, allow scalar Dissipation Rate measurements within 20%. The probability density functions of the mixture fraction, measured within a region of the order of 3 times the Batchelor length scale of the flow, are bimodal and skewed around a well-mixed radial location, which depends on the downstream distance and the time after the start of injection. The instantaneous distributions of the scalar Dissipation Rate reveal regions of high mixing at the jet periphery and at the developing vortex ring. The normalised probability density function (pdf) of the scalar Dissipation Rate at various flow positions and times after the start of injection has the same characteristic shape but differs from the usually suggested lognormal distribution at both low and high Dissipation values; the same, also, holds true for the pdf conditioned on different values of the mixture fraction. The mean of the scalar Dissipation Rate conditional on mixture fraction shows a variation across the mixture fraction range, which differs between flow locations and times after the start of injection; however, at later times and for larger downstream distances the conditional mean between flow locations has similar distributions. Implications of the measurements for the auto-ignition of gaseous jets are examined and demonstRate that near the nozzle exit or at earlier times conditions are un-favourable for auto-ignition.
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Scalar Dissipation Rate measurements in a starting jet
Experiments in Fluids, 2014Co-Authors: N. Soulopoulos, Yannis Hardalupas, Alexander TaylorAbstract:Measurements of the scalar Dissipation Rate are taken in an impulsively started gas jet, using planar laser-induced fluorescence. The measurements are well-resolved spatially. The deteriorating effect of experimental noise on this experiment is treated with a Wiener filter, which is shown to be applicable to this large-scale inhomogeneous flow. The accuracy of the scalar Dissipation Rate is within 20 %, as determined from an explicit calculation of the filtering errors. The residual fields that remain after the filtering are analysed in detail, and their statistical properties show that these resemble white noise to a good approximation. The level of corrections is minimal for the scalar field but it is of the order of 40 % for the scalar Dissipation Rate. An examination of the filtering operation using modelled spectra and the measured spatial resolution shows that the Wiener filter produces errors in the estimate of the scalar Dissipation Rate ∼30 %, for Taylor-scale Reynolds number up to 1,000. The implications of this modelling are discussed with respect to common experimental situations and point out the relative merits of improving the spatial resolution as compared to improvements in the signal-to-noise ratio.
Gerard Kiely - One of the best experts on this subject based on the ideXlab platform.
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convective scaling of the average Dissipation Rate of temperature variance in the atmospheric surface layer
Boundary-Layer Meteorology, 1996Co-Authors: Gerard Kiely, John D Albertson, Marc B Parlange, William EichingerAbstract:The flux of sensible heat from the land surface is related to the average Rate of Dissipation of temperature fluctuations in the atmospheric surface layer through the temperature variance budget equation. In many cases it is desirable to estimate the heat flux from measurement or inference of the Dissipation Rate. Here we study how the Dissipation Rate scales with atmospheric stability, using three inertial range methods to calculate the Dissipation Rate: power spectra, second order structure functions, and third order structure functions. Experimental data are analyzed from a pair of field experiments, during which turbulent fluctuations of velocity and temperature were measured over a broad range of neutral and unstable atmospheric flows. It is shown that the temperature Dissipation Rate scales with a single convective power law continuously from near-neutral to strongly unstable stratification. The Dissipation scaling is found to nearly match production in the near-neutral region, but to be consistently lower than production in the more convective regimes. The convective scaling is shown to offer a simplified means of computing sensible heat flux from the Dissipation Rate of temperature variance.
Zhenglun Alan Wei - One of the best experts on this subject based on the ideXlab platform.
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the advantages of viscous Dissipation Rate over simplified power loss as a fontan hemodynamic metric
Annals of Biomedical Engineering, 2018Co-Authors: Zhenglun Alan Wei, Mike Tree, Phillip M Trusty, Shelly Singhgryzbon, Ajit P YoganathanAbstract:Flow efficiency through the Fontan connection is an important factor related to patient outcomes. It can be quantified using either a simplified power loss or a viscous Dissipation Rate metric. Though practically equivalent in simplified Fontan circulation models, these metrics are not identical. Investigation is needed to evaluate the advantages and disadvantages of these metrics for their use in in vivo or more physiologically-accuRate Fontan modeling. Thus, simplified power loss and viscous Dissipation Rate are compared theoretically, computationally, and statistically in this study. Theoretical analysis was employed to assess the assumptions made for each metric and its clinical calculability. Computational simulations were then performed to obtain these two metrics. The results showed that apparent simplified power loss was always greater than the viscous Dissipation Rate for each patient. This discrepancy can be attributed to the assumptions derived in theoretical analysis. Their effects were also delibeRately quantified in this study. Furthermore, statistical analysis was conducted to assess the correlation between the two metrics. Viscous Dissipation Rate and its indexed quantity show significant, strong, linear correlation to simplified power loss and its indexed quantity (p 0.99) under certain assumptions. In conclusion, viscous Dissipation Rate was found to be more advantageous than simplified power loss as a hemodynamic metric because of its lack of limiting assumptions and calculability in the clinic. Moreover, in addition to providing a time-averaged bulk measurement like simplified power loss, viscous Dissipation Rate has spatial distribution contours and time-resolved values that may provide additional clinical insight. Finally, viscous Dissipation Rate could maintain the relationship between Fontan connection flow efficiency and patient outcomes found in previous studies. Consequently, future Fontan hemodynamic studies should calculate both simplified power loss and viscous Dissipation Rate to maintain ties to previous studies, but also provide the most accuRate measure of flow efficiency. Additional attention should be paid to the assumptions required for each metric.