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Seyed G. Saddoughi - One of the best experts on this subject based on the ideXlab platform.
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Local Isotropy in complex turbulent boundary layers at high reynolds number
Journal of Fluid Mechanics, 1997Co-Authors: Seyed G. SaddoughiAbstract:To continue our tests of the Local-Isotropy predictions of Kolmogorov's universal equilibrium theory in shear flows, we have taken hot-wire measurements of the velocity fluctuations in complex turbulent boundary layers at several Reynolds numbers. We have studied the plane-of-symmetry flow upstream of a 4 ft diameter, 6 ft long circular cylinder placed with its axis vertical in the zero-pressure-gradient turbulent boundary layer of the test-section ceiling in the 80 ft x 120 ft Full-Scale Aerodynamics Facility at NASA Ames Research Center. In the present experiments, the pressure rises strongly as the obstacle is approached and in and near the plane of symmetry of the flow the boundary layer is influenced by the effects of lateral divergence. In addition to the basic mean shear, ∂U/∂γ, the extra mean strain rates are ∂U/∂x, ∂V/∂γ and ∂W/∂z. During our experiments a full-scale F-18 fighter aircraft, set at an angle of attack of 50°, was present in the central region of the working section. To identify the effects of the aircraft on the boundary-layer characteristics upstream of the cylinder, we have also taken measurements when the wind tunnel was empty.
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Local Isotropy in turbulent boundary layers at high reynolds number
Journal of Fluid Mechanics, 1994Co-Authors: Seyed G. Saddoughi, Srinivas V. VeeravalliAbstract:To test the Local-Isotropy predictions of Kolmogorov's universal equilibrium theory, we have taken hot-wire measurements of the velocity fluctuations in the test-section-ceiling boundary layer of the 80×120 foot Full-Scale Aerodynamics Facility at NASA Ames Research Center, the world's largest wind tunnel. The maximum Reynolds numbers based on momentum thickness, R θ , and on Taylor microscale, R λ , were approximately 370 000 and 1450 respectively. These are the largest ever attained in laboratory boundary-layer flows. The boundary layer develops over a rough surface, but the Reynolds-stress profiles agree with canonical data sufficiently well for present purposes
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Local Isotropy in distorted turbulent boundary layers at high reynolds number
Stanford Univ. Annual Research Briefs 1993, 1993Co-Authors: Seyed G. SaddoughiAbstract:This is a report on the continuation of our experimental investigations of the hypothesis of Local Isotropy in shear flows. This hypothesis, which states that at sufficiently high Reynolds numbers the small-scale structures of turbulent motions are independent of large-scale structures and mean deformations, has been used in theoretical studies of turbulence and computational methods such as large-eddy simulation. Since Kolmogorov proposed his theory, there have been many experiments, conducted in wakes, jets, mixing layers, a tidal channel, and atmospheric and laboratory boundary layers, in which attempts have been made to verify - or refute - the Local-Isotropy hypothesis. However, a review of the literature over the last five decades indicated that, despite all these experiments in shear flows, there was no consensus in the scientific community regarding this hypothesis, and, therefore, it seemed worthwhile to undertake a fresh experimental investigation into this question.
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Local Isotropy in distorted turbulent boundary layers at high reynolds number
Annual Research Briefs 1992, 1993Co-Authors: Seyed G. SaddoughiAbstract:By Seyed G. Saddoughi1. Motivation and backgroundThis is a report on the continuation of our experimental investigations (Sad-doughi 1993; Saddoughi & Veeravalli 1993) of the hypothesis of Local Isotropy inshear flows. This hypothesis, which states that at sufficiently high Reynolds num-bers the small-scale structures of turbulent motions are independent of large-scalestructures and mean deformations (Kolmogorov 1941), has been used in theoreticalstudies of turbulence and computational methods such as large-eddy simulation.Since Kolmogorov proposed his theory, there have been many experiments, con-ducted in wakes, jets, mixing layers, a tidal channel, and atmospheric and labora-tory boundary layers, in which attempts have been made to verify - or refute -the Local-Isotropy hypothesis. However, a review oi the literature over the last fivedecades indicated that, despite all these experiments in shear flows, there was noconsensus in the scientific community regarding this hypothesis, and, therefore, itseemed worthwhile to undertake a fresh experimental investigation into this ques-tion.i.i Plane boundary layerIn our previous reports, we presented hot-wire measurements of the velocityfluctuations in the test-section-ceiling boundary layer of the 80- by 120-foot Full-Scale Aerodynamics Facility at NASA Ames Research Center, the world's largestwind tunnel. At our measurement location, the boundary-layer thickness, 6, wasabout 1.1 m, and the maximum Reynolds numbers based on momentum thickness,Ra, and on Taylor microscale, Rx, were approximately 370,000 and 1,450 respec-tively. These were the largest ever attained in laboratory boundary-layer flows.The boundary layer developed over a rough surface, but the Reynolds-stress pro-files agreed with canonical data sufficiently well for our purposes. Spectral andstructure-function relations for isotropic turbulence were used to test the Local-Isotropy hypothesis, and our results established the condition under which LocalIsotropy can be expected.Here we use a Cartesian co-ordinate system xi = (x,y,z) with x-axis alongthe flow direction, y-axis normal to the solid surface, and z-axis in the span-wise direction. The respective mean-velocity components in these directions areUi = (U, V, W), and the fluctuating components are ui = (u, v, w). Overbars de-note time averages.Our plane boundary-layer data showed that, to within the accuracy of measure-ment, the shear-stress co-spectral density El_(kl ), which is the most sensitive indi-cator of Local Isotropy, fell to zero at a wavenumber about a decade larger than thatPf_lOlil)lNG PAGE BLANK NOT FILMED PAt li_'_';i_":'":"_'__ .....
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A Note on Local Isotropy in High-Reynolds-Number Turbulence
New Approaches and Concepts in Turbulence, 1993Co-Authors: Srinivas V. Veeravalli, Seyed G. Saddoughi, Alexander A. Praskovsky, Peter BradshawAbstract:Detailed measurements of the velocity field were made in the wall boundary layer of the 80’ by 120’ facility at NASA Ames. The Reynolds Number Rλ, based on the Taylor microscale λ at the measurement location, was approximately 1450, one the largest attained in laboratory flows. The data indicate that to within measurement accuracy, the w-spectrum follows, but the v-spectrum deviates from, the isotropic relation in the inertial subrange. No definite statement can be made regarding Local Isotropy for the dissipating scales because the spectral measurements were contaminated by high-frequency electrical noise, but it appears that the inertial-subrange anIsotropy persist in the dissipation region.
Nelson Luis Dias - One of the best experts on this subject based on the ideXlab platform.
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reply to a comment by r j smalley and r a antonia on the Local Isotropy hypothesis and the turbulent kinetic energy dissipation rate in the atmospheric surface layer october b 2004 130 2733 2752
Quarterly Journal of the Royal Meteorological Society, 2006Co-Authors: Marcelo Chamecki, Nelson Luis DiasAbstract:In response to Smalley and Antonia's comment, we agree that our statement that closer-to-zero velocity–velocity-increment correlation for larger turbulence intensity values cannot be taken as a measure of homogeneity. We also explain that, although not aware that the velocity–velocity-increment correlation cannot be zero at finite r1, we concentrated most of our analysis on its tendency towards zero with decreasing r1, since this limit is well known for homogeneous turbulence. Copyright © 2006 Royal Meteorological Society
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the Local Isotropy hypothesis and the turbulent kinetic energy dissipation rate in the atmospheric surface layer
Quarterly Journal of the Royal Meteorological Society, 2004Co-Authors: Marcelo Chamecki, Nelson Luis DiasAbstract:We test the applicability of the Local Isotropy hypothesis to surface-layer turbulent flow; turbulent velocities measured with a three-dimensional sonic anemometer are used for this purpose, and the predictions of Local Isotropy for the spectra, second- and third-order structure functions are assessed against measured data. Also investigated are scale interactions via the correlation between velocities and velocity increments, and the ability of isotropic spectral models to reproduce measured spectra. In general, second-order structure functions display a narrower inertial range than the corresponding spectra; both the known effects of path-averaging and the predictions of the spectral models show that the sonic anemometer is unable to resolve the whole inertial range, even at a measurement frequency of 60 Hz. We confirm previous results that unstable runs tend to be more isotropic, but find that, for third-order statistics, Isotropy does not hold well for the data analysed. Turbulence intensity, and not atmospheric stability, plays a determining role on the correlation coefficient between velocities and velocity increments. The observed anisotropic behaviour has important implications for the calculation of the turbulent kinetic energy dissipation rate from Kolmogorov's four-fifths law, whose estimates are consistently smaller than those from the inertial range of the spectrum or the structure functions. Copyright © 2004 Royal Meteorological Society
R. A. Antonia - One of the best experts on this subject based on the ideXlab platform.
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towards Local Isotropy of higher order statistics in the intermediate wake
Experiments in Fluids, 2016Co-Authors: R. A. Antonia, L. Djenidi, S L Tang, Luminita Danaila, Tongming Zhou, Y ZhouAbstract:In this paper, we assess the Local Isotropy of higher-order statistics in the intermediate wake region. We focus on normalized odd moments of the transverse velocity derivatives, $${M_{2n + 1}}(\partial u/\partial z) = {{\overline{{{(\partial u/\partial z)}^{2n + 1}}} }}/{{{{\overline{{{(\partial u/\partial z)}^2}} }^{(2n + 1)/2}}}}$$ and $${N_{2n + 1}}(\partial u/\partial y) = {{\overline{{{(\partial u/\partial y)}^{2n + 1}}} }}/{{{{\overline{{{(\partial u/\partial y)}^2}} }^{(2n + 1)/2}}}}$$ , which should be zero if Local Isotropy is satisfied (n is a positive integer). It is found that the relation $$M_{2n+1}(\partial u/\partial z) \sim R_\lambda ^{-1}$$ is supported reasonably well by hot-wire data up to the seventh order ( $$n=3$$ ) on the wake centreline, although it is also dependent on the initial conditions. The present relation $$N_{3}(\partial u/\partial y) \sim R_\lambda ^{-1}$$ is obtained more rigorously than that proposed by Lumley (Phys Fluids 10:855–858, 1967) via dimensional arguments. The effect of the mean shear at locations away from the wake centreline on $$M_{2n+1}(\partial u/\partial z)$$ and $$N_{2n+1}(\partial u/\partial y)$$ is addressed and reveals that, although the non-dimensional shear parameter is much smaller in wakes than in a homogeneous shear flow, it has a significant effect on the evolution of $$N_{2n+1}(\partial u/\partial y)$$ in the direction of the mean shear; its effect on $$M_{2n+1}(\partial u/\partial z)$$ (in the non-shear direction) is negligible.
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towards Local Isotropy of higher order statistics in wakes
5th International Conference on Jets Wakes and Separated Flows: ICJWSF2015, 2016Co-Authors: R. A. Antonia, L. Djenidi, S L Tang, Luminita Danaila, Y Zhou, Tongming ZhouAbstract:In this paper, we test the Local Isotropy of higher order statistics in the intermediate wake region. We focus on normalized odd moments of the transverse velocity derivatives, \({M_{2n + 1}}(\partial u/\partial z) = {{\overline{{{(\partial u/\partial z)}^{2n + 1}}} }}/{{{{\overline{{{(\partial u/\partial z)}^2}} }^{(2n + 1)/2}}}}\) on the wake centreline, which should be zero if Local Isotropy is satisfied (n is a positive integer). It is found that the relation \(M_{2n+1}(\partial u/\partial z) \sim R_\lambda ^{-1}\) is supported reasonably well by hot-wire data up to the seventh-order (\(n=3\)), although it is also dependent on the initial conditions. In particular, the present data show that the higher the order (e.g. fifth- or seventh-order), the higher \(R_\lambda \) must be for Local Isotropy to be satisfied (i.e. \(M_{2n+1}(\partial u/\partial z)=0\)).
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statistics of the turbulent kinetic energy dissipation rate and its surrogates in a square cylinder wake flow
Physics of Fluids, 2014Co-Authors: N. Lefeuvre, L. Djenidi, F. Thiesset, R. A. AntoniaAbstract:A numerical simulation based on the lattice Boltzmann method is carried out in the wake of a square cylinder with the view to investigating possible surrogates for the instantaneous turbulent kinetic energy dissipation rate, e, as well as its mean value, e¯. Various surrogate approximations of e, based on Local Isotropy (eiso), Local axisymmetry along the streamwise direction x (ea, x) and the transverse direction y (ea, y), Local homogeneity (ehom), and homogeneity in the transverse plane, (e4x), are assessed. All the approximations are in agreement with e¯ when the distance downstream of the obstacle is larger than about 40 diameters. Closer to the obstacle, the agreement remains reasonable only for e¯a,x, e¯hom and e¯4x. The probability density functions (PDF) and joint PDFs of e and its surrogates show that e4x correlates best with e while eiso and ehom present the smallest correlation. The results indicate that e4x is a very good surrogate for e and can be used for correctly determining the behaviour...
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dynamical effect of the total strain induced by the coherent motion on Local Isotropy in a wake
Journal of Fluid Mechanics, 2013Co-Authors: Fabien Thiesset, Luminita Danaila, R. A. AntoniaAbstract:We assess the extent to which Local Isotropy (LI) holds in a wake flow for different initial conditions, which may be geometrical (the shape of the bluff body which creates the wake) and hydrodynamical (the Reynolds number), as a function of the dynamical effects of the large-scale forcing (the mean strain, $ \overline{S} $ , combined with the strain induced by the coherent motion, $\tilde {S} $ ). LI is appraised through either classical kinematic tests or phenomenological approaches. In this respect, we reanalyse existing LI criteria and formulate a new Isotropy criterion based on the ratio between the turbulence strain intensity and the total strain ( $ \overline{S} + \tilde {S} $ ). These criteria involve either time-averaged or phase-averaged quantities, thus providing a deeper insight into the dynamical aspect of these flows. They are tested using hot wire data in the intermediate wake of five types of obstacles (a circular cylinder, a square cylinder, a screen cylinder, a normal plate and a screen strip). We show that in the presence of an organized motion, Isotropy is not an adequate assumption for the large scales but may be satisfied over a range of scales extending from the smallest dissipative scale up to a scale which depends on the total strain rate that characterizes the flow. The Local value of this scale depends on the particular nature of the wake and the phase of the coherent motion. The square cylinder wake is the closest to Isotropy whereas the least Locally isotropic flow is the screen strip wake. For locations away from the axis, the study is restricted to the circular cylinder only and reveals that LI holds at scales smaller than those that apply at the wake centreline. Arguments based on self-similarity show that in the far wake, the strength of the coherent motion decays at the same rate as that of the turbulent motion. This implies the persistence of the same degree of anIsotropy far downstream, independently of the scale at which anIsotropy is tested.
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comment on the Local Isotropy hypothesis and the turbulent kinetic energy dissipation rate in the atmospheric surface layer by m chamecki and n l dias october b 2004 130 2733 2752
Quarterly Journal of the Royal Meteorological Society, 2006Co-Authors: R J Smalley, R. A. AntoniaAbstract:The velocity–velocity-increment correlation is shown to be an incorrect method for determining the interaction between the energy-containing and inertial-range scales of a turbulent flow. As a consequence, the correlation cannot be used as a means for testing the Local Isotropy hypothesis. Copyright © 2006 Royal Meteorological Society
K R Sreenivasan - One of the best experts on this subject based on the ideXlab platform.
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on Local Isotropy of passive scalars in turbulent shear flows
Proceedings of The Royal Society A: Mathematical Physical and Engineering Sciences, 1991Co-Authors: K R SreenivasanAbstract:An assessment of Local Isotropy and universality in high-Reynolds-number turbulent flows is presented. The emphasis is on the behaviour of passive scalar fields advected by turbulence, but a brief review of the relevant facts is given for the turbulent motion itself. Experiments suggest that Local Isotropy is not a natural concept for scalars in shear flows, except, perhaps, at such extreme Reynolds numbers that are of no practical relevance on Earth. Yet some type of scaling exists even at moderate Reynolds numbers. The relation between these two observations is a theme of this paper.
Marcelo Chamecki - One of the best experts on this subject based on the ideXlab platform.
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reply to a comment by r j smalley and r a antonia on the Local Isotropy hypothesis and the turbulent kinetic energy dissipation rate in the atmospheric surface layer october b 2004 130 2733 2752
Quarterly Journal of the Royal Meteorological Society, 2006Co-Authors: Marcelo Chamecki, Nelson Luis DiasAbstract:In response to Smalley and Antonia's comment, we agree that our statement that closer-to-zero velocity–velocity-increment correlation for larger turbulence intensity values cannot be taken as a measure of homogeneity. We also explain that, although not aware that the velocity–velocity-increment correlation cannot be zero at finite r1, we concentrated most of our analysis on its tendency towards zero with decreasing r1, since this limit is well known for homogeneous turbulence. Copyright © 2006 Royal Meteorological Society
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the Local Isotropy hypothesis and the turbulent kinetic energy dissipation rate in the atmospheric surface layer
Quarterly Journal of the Royal Meteorological Society, 2004Co-Authors: Marcelo Chamecki, Nelson Luis DiasAbstract:We test the applicability of the Local Isotropy hypothesis to surface-layer turbulent flow; turbulent velocities measured with a three-dimensional sonic anemometer are used for this purpose, and the predictions of Local Isotropy for the spectra, second- and third-order structure functions are assessed against measured data. Also investigated are scale interactions via the correlation between velocities and velocity increments, and the ability of isotropic spectral models to reproduce measured spectra. In general, second-order structure functions display a narrower inertial range than the corresponding spectra; both the known effects of path-averaging and the predictions of the spectral models show that the sonic anemometer is unable to resolve the whole inertial range, even at a measurement frequency of 60 Hz. We confirm previous results that unstable runs tend to be more isotropic, but find that, for third-order statistics, Isotropy does not hold well for the data analysed. Turbulence intensity, and not atmospheric stability, plays a determining role on the correlation coefficient between velocities and velocity increments. The observed anisotropic behaviour has important implications for the calculation of the turbulent kinetic energy dissipation rate from Kolmogorov's four-fifths law, whose estimates are consistently smaller than those from the inertial range of the spectrum or the structure functions. Copyright © 2004 Royal Meteorological Society