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  • 1
    Electronic Resource
    Electronic Resource
    On temperature spectra in grid turbulence (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 6 (1994), S. 306-312 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: This paper reports wind tunnel measurements of passive temperature spectra in decaying grid generated turbulence both with and without a mean transverse temperature gradient. The measurements cover a turbulence Reynolds number range 60〈Rel〈1200. It is shown that for a linear temperature profile the temperature spectrum has a scaling exponent close to −5/3 and that its dilation is consistent with Kolmogorov–Obukhov–Corrsin (KOC) scaling, i.e., the width of the scaling region increases approximately as Re3/4l. The remarkably low Reynolds number onset (Rel∼70) of Kolmogorov–Obukhov–Corrsin scaling in isotropic grid turbulence is contrasted to the case of scalars in (anisotropic) shear flows where KOC scaling only appears at very high-Reynolds numbers (Rel∼105). It is also shown that when the temperature fluctuations are inserted very close to the grid in the absence of a gradient (by means of a mandoline), the temperature spectrum behaves in a similar way to the linear gradient case, i.e., a spectrum with a scaling exponent close to −5/3 is observed, a result noted earlier in heated grid experiments. However, when the scalar is inserted farther downstream of the grid (in the fully developed turbulence), the spectrum has a scaling region of −1.3 and its dilation with Re is less well defined than for the other cases. The velocity spectrum is also shown to have a scaling region, of slope −1.3, and its onset occurs at higher Reynolds number than for the case of the scalar experiments that exhibit the KOC scaling.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.868085
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  • 2
    Electronic Resource
    Electronic Resource
    Conditionally filtered scalar dissipation, scalar diffusion, and velocity in a turbulent jet (2002)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 14 (2002), S. 2170-2185 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Conditionally filtered conserved scalar (temperature) dissipation (CFD), diffusion, and conditionally filtered velocity are studied experimentally in the fully developed region of a turbulent jet with a jet Reynolds number Rej=UjDj/ν of 40 000. These variables are the unclosed terms in the transport equation of the conserved scalar filtered density function. One-dimensional box filters of widths Δ ranging from 30 to 496 scalar dissipation scales (ηφ) as well as a two-dimensional box filter (Δ/ηφ=90) which consists of three discrete sensors are used to obtain filtered variables. Taylor's hypothesis is used to perform the streamwise filtering. The means of these conditionally filtered variables conditional on the resolvable-scale scalar fluctuations 〈φ〉L and the subgrid-scale (SGS) variance 〈φ″2〉L indicate two regimes of the SGS scalar mixing. For large SGS variance (〈φ″2〉L/〈φ″2〉〉1) the SGS scalar exhibits similarities to scalars in initially binary mixing at early times, suggesting existence of diffusion-layer-like structure and nonequilibrium SGS scalar. The nonequilibrium is further evidenced by the observed large SGS mechanical-to-scalar time scale ratio and large SGS variance production-to-dissipation ratio. These characteristics have not been previously observed in fully developed flows. For small SGS variance the conditionally filtered variables indicate well-mixed SGS scalar. The results also show that the SGS advection is only significant at large SGS variance. Our measurements also suggest that the dependence of the scalar dissipation (and the filtered dissipation) on large-scale scalar fluctuations is caused primarily by the fluctuations in the spectral transfer and that the conditional CFD is directly affected mostly by scales near the filter scale. The present study suggests that the nonequilibrium effects related to sheet-like structures are important for modeling the conditionally filtered variables. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1481744
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  • 3
    Electronic Resource
    Electronic Resource
    Turbulence suppression in a jet by means of a fine ring (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 6 (1994), S. 328-333 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Studied are the effects on the velocity field of a fine circular wire ring placed axisymmerically downstream of an air jet of Reynolds number 1.4×104. When placed close to the jet exit (x/Dj〈3 where Dj is the jet diameter) the ring caused a pronounced reduction in the rms longitudinal velocity fluctuations, the maximum reduction (of approximately 30% based on the peak value) occurring when the ring is very close to the jet (x/Dj=0.07) and in the shear layer although rings of smaller diameter located in the core also produced the same qualitative effect. The ring also caused a shift in the virtual origin of the mean flow and an increase of the momentum thickness of the shear layer. There resulted a reduction of the magnitude of the fundamental roll-up frequency, and it saturated later than for the undisturbed flow. The spatial growth rate of the disturbances in the shear layer were also drastically reduced. Spectra showed that vortex pairing was inhibited by the ring and there was an earlier transition to fully developed turbulence. These effects are attributed to the ring causing a change in the form of the mean velocity profile in the shear layer. The effects of vortex shedding by the ring were discounted by showing that the results did not qualitatively change when the ring wire Reynolds number, Rew, was varied from sub to super critical values (30〈Rew〈120). These findings are compared with previous results, using acoustic excitation to reduce the turbulence intensity in a jet, with which they have much in common.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.868087
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  • 4
    Electronic Resource
    Electronic Resource
    Measurements of conserved scalar filtered density function in a turbulent jet (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 13 (2001), S. 2923-2937 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Conserved scalar (temperature) filtered density function (FDF) is studied experimentally in the fully developed region of a turbulent jet with Taylor-microscale Reynolds numbers of 293 and 190. We obtain the FDFs using one-dimensional box filters of widths Δ ranging from 30 to 248 scalar dissipation scales (ηφ) as well as a two-dimensional box filter (Δ/ηφ=90) which consists of three discrete sensors. Taylor's hypothesis is used to perform streamwise filtering operations. The mean conserved scalar FDF conditioned on the resolvable-scale scalar fluctuations 〈φ〉L and the subgrid scale (SGS) variance 〈φ″2〉L (log-normally distributed) is found to be bimodal when 〈φ″2〉L/〈φ″2〉 is large, indicating that the conditional SGS mixing is nearly binary. For small 〈φ″2〉L/〈φ″2〉 (〈1) the conditional FDF is approximately Gaussian. The kurtosis of the conditional FDF decreases with increasing SGS variance and is independent of the filter widths for large SGS variance. The bimodal distribution can be symmetric or asymmetric depending on the curvature of the resolvable-scale scalar. As the SGS variance increases, the conditional scalar differences for separations comparable to the filter widths also change from Gaussian to bimodal distributions. At the same time the conditional SGS scalar changes from approximately isotropic to strongly anisotropic. The results show that the contributions to the bimodal distributions come primarily from the SGS scales comparable to the filter width. It is remarkable that similarities exist between the bimodal conditional FDFs obtained here in a fully developed jet and bimodal probability density functions observed in the early stages of binary scalar mixing. The present study provides a physical basis for the assumed FDF method for conserved scalars used in large-eddy simulation of turbulent combustion. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1402171
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  • 5
    Electronic Resource
    Electronic Resource
    On passive scalar derivative statistics in grid turbulence (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 6 (1994), S. 2165-2176 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The probability density function, and related statistics, of scalar (temperature) derivative fluctuations in decaying grid turbulence with an imposed cross-stream, passive linear temperature profile, is studied for a turbulence Reynolds number range, Rel, varying from 50 to 1200, (corresponding to a Taylor Reynolds number range 30〈Rλ〈130). It is shown that the temperature derivative skewness in the direction of the mean gradient, Sθy has a value of 1.8±0.2 (twice the value observed in shear flows), and has no significant variation with Reynolds number. The ratio of the temperature derivative standard deviation along the gradient to that normal to it is approximately 1.2±0.1 also, with no variation with Re. The kurtosis of the derivatives increases approximately as Re0.2l. The results show that the rare, intense temperature deviations that produce the skewed scalar derivative, increase in frequency, but their area fraction (of the total field) becomes smaller as the Reynolds number increases. Thus, since Sθy remains constant, they become sharper and more intense, occurring deeper in the tails of the probability density function. Measurements in a thermal mixing layer, which has a nonlinear mean temperature profile, are also presented, and these show a similar value of Sθy to the linear profile case. The experiments broadly confirm the two-dimensional numerical simulations of Holzer and Siggia [Phys. Fluids (in press)], as well as other recent simulations, although there are some differences.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.868219
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  • 6
    Electronic Resource
    Electronic Resource
    Two-point coherence in the atmospheric surface layer (1996)
    Springer
    Boundary layer meteorology 81 (1996), S. 105-121 
    Details
    ISSN: 1573-1472
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Two-point, one-dimensional coherence in horizontally homogeneous atmospheric turbulence is studied, both by experiment and analysis. Measurements are carried out using horizontally spaced sensors with the separation perpendicular to the mean velocity. Two-dimensional spectral models and three-dimensional inertial-range spectral tensors are used in the coherence calculations. The one-dimensional coherence for both velocity and scalar fluctuations is found to roll off at a wavenumber much smaller than we would expect from the classical notion of eddy correlation. This is a consequence of the cancellation of Fourier components aliased from the direction of the sensor separation into the streamwise direction. However, the coherence for the three velocity components behaves somewhat differently, reflecting the relative orientations of the velocity component, sensor separation and the mean velocity. These features are well predicted by the calculation. The analysis is also extended to calculate the two-point scalar-vertical velocity cospectrum and the results are in good agreement with our experimental data. The ratio of two- to one-point cospectra decreases at slightly larger wavenumber than the two-point scalar coherence does.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00119061
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  • 7
    Electronic Resource
    Electronic Resource
    The Effects of Probe-Induced Flow Distortion on Velocity Covariances: Field Observations (1999)
    Springer
    Boundary layer meteorology 91 (1999), S. 483-493 
    Details
    ISSN: 1573-1472
    Keywords: Turbulence ; Flow distortion ; Sonic anemometers
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract We carried out measurements to test a simple theory of the effect of probe-induced flow distortion on turbulence measurements. We used two three-component sonic anemometers mounted 1.8m apart at a height of 6.7 m. Behind one was a horizontal circular cylinder of radius 0.15 m and length 1.2 m, chosen to model two-dimensional probe-induced flow distortion in the limit where the scale of the turbulence is very large compared to the scale of the probe. The second sonic anemometer measured the undistorted flow. The measured flow-distortion effects on the Reynolds shearing stress and the variances of streamwise and vertical velocity agree well with the theory.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1001868030313
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  • 8
    Electronic Resource
    Electronic Resource
    Taylor’s hypothesis and two-point coherence measurements (1996)
    Springer
    Boundary layer meteorology 81 (1996), S. 399-410 
    Details
    ISSN: 1573-1472
    Keywords: Turbulence ; Coherence ; Taylor’s hypothesis ; Surface layer
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Experimentally obtained time coherence has traditionally been interpreted as streamwise one-dimensional spatial coherence through Taylor’s hypothesis. We calculate corrections to the highwavenumber part of the coherence to account for the errors caused by the deviation from Taylor’s hypothesis in high-intensity turbulent flows. The small-scale turbulence is assumed to be frozen and convected by a fluctuating convection velocity. Both Lumley’s two-term approximation and the Gaussian approximation are used in the calculations. In general, we find that the coherence for crossstream separations is significantly overestimated by the direct use of Taylor’s hypothesis, the error increasing with wavenumber; that for streamwise separations is underestimated. The analyses are compared with cross-stream coherence measurements in the atmospheric surface layer. Our results indicate that predictions from Lumley’s approximation yield better agreement with experimental data for cross-stream separations than those from the Gaussian model. Our study suggests that reliable measurement of two-point spatial coherence can be achieved only for scales not too small compared to the sensor separation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02430337
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  • 9
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    A conditional sampling-based method for noise and resolution corrections for scalar dissipation rate measurements (2009)
    American Institute of Physics
    Details
    Publication Date: 2009-06-01
    Print ISSN: 1070-6631
    Electronic ISSN: 1089-7666
    Topics: Physics
    Published by American Institute of Physics
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  • 10
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    Whirlwinds and Hairpins in the Atmospheric Surface Layer (2016)
    American Meteorological Society
    Details
    Publication Date: 2016-11-28
    Description: Vortices in the atmospheric surface layer are characterized using observations at unprecedented resolution from a fixed array of 31 turbulence sensors. During the day, these vortices likely are dust devils, though no visual observations are available for confirmation. At night, hairpin vortices appear to have been observed. The structure and dynamics of several types of vortices are described and related to other vortex investigations, including tornadoes and hurricanes.
    Print ISSN: 0022-4928
    Electronic ISSN: 1520-0469
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
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