Forced Rayleigh scattering in turbulent plane Poiseuille flows. Part 1. Study of the transverse velocity-gradient component
TL;DR: In this article, an optical set-up designed to measure directly velocity gradients (strophometry) is described, based on the analysis of the distortions of a fringe pattern "written" instantaneously in a flow field.
Abstract: In this article, we describe an optical set-up designed to measure directly velocity gradients (strophometry). This strophometer is based on the analysis of the distortions of a fringe pattern ‘written’ instantaneously in a flow field. We apply it to study the transverse velocity-gradient component ∂u/∂y in a plane Poiseuille flow at moderate Reynolds numbers. Mean values and different moments of the fluctuating gradient distribution related to viscous dissipation, vorticity dynamics and intermittency are obtained. These results are interpreted in terms of the large-scale structures which are present in the flow.
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01 Jan 1989
TL;DR: In this article, the significance of vorticity and the importance of its temporally and spatially resolved measurement in turbulent flows is discussed and the measurement methods are categorized as those which use multipoint measurements of velocity to determine the required velocity gradients or circulation and those which sense these gradients directly.
Abstract: The significance of vorticity and the importance of its temporally and spatially resolved measurement in turbulent flows is discussed. The measurement methods are categorized as those which use multipoint measurements of velocity to determine the required velocity gradients or circulation and those which sense these gradients directly. The characteristics and performance of these vorticity measurement methods are evaluated. Where available, measurements of some vorticity field statistics by two or more methods are compared.
35 citations
TL;DR: In this paper, a time-resolved dynamic light scattering technique is presented for measuring the velocity gradient in transient but repeatable flows, and the experimental technique is verified via measurements for a Newtonian fluid undergoing a known time-dependent flow.
Abstract: A time‐resolved dynamic light scattering technique is presented for measuring the velocity gradient in transient but repeatable flows. The experimental technique is verified via measurements for a Newtonian fluid undergoing a known time‐dependent flow. The method is then applied to the creeping flow of a Newtonian fluid in a corotating two‐roll mill. It is demonstrated that the flow near the stagnation point can be accurately described by an analytical creeping flow solution for a two‐roll mill in an unbounded fluid. The time dependence of the velocity gradient for a concentrated polymer solution in the startup of the two‐roll mill has also been measured, it is believed, for the first time. The measurement provides direct evidence of the modification of the flow for the viscoelastic polymer liquid, and will ultimately lead to significant insights into the polymer dynamics for concentrated solutions in strong, extension‐like flows. A second significant feature of the dynamic light scattering experiment is that the initial magnitude of the correlation function is related to the degree of optical anisotropy of the polymer molecules, i.e., to the geometric configuration of the polymer chains. Thus, it yields information on the time‐dependent degree of polymer orientation and stretch that is equivalent to birefringence, but is obtained at the ‘‘point’’ occupied by the scattering volume rather than as a two‐dimensional average across the whole fluid as in birefringence. This measurement of polymer configuration is compared with birefringence data for the exact same flow.
21 citations
TL;DR: In this paper, a light scattering technique is described which allows the non-invasive measurement of instantaneous velocity gradient components in fluid flow, and statistical and systematic errors are investigated both experimentally and theoretically.
Abstract: A light scattering technique is described which allows the non-invasive measurement of instantaneous velocity gradient components in fluid flow. Statistical and systematic errors are investigated both experimentally and theoretically. The method represents a basis for the study of temporal evolution of velocity gradients in fluid flow.
14 citations
TL;DR: In this article, a method of measuring velocity gradients on the basis of a photon correlation technique has been developed for laminar flow without any knowledge of the flow symmetry, which is also applicable to turbulent flow.
Abstract: A method of measuring velocity gradients on the basis of a photon correlation technique has been developed. Depending on the scattering geometry, particular rows of the velocity gradient tensor Gamma can be determined for laminar flow without any knowledge of the flow symmetry. The method is also applicable to turbulent flow. With the assumption of a normal distribution of the tensor elements of one row of Gamma , the measured correlation functions for a tube flow can be fitted satisfactorily. Limits of the applicability of the method are discussed.
11 citations
TL;DR: In this article, three different measurement schemes for the analysis of the scattered light were presented, and the experimental results refer to mean gradients obtained from measurements on turbulent channel flows, which is the basic physical effect of a non-invasive method that allows the measurement of velocity gradients with high spatial and temporal resolution down to 75 μm and 1 ms, respectively.
Abstract: The motion of the speckle pattern in the far field limit is uniquely determined by the velocity gradient of the light scattering medium. This is the basic physical effect of a non-invasive method that allows the measurement of velocity gradients with high spatial and temporal resolution down to 75 μm and 1 ms, respectively. We present three different measurement schemes for the analysis of the scattered light. The experimental results refer to mean gradients obtained from measurements on turbulent channel flows.
8 citations
References
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01 Jan 1972
TL;DR: In this article, the authors discuss the Reynolds equations and estimate of the Reynolds stress in the kinetic theory of gases, and describe the effects of shear flow near a rigid wall.
Abstract: This chapter contains sections titled: The Reynolds equations, Elements of the kinetic theory of gases, Estimates of the Reynolds stress, Turbulent heat transfer, Turbulent shear flow near a rigid wall
3,270 citations
01 Jan 1972
TL;DR: In this article, the probability density, Fourier transforms and characteristic functions, joint statistics and statistical independence, Correlation functions and spectra, the central limit theorem, and the relation functions are discussed.
Abstract: This chapter contains sections titled: The probability density, Fourier transforms and characteristic functions, Joint statistics and statistical independence, Correlation functions and spectra, The central limit theorem
3,260 citations
TL;DR: In this paper, the scale of turbulence is defined in terms of the correlation between the velocity of a particle at one time and that of the same particle at a later time, or between simultaneous velocities at two fixed points.
Abstract: Since the time of Osborne Reynolds it has been known that turbulence produces virtual mean stresses which are proportional to the coefficient of correlation between the components of turbulent velocity at a fixed point in two perpendicular directions. The significance of correlation between the velocity of a particle at one time and that of the same particle at a later time, or between simultaneous velocities at two fixed points was discussed in 1921 by the present writer in a theory of “Diffusion by Continuous Movements.” The recent improvements in the technique of measuring turbulence have made it possible actually to measure some of the quantities envisaged in the theory and thus to verify some of the relationships then put forward. The theory has also been developed in several directions which were not originally contemplated. The theory, as originally put forward, provided a method for defining the scale of turbulence when the motion is defined in the Lagrangian manner, and showed how this scale is related to diffusion. It is now shown that it can be applied either to the Lagrangian or to the Eulerian conceptions of fluid flow.
1,367 citations
TL;DR: In this paper, the structure of the flat plate incompressible smooth-surface boundary layer in a low-speed water flow is examined using hydrogen-bubble measurements and also hot-wire measurements with dye visualization.
Abstract: The structure of the flat plate incompressible smooth-surface boundary layer in a low-speed water flow is examined using hydrogen-bubble measurements and also hot-wire measurements with dye visualization. Particular emphasis is placed on the details of the process of turbulence production near the wall. In the zone 0 < y+ < 100, the data show that essentially all turbulence production occurs during intermittent ‘bursting’ periods. ‘Bursts’ are described in some detail.The uncertainties in the bubble data are large, but they have the distinct advantage of providing velocity profiles as a function of time and the time sequences of events. These data show that the velocity profiles during bursting periods assume a shape which is qualitatively distinct from the well-known mean profiles. The observations are also used as the basis for a discussion of possible appropriate mathematical models for turbulence production.
1,004 citations
TL;DR: A review of organized motion in turbulent flow indicates that the transport properties of most shear flows are dominated by large-scale vortex nonrandom motions as mentioned in this paper, and the boundary layer coherent structure was isolated by the correlation methods of Townsend (1956) and flow visualization by direct observations of complex unsteady turbulent motions.
Abstract: A review of organized motion in turbulent flow indicates that the transport properties of most shear flows are dominated by large-scale vortex nonrandom motions. The mean velocity profile of a turbulent boundary layer consists of a viscous sublayer, buffer layer, and a logarithmic outer layer; an empirical formula of Coles (1956) applies to various pressure gradients. The boundary layer coherent structure was isolated by the correlation methods of Townsend (1956) and flow visualization by direct observations of complex unsteady turbulent motions. The near-wall studies of Willmart and Wooldridge (1962) used the space-time correlation for pressure fluctuations at the wall under a thick turbulent boundary layer; finally, organized motion in free shear flows and transition-control of mixing demonstrated that the Reynolds number invariance of turbulence shows wide scatter.
826 citations