The pertinent correlation function for nmr dipolar relaxation of 31P by its neighboring protons in DNA is derived for a comparatively realistic model of the internal Brownian motions. These motions include the collective torsional deformation modes of the elastic filament, uncoupled local overdamped reorienting motions of the P-H vectors in harmonic potential wells within the nucleotide unit, and the compartively slow end-over-end rotations of the local helix axis. These latter slow axial tumbling motions essentially completely determine T2 but have virtually no effect on T1 or the nuclear Overhauser effect (NOE), which are governed almost exclusively by rapid torsional deformations and local reorientations on the nanosecond time scale. The essential behavior of the relevant correlation function for the collective torsional motions has recently been determined experimentally in this laboratory using the decay of fluorescence polarization anisotropy of bound ethidium dye [J. C. Thomas et al. (1980) Biophys. Chem.12, 177–180]. By using that result to carry out nmr relaxation calculations for various amplitudes and time constants of the uncoupled local motions and comparing them with the experimental data, it can be demonstrated that (within this model of purely dipolar relaxation), only rather small rms amplitudes of local reorientations (<7°) occur and that their relaxation times are near 1 ns. Contrary to previous conclusions in the literature, the collective torsional deformation modes actually make the dominant contribution to T1 and NOE. At t = 1 ns the total rms azimuthal displacement of the P-H vector in this model is 19.5°, which results from a superposition of torsional deformations with rms displacement 18.2° and uncoupled local motions with rms displacement 7°. The contribution of pure chemical shift anisotropy (CSA) to the 1/T1 relaxation rate is calculated for the first time for the case when torsional deformation modes predominate, and it is predicted to be 46% of the corresponding dipolar relaxation rate or 31% of the total relaxation rate. Unusual magnetic field strength dependence of the pure CSA and dipolar contributions is predicted to arise as a consequence of the collective torsional deformation modes. This seriously weakens empirical arguments in favor of a small (<10%) CSA contribution. In any case, a detailed interpretation of T1 and NOE incorporating both dipolar and CSA relaxation must await the evaluation of the CSA:dipolar interference term, or crossterm, contribution to the relaxation rate. The contribution of pure CSA to 1/T2 relaxation is likewise calculated for the case when local reorienting motions are negligible; it is found to be ≲16% of the corresponding dipolar relaxation rate for the comparatively short (300–600-base-pair) DNA fragments of interest. For high-molecular-weight DNAs we predict that the slow Rouse-Zimm coil-deformation modes will dominate 1/T2 relaxation and the linewidth. Dynamic light-scattering and other evidence is presented that the remarkable loss of nmr signal from DNA on addition of ethidium bromide, as reported by Hogan and Jardetzky, is actually a consequence of phase separation in such concentrated solutions. A pronounced decrease in T2, due to greatly hindered axial tumbling in the more concentrated phase, is advanced as a plausible line-broadening explanation for the apparent loss of nmr signal from DNA in that phase.

NMR relaxation in DNA. I. The contribution of torsional deformation modes of the elastic filament.

Experimental methods which have been developed over the last three decades to measure one or more components of the vorticity vector are described and assessed. These include methods which estimate the velocity gradients comprising the various vorticity components, methods which sense fluid rotation directly, and methods which sense other fluid properties which can be related to vorticity.

Methods of measuring vorticity in turbulent flows

Quasielastic light scattering from polymers, colloids and gels

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.

The measurement of vorticity in transitional and fully developed turbulent flows

Poursuivant la realisation de methodes de caracterisation d'un fluide en ecoulement par diffusion Rayleigh forcee, nous avons mis au point une technique de marquage d'une figure bidimensionnelle quelconque dans le fluide par imagerie optique. L'impression de ce reseau et la visualisation directe de sa deformation permettent d'obtenir une mesure directe des composantes du gradient de vitesse.

https://hal.archives-ouvertes.fr/jpa-00244976/document

Nouvelle technique de marquage d'écoulement par utilisation de molécules photochromes

2014 We present a new visualization technique of an hydrodynamic flow using a thermal grid, « written » in the liquid by means of a pulsed laser, and whose image is subsequently formed, using standard optics, by illumination by a continuous laser. The experiment gives access to a new Doppler-anemometric technique which does not involve the presence of particles in the fluid and to an instantaneous image of a finite volume of the flow. J. Physique LETTRES 41 (1980) L-519 L-521 1 Classification Physics Abstracts 47.80 1 er NOVEMBRE 1980. The instantaneous visualization and velocity measurement of a finite domain of flow is of considerable interest for unsteady or turbulent velocity field u(r, t). Several methods are based on the periodic injection Fig. 1. In the Poiseuille flow cell (C) a thermal grating G is formed at initial time (t = 0) by the interference of two beams coming from a pulsed Nd YAG laser Lp. The second laser La illuminates this grid and is diffracted (orders 0, 1, 1). The Doppler shifted signals 1, 1 can be used to set an anemometric measurement by standard heterodyne technique. Another possibility is the formation of an image of the grid in the plane I conjugate of G through the lens L. If the laser La is switched on (by means of an acoustooptic modulator) for a short time period (t, t + At), the image reflects the integrated effect of the flow field between t = 0 and t. of lines of lagrangian markers (dye, solid particles, bubbles) in the fluid [1]. The lines distort as they move with the flow, due to the velocity gradients; a measurement of the integrated displacement and of u(r, t) is possible as long as the diffusion has not washed away the lines. The letter discusses a related technique in which a thermal grid is « written » in a very short time (tp 10 ns) within a liquid using the pattern formed by the interference of two beams from a pulsed laser Lp [2] (Fig. 1). In a previous paper [3] we have shown some applications of the forced Rayleigh technique [4] to hydrodynamics : if the thermal grid is « read » by a second laser La (cw HeNe 10 mW) incident at right angle with the wave vector of the index grating formed by the thermal grid, the exponential time decay of the intensity of the diffracted peaks gives access to the thermal diffusivity in the flow (Fig. 2a) as in static experiments. Moreover, the rotation and distortion of the grid caused by the velocity gradients are measured through the corresponding changes of the diffraction pattern. The average velocity u (in the grid volume) is automatically subtracted by the diffraction process; this is of particular interest for the study of the structure of turbulence which is determined in terms of the gradient components. Moreover, this experiment gives an average information over the size of the laser La. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyslet:019800041021051900 L-520 JOURNAL DE PHYSIQUE LETTRES Fig. 2. a) Classical decay of a forced Rayleigh signal : intensity of the first diffracted order (with an exponential time constant T = p2/(8 ~2 K) where p is the wavelength of the grid, K the thermal diffusivity). b) The output signal Ia of a photodetector placed in the image of the grid : a periodic modulation is superimposed to the exponential decay with a frequency 2 Av proportional to u. The present extension of the technique is quite complementary : i) it permits to observe the thermal grid not only in the Fourier space but also in real space; thus it has spatial resolution; ii) it can give a measure of u which, in the forced Rayleigh experiment, was « memorized » as a Doppler shift of the frequency of the diffracted order ± 1 as Av = plii [1] (where p = period of the thermal grid). The information in u can be extracted by mixing the diffracted beams ± 1. This is the principle of the experiment sketched in the right part of figure 1 : the two beams ± 1 are recombined to form the image of the grid in a plane I, conjugate of the index grating, through the lens L (the image is magnified by a ratio m = q’/q ’\" 10). In the focal plane of L, the screen S acts as an aperture stop in the Fourier space and suppresses the central beam. The strioscopic technique gives an image (amplitude grating) of 100 % contrast. The motion of the image at a velocity mu(r, t) follows that of the object grid. The photographs (Fig. 3) shows a sequence of two images of a flow. The grid is initially (time t = 0) perpendicular to the plates of a plane Poiseuille flow cell (the flow being uniform along the direction y of the laser La). At a subsequent time (t = 300 ~s), the parabolic velocity profile of the laminar flow is seen from the integrated motion of the grid. Fig. 3. Photographs of the grid in the image plane I at two different times : a) t = 0 : right when the grid was formed in the flow cell; b) t = 300 us : grid convected by the Poiseuille flow. We also notice the decay of intensity due to thermal diffusion between the photographs taken with equal exposure time At = 20 us. One can extend the technique to non uniform Poiseuille flows and suppress the integration along the y direction by replacing the spherical lens used to form the index grating by a cylindrical one. In particular for the visualization of small turbulent structures [5] the thickness of the vertical « ribbon » of pulsed laser light should be kept comparable to the Kolmogorov scale. The present experiment is limited by the sensitivity of the recording film [6]. In order to avoid fuzzy images, the exposure time has to be kept of the order of T/I0, where the period of displacement of the grid is T ~ 10-4 s for a grating of period p ~ 100 Ilm and u ~ 1 m/s. The limit is quite severe for turbulent flows where the attenuation of the grid intensity by turbulent diffusion can be much faster than in laminar flows. It is also possible to place a small (of extent p’/2 where p’ is the period of the image grid) photodetector in a prescribed point of the image flow field. The variaL-521 AN ANEMOMETRIC AND FLOW VISUALIZATION TECHNIQUE tion of intensity with time is given in the figure 2b. In addition to exponential diffusive decay, the periodic modulation at a frequency 2 Av (given by [1]) is a measure of the local velocity u(r) during the lifetime of the grid. This laser Doppler anemometry technique is complementary to the classical LDA one [7] : in the latter case, particles move within the flow through a fixed grid ; in the present experiment, the grid moves as a whole through the reference line of the La beam. Our experiment is not a true lagrangian one because different parts of the grid pass through the laser beam. However, it offers several advantages : i) it does not require the presence of tracer particles in the flow ; ii) it allows the simultaneous measurement of the diffusivity and of one velocity component. Such a joint information can be of interest in particular in the study of two-dimensional turbulent flow fields where it is known that the turbulent mixing can be controled by the large eddy structures [8] ; iii) measurement of the velocity in different points of the flow field can be done simultaneously or sequentially by using several small detectors in the image plane I. Let us finally note that a simpler LDA measurement can be obtained by mixing one of the diffracted beams with unshifted diffused light from the central beam. However in this case we lose the dual information on the map of velocity field obtained by means of the grid image.

/pdf/a-non-perturbative-anemometric-and-flow-visualization-1i8tda1ego.pdf

A non-perturbative anemometric and flow visualization technique

We applied the forced Rayleigh scattering technique to study laminar and turbulent flows. We recorded the optical Fourier transform of a transient grating created in a flowing liquid. Depending on the type of detection used, FRS provides either a direct measurement of velocity gradients or the determination of an effective turbulent diffusivity. Experimental results in a plane Poiseuille flow are given.

J.C. Charmet

Papers

A non-perturbative anemometric and flow visualization technique

Application of forced Rayleigh scattering to hydrodynamic measurements