Visualization of turbulence anisotropy by single exposure speckle photography

A new technique based on single exposure speckle photography is described that allows to measure the degree of anisotropy of turbulence in a flow with fluctuating fluid density or temperature.Since anisotropy is linked to shear in a flow, the method is also a means for discriminating regions of different shear rate.The research described in this publication was made possible in part by Grant RWB 000 from the International Science Foundation. The authors are also grateful to Deutsche Forschungsgemeinschaft (DFG) for partial financial support (Me 484/18).     

Beschleunigung eines Kolbens in einem Stosswellenrohr mit veränderlichem Querschnitt

Zusammenfassung Es wird ein vereinfachtes, eindimensionales Modell beschrieben für die Beschleunigung eines Kolbens in einem Stosswellenrohr, dessen Querschnitt sich nach einer gewissen Lauflänge sprunghaft erweitert. Numerische Rechnungen werden mit Messungen im Stosswellenrohr verglichen.

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Summary A simplified, one-dimensional model is developed to describe the acceleration of a piston in a shock tube having a discontinuous increase in cross-section. Numerical calculations are compared with shock tube experiments.

     

Turbulence microscale variation due to interaction with shock wave

A speckle photographic technique is used for visualizing the planar distribution of the refractive deflection angles of light transmitted through a compressible turbulent flow. Double exposure speckle photography is applied for the statistical analysis of such flows. Quantitative visualization of the planar distribution of the refractive deflection angles of the light transmitted through the compressible turbulent flow is performed. Turbulent structures are visible in the patterns of the deflection angles isolines. The 2-D correlation functions of these deflection angles are constructed and analyzed. A mathematical procedure for solving Erbeck-Merzkirch integral transformation is described and the 3-D density correlation functions are evaluated using 2-D correlation functions of deflection angles of the light passed through the turbulence. Received 14 November 1999 / Accepted 3 June 2000     

Digital speckle photography and speckle tomography in heat transfer studies

The line-of-sight speckle photography of transparent media is used for quantitative measurements of the instantaneous temperature fields in 3D unsteady flows. Both electronic and photographic methods are employed for specklegram recording. The subsequent specklegram treatment uses the Young's fringes method as well as cross-correlation analysis of small interrogation areas of the recordings. Experimental data for three different heat transfer configurations are obtained and discussed. The first one is natural convection over extended vertical heated plates with forward facing steps, the second is unsteady 3D convective flow around a suddenly heated vertical thin wire, and the third one is a convective plume above a multi-jet flame. Both local and global Nusselt numbers are determined via measuring local surface temperature gradients for these convective flows. The results are compared with Ostrach's theory for a single vertical plate and with the data obtained by Mach–Zehnder interferometry. The 3D temperature fields are reconstructed for axisymmetric convective flows around a suddenly heated vertical wire using quasi-double projection measurement and the Radon inversion. 3D temperature distributions above the combustion zone are reconstructed using multi-projection speckle photography measurements and computerised tomography.     

Interaction of a normal shock wave with a compressible turbulent flow

A speckle photographic method, which is sensitive to changes of gradients in fluid density, is applied for analyzing a compressible turbulent air flow with density fluctuations. Spatial correlation coefficients, turbulent length scales, and energy spectra are determined under the assumption of homogeneous isotropic turbulence. The experiments are performed in a shock tube where the flow is passed through a turbulence grid. Measurements are taken before and after the turbulent regime interacts with the normal shock wave reflected from the tube's end wall. Amplification of the turbulence intensity by the shock interaction process is verified quantitatively and is shown to be restricted to the lower wave numbers in the spectrum.A version of this paper was presented at the 11th Symposium on Turbulence, University of Missouri-Rolla, Oct. 17–19, 1988.To Professor Dr.-Ing. Klaus Gersten on the occasion of his 60th birthday