Tollmien-Schlichting wave generation by flow turbulence

The problem of freestream-turbulence-generated instability waves in the flat-plate boundary layer is solved on the basis of a nonlinear turbulence model admitting the deviation of the speed of propagation of vortex disturbances from the flow velocity. The solution obtained well describes the experimental dependence of the laminar-turbulent transition Reynolds number on the freestream turbulence degree.     

Turbulent heat transfer for pipe flow with uniform heat generation

An Analytic solution is presented of the problem of turbulent heat transfer in pipes with internal heat generation and insulated wall by applying a recently-developed eddy conductivity model. The results agree closely with available experimental data for a wide range of Prandtl number (0.02–10.5).     

The self-preservation of homogeneous shear flow turbulence

An analysis of the equations governing homogeneous shear flow shows the possibility of solutions which are self-preserving at all scales of motion, and that these solutions are dependent on the initial conditions. The appropriate velocity scale is the one obtained from the turbulence kinetic energy, q ²/2, while the length scale is the Taylor microscale, . Two cases of self-preserving flow are identified: one corresponding to constant mean shear, the other to a mean shear which is inversely proportional to time. For the first case (the only one considered in detail) the principal results of the postulated similarity are that is constant, while q ² varies exponentially with time. The ratio of the turbulence energy production rate to its dissipation rate remains constant. It is also shown that the energy spectra scale over all wavenumbers with q ² and , and that they have shapes determined by the initial conditions. The experimental evidence is generally consistent with the theory.     

Grid turbulence in compressible flow

 This paper describes an experimental investigation of the effects of mean compressibility on the structure and the decay characteristics of grid turbulence. The experiments comprise laser Doppler velocimetry measurements of an approximately homogeneous, isotropic turbulence field in subsonic compressible flow, with the Mach number ranging from 0.15 to 0.7. The results reveal that the turbulence intensity and decay characteristics are influenced by mean compressibility. Experience with the high subsonic and supersonic regimes is also reported. Received: 27 February 1996/Accepted: 13 February 1997     

On the Lighthill relationship and sound generation from isotropic turbulence

In 1952 Lighthill (1952) developed a theory for determining the sound generated by a turbulent motion of a fluid. With some statistical assumptions, Proudman (1952) applied this theory to estimate the acoustic power of isotropic turbulence. Recently, Lighthill established a simple relationship that relates the fourth-order retarded-time and space covariance of his stress tensor to the corresponding second-order covariance and the turbulent flatness factor, without making statistical assumptions for a homogeneous turbulence. Lilley (1994) revisited Proudman's work and applied the Lighthill relationship to evaluate the radiated acoustic power directly from isotropic turbulence. After choosing the time separation dependence in the two-point velocity time and space covariance based on the insights gained from direct numerical simulations, Lilley concluded that the Proudman constant is determined by the turbulent flatness factor and the second-order spatial velocity covariance. In order to estimate the Proudman constant at high Reynolds numbers, we analyzed a unique data set of measurements in a large wind tunnel and atmospheric surface layer that covers a range of the Taylor microscale-based Reynolds number 2.0×10³R 12.7×10³. Our measurements demonstrate that the Lighthill relationship is a good approximation, providing additional support to Lilley's approach. The flatness factor is found between 2.7 and 3.3 and the second-order spatial velocity covariance is obtained. Based on these experimental data, the Proudman constant is estimated to be 0.68–3.68.This research was supported by the National Aeronautics and Space Administration under NASA Contract No. NAS1-19480 while the first author was in residence at the Institute for Computer Applications in Science and Engineering (ICASE), NASA Langley Research Center, Hampton, VA 23665, U.S.A.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Instability of uniform flow

We consider uniform flow of a Newtonian fluid trasverse to a domain bounded by parallel planes. We investigate the possibility of introducing instabilities in this flow by the choice of inflow and outflow conditions. Some instabilities of this kind are found.     

The turbulence structure of drag-reduced boundary layer flow

The structure of turbulence in a drag-reduced flat-plate boundary layer flow has been studied with particle image velocimetry (PIV). Drag reduction was achieved by injection of a concentrated polymer solution through a spanwise slot along the test wall at a location upstream of the PIV measurement station. Planes of velocity were measured parallel to the wall (x–z plane), for a total of 30 planes across the thickness of the boundary layer. For increasing drag reduction, we found a significant modification of the near-wall structure of turbulence with a coarsening of the low-speed velocity streaks and a reduction in the number and strength of near-wall vortical structures.     

The generation of nearly isotropic turbulence by means of grids

Grids have been used as a means of generating nearly isotropic turbulence for about fifty years. Even so, there does not appear to be a single document which gives adequate and simple rules for the design of such devices in an airflow installation. This paper attempts to fill this gap by means of a synthesis of experimental data with simple analyses, such that useful design guidelines are derived. Pressure losses, turbulence intensities, spectra, correlation functions and length scales are all examined. The present results are found to agree well with other data published in the literature.     

The generation of nearly isotropic turbulence downstream of streamwise tube bundles

Tube bundles were used to generate a field of nearly isotropic turbulence. A wide range of geometric and aerodynamic parameters were systematically studied, with measurements being made of pressure losses, turbulence intensities, spectra. auto-correlations and length scales. Simple analyses were utilized to correlate effectively these results and to aid in their understanding. The results agree well with other (limited) results published in the literature.     

On the generation of large-scale homogeneous turbulence

An active turbulence generating grid, based on the rotating-vane design of Makita (1991), was developed for a large wind tunnel. At 2.14 m square, the grid is the largest of this type ever developed. To improve the isotropy of the turbulence generated, the grid was placed in the wind tunnel contraction. Measurements show that the grid produces a closely uniform mean flow and homogeneous isotropic turbulence to within two integral scales from the wall. By systematically varying the flow speed and parameters controlling the random motion of the vanes, grid turbulence with a wide variety of characteristics was produced and the dependence of those characteristics on the operating parameters of the grid revealed. Taylor Reynolds numbers of the grid turbulence varied from 100 to 1,360 and integral scales from 5 to almost 70 cm. The extreme cases represent some of the highest Reynolds number and largest scale homogeneous turbulent flows ever generated in a wind tunnel.     

The velocity fluctuation spectrum of a gas with particles in the case of uniform isotropic turbulence

In the present paper, an analysis of the influence of particles on the fluctuation spectrum of a gas is based on equations for the two-point moments of the velocity component fluctuations of the solid and carrier phases. For an approximation of the terms describing the transfer of fluctuation energy through the spectrum, we use Heisenberg's hypothesis [8], modified to take into account the influence of the particles on the rate of redistribution of turbulent energy between vortices of different dimensions.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 1, pp. 69–74, January–February, 1987.     

The onset of turbulence in physiological pulsatile flow in a straight tube

 An empirical correlation for the onset of turbulence in physiological pulsatile flow is presented. We pumped three different test fluids of kinematic viscosity 0.008–0.035 cm²/s through four straight tubes 0.4–3.0 cm in diameter. A Scotch yoke mechanism provided an oscillatory sine wave flow component of known stroke volume and frequency. We adjusted the mean flow independently until we detected signal instabilities from hot film wall shear stress probes. The critical peak Reynolds number was found to correlate with the Womersley parameter and the Strouhal number as a power law function with a root-mean-square (rms) error of 15.2%. Experimental measurements of the laminar velocity profile are compared to theoretical predictions from Poiseuille’s law and Womersley’s solution. Received: 30 October 1995/Accepted: 7 April 1997     

Large-scale vortex generation driven by nonequilibrium turbulence

The development of large-scale perturbations in a heated layer of rotating fluid is studied within the framework of the nonequilibrium turbulence model with asymmetric Reynolds stress tensor. It is shown that, as in spiral turbulence, when there is no equilibrium on one of the boundaries of the layer large-scale structures develop. Conditions under which both perfect intrinsic matching of turbulence and convection and internal resonance development exist are determined. It is shown that the manifestation in a turbulent medium of properties of the convective vector field such as spirality may be caused by constraints imposed by the angular momentum conservation law.Novosibirsk. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, pp. 47–55, January–February, 1996.     

Vapor core turbulence in annular two-phase flow

 This paper reports a new technique to measure vapor turbulence in two-phase flows using hot-film anemometry. Continuous vapor turbulence measurements along with local void fraction, droplet frequency, droplet velocity and droplet diameter were measured in a thin, vertical duct. By first eliminating the portion of the output voltage signal resulting from the interaction of dispersed liquid droplets with the HFA sensor, the discrete voltage samples associated with the vapor phase were separately analyzed. The data revealed that, over the range of liquid droplet sizes and concentrations encountered, the presence of the droplet field acts to enhance vapor turbulence. In addition, there is evidence that vapor turbulence is significantly influenced by the wall-bounded liquid film. The present results are qualitatively consistent with the limited data available in the open literature. Received: 17 August 1998/Accepted: 12 April 1999     

Subharmonic transition to turbulence in channel flow

A direct numerical simulation of subharmonic transition to turbulence in channel flow has been performed. The stages of primary and secondary instability have been identified in the results leading to a staggered pattern of A-shaped vortices. The associated staggered symmetry has been found to persist up to rather late stages of the breakdown process. This symmetry rapidly disappears in the final stage of transition and a developed turbulent flow is attained. A pronounced difference in the development between the two channel halves is observed which is consistent with the predictions of secondary instability theory.     

Numerical modeling of the generation of internal waves by uniform stratified flow over a thin vertical barrier

Numerical simulations of two‐dimensional stratified flow past an obstacle (thin vertical strip) were performed at relatively low Reynolds numbers. A finite differences solver was adopted to simultaneously solve Navier–Stokes equations together with transport equations for salinity (stratifying agent), and the standard Smagorinsky turbulent closure scheme was called in whenever necessary to account for turbulence. The emphases were on the evaluation of code for unsteady stratified flow applications as well as identification of transient and steady internal‐wave processes during flow past obstacles. Simulations were compared with laboratory experiments, where observations were made using a high resolution Schlieren technique and conductivity probes. Blocking was observed upstream of the obstacle, surrounded by near‐zero frequency internal waves, the phase lines of which joined those of lee waves through a transition zone in the proximity of the obstacle. This pattern was preceded by initial transients of the starting flow in which propagating internal waves played a dominant role. Confluence of isopycnals passing over/under the obstacle in the wake led to interesting flow phenomena, including the radiation of internal waves. The numerical simulations were in good agreement with observations, except that some phenomena could not be captured due to resolution issues of either numerical or experimental techniques. The efficacy of the code in point for stratified flow calculations with relevance to the atmosphere and oceans was confirmed. Copyright © 2011 John Wiley & Sons, Ltd.     

Vorticity shedding from a lentil-shaped body at large incidence in uniform flow

The 2D flow about a lentil-shaped body at high incidence in uniform stream is numerically simulated for high Reynolds numbers. The separation points are fixed on the body edges and the vorticity released through a Kutta-like condition is spatially discretized by point vortices. Their Lagrangianinviscid dynamics is described by using both aconformal mapping and aboundary elements method. The present results are in good agreement with previous calculations but still not satisfactory when compared with the experiments.The aim of the present work is to investigate the effect of different Kutta conditions and the effect of different amalgamation procedures in the far wake on global quantities such as theStrouhal number or the total shed circulation. Other remarkable effects induced either by the action of viscosity or by the rising of secondary 3D motion, may not be analyzed by the present simplified model.

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Sommario Si studia il flusso attorno ad una lente ad elevata incidenza nelle ipotesi di elevato numero di Reynolds e flusso bidimensionale. I punti di separazione sono fissati sulle estremità della lente e la produzione di vorticità da questi è realizzata con opportune condizioni di Kutta. La dinamica dei vortici è descritta con un modello Lagrangiano non viscoso. I risultati, ottenuti sia con l'uso di una trasformazione conforme che con un metodo agli elementi di contorno, sono in ottimo accordo con i risultati di precedenti simulazioni numeriche, ma non sono ancora soddisfacenti se confrontati con gli esperimenti.Si analizza in termini di grandezze globali del flusso, quali il numero di Strouhal e la circolazione totale rilasciata, l'effetto di due diverse condizioni di Kutta e dell'uso di procedure di compattazione della scia lontana. Si conclude che l'uso di diverse condizioni di Kutta produce risultati pressoché identici mentre l'utilizzo di procedure di compattazione della scia può alterare in modo significativo il numero di Strouhal del flusso. Rimangono da analizzare altre possibili cause del disaccordo con le sperimentazioni quali la diffusione viscosa e la probabile presenza di moti tridimensionali.

     

Characterization of flow turbulence in large-scale bubble-plume experiments

Flow turbulence generated by a bubble plume in a large tank is characterized. Two different turbulence mechanisms contributing to the mixing and transport process are identified on velocity signals recorded outside of the bubble-plume core: a macro-scale process governed by the wandering motion of the bubble-plume; and an intermediate- and micro-scale process represented by the Kolmogorov power spectrum. A methodology is presented to characterize the different processes and their contributions to the turbulence parameters. The results help to understand the bubble-plume phenomenon and provide a basis to validate numerical models of bubble-plumes used in the design of combined-sewer-overflow reservoirs.     

Experimental investigation of turbulence in a supersonic flow

The structure of disturbances carried by the flow into the working section of a supersonic wind tunnel has been investigated by means of a constant-current hot-wire anemometer. In order to generate the disturbances grids consisting of round rods were introduced upstream from the nozzle throat. It was found that in the working section the disturbances consist of non-correlating vortex, entropy and acoustic modes. The latter is generated by the boundary layer on the nozzle walls and the first two by the grids. The spectral compositions of the various modes are compared. Because of the presence of grid turbulence the point of laminar-turbulent transition in the boundary layer on a flat plate varied widely.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 119–124, May–June, 1990.