Determinations of turbulent velocity fluctuations and mean particle radii in flames using scattered laser-power spectra

Author(s):  
P.H.P. Chang ◽  
S.S. Penner
2002 ◽  
Vol 4 (1) ◽  
pp. 39-51
Author(s):  
Helen Kettle ◽  
Keith Beven ◽  
Barry Hankin

A method has been developed to estimate turbulent dispersion based on fuzzy rules that use local transverse velocity shears to predict turbulent velocity fluctuations. Turbulence measurements of flow around a rectangular dead zone in an open channel laboratory flume were conducted using an acoustic Doppler velocimeter (ADV) probe. The mean velocity and turbulence characteristics in and around the shear zone were analysed for different flows and geometries. Relationships between the mean transverse velocity shear and the turbulent velocity fluctuations are encapsulated in a simple set of fuzzy rules. The rules are included in a steady-state hybrid finite-volume advection–diffusion scheme to simulate the mixing of hot water in an open-channel dead zone. The fuzzy rules produce a fuzzy number for the magnitude of the average velocity fluctuation at each cell boundary. These are then combined within the finite-volume model using the single-value simulation method to give a fuzzy number for the temperature in each cell. The results are compared with laboratory flume data and a computational fluid dynamics (CFD) simulation from PHOENICS. The fuzzy model compares favourably with the experiment data and offers an alternative to traditional CFD models.


2012 ◽  
Vol 7 (1) ◽  
pp. 53-69
Author(s):  
Vladimir Dulin ◽  
Yuriy Kozorezov ◽  
Dmitriy Markovich

The present paper reports PIV (Particle Image Velocimetry) measurements of turbulent velocity fluctuations statistics in development region of an axisymmetric free jet (Re = 28 000). To minimize measurement uncertainty, adaptive calibration, image processing and data post-processing algorithms were utilized. On the basis of theoretical analysis and direct measurements, the paper discusses effect of PIV spatial resolution on measured statistical characteristics of turbulent fluctuations. Underestimation of the second-order moments of velocity derivatives and of the turbulent kinetic energy dissipation rate due to a finite size of PIV interrogation area and finite thickness of laser sheet was analyzed from model spectra of turbulent velocity fluctuations. The results are in a good agreement with the measured experimental data. The paper also describes performance of possible ways to account for unresolved small-scale velocity fluctuations in PIV measurements of the dissipation rate. In particular, a turbulent viscosity model can be efficiently used to account for the unresolved pulsations in a free turbulent flow


Author(s):  
Yuichi Kaiho ◽  
Shumpei Hara ◽  
Takahiro Tsukahara ◽  
Yasuo Kawaguchi

It is known as the Toms effect that the wall friction coefficient is reduced by adding a small amount of polymer or surfactant into a water flow. In the drag-reducing flow, it is expected that a time scale of turbulent velocity fluctuation is changed by relaxation time due to viscoelasticity. In the present study, experimental analysis of the turbulent velocity fluctuation was performed with temporal characteristics in surfactant solution flow. The velocity fluctuations were measured by using a two-component laser Doppler velocimeter system on turbulent channel flow. And then, we performed statistical operation on those data and examined the time scale. From spectra analysis, it was found that very low frequency velocity fluctuations existed near the wall region in the surfactant solution flow. It was also revealed that the strong anisotropy occurred not only with the intensity but also with frequency distribution in turbulent velocity fluctuations. Moreover, the turbulence contributes nothing to the Reynolds shear stress and behaves as a wave motion. It was concluded that the turbulent eddies and viscoelasticity were two factors contributing to turbulent generation in the viscoelastic turbulent flow, with each factor having its own time scale.


2019 ◽  
Vol 19 (02) ◽  
pp. 2050017
Author(s):  
Roumen Tsekov

A theoretical parallel between the classical Brownian motion and quantum mechanics is explored via two stochastic sources. It is shown that, in contrast to the classical Langevin force, quantum mechanics is driven by turbulent velocity fluctuations with diffusive behavior. In the case of simultaneous action of the thermal and quantum noises, the quantum Brownian motion is described as well.


2007 ◽  
Vol 2007 ◽  
pp. 1-28 ◽  
Author(s):  
T. F. Oliveira ◽  
R. B. Miserda ◽  
F. R. Cunha

A statistical approach for the treatment of turbulence data generated by computer simulations is presented. A model for compressible flows at large Reynolds numbers and low Mach numbers is used for simulating a backward-facing step airflow. A scaling analysis has justified the commonly used assumption that the internal energy transport due to turbulent velocity fluctuations and the work done by the pressure field are the only relevant mechanisms needed to model subgrid-scale flows. From the numerical simulations, the temporal series of velocities are collected for ten different positions in the flow domain, and are statistically treated. The statistical approach is based on probability averages of the flow quantities evaluated over several realizations of the simulated flow. We look at how long of a time average is necessary to obtain well-converged statistical results. For this end, we evaluate the mean-square difference between the time average and an ensemble average as the measure of convergence. This is an interesting question since the validity of the ergodic hypothesis is implicitly assumed in every turbulent flow simulation and its analysis. The ergodicity deviations from the numerical simulations are compared with theoretical predictions given by scaling arguments. A very good agreement is observed. Results for velocity fluctuations, normalized autocorrelation functions, power spectra, probability density distributions, as well as skewness and flatness coefficients are also presented.


1994 ◽  
Vol 116 (3) ◽  
pp. 631-642 ◽  
Author(s):  
M. Matovic ◽  
S. Oka ◽  
F. Durst

Laser-Doppler measurements of axial mean velocities and the corresponding rms values of turbulent velocity fluctuations are reported for premixed, axisymmetric, acetylene flames together with the probability density distributions of the turbulent velocity fluctuations. All this information provides an insight into the structure of the flow field. Characteristic zones of the flow field are defined that show common features for all acetylene flames studied by the authors. These features are discussed in the paper and are suggested to characterize, in general, interesting parts of the flames.


1995 ◽  
Vol 7 (8) ◽  
pp. 1949-1955 ◽  
Author(s):  
Stein Malerud ◽  
Knut Jo/rgen Målo/y ◽  
Walter I. Goldburg

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