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Turbulence Scales (turbulence + scale)
Selected AbstractsPIV measurement and turbulence scale in turbulent combustionHEAT TRANSFER - ASIAN RESEARCH (FORMERLY HEAT TRANSFER-JAPANESE RESEARCH), Issue 7 2006Kazuhiro Yamamoto Abstract We have investigated turbulent combustion by PIV (Particle Image Velocimetry) technique. Comparing with LDV data, the validity of PIV measurements has been confirmed. Particularly, the conditions of sampling number and spatial resolution have been shown to yield reliable data using PIV. Based on the velocity fields in cold flow and combustion, the interaction between flame and flow has been discussed. It was observed that the flow field is changed by combustion and the turbulence is reduced. In order to determine statistical quantities such as mean velocity and RMS of velocity fluctuation, a sampling number of 1000 is needed. Moreover, the velocity correlation coefficient was evaluated to obtain the integral length scale of the flow. For both cold flow and combustion, the PIV estimated scale is very close to that of LDV based on the assumption of Taylor's hypothesis. As a result, the spatial resolution in this study is about 6 times smaller than the integral length scale. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(7): 501,512, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20129 [source] On the effect of the local turbulence scales on the mixing rate of diffusion flames: assessment of two different combustion modelsINTERNATIONAL JOURNAL OF ENERGY RESEARCH, Issue 10 2002Jose Lopes Abstract A mathematical model for the prediction of the turbulent flow, diffusion combustion process, heat transfer including thermal radiation and pollutants formation inside combustion chambers is described. In order to validate the model the results are compared herein against experimental data available in the open literature. The model comprises differential transport equations governing the above-mentioned phenomena, resulting from the mathematical and physical modelling, which are solved by the control volume formulation technique. The results yielded by the two different turbulent-mixing physical models used for combustion, the simple chemical reacting system (SCRS) and the eddy break-up (EBU), are analysed so that the need to make recourse to local turbulent scales to evaluate the reactants' mixing rate is assessed. Predictions are performed for a gaseous-fuelled combustor fired with two different burners that induce different aerodynamic conditions inside the combustion chamber. One of the burners has a typical geometry of that used in gaseous fired boilers,fuel firing in the centre surrounded by concentric oxidant firing,while the other burner introduces the air into the combustor through two different swirling concentric streams. Generally, the results exhibit a good agreement with the experimental values. Also, NO predictions are performed by a prompt-NO formation model used as a post-processor together with a thermal-NO formation model, the results being generally in good agreement with the experimental values. The predictions revealed that the mixture between the reactants occurred very close to the burner and almost instantaneously, that is, immediately after the fuel-containing eddies came into contact with the oxidant-containing eddies. As a result, away from the burner, the SCRS model, that assumes an infinitely fast mixing rate, appeared to be as accurate as the EBU model for the present predictions. Closer to the burner, the EBU model, that establishes the reactants mixing rate as a function of the local turbulent scales, yielded slightly slower rates of mixture, the fuel and oxidant concentrations which are slightly higher than those obtained with the SCRS model. As a consequence, the NO concentration predictions with the EBU combustion model are generally higher than those obtained with the SCRS model. This is due to the existence of higher concentrations of fuel and oxygen closer to the burner when predictions were performed taking into account the local turbulent scales in the mixing process of the reactants. The SCRS, being faster and as accurate as the EBU model in the predictions of combustion properties appears to be more appropriate. However, should NO be a variable that is predicted, then the EBU model becomes more appropriate. This is due to the better results of oxygen concentration yielded by that model, since it solves a transport equation for the oxidant concentration, which plays a dominant role in the prompt-NO formation rate. Copyright © 2002 John Wiley & Sons, Ltd. [source] Effects of fish size, time-to-fatigue and turbulence on swimming performance: a case study of Galaxias maculatusJOURNAL OF FISH BIOLOGY, Issue 6 2003V. I. Nikora A simple relationship for the inanga Galaxias maculatus swimming velocity is suggested and tested in low and high turbulence channels. The relationship connects the swimming velocity with fish Reynolds and Froude numbers and can be used in both ecological analysis (e.g. habitat requirements) and management strategies (e.g. fishways design). Contrary to some previous studies and intuition, effects of turbulence on swimming performance appeared to be negligible. The most likely explanation for this result relates to mechanics of fish,turbulence interactions that may be dependent on both the turbulence scales and energy. The data suggest that future studies of turbulence effects on fish behaviour should involve, in addition to turbulence energetics, consideration of fish dimensions in relation to the spectrum of turbulence scales. [source] Air,sea exchanges in the equatorial area from the EQUALANT99 dataset: Bulk parametrizations of turbulent fluxes corrected for airflow distortionTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 610 2005A. Brut Abstract Turbulent fluxes of momentum, sensible heat and water vapour were calculated using both the eddy covariance (EC) and the inertial dissipation (ID) methods applied to data collected on board the research vessel La Thalassa during 40 days of the EQUALANT99 oceanographic campaign. The aim of this experiment was to establish accurate parametrizations of air,sea fluxes for the equatorial Atlantic area from a large dataset. However, the accuracy of turbulent fluxes measured aboard ships is strongly affected by the distortion of airflow patterns generated by obstacles such as the ship and mast. For the EQUALANT99 experiment, the effects of airflow distortion were estimated using physical simulations in a water channel. To reproduce the conditions of the campaign, a neutral boundary layer was simulated in the water channel and a detailed model of the ship La Thalassa was built. Correction coefficients for the mean wind speed were evaluated from these physical simulations. They show a dependence on both the azimuth angle of the flow (i.e. the horizontal direction of the flow with respect to the ship's longitudinal axis) and the angle of incidence of the wind. The correction for airflow distortion was applied to the measured wind speed and also included in the flux computation using the ID method. Compared with earlier studies which applied a single correction per flux sample, it appears that our results for the corrected transfer coefficients present greater dependence on neutral wind speed than the previous parametrizations; the method also shows encouraging results, with a decrease in the scatter of the transfer coefficients parametrization. However, the distortion could not be corrected for in the fluxes calculated using the EC method, because this technique integrates a wide range of turbulence scales for which the airflow distortion cannot be simulated in a water channel. Fluxes computed using the ID and EC methods are presented and compared in order to determine which method, in the configuration of the EQUALANT99 experiment, provides the best resulting transfer coefficients. According to the results, fluxes of momentum and latent heat computed by ID were better for deriving the drag and humidity coefficients. The EC method seemed better adapted to calculate sensible-heat fluxes than the ID method, although a high scatter remained on the Stanton neutral number. Copyright © 2005 Royal Meteorological Society [source] | |||||||||||||