The Experts below are selected from a list of 321 Experts worldwide ranked by ideXlab platform
Alain Kondjoyan - One of the best experts on this subject based on the ideXlab platform.
-
effect of turbulent Integral Length Scale on heat transfer around a circular cylinder placed cross to an air flow
Experimental Thermal and Fluid Science, 2002Co-Authors: Frederic Peyrin, Alain KondjoyanAbstract:Abstract The effect of air turbulence on heat transfer around a cylinder is known to be dramatic. Relations based on the intensity of turbulence have been developed to take it into account. However the range of validity of these relations is under debate since they do not take into consideration the Scale of turbulence which should also have an effect. This debate is not only theoretical because if the turbulent Length Scale is important most experiments usually performed at the laboratory Scale would probably be disputable in practical conditions. This paper aims at providing some experimental arguments on the effect of the turbulent Length Scale on the average transfer coefficient value. The effect of a twofold multiplication of this Scale from 0.05 to 0.10 m on the heat transfer coefficient has been measured around a circular cylinder placed in the cross-flow of air whose free stream turbulence intensity was 14%. In this range, the effect of the turbulent Integral Length Scale on the average transfer coefficient value is very small and cannot be compared with the dramatic influence of the turbulence intensity.
M K Behringer - One of the best experts on this subject based on the ideXlab platform.
-
flame front analysis of ethanol butanol iso octane and gasoline in a spark ignition engine using laser tomography and Integral Length Scale measurements
Combustion and Flame, 2015Co-Authors: P G Aleiferis, M K BehringerAbstract:Abstract Direct-injection spark-ignition engines have become popular due to their flexibility in injection strategies and higher efficiency; however, the high-pressure in-cylinder injection process can alter the airflow field by momentum exchange, with different effects for fuels of diverse properties. The current paper presents results from optical studies of stoichiometric combustion of ethanol, butanol, iso-octane and gasoline in a direct-injection spark-ignition engine run at 1500 RPM with 0.5 bar intake plenum pressure and early intake stroke fuel injection for homogeneous mixture preparation. The analysis initially involved particle image velocimetry measurements of the flow field at ignition timing with and without fuelling for comparison. Flame chemiluminescence imaging was used to characterise the global flame behaviour and double-pulsed Laser-sheet flame tomography by Mie scattering to quantify the local topology of the flame front. The flow measurements with fuel injection showed Integral Length Scales of the same order to those of air only on the tumble plane, but larger regions with Scales up to 9 mm on the horizontal plane. Averaged Length Scales over both measurement planes were between 4 and 6 mm, with ethanol exhibiting the largest and butanol the smallest. In non-dimensional form, the Integral Length Scales were up to 20% of the clearance height and 5–12% of the cylinder bore. Flame tomography showed that at radii between 8 and 12 mm, ethanol was burning the fastest, followed by butanol, iso-octane and gasoline. The associated turbulent burning velocities were 4.6–6.5 times greater than the laminar burning velocities and about 13–20% lower than those obtained by flame chemiluminescence imaging. Flame roundness was 10–15% on the tomography plane, with largest values for ethanol, followed by butanol, gasoline and iso-octane; chemiluminescence imaging showed larger roundness (18–25%), albeit with the same order amongst fuels. The standard deviation of the displacement of the instantaneous flame contour from one filtered by its equivalent radius was obtained as a measure of flame brush thickness and correlated strongly with the equivalent flame radius; when normalised by the radius, it was 4–6% for all fuels. The number of crossing points between instantaneous and filtered flame contour showed a strong negative correlation with flame radius, independent of fuel type. The crossing point frequency was 0.5–1.6 mm−1. The flame brush thickness was about 1/10th of the Integral Length Scale. A positive correlation was found between Integral Length Scale and flame brush thickness and a negative correlation with crossing frequency.
Mani Fathali - One of the best experts on this subject based on the ideXlab platform.
-
Impact of Initial Flow Parameters on a Temporal Mixing Layer Evolution
Direct and Large-Eddy Simulation VI, 2020Co-Authors: Mani Fathali, Johan Meyers, Chris Lacor, Martine BaelmansAbstract:This paper aims at determining the most influential initial flow field parameters on the evolution of turbulent mixing phenomena. To this end a stochastic method is developed to generate a divergence-free random velocity field with a prescribed energy distribution in physical and wave-number space. In addition, predetermined Integral Length Scales are established. Eight direct numerical simulations of a temporally evolving turbulent mixing layer are examined in detail. Simulation results show a large disparity in mean and instantaneous turbulent quantities, mainly effected by the energy spectrum, the Integral Length Scale and the divergence freeness of the initial field.
-
sensitivity of the two dimensional shearless mixing layer to the initial turbulent kinetic energy and Integral Length Scale
Physical Review E, 2016Co-Authors: Mani Fathali, Khoshnami M DeshiriAbstract:: The shearless mixing layer is generated from the interaction of two homogeneous isotropic turbulence (HIT) fields with different Integral Scales l_{1} and l_{2} and different turbulent kinetic energies E_{1} and E_{2}. In this study, the sensitivity of temporal evolutions of two-dimensional, incompressible shearless mixing layers to the parametric variations of l_{1}/l_{2} and E_{1}/E_{2} is investigated. The sensitivity methodology is based on the nonintrusive approach; using direct numerical simulation and generalized polynomial chaos expansion. The analysis is carried out at Re_{l_{1}}=90 for the high-energy HIT region and different Integral Length Scale ratios 1/4≤l_{1}/l_{2}≤4 and turbulent kinetic energy ratios 1≤E_{1}/E_{2}≤30. It is found that the most influential parameter on the variability of the mixing layer evolution is the turbulent kinetic energy while variations of the Integral Length Scale show a negligible influence on the flow field variability. A significant level of anisotropy and intermittency is observed in both large and small Scales. In particular, it is found that large Scales have higher levels of intermittency and sensitivity to the variations of l_{1}/l_{2} and E_{1}/E_{2} compared to the small Scales. Reconstructed response surfaces of the flow field intermittency and the turbulent penetration depth show monotonic dependence on l_{1}/l_{2} and E_{1}/E_{2}. The mixing layer growth rate and the mixing efficiency both show sensitive dependence on the initial condition parameters. However, the probability density function of these quantities shows relatively small solution variations in response to the variations of the initial condition parameters.
-
numerical study of the impact of the initial turbulent Integral Length Scale on the dynamics of a two dimensional shear free turbulent mixing layer
2014Co-Authors: Mostafa Khoshnami Deshiri, Mani FathaliAbstract:دییامن هدافتسا لیذ ترابع زا هلاقم نیا هب عاجرا يارب : Please cite this article using: M. Khoshnami Deshiri, M. Fathali, Numerical study of the impact of the initial turbulent Integral Length Scale on the dynamics of a two dimensional shear-free turbulent mixing layer, Modares Mechanical Engineering, Vol. 14, No. 6, pp. 113-123, 2014 (In Persian) يددع یسررب کیمانید يور هیلوا هتفشآ نادیم هصخشم لوط ریثات هتفشآ طلاتخا هیلا يدعبود شرب نودب
-
Sensitivity analysis of initial condition parameters on the transitional temporal turbulent mixing layer
Journal of Turbulence, 2008Co-Authors: Mani Fathali, Johan Meyers, Gustavo Rubio, Sergey Smirnov, Martine BaelmansAbstract:This paper aims at determining the most influential inlet turbulence parameters on the downstream transitional mixing region. To this end, a stochastic method is developed to generate a divergence-free random velocity field with a prescribed energy distribution in physical and wave-number space. In addition, predetermined Integral Length Scales can be established. Ten direct numerical simulations of a temporally evolving transitional turbulent mixing layer are examined in detail. Simulation results show a large disparity in mean and instantaneous turbulent quantities, mainly effected by the energy spectrum, the Integral Length Scale and the divergence freeness of the initial field.
J žiugžda - One of the best experts on this subject based on the ideXlab platform.
-
analysis of influence of free stream turbulence intensity and Integral Length Scale on skin friction and heat transfer of a circular cylinder
Experimental Thermal and Fluid Science, 1993Co-Authors: A žukauskas, P Vaitiekunas, J žiugždaAbstract:ABSTRACT A theoretical model for the analysis and approximation of experimental results has been proposed for the momentum eddy diffusivity induced by free turbulence intensity and Integral Length Scale. There eddy diffusivity model is applied to a cylinder situated in a uniform crossflow in the presence of free stream turbulence. Numerical solution of the governing momentum and energy equations with the proposed eddy diffusivity model yielded results for the skin friction coefficient and the Nusselt number. The analysis is performed Reynolds numbers in the range of 104 – 106, Prandtl numbers 0.71 – 112, the turbulence intensities Tu = 0.35 – 8.9% and dimensionless Integral Length Scale L' = (L / d)Re½ =0-50. The agreement between them is seen to be very good considering the scatter in the experimental data points.
Dong Kee Sohn - One of the best experts on this subject based on the ideXlab platform.
-
simultaneous effects of free stream trubulence intensity and Integral Length Scale on mass transfer about cylinder
Experimental Thermal and Fluid Science, 1994Co-Authors: Dong Kee SohnAbstract:Abstract The present study is concerned with an experimental investigation of the effects of both the cross-flow tubulence intensity and Length Scale on the mass transfer around a cylinder surface, where the flow of interest shows no transition to turbulent boundary layer and thus belongs to the subscritical regime. Turbulence quantities, such as turbulence intensity and Length Scale, were measured by time-averaging about 4 × 10 4 samples that were signaled by a hot-wire anemometer. Turbulence intensities considered here range from 0.5 to 10%,a nd Integral Length Scales from 4 to 23 mm at a free-stream mean velocity of 10 m/s. This yields a Reynolds number of about 49,000, based on the cylinder diameter. Turbulence characteristics were controlled by varying grids as well as by adjusting the grid-to cylinder distance. Under the turbulence conditions described, the napthalene sublimation technique was employed to measure the circumferential distributions of the local Sherwood number. Two distinc features were observed depending on the intensities of turbulence. At lower turbulence intensities, the local Sherwood numver distribution qualitatively exhibited the trend of the typical subcritical flow with no turbulence, although quantitative disaggrement was detected. By contrast, at higher turbulence levels there occurred two minimal values in the distribution of the local Sherwood number, which was of close resemblance with that found in the supercitical flow regime.