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Shiyi Chen - One of the best experts on this subject based on the ideXlab platform.
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Local Reynolds Number and thresholds of transition in shear flows
Science China Physics Mechanics and Astronomy, 2013Co-Authors: Jianjun Tao, Shiyi ChenAbstract:Recent experimental and numerical investigations reveal that the onset of turbulence in plane-Poiseuille flow and plane-Couette flow has some similar stages separated with different threshold Reynolds Numbers. Based on these observations and the energy equation of a disturbed fluid element, a Local Reynolds Number ReL is derived to represent the maximum ratio of the energy supplement to the energy dissipation in a cross section. It is shown that along the sequence of transition stages, which include transient Localized turbulence, “equilibrium” Localized turbulence, spatially intermittent but temporally persistent turbulence and uniform turbulence, the corresponding thresholds of ReL for plane-Couette flow, Hagen-Poiseuille flow and plane-Poiseuille flow are consistent, indicating that the critical (threshold) states during the laminar-turbulent transition are determined by the Local properties of the base flow and are independent of global features, such as flow geometries (pipe or channel) and types of driving forces (shear driving or pressure driving).
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Universal threshold of the transition to Localized turbulence in shear flows
Theoretical and Applied Mechanics Letters, 2011Co-Authors: Jianjun Tao, Shiyi ChenAbstract:Abstract Experimental and numerical studies have shown similarities between Localized turbulence in channel and pipe flows. By scaling analysis of a disturbed-flow model, this paper proposes a Local Reynolds Number Re M to characterize the threshold of transition triggered by finite-amplitude disturbances. The Re M represents the maximum contribution of the basic flow to the momentum ratio between the nonlinear convection and the viscous diffusion. The lower critical Re M observed in experiments of plane Poiseuille flow, pipe Poiseuille flow and plane Couette flow are all close to 323, indicating the uniformity of mechanism governing the transition to Localized turbulence.
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examination of hypotheses in the kolmogorov refined turbulence theory through high resolution simulations part 1 velocity field
Journal of Fluid Mechanics, 1996Co-Authors: Lianping Wang, Shiyi Chen, James G Brasseur, John C WyngaardAbstract:The fundamental hypotheses underlying Kolmogorov-Oboukhov (1962) turbulence theory (K62) are examined directly and quantitutivezy by using high-resolution numerical turbulence fields. With the use of massively parallel Connection Machine-5, we have performed direct Navier-Stokes simulations (DNS) at 5123 resolution with Taylor microscale Reynolds Number up to 195. Three very different types of flow are considered : free-decaying turbulence, stationary turbulence forced at a few large scales, and a 2563 large-eddy simulation (LES) flow field. Both the forced DNS and LES flow fields show realistic inertial-subrange dynamics. The Kolmogorov constant for the k-5/3 energy spectrum obtained from the 5123 DNS flow is 1.68 kO.15. The probability distribution of the Locally averaged disspation rate E, over a length scale r is nearly log-normal in the inertial subrange, but significant departures are observed for high-order moments. The intermittency parameter p, appearing in Kolmogorov's third hypothesis for the variance of the logarithmic dissipation, is found to be in the range of 0.20 to 0.28. The scaling exponents over both E, and r for the conditionally averaged velocity increments ~,u(E, are quantified, and the direction of their variations conforms with the refined similarity theory. The dimensionless averaged velocity increments (&.PI~,)/(E,~)"/ ~ are found to depend on the Local Reynolds Number Recr = ~f/~r~/~/v in a manner consistent with the refined similarity hypotheses. In the inertial subrange, the probability distribution of ~5,u/(e,r)'/~ is found to be universal. Because the Local Reynolds Number of K62, I&, = ~:'~r~/~/v, spans a finite range at a given scale r as compared to a single value for the Local Reynolds Number R,. = Z1/3r4/3/v in Kolmogorov's (1941a,b) original theory (K41), the inertial range in the K62 context can be better realized than that in K41 for a given turbulence field at moderate Taylor microscale (global) Reynolds Number RA. Consequently universal constants in the second refined similarity hypothesis can be determined quite accurately, showing a faster-than-exponential growth of the constants with order n. Finally, some consideration is given to the use of pseudo-dissipation in the context of the K62 theory where it is found that the probability distribution of Locally averaged pseudo-dissipation ei deviates more from a log-normal model than the full dissipation
Emmanuel Sanjayanand - One of the best experts on this subject based on the ideXlab platform.
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Viscoelastic MHD flow and heat transfer over a stretching sheet with viscous and ohmic dissipations
Communications in Nonlinear Science and Numerical Simulation, 2008Co-Authors: M. Subhas Abel, Emmanuel Sanjayanand, Mahantesh M. NandeppanavarAbstract:Abstract A mathematical analysis has been carried out on momentum and heat transfer characteristics in an incompressible electrically conducting viscoelastic boundary layer fluid flow over a linear stretching sheet. Momentum boundary layer equation takes into account the effect of transverse magnetic field and electric field. Thermal boundary layer equation takes into account the viscous dissipation and Ohmic dissipation due to transverse magnetic field and electric field. Highly non-linear momentum boundary layer equation and thermal boundary layer equation are converted into similarity equations and then solved numerically by employing fifth order Runge–Kutta–Fehlberg method with shooting. The results are analysed for the situation when stretching boundary is prescribed by non-isothermal temperature, namely, prescribed surface temperature (PST) which varies quadratically with the flow directional coordinate x. The effects of various physical parameters like viscoelastic parameter, Prandtl Number, Local Reynolds Number, Eckert Number Hartmann Number and electric parameter on various momentum and heat transfers characteristics are analysed. Some of the important findings of this paper are (i) The combined effect of increasing the values of Local Reynolds Number Re x and Local electric parameter (E1) is to decrease skin-friction coefficient Cf largely. (ii) In presence of magnetic field the effect of electric field is to decrease temperature near the stretching boundary and increase the same significantly away from the stretching sheet. (iii) If electric field is present and the Prandtl Number is lower then there would be a significant decrease of temperature near the boundary sheet in case of viscoelastic fluid. (iv) The presence of electric field reverses the direction of heat transfer on the boundary stretching sheet more significantly in case of viscoelastic fluid.
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on heat and mass transfer in a viscoelastic boundary layer flow over an exponentially stretching sheet
International Journal of Thermal Sciences, 2006Co-Authors: Emmanuel Sanjayanand, Sujit Kumar KhanAbstract:A comprehensive mathematical analysis has been carried out on momentum, heat and mass transfers in a viscoelastic boundary layer fluid flow over an exponentially stretching continuous sheet. The contributions of the viscous dissipation and elastic deformation have been taken into account in this study. Zeroth order analytical Local similar solution of the highly non-linear stream function equation and confluent hypergeometric solutions of heat and mass transfer equations are obtained. These solutions involve an exponential dependence of stretching velocity, prescribed boundary temperature and concentration, prescribed boundary heat flux and concentration flux on the flow directional coordinate. The contribution of elastic deformation on various heat transfer characteristics is examined. The effects of various physical parameters like Local viscoelastic parameter, Prandtl Number, Local Reynolds Number, Local Eckert Number and Schmidt Number on various momentum, heat and mass transfers characteristics are presented in this paper.
Sujit Kumar Khan - One of the best experts on this subject based on the ideXlab platform.
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on heat and mass transfer in a viscoelastic boundary layer flow over an exponentially stretching sheet
International Journal of Thermal Sciences, 2006Co-Authors: Emmanuel Sanjayanand, Sujit Kumar KhanAbstract:A comprehensive mathematical analysis has been carried out on momentum, heat and mass transfers in a viscoelastic boundary layer fluid flow over an exponentially stretching continuous sheet. The contributions of the viscous dissipation and elastic deformation have been taken into account in this study. Zeroth order analytical Local similar solution of the highly non-linear stream function equation and confluent hypergeometric solutions of heat and mass transfer equations are obtained. These solutions involve an exponential dependence of stretching velocity, prescribed boundary temperature and concentration, prescribed boundary heat flux and concentration flux on the flow directional coordinate. The contribution of elastic deformation on various heat transfer characteristics is examined. The effects of various physical parameters like Local viscoelastic parameter, Prandtl Number, Local Reynolds Number, Local Eckert Number and Schmidt Number on various momentum, heat and mass transfers characteristics are presented in this paper.
Andrew Pollard - One of the best experts on this subject based on the ideXlab platform.
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the velocity spectra and turbulence length scale distributions in the near to intermediate regions of a round free turbulent jet
Physics of Fluids, 2009Co-Authors: Hachimi Fellouah, Andrew PollardAbstract:Stationary and flying single hot-wire measurements were made to investigate the near to intermediate field in a round free turbulent jet. Measurements were carried out at Reynolds Numbers ReD based on the jet exit mean velocity and the nozzle diameter, between 6000 and 100 000. The objective of this study was to investigate the concept of a mixing transition proposed by Dimotakis [“The mixing transition in turbulent flows,” J. Fluid Mech. 409, 69 (2000)]. This was done through the measurement of the velocity spectra and the calculation of Kolmogorov λK, Taylor microscale λT, the Liepmann–Taylor microscale λL, and the viscous length scale λν at different positions in the near to intermediate regions of a round free jet. The present results demonstrate the Local nature of mixing transition. It is found that the Kolmogorov, Taylor, and viscous length scales all decrease in magnitude with the Local Reynolds Number Reδ based on the Local center line mean velocity and the Local time-averaged diameter of the jet...
Michael Jason Gourlay - One of the best experts on this subject based on the ideXlab platform.
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equilibrium similarity effects of initial conditions and Local Reynolds Number on the axisymmetric wake
Physics of Fluids, 2003Co-Authors: Peter B V Johansson, William K George, Michael Jason GourlayAbstract:Equilibrium similarity considerations are applied to the axisymmetric turbulent wake, without the arbitrary assumptions of earlier theoretical studies. Two solutions for the turbulent flow are found: one for infinite Local Reynolds Number which grows spatially as x1/3; and another for small Local Reynolds Number which grows as x1/2. Both solutions can be dependent on the upstream conditions. Also, the Local Reynolds Number diminishes with increasing downstream distance, so that even when the initial Reynolds Number is large, the flow evolves downstream from one state to the other. Most of the available experimental data are at too low an initial Reynolds Number and/or are measured too near the wake generator to provide evidence for the x1/3 solution. New results, however, from a laboratory experiment on a disk wake and direct numerical simulations (DNS) are in excellent agreement with this solution, once the flow has had large enough downstream distance to evolve. Beyond this the ratio of turbulence inten...