Ferrohydrodynamics Mixed Convection of a Ferrofluid in a Vertical Channel with Porous Blocks of Various Shapes

Numerical simulations of (water-Fe3O4) ferrohydrodynamics (FHD) mixed convection inside a vertical channel are performed. The magnetic field is produced by three sources positioned outside the channel’s right wall. The latter is provided with localized heat sources surmounted by variously shaped porous blocks: rectangular, trapezoidal, and triangular. The general model of Darcy-Brinkman-Forchheimer is employed to describe the fluid flow in the porous regions, and the resulting equations are numerically solved by the finite volume approach. The influence of significant parameters, including the magnetic number (Mn), the Richardson number (Ri), and the shape of blocks, is examined. The results essentially reveal that the enhanced heat transfer brought by the magnetic field and its intensity increase is suppressed by the augmentation of Ri until a critical value, rising with Mn, beyond which the global Nusselt number increases again. The mean friction coefficient increases with increased Mn and reduced Ri. Compared to the case with no magnetic field, the maximum enhancement in heat transfer rate is around 132% for the rectangular blocks, 146% for the trapezoidal blocks, and 160% for the triangular blocks, while the maximum increase in pressure drop is approximately 45% for all the shapes. The triangular shape seems the most efficient because it leads to high heat transfer rates and low mean friction coefficients; its performance factor is 2.32 for a dominant magnetic field and 2.62 for a dominant buoyancy force. The current research's conclusions will help optimize the operation of various thermal engineering systems, including electronic devices, where the improved heat removal rate will keep the electronic components at a safe operating temperature

Experimental assessment of a new form of scaling law for near-wall turbulence

Scaling laws and intermittency in the wall region of a turbulent flow are addressed by analyzing moderate Reynolds number data obtained by single component hot wire anemometry in the boundary layer of a flat plate. The paper aims in particular at the experimental validation of a new form of refined similarity recently proposed for the shear dominated range of turbulence, where the classical Kolmogorov-Oboukhov inertial range theory is inappropriate. An approach inspired to the extended self-similarity allows for the extraction of the different power laws for the longitudinal structure functions at several wall normal distances. A double scaling regime is found in the logarithmic region, confirming previous experimental results. Approaching the wall, the scaling range corresponding to the classical cascade-dominated range tends to disappear and, in the buffer layer, a single power law is found to describe the available range of scales. The double scaling is shown to be associated with two different forms of refined similarity. The classical form holds below the shear scale L s . The other, originally introduced on the basis of DNS data for a turbulent channel, is experimentally confirmed to set up above L s . Given the experimental diffulties in the evaluation of the instantaneous dissipation rate, some care is devoted to check that its one-dimensional surrogate does not bias the results. The increased intermittency as the wall is approached is experimentally found entirely consistent with the failure of the refined Kolmogorov-Oboukhov similarity and the establishment of its new form near the wall.Comment: 27 pages, 9 figure

Transverse velocity structure functions in developed turbulence

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MHD mixed convection and entropy generation of a nanofluid in a vertical porous channel

International audienceA numerical study of entropy generation and MHD mixed convection flow of a nanofluid in a vertical porous channel is made. The left plate is thermally insulated, whereas four discrete heat sources dissipating a uniform heat flux are mounted on the right wall which is adiabatic elsewhere. Both assisting and opposing flows are considered. The Darcy-Brinkman-Forchheimer model with the Boussinesq approximation is adopted and the finite volume method is used to solve the governing equations with the appropriate boundary conditions. The influence of the magnetic field strength (Hartmann number), Joule heating effect (Eckert number), buoyancy force intensity (Richardson number), nanoparticles volume fraction, as well as porous medium permeability (Darcy number) on velocity profiles, isotherms, isentropic lines, global Nusselt number and total entropy generation are analyzed. The results showed an enhancement on heat transfer rate by using a porous medium, a nanofluid, a magnetic field without taking into account the Joule heating and when mixed convection is assisted. Globally, entropy generation increases with the parameters cited above

Intermittency and Reynolds number

cited By 41International audienceHot wire measurements of longitudinal and transverse increments are performed in three different types of flows on a large range of Reynolds numbers (100≲Rλ≲3000). An improved technique based on cumulant expansion of velocity structure functions is used to estimate the spreading of the pdfs and to study their scaling properties in the inertial range. Thus, the rate of intermittency depth through the scales of flow, called here β(Rλ), is experimentally introduced, and it is shown that β(Rλ) has a universal behavior on a very large Reynolds numbers range. © 1998 American Institute of Physics

A statistical estimator of turbulence intermittency in physical and numerical experiments

cited By 32International audienceThe velocity increments statistic in various turbulent flows is analysed through the hypothesis that different scales are linked by a multiplicative process, of which multiplier is infinitely divisible. This generalisation of the Kolmogorov-Obukhov theory is compatible with the finite Reynolds number value of real flows, thus ensuring safe extrapolation to the infinite Reynolds limit. It exhibits a β estimator universally depending on the Reynolds number of the flow, with the same law either for Direct Numerical Simulations or experiments, both for transverse and longitudinal increments. As an application of this result, the inverse dependence Rλ = f(β) is used to define an unbiased Rλ value for a Large Eddy Simulation from the resolved scales velocity statistics. However, the exact shape of the multiplicative process, though independent of the Reynolds number for a given experimental setup, is found to depend significantly on this setup and on the nature of the increment, longitudinal or transverse. The asymmetry of longitudinal velocity increments probability density functions exhibits similarly a dependence with the experimental setup, but also systematically depends on the Reynolds number

A statistical estimator of turbulence intermittency in physical and numerical experiments

PACS. 47.27.-i Turbulent flows, convection, and heat transfer - 47.27.Gs Isotropic turbulence; homogeneous turbulence - 47.27.Jv High-Reynolds-number turbulence,