N.S. Vlachos

Natural convection in a 2D enclosure with sinusoidal upper wall temperature

Natural convection in a two-dimensional, rectangular enclosure with sinusoidal temperature profile on the upper wall and adiabatic conditions on the bottom and sidewalls is numerically investigated. The applied sinusoidal temperature is symmetric with respect to the midplane of the enclosure. Numerical calculations are produced for Rayleigh numbers in the range 10 2 to 10 8 , and results are presented in the form of streamlines, isotherm contours, and distributions of local Nusselt number. The circulation patterns are shown to increase in intensity, and their centers to move toward the upper wall corners with increasing Rayleigh number. As a result, the thermal boundary layer is confined near the upper wall regions. The values of the maximum and the minimum local Nusselt number at the upper wall are shown to increase with increasing Rayleigh number. Finally, an increase in the enclosure aspect ratio produces an analogous increase of the fluid circulation intensity.

Measurement and calculations of laminar flow in a ninety degree bifurcation

Measurements and numericaL calculations of laminar flow in a plane 90 degrees bifurcation are presented. The corresponding two-dimensional steady flow Navier-Stokes equations solved by a finite-difference procedure employing pressure and velocity as dependent variables. The influence of Reynolds number and mass flow ratio on the velocity field, streamlines, local shear stress and pressure drop are quantified and shown to be substantial. The circulation patterns and shear stresses are examined in view of available data regarding the formation of atherotic plaques in the human circulatory system. The calculated velocity profiles are compared with measurements obtained with laser Doppler anemometry and the agreement is shown to be satisfactory. Calculations outside the range of measurements which are of value to biomechanics are also presented.

On the Limits of Validity of the Low Magnetic Reynolds Number Approximation in MHD Natural-Convection Heat Transfer

In the majority of magnetohydrodynamic (MHD) natural-convection simulations, the Lorentz force due to the magnetic field is suppressed into a damping term resisting the fluid motion. The primary benefit of this hypothesis, commonly called the low-R m approximation, is a considerable reduction of the number of equations required to be solved. The limitations in predicting the flow and heat transfer characteristics and the related errors of this approximation are the subject of the present study. Results corresponding to numerical solutions of the full MHD equations, as the magnetic Reynolds number decreases to a value of 10−3, are compared with those of the low-R m approximation. The influence of the most important parameters of MHD natural-convection problems (such as the Grashof, Hartmann, and Prandtl numbers) are discussed according to the magnetic model used. The natural-convection heat transfer in a square enclosure heated laterally, and subject to a transverse uniform magnetic field, is chosen as a case study. The results show clearly an increasing difference between the solutions of the full MHD equations and low-R m approximation with increasing Hartmann number. This difference decreases for higher Grashof numbers, while for Prandtl numbers reaching lower values like those of liquid metals, the difference increases.

Dynamical behaviour of drag-reducing polymer solutions

Abstract Related to the problem of the drag-reduction mechanism due to the elastic properties of macromolecular additives, we study theoretically and experimentally the dynamically behaviour of the molecules and the corresponding flow field. Optical properties, orientation, configuration or change of shape are studied by flow birefringence in classical or original, elongational flow devices. The dependence of the birefringence threshold on the molecular weight, nature, ionic character, concentration and level of degradation of the polymer is studied. The flow field is determined experimentally and by computation using a simple rheological model. A correlation with drag-reduction measurement in capillaries is tried.

Numerical study of educed coherent structures in the near-wall region of a particle-laden channel flow

The interaction of small heavy solid particles with turbulence near the wall of a vertical downward channel flow is investigated by using direct numerical simulation (DNS) and Lagrangian particle tracking. The interest is focused on the effect of the particles on the near-wall coherent structures obtained by conditional sampling of DNS results of a particle-laden turbulent channel flow. The coherent structures are detected from instantaneous flow fields by using the vortex definition of Jeong and Hussain [J. Fluid Mech. 285, 69 (1995)]. The Reynolds number of the particle-free flow is Reτ≈180 based on the friction velocity and the wall half distance. The particle response time is 200 wall units and the average mass and volume fractions φm=0.5 and φv=6.8×10−5, respectively. The particle diameter is smaller than the Kolmogorov length scale and the grid spacing, the latter being small enough to adequately resolve the smaller fluid flow scales. The feedback effect of the particles on the carrier phase is take...

Numerical investigation of momentum exchange between particles and coherent structures in low Re turbulent channel flow

The interaction between particles and coherent structures is studied by using discrete particle simulation combined with direct numerical simulation of gaseous flow in a vertical channel. A conditional sampling scheme is used to examine the modifications of the near-wall quasistreamwise vortices by the momentum exchange between the phases. The particle effect on the fluid flow is modeled by a point-force approximation. The particle diameters are smaller than both the smallest flow length scales and the computational grid spacing. Results are obtained for particle ensembles with four response times ranging from 10 to 200 wall units in numerical simulations with and without gravitational settling in the streamwise direction and interparticle collisions. It is found that the size of the quasistreamwise vortices is increased up to 25% in the presence of particles. The increase is larger for the smallest inertia particles studied, which is partly due to their locally nonuniform spatial distribution. The underl...

Effects of Flow and Geometry Boundary Conditions on Fluid Motion in a Motored IC Model Engine

Measurements of ensemble-averaged axial velocities and the r.m.s. of the corresponding fluctuations, obtained by laser-Doppler anemometry, are reported for axisymmetric flow in a non-compressing piston-cylinder assembly motored at 200 rev/min simulating an IC engine. The inlet geometry comprised an open valve, located centrally and flush with the cylinder head, with seat angles of 30° and 60° and incorporating 30° swirl vanes. Results are presented for bore-to-stroke ratios of 0.83 and 1.25 and swept-to-clearance volume ratios of 2,3 and 9. The results indicate strong similarities between the flow structures for different stroke and clearance; a system of vortices is formed with a large vortex occupying most of the flow space and with smaller vortices in the corners between the wall, piston and cylinder head. The influence of valve seat angle is more pronounced and results, for the 30° angle, in adherence of the incoming jet to the cylinder head with increase of the overall turbulence levels and creation ...

LAMINAR FLOW AND CONJUGATE HEAT TRANSFER IN RIB ROUGHENED TUBES

A numerical study of fluid flow and heat transfer in circular tubes roughened with an internal circumferential rib is presented. Laminar flow with constant wall temperature is considered while the influence of heat conduction in the tube wall is also incorporated in the analysis. It was found that heat transfer is affected by the ratio of thermal conductivity of the wall to that of the fluid. Significant increase in heat transfer is achieved only for high Prandtl number fluids in the ribbed tube. The higher pressure drop in a ribbed tube should also be considered in a cost-effective design. There are no experimental results available in the open literature to compare with the present computations.

Effects of turbulence model constants on computation of confined swirling flows

The SIMPLE turbulence model procedure of Patankar (1980) is presently used to systematically assess the applicability of the standard k-epsilon turbulence model and one of its variations to decaying swirl prediction in developing turbulent pipe-flow. It is found that the trends and extent of the decaying tangential velocity are well-predicted by the standard k-epsilon model; the modified turbulence model results in slower decay, but with no substantial improvement over the standard model. 6 refs.

A solution method for three-dimensional turbulent boundary layers on bodies of arbitrary shapes

Abstract This paper is concerned with the prediction of the three-dimensional turbulent flows around bodies of irregular but basically cylindrical shape. Examples of such bodies are: ship or submarine hulls or an aircraft fuselage. The solution method described uses a non-orthogonal coordinate system in which the surface of the body is arranged to coincide with a coordinate surface. The velocity components solved for are the axial, radial and circumferential components of the cylindrical system from which the coordinates are derived. The equations are solved by the finite difference method of [1] for three-dimensional, parabolic flows. Turbulence is accounted for via a two-equation model of turbulence developed in [2] and modelled in [3]. Some preliminary solutions are presented for flow around the rear part of a ships hull. The results seem to be qualitatively correct. However, no comparison with experimental data has yet been made. The present method can be easily extended to handle partially parabolic effects as described in [4].