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Flows in Complex Systems

J. Fluids Eng. 2012;134(9):091101-091101-9. doi:10.1115/1.4007268.

Numerical simulations for mixed convection flow of micropolar fluid in an open ended arc-shape cavity have been carried out in this study. Computation is performed using the alternate direct implicit (ADI) method together with the successive over relaxation (SOR) technique for the solution of governing partial differential equations. The flow phenomenon is examined for a range of values of Rayleigh number 102  ≤ Ra ≤ 106 , Prandtl number 7 ≤ Pr ≤ 50, and Reynolds number 10 ≤ Re ≤ 100. The study is mainly focused on how the micropolar fluid parameters affect the fluid properties in the flow domain. It was found that despite the reduction of flow in the core region, the heat transfer rate increases, whereas the skin friction and microrotation decrease with the increase in the vortex viscosity parameter Δ.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2012;134(9):091102-091102-11. doi:10.1115/1.4007316.

Electromagnetic fields may be used to control the flow separation during the flow of electrically conducting fluids around bluff obstacles. The steady separated flow around bluff bodies at low Reynolds numbers almost behaves as a creeping flow at a certain field strength. This phenomena, although already known, is exactly quantified through numerical simulation and the critical field strength of an externally applied magnetic field is obtained, for which the flow separation is completely suppressed. The flow of a viscous, incompressible, and electrically conducting fluid (preferably liquid metal or an electrolyte solution) at a Reynolds number range of 10–40 and at a low magnetic Reynolds number is considered in an unbounded medium subjected to uniform magnetic field strength along the transverse direction. Circular and square cross sections of the bluff obstacles are considered for simulation purposes. Fictitious confining boundaries are chosen on the lateral sides of the computational domain that makes the blockage ratio (the ratio of the cylinder size to the width of the domain) 5%. The two-dimensional numerical simulation is performed following a finite volume approach based on the semi-implicit method for pressure linked equations (SIMPLE) algorithm. The major contribution is the determination of the critical Hartmann number for the complete suppression of the flow separation around circular and square cylinders for the steady flow in the low Reynolds number laminar regime. The recirculation length and separation angle are computed to substantiate the findings. Additionally, the drag and skin friction coefficients are computed to show the aerodynamic response of the obstacles under imposed magnetic field conditions.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2012;134(9):091103-091103-9. doi:10.1115/1.4007257.

Magnetorheological (MR) fluids are widely used in the industrial world; however, sometimes their properties fail to meet system requirements. In shear mode, MR fluids have been found to exhibit a pressure dependency called squeeze strengthen effect. Since a lot of MR fluid based devices work in flow mode (i.e., dampers), this paper investigates the behavior in flow mode under pressure. The system design consists of three steps: the hydraulic system, the magnetic circuit, and the design of experiment method. The experimental apparatus is a cylinder in which a piston displaces the fluid without the use of standard gear pumps, which are incompatible with MR fluids. The experimental apparatus measures the yield stress of the MR fluid as a function of the pressure and magnetic field, thus, enabling the yield shear stress to be calculated. A statistical analysis of the results shows that the squeeze strengthen effect is also present in flow mode, and that the internal pressure enhances the performance of MR fluids by nearly five times.

Commentary by Dr. Valentin Fuster

Fundamental Issues and Canonical Flows

J. Fluids Eng. 2012;134(9):091201-091201-9. doi:10.1115/1.4007157.

The instability characteristics of a liquid jet discharging from a nozzle into a stagnant gas are investigated using the linear stability theory. Starting with the equations of motion for incompressible, inviscid, axisymmetric flows in cylindrical coordinates, a dispersion relation is obtained, where the amplification factor of the disturbance is related to its wave number. The parameters of the problem are the laminar velocity profile shape parameter, surface tension, fluid densities, and electrical charge of the liquid jet. The dispersion relation is numerically solved as a function of the wave number. The growth of instabilities occurs in two modes, the Rayleigh and atomization modes. For rWe<1 (where We represents the Weber number and r represents the gas-to-liquid density ratio) corresponds to a Rayleigh or long wave instability, where atomization does not occur. On the contrary, for rWe>>1 the waves at the liquid-gas interface are shorter and when they reach a threshold amplitude the jet breaks down or atomizes. The surface tension stabilizes the flow in the atomization regime, while the density stratification and electric charges destabilize it. Additionally, a fully developed flow is more stable compared to an underdeveloped one. For the Rayleigh regime, both the surface tension and electric charges destabilize the flow.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2012;134(9):091202-091202-9. doi:10.1115/1.4004904.

The flow behaviors around a square cylinder were modulated using the passive mesh fence. The effects of Reynolds number (Re) and rotation angle (θ) on the square-cylinder flow fields using different turbulence intensity (TI) were also investigated. Additionally, various steel mesh fences with different mesh densities were installed between the nozzle outlet and the test-section inlet to adjust the free-stream TI. The Reynolds number and turbulence intensity used in this investigation are 3.0 × 104  ≤ Re ≤ 1.0 × 105 and 0.32% ≤ TI ≤ 0.82%. The flow fields are visualized using the surface oil-flow visualization scheme. Furthermore, the flow patterns are classified as—leading-edge bubble, separation bubble, separation, leading-edge separation, and boundary-layer attached modes. Specifically, the leading-edge bubble mode does not exist while θ and TI are low. Moreover, a hot-wire anemometer was placed in the wake to detect the vortex-shedding frequency. The experimental results indicate that Strouhal number (St) decreases with increasing the free-stream TI while TI < 0.45%. However, St approaches a constant as TI > 0.45%. Furthermore, the surface pressure was detected using a pressure scanner and the drag coefficient (CD ) was obtained using the surface-pressure profile. The experimental results also reveal that CD decreases with increasing the free-stream TI. However, the change rate of CD for TI < 0.45% exceeds that for TI > 0.45%.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2012;134(9):091203-091203-23. doi:10.1115/1.4007015.

The influence of rounded corners on the aerodynamic forces and flow interference has been studied in detail for a uniform flow past two side-by-side arranged square cylinders. The Reynolds number (Re) based on the cylinder diameter (D) and free stream velocity (U∞ ) is 100. Numerical simulations are carried out for seven different transverse gap ratios (T/D), each with a minimum and maximum corner radius. An inbuilt finite difference code with staggered arrangement of flow variables is used to discretize the governing equations. The concept of immersed boundary method (IBM) is employed to simulate flow around rounded corners using the regular Cartesian grids. The computational code was validated for flow past an isolated circular cylinder, square cylinder, and two equal sized circular cylinders and the results were found to be in very good agreement with available literatures. In the present study, results in terms of the mean and rms values of lift and drag coefficients, Strouhal number, phase diagrams, and contours of streamlines and vorticity are presented. As the corner radius is increased, a reduction in the drag force is observed. There exists a significant effect of gap ratio and corner radius on the phase angle of lift and drag coefficients. Three different flow patterns, namely the single bluff body flow, biased gapside flow, and two independent bluff body flows, were observed from this study.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2012;134(9):091204-091204-9. doi:10.1115/1.4007156.

Correlations predicting the pressure loss coefficient along with the laminar, transitional, and turbulent limiting Reynolds numbers with the β ratio are presented for short square-edged orifice plates. The knowledge of pressure losses across orifices is a very important industrial problem while predicting pressure losses in piping systems. Similarly, it is important to define stable operating regions for the application of a short orifice at lower Reynolds numbers. This work experimentally determined pressure loss coefficients for square-edged orifices for orifice-to-diameter ratios of β = 0.2, 0.3, 0.57, and 0.7 for Newtonian and non-Newtonian fluids in both laminar and turbulent flow regimes.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2012;134(9):091205-091205-12. doi:10.1115/1.4007158.

In this paper, a numerical model is developed that can simulate the unsteady axisymmetric free-surface flow of a perfectly conductive liquid under an electrostatic field. The effect of the electrostatic field is modeled by a force distributed on the liquid free surface. Assuming the liquid as a perfect conductor makes it possible to reduce the general electromagnetic equations to electrostatic equations. The Navier–Stokes equations are solved to find the velocity and pressure fields. The free surface advection and reconstruction are performed based on the volume-of-fluid method using Youngs’ algorithm. To evaluate the effect of the electric field on the free surface, the electrostatic potential is first solved for the entire computational domain. Next, the electric field intensity and the surface density of the electric charge are calculated on the free surface after which the electric force can be determined. The computational method for treating this force is similar to that of the surface tension using the continuum surface force method. The developed model is validated by a comparison between the calculated results with those of the analytics as well as experiments for an electrowetting scenario.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2012;134(9):091206-091206-9. doi:10.1115/1.4007269.

After three decades of accumulated experimental and numerical results, a comprehensive understanding of the spatial evolution of axisymmetric turbulent boundary layers (ATBL) along long thin cylinders still eludes both scientists and engineers. While experimentalists dealt with axial alignment complexities, computationalists lacked proper inflow boundary conditions. Herein, we correct this latter deficiency and initiate an investigation of the thin cylinder turbulence under low Reynolds numbers and high transverse curvatures (boundary layer thicknesses to radius). Using the large-eddy simulation (LES) methodology, we are particularly interested in the radial propagation of the transverse curvature on the ATBL statistics. A ten-simulation matrix was constructed to examine these effects with validation against the experimental evidence. These tests investigated the ATBL maturity up to transverse curvatures approaching 2 orders of magnitude. A recently developed turbulent inflow procedure for the thin cylinder was implemented that couples a dynamic form of Spalding’s expression for rescaling the mean streamwise velocity with recycling of all superimposed turbulent fluctuations. The technique specifically circumvents intensive computations from the cylinder leading edge, and the rescaling-recycling combination minimizes the inflow turbulent regeneration length under very high transverse curvatures. After the initial transition phase in each LES computation, the respective numerical uncertainty was quantified to ensure sufficient spatial resolution within the discretized domain for resolving the energy-bearing scales of the turbulent motion. For the present low-Re conditions, the strength of the log layer steadily diminishes under continuous rise in the transverse curvature whereas the scaled fluctuating intensities elevate (except for the dominate shear stresses) with no sign towards full maturity. Each simulation reveals a boundary layer thickness that grows downstream by a factor of 7 relative to the momentum thickness with a linear influence of the transverse curvature on the wall-shear stress coefficient.

Commentary by Dr. Valentin Fuster

Multiphase Flows

J. Fluids Eng. 2012;134(9):091301-091301-14. doi:10.1115/1.4007267.

Two-phase flow pressure drops through thin and thick orifices have been numerically investigated with air–water flows in horizontal pipes. Two-phase computational fluid dynamics (CFD) calculations, using the Eulerian–Eulerian model have been employed to calculate the pressure drop through orifices. The operating conditions cover the gas and liquid superficial velocity ranges Vsg  = 0.3–4 m/s and Vsl  = 0.6–2 m/s, respectively. The local pressure drops have been obtained by means of extrapolation from the computed upstream and downstream linearized pressure profiles to the orifice section. Simulations for the single-phase flow of water have been carried out for local liquid Reynolds number (Re based on orifice diameter) ranging from 3 × 104 to 2 × 105 to obtain the discharge coefficient and the two-phase local multiplier, which when multiplied with the pressure drop of water (for same mass flow of water and two phase mixture) will reproduce the pressure drop for two phase flow through the orifice. The effect of orifice geometry on two-phase pressure losses has been considered by selecting two pipes of 60 mm and 40 mm inner diameter and eight different orifice plates (for each pipe) with two area ratios (σ = 0.73 and σ = 0.54) and four different thicknesses (s/d = 0.025–0.59). The results obtained from numerical simulations are validated against experimental data from the literature and are found to be in good agreement.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2012;134(9):091302-091302-6. doi:10.1115/1.4007214.

Preservation of the live storage of reservoirs is a serious challenge for most of the countries that encounter drought. Flushing of deposited sediments through bottom outlets in a dam is one of the suitable means to transport sediments toward downstream of the dam. Due to complicated conditions of flow through bottom outlets, we are facing a lack of accurate information about the various phenomena that occurs in bottom outlets such as cavitation, corrosion, and abrasion. Cavitation is the most important problem that causes remarkable damages on the lining of these tunnels. A numerical model based on the finite volume method for fully three-dimensional (3D) open channel flow equations is incorporated into a modeling cavitation and aeration system along the dam outlets. In this study, complex two-phase turbulent flow is simulated using the K-ɛ model. The pressure and velocity distribution under different conditions of gate opening and reservoir water level are computed along the tunnel and validation of the work has been obtained by comparison with measurements of a laboratory model. For both nonaerated and aerated flow, cavitation index in flow direction have been estimated and compared with each other.

Commentary by Dr. Valentin Fuster

Technical Briefs

J. Fluids Eng. 2012;134(9):094501-094501-3. doi:10.1115/1.4007232.

Slip flow in ducts is important in numerous contemporary applications, especially microchannel flow. This Note reviews the existing exact solutions for slip flow. These solutions serve as accuracy standards for approximate methods including numerical or semi-numerical means. Some new solutions are also introduced.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2012;134(9):094502-094502-6. doi:10.1115/1.4007266.

This paper deals with the study of a submerged jet for the suction of unwanted fluid. This submerged jet is caused by the fluid coming out from a source. The presence of a sink in front of this source facilitates the suction of the fluid depending upon the source and sink flow rates, the axial and lateral separations of the source and sink, and the angle between the axes of the source and sink. The main purpose is the determination of the sink flow rate for 100% removal of the source fluid as a function of these parameters. The experiments have been carried using a source nozzle 6 mm in diameter and two sizes for the sink pipe diameter: 10 mm and 20 mm. The main diagnostics used are flow visualization using dye and particle image velocimetry (PIV). The dependence of the required suction flow rate to obtain 100% effectiveness on the suction tube diameter and angle is relatively weak compared to the lateral separation.

Commentary by Dr. Valentin Fuster

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