Constant Turbulent Viscosity Research Articles

Comparative study between turbulence models in curved channels

ABSTRACT This paper presents a comparative study between results obtained in two-dimensional computational simulations performed with three different turbulence models: constant viscosity; Elder Model and k˗Ꜫ Model. The simulations were performed using the software Telemac 2D. These results were compared to data obtained from a study in experimental channel with trapezoidal cross-section and composed of straight stretches and curves. The main objective of this comparison is to explore how turbulence models affect the general behavior of the simulated flow. To support these comparisons, statistical analysis were adopted to quantify the differences between the velocity fields obtained in the simulations and that observed in the experimental channel. The results showed that, despite the theoretical limitations, the use of the simpler turbulence closure model, that is the constant turbulent viscosity, can lead to results as good as or better than those obtained with more sophisticated models.

Surface signatures of submerged heated jet

The flow induced at the surface of a water body by a submerged heated horizontal turbulent jet was investigated experimentally with the aim of developing parameterizations for surface mean temperature/velocity fields. The jet nozzle diameter was fixed, the depth of the jet beneath the free surface was varied, and two jet Reynolds numbers (5020, 11300) were considered. The surface temperature was measured using a highly sensitive infrared camera, and the near-surface horizontal velocity field was measured using particle image velocimetry. The experimental results were explained using a model based on similarity solutions with variable turbulent viscosity. While classical Schlichting’s solution with constant turbulent viscosity predicts complete similarity for transverse velocity/temperature distributions only in a plane that coincides with the flow axis, the present solution predicts similarity in an arbitrary plane parallel to the flow axis, which was confirmed using data collected at the surface. Comparisons of present data with available previous results also showed general agreement.

Experimental Study and Numerical Simulation of Sediment Transport in a Shallow Reservoir

The prediction of sedimentation is an important aspect of reservoir planning and design. Such prediction can be supported by detailed analyses of flow patterns and sediment transport inside reservoirs, usually conducted through numerical simulation. This research compares laboratorial sedimentation experiments in a shallow reservoir and predictions using a 2D numerical model with depth-average Navier-Stokes equations and a sediment transport code. A number of sediment transport equations were tested, among which the Engelund and Fredsoe formulation better represented the measured data. Flows without sediment transport or without bed dunes could be simulated using Smagorinski’s turbulence model, while flows with sediment occurring over dunes needed the use of a constant turbulent viscosity. The similarity obtained between experimental data and numerical results, for both flow pattern and sediment deposition, confirms that the models and numerical codes used in this work are useful for the analysis and prediction of reservoir sedimentation.

Emergence and Secondary Instability of Ekman Layer Rolls

Abstract The authors revisit the idealized scenario by which long-lived rolls are believed to emerge in the neutral planetary boundary layer, that is, through the saturation of the shear instability of the neutrally stratified Ekman flow. First, the nonlinear stages of the primary instability are studied, using a constant turbulent viscosity with Reynolds numbers up to 1000. Two-dimensional equilibrated rolls are found to exist, as predicted earlier based on a weakly nonlinear expansion. However, the flow may not saturate into those equilibrated rolls if the turbulent Reynolds number is too high. Second, a linear stability analysis of these equilibrated rolls is performed, which finds that they are subject to a three-dimensional instability. The growth rate of the most unstable mode is comparable to the growth rate of the primary instability; the selected horizontal length scale is about 4 times shorter. The unstable mode draws its energy by interacting with both across-roll and along-roll shear, the latter interaction being stronger. The latitude and the direction of the geostrophic wind affect the dynamics through the horizontal component of the Coriolis vector; their influence is investigated in both studies. At Reynolds numbers sufficiently higher than the threshold of the primary instability, the saturated rolls depend negligibly on latitude and wind direction. However, the growth rate of the secondary instability depends substantially on latitude and wind direction over the range of Reynolds numbers considered.

Force Distributions Produced by an Electromagnetic Mold

The electromagnetic force and consequent fluid flow patterns that can occur in a conventional electromagnetic mold for continuous casting of aluminum are described. The role of system geometry, specifically the placement of the confinement coil and shield, is examined in detail. The distribution of electromagnetic pressure as well as the curl of the lorentz force are presented. The effect of frequency is also considered. The coupled circuit method is used to predict the current density distribution for a specific mold geometry. The induced currents are then used to predict the pressures and forces. Nonlinear stream function/vorticity equations are solved under the assumption of constant turbulent viscosity to obtain the flow patterns. The role of the screen in shaping the force patterns is critically examined.

Recirculating turbulent flow of liquid in gas-liquid-solid fluidized bed.

Axial and radial distributions of gas holdup and liquid velocity were studied in a gas-liquid-solid fluidized bed of 0.12 m i.d. Air, water and glass beads (0.58, 1.2 and 2.2 mm in diameter) were used as gas, liquid and solid particle phases, respectively. Local gas holdup was meaured by using an electric conductivity probe, and local liquid velocity was obtained by the electrolyte tracer method. The value of local gas holdup was in proportion to the square of the distance from the column wall. The liquid flow pattern was similar to that of the bubble column, showing an up-flow in the central region and a down-flow in the peripheral region with respect to the interstitial mean liquid flow. Liquid velocity at the central axis increased with increasing superficial gas velocity and with decreasing particle diameter. The results were analyzed on the basis of the circulating flow model, where a constant turbulent viscosity and a constant composition of the liquid-solid mixtures were assumed. The turbulent viscosity for fully developed fluidization was correlated as a function of particle diameter and superficial gas velocity, and was compared to that of the bubble column.