R. M. Barron

Computation of incompressible potential flow using von Mises coordinates

Incompressible potential flow over symmetric airfoils has been formulated in terms of von Mises coordinates. This formulation provides a rectangular computational domain with Dirichlet boundary conditions for the single unknown y(x, ψ). The governing second order nonlinear pde is solved by SLOR on uniform and clustered grids. The method proves to be accurate, efficient and easy to code.

Numerical simulation of high-speed turbulent water jets in air

Numerical simulation of high-speed turbulent water jets in air and its validation with experimental data has not been reported in the literature. It is therefore aimed to simulate the physics of these high-speed water jets and compare the results with the existing experimental works. High-speed water jets diffuse in the surrounding atmosphere by the processes of mass and momentum transfer. Air is entrained into the jet stream and the entire process contributes to jet spreading and subsequent pressure decay. Hence the physical problem is in the category of multiphase flows, for which mass and momentum transfer is to be determined to simulate the problem. Using the Eulerian multiphase and the k–ε turbulence models, plus a novel numerical model for mass and momentum transfer, the simulation was achieved. The results reasonably predict the flow physics of high-speed water jets in air.

Numerical simulation of scour by a free falling jet

In this paper the numerical simulation of scour by a free falling jet is presented. It is assumed that the flow is two-dimensional, the alluvium is cohesionless, and that at the beginning of a run, i.e. at t = 0, the movable bed is flat. The scour simulation involves three basic steps: simulation of a turbulent flow in the stilling basin of a free falling jet, determination of distribution of sand concentration, and computation of bed deformation. The flow simulation rests on the momentum equations, the continuity equation, and on the k-ε equations for turbulent flows. A general 2-D non-orthogonal curvilinear computational domain is used; the solution is by the finite volume method, with a non-staggered grid arrangement. The SIMPLE scheme is used for pressure correction and the checkerboard problem is solved with a momentum interpolation scheme. The distribution of sand concentration is determined on the basis of the convection-diffusion equation. An appropriate boundary condition for concentration at the bed, which takes into account the effect of bed-load, is implemented. The bed deformation is computed with the aid of the sediment continuity equation. The aforementioned steps are repeated until the equilibrium bed surface (i.e. the equilibrium scour hole) is reached. The results of the numerical simulation appear to compare favourably with experiment.

Reynolds Number Effects on the Near-Exit Region of Turbulent Jets

In this paper, attention has been focused on the near-exit region of a round turbulent free jet to study the large-scale coherent structures and to document the signatures of the vortices over a range of Reynolds numbers. Particle image velocimeter measurements were conducted at three jet exit Reynolds numbers of 10,000, 30,000, and 55,000. The large-scale structures in the near field ( X/D<12 ) were investigated by performing a proper orthogonal decomposition analysis of the velocity fields. A vortex identification algorithm was complemented by swirling strength maps to identify the vortex centers, rotational sense, size, and circulation of the vortices. The influence of the Reynolds number on the distribution of the number, size, and circulation of the identified vortices was studied. Proper orthogonal decomposition of the velocity fields revealed that Reynolds number has a strong influence on the mean circulation of vortices. The present results show that the axial location where vortices first appear ...

CFD Study of Effects of Geometry Variations on Flow in a Nozzle

Abstract Reynolds-Averaged Navier-Stokes simulations have been performed to investigate the effect of nozzle geometry on the turbulence characteristics of incompressible fluid flow through nozzles at Reynolds number of approximately 50,000. Four nozzles have been considered: a baseline nozzle and three modified nozzles (extended, grooved and ringed). The flow in these nozzles has been simulated using different turbulence closure models, including Spalart-Allmaras, variants of k - ε and k - ω, and the Reynolds Stress Model (RSM). By comparison to experimental data, it is shown that the RSM produces more accurate results for the prediction of turbulent fluctuations. The presence of a ring significantly increases both the turbulence intensity and mean velocity at the exit, and requires a much higher inlet pressure to move the fluid through the nozzle. On the other hand, cutting a groove near the exit or extending the nozzle has little effect on the exit flow characteristics.

ANALYSIS OF ENTRAINMENT AT THE TURBULENT/NON-TURBULENT INTERFACE OF A SQUARE JET

Particle image velocimetry measurements are carried out to study the entrainment at the interface between the non-turbulent and turbulent regions in a square jet. Jet Reynolds number based on the hydraulic diameter of the jet is 50,000. Measurements cover up to 25 diameters downstream of the nozzle exit using five horizontal field-of-views in the central plane of the jet. The turbulent/non-turbulent interface is identified using a velocity criterion and a suitable thresholding method. Using vorticity and swirling strength it is shown that the turbulent/non-turbulent interface separates the rotational and irrotational regions of the flow. Instantaneous velocity vector field superimposed with the turbulent/non-turbulent interface are presented. The relation between the vortex cores in the vicinity of the turbulent/non-turbulent interface and the contractions and expansions noticed in the jet velocity field are explained. Entrainment into the jet is evaluated at each axial distance by identifying the points falling inside the turbulent region of the jet. Compared to a round jet, the square jet entrains more ambient fluid. In addition, normal volume fluxes going through the turbulent/non-turbulent interface of the square jet are found to be larger compared to that of a round jet.Copyright

Characteristics of Flow Structures in the Wake of a Bed-Mounted Bluff Body in Shallow Open Channels

The characteristics of the flow structures observed in the wake of a bluff body mounted vertically on the bed and normal to the flow in a shallow open channel are investigated using detached eddy simulation (DES). The flow structures in the shallow wake are identified using the λ2-criterion. A distinctive feature in the time-averaged flow field, referred to as the owl face of the first kind, is observed. The position of this spiraling structure is stable at locations close to the bed, while its rotation sense switches from stable inward to unstable outward spiraling as it moves toward the free surface, where the bed friction becomes insignificant and the flow develops into a traditional two-dimensional (2D) wake. A three-dimensional (3D) structure resulting from a horizontally oriented secondary roll-up process is observed immediately downstream of the base of the bluff body in the center of the near-wake region. In addition to the horseshoe vortex, a new structure that wraps around the bluff body in the toe region is identified, referred to as a collar vortex. The presence of the coherent structures in the near-bed region is highlighted and their influence on the wake region is discussed.

Heat Transfer Due to an Impinging Jet in a Confined Space

A numerical investigation using unsteady three-dimensional Reynolds-averaged Navier–Stokes (RANS) equations with the k-ω SST (shear stress transport) turbulent model was conducted to determine the flow and thermal characteristics of an unsubmerged axisymmetric oil jet in air, impinging normally on to a heated flat disk with finite radius, bounded by cylindrical walls kept at constant temperature. A 10 mm thick disk subjected to a high uniform heat flux was located at impingement distances ranging from 40 to 80 mm from the nozzle exit, for nozzle exit diameters of d = 1.0, 2.0, and 4.0 mm. The volume of fluid (VOF) method with a high-resolution interface-capturing (HRIC) scheme was implemented in STAR-CCM+. A new methodology was developed to predict the stagnation zone and local heat transfer coefficients. Contrary to previous research, it is shown that the radial extent of the stagnation zone is not fixed but depends on the gradient of radial velocity along the disk. The normalized local Nusselt number profile along the disk radius is found to be weakly dependent on Reynolds number for a given nozzle size. It is also shown that the local Nusselt number is not uniform in the stagnation region as reported by experimental studies but depends on the distribution of the near-wall radial velocity gradient. Using the computational results, new correlations to predict the dimensionless radial velocity gradient and Nusselt number have been developed. The present correlations are dimensionally balanced, eliminating a deficiency in earlier correlations noted in the literature.

2-D transonic calculations on a flow-based grid system

Lifting transonic full-potential flow is investigated using a streamfunction formulation which combines grid generation and flow physics into a single equation for the transformation metric. Supercritical calculations are carried out using the modified density method to provide numerical dissipation. The formulation is inherently simple and leads to easy coding and efficient computation.

Large eddy simulation of the near-field vortex dynamics in starting square jet transitioning into steady state

Large eddy simulation (LES) is carried out to study the vortex dynamics in the near-field of a starting turbulent square jet as well as its evolution into a developed steady jet. Simulations are conducted at Reynolds numbers (Re = UjD/υ) of 8000 and 45 000 based on the nozzle hydraulic diameter D and jet velocity (Uj). A Reynolds stress model was used to simulate the internal flow in the nozzle which provided the inlet conditions for the LES of the jet. To validate the simulations, turbulence statistics are compared with experimental results available for a steady square jet. Evaluation of the probability density function, skewness, and flatness of the centerline streamwise velocity (Uc) shows deviation from the Gaussian distribution in the near-field. Evolution of the self-induced deformation of the leading vortex ring is investigated to further clarify the role of axis-switching. The axis-switching is initiated earlier at low Reynolds number while the completion of the axis-switching process occurred at...