Reynolds-averaged Navier-Strokes Research Articles

Numerical analysis on thermal environment of liquid rocket with afterburning under different altitudes

To achieve a detailed understanding of the characteristics of the liquid hydrogen-oxygen (LH2/LOX) rocket exhaust plume with afterburning, the three-dimensional compressible Reynolds-averaged Navier-Strokes (RANS) equation with the realizable k-Ɛ turbulence model is applied to calculate the flow field of the supersonic exhaust gas. An optimized 9-species and 14-steps chemical mechanism is used for the afterburning effects. Multi-grid method is adopted to establish structural grid of 8.87 million cells for the finite volume computation. The afterburning model is established under six inflow conditions and then validated by experimental data. The validated model is applied to compare the under-expanded supersonic plume with and without afterburning reaction at different flight altitudes. The simulation results indicate that afterburning mainly occurs in the mixed layer. Compared to the frozen flow, the peak temperature of the reaction flow has increased from 123 K to 318 K, the mole concentrations of hydrogen and water vapor change in the reacting plume. The afterburning has significant influence on the thermal environment of the exhaust plume at low altitude, but its effect becomes weak with the altitude increase. The results have provided a relevant evaluation of the flow field characteristics of supersonic plume and offer some usefulness to further numerical simulations of the liquid rocket exhaust jet with afterburning.

Groyne spacing role on the effective control of wall shear stress in open-channel flow

ABSTRACTThe objective of an effective configuration for a series of groynes in an open channel is the attainment of large bed shear stress in the main channel for channel deepening and small sidewall shear stress in the groyne fields for bank protection with the lowest cost, i.e. with the largest spacing between groynes. A Reynolds-averaged Navier–Strokes, finite-volume, numerical model was used and it was validated against experimental data for the case of a single groyne. For series of groynes with uniform spacing between them, the most effective configuration was the one with spacing equal to six groyne lengths. For the range of parameters considered, a substantial improvement of the effectiveness was achieved by implementing a novel non-uniform configuration where the spacing between groynes was reduced to one and a half groyne lengths in the first four groyne fields, and remained equal to six groyne lengths in the subsequent ones.

Evaluation of computational and physical parameters influencing CFD simulations of pollutant dispersion in building arrays

Many CFD studies have investigated the influence of computational parameters on the predicted concentration distribution of pollutants around isolated buildings, but such studies for building arrays are still lacking. This study systematically evaluated the influence of four computational and two physical parameters on pollutant dispersion in building arrays, including turbulence models, grid resolution, discretization of time step size Δt, length of sampling period, aspect ratio of the arrays, and release rate of tracer gas. Throughout these evaluations, a set of published wind tunnel experimental data was used to validate the CFD models. For concentration simulations, the Large Eddy Simulation (LES) model gave the most accurate results but still had limitations in areas near the source, whereas the Detached Eddy Simulation (DES) and the Reynolds Averaged Navier-Strokes (RANS) RNG k−ε models underperformed in some areas. The results of the LES and DES simulations varied with changes in Δt∗ and sampling length until Δt∗ was less than 0.24 and the sampling length was higher than 2400 Δt∗ for LES and 1200 Δt∗ for DES. A larger aspect ratio did not necessarily result in a higher concentration field than a smaller ratio. An increase in the tracer gas release rate did not change the general dispersion characteristics, but it still affected the concentration distribution in the areas near the source and resulted in a larger polluted area. The findings of this study are intended to contribute to improvements in the quality of CFD simulations of pollutant dispersion in building arrays.

Numerical model of liquid metal flow in steel making tundish with flow modifiers

The optimum condition for clean steel production in the tundish of a continuous casting process reactor can be obtained using numerical modelling. Five different arrangements of flow modifier in the form of impact pad systems deployed in an eight ton, delta shaped, and two strand bloom caster tundish are analysed and optimum design of the impact pad to improve the inclusion removal efficiency is evolved. Reynolds Averaged Navier-Strokes (RANS) equations with standard k-ε model of turbulence and energy equation are used to study fluid flow and inclusion flotation in the tundish. The inclusion separation efficiency is evaluated by solving the inclusion transport equation. Height variations along with additional notch amongst different impact pads yield best micro inclusion separation efficiency.

An Entropy-Assisted Shielding Function in DDES Formulation for the SST Turbulence Model

The intent of shielding functions in delayed detached-eddy simulation methods (DDES) is to preserve the wall boundary layers as Reynolds-averaged Navier–Strokes (RANS) mode, avoiding possible modeled stress depletion (MSD) or even unphysical separation due to grid refinement. An entropy function fs is introduced to construct a DDES formulation for the k-ω shear stress transport (SST) model, whose performance is extensively examined on a range of attached and separated flows (flat-plate flow, circular cylinder flow, and supersonic cavity-ramp flow). Two more forms of shielding functions are also included for comparison: one that uses the blending function F2 of SST, the other which adopts the recalibrated shielding function fd_cor of the DDES version based on the Spalart-Allmaras (SA) model. In general, all of the shielding functions do not impair the vortex in fully separated flows. However, for flows including attached boundary layer, both F2 and the recalibrated fd_cor are found to be too conservative to resolve the unsteady flow content. On the other side, fs is proposed on the theory of energy dissipation and independent on from any particular turbulence model, showing the generic priority by properly balancing the need of reserving the RANS modeled regions for wall boundary layers and generating the unsteady turbulent structures in detached areas.

Performance Optimization of a Six-Strand Tundish

The aims of the present study are to predict and improve inclusion separation capacity of a six strand tundish by employing flow modifiers (dams and weirs) and to assess the influence of inclusion properties (diameter and density) together with velocity of liquid steel at the inlet gate on the inclusion removal efficiency of a six-strand tundish. Computational solutions of the Reynolds-Averaged Navier-Strokes (RANS) equations together with the energy equation are performed to obtain the steady, three-dimensional velocity and temperature fields using the standard k-e model of turbulence. These flow fields are then used to predict the inclusion sepapration by numerically solving the inclusion transport equation. To account for the effects of turbulence on particle paths a discrete random walk model is employed. It was observed that with the employment of flow modifiers, the inclusion separation capacity of tundish increases without any large variation in the outlet temperatures. It is shown that inclusion properties and velocity are important parameters in defining the operating conditions of a six-strand tundish.

Numerical Study of Flow Characteristics due to Interaction Between a Pair of Vortices in a Turbulent Boundary Layer

This paper represents a numerical study of the flow field due to the interactions between a pair of vortices produced by vortex generators in a rectangular channel flow. In order to analyze longitudinal vortices induced by the vortex generators, the pseudo-compressibility method is introduced into the Reynolds-averaged Navier-Strokes equations of a 3-dimensional unsteady, incompressible viscous flow. A two-layer k-e turbulence model is applied to a flat plate 3 dimensional turbulence boundary to predict the flow structure and turbulence characteristics of the vortices. The computational results predict accurately the vortex characteristics related to the flow field, the Reynolds shear stresses and turbulent kinetic energy. Also, in the prediction of skin friction characteristics the computational results are reasonably close to those of the experiment obtained from other researchers.

The effect of rotation and buoyancy on flow development in a rotating circular coolant channel with radially inward flow

The influence of rotation on the development of pressure-driven air flow in a circular-sectioned coolant channel is presented in this article. The channel is rotating about an axis perpendicular to its own centerline with an eccentric orientation, representing a typical configuration in turbine blade cooling design. As a first step, radially inwad directed flow has been examined. The opposite case, radially outward directed flow, is still under investigation and will be presented in the enar future. The flow field of the duct is investigated by means of experiments and numerical calculations that demonstrate rotation effects under both isothermal and heated flow conditions. Coriolis forces generate two counterrotating vortices in the flow and change the distributions of flow velocity as well as turbulence instensity. In the presence of density variations across the channel induced by thermal fluxes, buoyancy forces arise; significantly influencing the turbulence. Turbulent pipe flow with Reynolds numbers up to 60,000 is covered by the experiments, with the rotation number in the 0 to 0.1 range. The buoyancy parameter was chosen to be 0.04. Nonintrusive flow measurements were performed with an advanced Laser-2-Focus velocimeter (L2F). A finite-element flow analysis code is adapted to this flow situation, solving the three-dimensional Reynolds-averaged Navier-Strokes equations. The experimental data help to understand the Coriolis and buoyancy force affected flow phenomena and provide test data for the validation of numerical flow prediction.