Computation Of Turbulent Flows Research Articles

Innovative bi-fluid atomizer inner flow characterization and outer spray diffusion analysis

We developed an atomizer nozzle equipping a medical device used for airborne disinfection of medical rooms. The diffusion technology of the equipment is based on the spraying of fine liquid droplets of disinfectant into the volume to be treated. The liquid phase is expulsed thanks to an air assist atomizer we designed, which originality comes from the geometry we give to the throat of the micro-venturi, inner part of the atomizer nozzle. The micro-venturi throat is deviated of angle of 4° and will permit a homogeneous diffusion.We computed three dimensional numerical calculations of the inner compressible turbulent air flow through the atomizer we designed and compared the results obtained with the ones computed for a symmetrical atomizer. The modeling was done with the CFD codes STARCCM+ and Fluent, choosing the k-omega turbulent model. The modeling has been validated especially by one dimensional analytical calculations and experimental measurements of the mean axial velocity and mass flow rate circulating through the atomizer. Three dimensional numerical calculations show the vertical deviation of the flow at throat level and swirl effect generated by the deviated inner throat of the micro-venturi. These calculations allowed understanding the nature of the spray observed in experimental conditions, and the advantages to use a deviated micro-venturi throat.Indeed, micro bacteriological tests showed that the quality and the effectiveness of the diffusion are enhanced in comparison to equipments with a symmetrical micro-venturi.

3D numerical analysis of flow in Francis turbine in sandy river

Three-dimensional turbulent flow calculation of the flow in Francis turbine in sandy river was conducted by using body-fitted coordinates, tetrahedral grid system and SIMPLE algorithm based on the solid-liquid two-phase flow equation of motion in Eulerian coordinate system. The flow mechanism of sandy water in the turbine was analyzed through numerical simulation of flow in the turbine in several different sand water concentrations. In sandy water with various concentrations, the erosion area of turbine are basically concentrated at the lateral wall of the whole flow passage and the heads of guide vanes and blades.

Discrete filter operators for large‐eddy simulation using high‐order spectral difference methods

SUMMARYThe combination of a high‐order unstructured spectral difference (SD) spatial discretization scheme with sub‐grid scale (SGS) modeling for large‐eddy simulation is investigated with particular focus on the consistent implementation of a structural mixed model based on the scale similarity hypothesis. The difficult task of deriving a consistent formulation for the discrete filter within the SD element of arbitrary order led to the development of a new class of three‐dimensional constrained discrete filters. The discrete filters satisfy a set of selected criteria and are completely local within the SD element. Their weights can be automatically computed at run time from the number of solution points within each element and the expected filter cutoff length scale. The novel discrete filters can be applied to any SGS model involving explicit filtering and to a broad class of high‐order discontinuous finite element numerical schemes. The code is applied to the computation of turbulent channel flows at three Reynolds numbers, namely Reτ = 180, 395, and 590 (based on the friction velocity uτ and channel half‐width δ). Results from computations with and without the SGS model are compared against results from direct numerical simulation. The numerical experiments suggest that the results are sensitive to the use of the SGS model, even when a high‐order numerical scheme is used, especially when the grid resolution is kept relatively low and mostly in terms of resolved Reynolds stresses. Results obtained using existing filters based on the projection of the solution over lower‐order polynomial bases are also shown and demonstrate that these filters are inadequate for SGS modeling purposes, mostly because of their inability to enforce the selected cutoff length scale with sufficient accuracy. The use of the similarity mixed formulation proved to be particularly accurate in reproducing SGS interactions, confirming that its well‐known potential can be realized in conjunction with state‐of‐the‐art high‐order numerical schemes.Copyright © 2012 John Wiley & Sons, Ltd.

Simulation of laminar and turbulent concentric pipe flows with the isogeometric variational multiscale method

We present an application of the residual-based variational multiscale modeling methodology to the computation of laminar and turbulent concentric annular pipe flows. Isogeometric analysis is utilized for higher-order approximation of the solution using Non-Uniform Rational B-Splines (NURBS). The ability of NURBS to exactly represent curved geometries makes NURBS-based isogeometric analysis attractive for the application to the flow through annular channels. We demonstrate the applicability of the methodology to both laminar and turbulent flow regimes.

Computations of Turbulent Flow and Heat Transfer in Circular Pipes Using a Low Reynolds Number Two-Equation Model of Turbulence Without Use of Wall Function

The present paper is concerned with the numerical simulations of the turbulent flow and heat transfer in circular pipes using a low Reynolds number model for turbulence kinetic energy and its dissipation rate. Unlike the high Reynolds number turbulence models, the present solution procedure requires no special treatments near the pipe walls; i.e., no wall functions are needed to simulate the turbulent flow and heat transfer at the pipe walls. It is well known that the type of the wall function has been criticized on the basis that the obtained solution is essentially dependent on some adjustable tuning model constants. The present low Reynolds number model takes the solution up to the walls without any abrupt changes in the axial velocity and temperature profiles. Finite-volume equations are derived for the conservation equations of the mass continuity, axial and radial velocity components, temperature, turbulence kinetic energy and its dissipation rate. The resulting finite-volume equations are solved iteratively using a tri-diagonal matrix algorithm. The obtained results are considered converged when the errors are less than 0.1 percent. The converged radial profiles of the gas temperature, the axial velocity and the axial profiles of the Nusselt number, mean bulk temperature, and wall heat flux are presented for six fixed values of the Reynolds numbers. Moreover, the axial profiles of the Nusselt number are presented for six values of the Reynolds number. The agreement between the present results and the corresponding standard analytical and numerical data is very good.

Large eddy simulation and a simple wall model for turbulent flow calculation by a particle method

SUMMARYA turbulent channel flow and the flow around a cubic obstacle are calculated by the moving particle semi‐implicit method with the subparticle‐scale turbulent model and a wall model, which is based on the zero equation RANS (Reynolds Averaged Navier‐Stokes). The wall model is useful in practical problems that often involve high Reynolds numbers and wall turbulence, because it is difficult to keep high resolution in the near‐wall region in particle simulation. A turbulent channel flow is calculated by the present method to validate our wall model. The mean velocity distribution agrees with the log‐law velocity profile near the wall. Statistical values are also the same order and tendency as experimental results with emulating viscous layer by the wall model. We also investigated the influence of numerical oscillations on turbulence analysis in using the moving particle semi‐implicit method. Finally, the turbulent flow around a cubic obstacle is calculated by the present method to demonstrate capability of calculating practical turbulent flows. Three characteristic eddies appear in front of, over, and in the back of the cube both in our calculation and the experimental result that was obtained by Martinuzzi and Tropea. Mean velocity and turbulent intensity profiles are predicted in the same order and have similar tendency as the experimental result. Copyright © 2012 John Wiley & Sons, Ltd.

Parallel Algorithms for Compressible Turbulent Flow Simulation Using Direct Numerical Method

In this study, SPMD parallel computation of compressible turbulent jet flow with an explicit finite difference method by direct numerical method is performed on the IBM Linux Cluster. The conservation equations, boundary conditions including NSCBC (charactering boundary conditions), grid generation method, and the solving processing are carefully presented in order to give other researchers a clear understanding of the large scale parallel computing of compressible turbulent flows using explicit finite difference method, which is scarce in the literatures. The speedup factor and parallel computational efficiency are presented with different domain decomposition methods. In order to use our explicit finite method for large scale parallel computing, the grid size imposed on each processor, the speedup factor, and the efficiency factor should be carefully chosen in order to design an efficient parallel code. Our newly developed parallel code is quite efficient from that of implicit finite difference method or spectral method on parallel computational efficiency. This is quite useful for future research for gas and particle two-phase flow, which is still a problem for highly efficient code for two-phase parallel computing.

Development and application of a high-resolution technique for jet flow computation using large eddy simulation

A high-resolution technique of large eddy simulation with the implicit subgrid model (ILES) is presented for computation of compressible turbulent flows. The technique is applied to computation of jet flows. The nozzle and jet flows were computed jointly for obtaining realistic flow parameters at the nozzle exit section. Two approaches to the flow simulation in the boundary layer and in the nozzle are considered. The Reynolds-averaged Navier-Stokes (RANS) equations are used for the boundary layer description in the first approach. For the RANS/ILES method, the influence of the order of difference approximation of convection terms in the Navier-Stokes equations on the resolvability of the developed method is shown. The computations were performed for subsonic and off-design supersonic jets on a grid containing 1.1 × 106 cells. It is found that an increase in the order of the difference approximation from five to nine enables the accuracy of results to be improved without increasing the number of computation grid cells. The high workability of the method is demonstrated for computation of supersonic flows with separations. In the second approach, the largest eddies in the boundary layer were resolved explicitly, and the flow near the wall was described by the simplified wall model (WM). Use of the WMILES method for joint computation of the nozzle and jet flows made it possible to refine the description of the turbulent mixing layer near the nozzle exit section and improve the accuracy of agreement with the experiment. The flows in different-shape chevron nozzles and their jets were jointly computed using the WMILES method on the grids with (2.3−2.5) × 106 cells. The agreement of the computations with the experiment for the level of maximum fluctuations in the mixing layer was improved appreciably as compared to the computations using the RANS/ILES method. A possible explanation is found for the growth in high-frequency noise in the investigated chevron nozzles, which is observed in the experiment.

Large-Eddy Simulations of Turbulent Incompressible Flows on GPU Clusters

A dual-level parallel incompressible flow solver accelerates turbulent flow computations on GPU clusters. This approach solves the pressure Poisson equation with a full-depth, amalgamated parallel geometric multigrid method, and implements a Lagrangian dynamic model for subgrid-scale turbulence modeling.

Flow Characteristics of Gas-Solid Two-Phase Flow in Annular Pipe of Gas Drilling

Air drilling technology has been widely used in the oil and gas exploration, coal, geothermal, geological exploration, nuclear industry and other fields due to its high drilling rate and low cost. However, the design of the pneumatic conveying system for the mineral detritus is still largely based on empiricism. The paper was set in the background of gas drilling, mainly studied the gas-solids two-phase flow characteristics in 90 degree bent annular pipe and backward-facing step of an annular pipe, which are very important parts of air drilling. They refer to the bent part and backward-facing step of an annular channel formed by the drill pipe and the borehole wall. A detailed numerical simulation and experimental studies were carried out for the flow structure and pressure losses of gas-solid two-phase in the annular pipe of gas drilling. Since a unified theory has not been developed for the two-phase flow in annular pipe, a lot of experimental work should be conducted. In the experimental research, the paper independently designed and built an annular pipe pneumatic conveying system with 90 degree bend and backward-facing step, including designing material screw feeder, material receiving hopper, pipeline, control system, data acquisition system, and etc. As known, many parameters, such as gas velocity, diameter and density of the particle, and solids loading ratio, can influence the conveying process. How these primordial influence factors act on the pressure losses of two-phase flow in annular pipe was analyzed in this paper. In the numerical simulation research, turbulent two-phase flow calculations were performed with a commercial CFD computer code referred to as FLUENT to study the gas-solid two phase flow in the sections of backward-facing step and 90 degree bent pipe respectively by using Euler-Lagrange method. The RNG κ-ε model and stochastic tracking were involved in the calculation of turbulence dispersion of two phases. The discrete phase model was performed for the solid phase. In the end, the numerical study 3-D results were translated to 1-D results using the standard averaging transformation to compare with experimental results. Predicted results obtained for pressure drop and velocity variations in full developed flows in the cases examined are in good qualitative agreement and are not in quantitative agreement with experimental data. The deviations between the simulations and experimental data lie in the range of 20%-30%. These results suggest commercial CFD codes such as FLUENT can be used productively for investigations into gas-solid two-phase flow phenomena and as an aid in pneumatic conveying design. The studies of the two-phase flow characteristics in the paper will contribute to reliable determination of the optimal condition of pneumatic conveying in gas drilling.

Effects of Quenching Heat Preserving Time on Structure and Properties of Boron-Containing High-Cr White Cast Iron

Air drilling technology has been widely used in the oil and gas exploration, coal, geothermal, geological exploration, nuclear industry and other fields due to its high drilling rate and low cost. However, the design of the pneumatic conveying system for the mineral detritus is still largely based on empiricism. The paper was set in the background of gas drilling, mainly studied the gas-solids two-phase flow characteristics in 90 degree bent annular pipe and backward-facing step of an annular pipe, which are very important parts of air drilling. They refer to the bent part and backward-facing step of an annular channel formed by the drill pipe and the borehole wall. A detailed numerical simulation and experimental studies were carried out for the flow structure and pressure losses of gas-solid two-phase in the annular pipe of gas drilling. Since a unified theory has not been developed for the two-phase flow in annular pipe, a lot of experimental work should be conducted. In the experimental research, the paper independently designed and built an annular pipe pneumatic conveying system with 90 degree bend and backward-facing step, including designing material screw feeder, material receiving hopper, pipeline, control system, data acquisition system, and etc. As known, many parameters, such as gas velocity, diameter and density of the particle, and solids loading ratio, can influence the conveying process. How these primordial influence factors act on the pressure losses of two-phase flow in annular pipe was analyzed in this paper. In the numerical simulation research, turbulent two-phase flow calculations were performed with a commercial CFD computer code referred to as FLUENT to study the gas-solid two phase flow in the sections of backward-facing step and 90 degree bent pipe respectively by using Euler-Lagrange method. The RNG κ-ε model and stochastic tracking were involved in the calculation of turbulence dispersion of two phases. The discrete phase model was performed for the solid phase. In the end, the numerical study 3-D results were translated to 1-D results using the standard averaging transformation to compare with experimental results. Predicted results obtained for pressure drop and velocity variations in full developed flows in the cases examined are in good qualitative agreement and are not in quantitative agreement with experimental data. The deviations between the simulations and experimental data lie in the range of 20%-30%. These results suggest commercial CFD codes such as FLUENT can be used productively for investigations into gas-solid two-phase flow phenomena and as an aid in pneumatic conveying design. The studies of the two-phase flow characteristics in the paper will contribute to reliable determination of the optimal condition of pneumatic conveying in gas drilling.

Unicorn: Parallel adaptive finite element simulation of turbulent flow and fluid–structure interaction for deforming domains and complex geometry

We present a framework for adaptive finite element computation of turbulent flow and fluid–structure interaction, with focus on general algorithms that allow for complex geometry and deforming domains. We give basic models and finite element discretization methods, adaptive algorithms and strategies for efficient parallel implementation. To illustrate the capabilities of the computational framework, we show a number of application examples from aerodynamics, aero-acoustics, biomedicine and geophysics. The computational tools are free to download open source as Unicorn, and as a high performance branch of the finite element problem solving environment DOLFIN, both part of the FEniCS project.

On the accuracy of high-order discretizations for underresolved turbulence simulations

In this paper, we investigate the accuracy of a high-order discontinuous Galerkin discretization for the coarse resolution simulation of turbulent flow. We show that a low-order approximation exhibits unacceptable numerical discretization errors, whereas a naive application of high-order discretizations in those situations is often unstable due to aliasing. Thus, for high-order simulations of underresolved turbulence, proper stabilization is necessary for a successful computation. Two different mechanisms are chosen, and their impact on the accuracy of underresolved high-order computations of turbulent flows is investigated. Results of these approximations for the Taylor–Green Vortex problem are compared to direct numerical simulation results from literature. Our findings show that the superior discretization properties of high-order approximations are retained even for these coarsely resolved computations.

Computation of Turbulent Flows in Natural Gas Pipes with Different Rectifiers

Computation of Turbulent Flows in Natural Gas Pipes with Different Rectifiers

Computation of Turbulent Flows in Natural Gas Pipes with Different Rectifiers

With the development of natural gas industry, improving the accuracy of the flowmeter systems is becoming the common concerned problem both corporations and customers. Many experiments demonstrate that the different flow field structures in pipes with diverse baffle installations have great influence on the measure accuracy of flowmeters. In order to measure the flux in pipes more accurate, the flowmeters must be installed downstream the baffles far away so that the irregular flows induced by baffles have less influence on the flowmeters. However, the installation lengths downstream baffles for flowmeters in ISO are are usually much longer than the allowable lengths for the local flowmeters. To decrease the installation lengths and loss measure accuracy of flowmeters as little as possible, it is necessary to study the flows in pipes with baffles. CFD is a good tool for the study. Here, the flows in a diffusive pipe with various rectifiers are computed as examples. The lengths of the irregular flows induced by the diffuser and rectifier are shown. By analyzing and comparison the numerical results, the installation lengths for flowmeters and the rectification of the different rectifiers are confirmed. The study conclusions will be very useful for guiding the local flowmeter selection and installation in the future.

Shear‐improved Smagorinsky modeling of turbulent channel flow using generalized Lattice Boltzmann equation

AbstractGeneralized Lattice Boltzmann equation (GLBE) was used for computation of turbulent channel flow for which large eddy simulation (LES) was employed as a turbulence model. The subgrid‐scale turbulence effects were simulated through a shear‐improved Smagorinsky model (SISM), which is capable of predicting turbulent near wall region accurately without any wall function. Computations were done for a relatively coarse grid with shear Reynolds number of 180 in a parallelized code. Good numerical stability was observed for this computational framework. The results of mean velocity distribution across the channel showed good correspondence with direct numerical simulation (DNS) data. Negligible discrepancies were observed between the present computations and those reported from DNS for the computed turbulent statistics. Three‐dimensional instantaneous vorticity contours showed complex vortical structures that appeared in such flow geometries. It was concluded that such a framework is capable of predicting accurate results for turbulent channel flow without adding significant complications and the computational cost to the standard Smagorinsky model. As this modeling was entirely local in space it was therefore adapted for parallelization. Copyright © 2010 John Wiley & Sons, Ltd.

Mathematically consistent boundary conditions and turbulence matching at block interfaces for computational aeroacoustics

The method of characteristics is used to implement the various boundary conditions (e.g. wall and interface) in a high-order computational aeroacoustic (CAA) code developed by the first author. Most characteristic methods do not satisfy Pfaff's condition (which needs to be satisfied for any mathematical relation to be valid). A mathematically consistent and valid method is used in this work to derive the characteristic boundary conditions. Also, a robust and efficient approach for the matching of turbulence quantities at multi-block interfaces is proposed. Various numerical simulation cases were run to validate the concepts. The computed results show that the proposed method is accurate, robust and is in excellent agreement with experimental data. The results also indicate that the matching of turbulence quantities is essential for accurate turbulent flow calculations.

2D and 3D numerical simulation of the wind-rotor/nacelle interaction in an atmospheric boundary layer

Two-dimensional axisymmetric and three-dimensional steady turbulent flow computations around two horizontal-axis wind turbines (Nordex N80 and Jeumont J48) are carried out to investigate the wind-rotor/nacelle interaction and quantify its effects on the wind speed at the nacelle anemometry. The actuator disk concept has been used to model the action of the blades. For both turbines, the geometry of the nacelle was reproduced as faithfully as possible. The terrain was represented by an appropriate law of the wall to account for roughness with particular attention paid to the boundary conditions in order to reproduce the neutral atmospheric boundary layer. The calculated velocity field in the vicinity of the nacelle exhibits good agreement with available experimental data. The results also show that for a complex nacelle geometry, like that of the N80, a three-dimensional calculation is necessary to obtain a good prediction of the velocity field in the near wake. The hub height effect is evaluated for the J48 by raising the nacelle from a height of 36 to 60 m. No significant impact is noted on the ratio nacelle wind speed/freestream wind speed.

Scale separation for implicit large eddy simulation

With implicit large eddy simulation (ILES) the truncation error of the discretization scheme acts as subgrid-scale (SGS) model for the computation of turbulent flows. Although ILES is comparably simple, numerically robust and easy to implement, a considerable challenge is the design of numerical discretization schemes resulting in a physically consistent SGS model. In this work, we consider the implicit SGS modeling capacity of the adaptive central-upwind weighted-essentially-non-oscillatory scheme (WENO-CU6) [X.Y. Hu, Q. Wang, N.A. Adams, An adaptive central-upwind weighted essentially non-oscillatory scheme, J. Comput. Phys. 229 (2010) 8952–8965] by incorporating a physically-motivated scale-separation formulation. Scale separation is accomplished by a simple modification of the WENO weights. The resulting modified scheme maintains the shock-capturing capabilities of the original WENO-CU6 scheme while it is also able to reproduce the Kolmogorov range of the kinetic-energy spectrum for turbulence at the limit of infinite Reynolds number independently of grid resolution. For isentropic compressible turbulence the pseudo-sound regime of the dilatational kinetic-energy spectrum and the non-Gaussian probability-density function of the longitudinal velocity derivative are reproduced.

A modified density based cavitation model for time dependent turbulent cavitating flow computations

The predictive capability of transport equation-based cavitation models including the Kubota cavitation model (Model-1) and interfacial dynamics cavitation model (Model-2), is evaluated for the attached turbulent cavitating flows. In this study, the test problem is the unsteady cloud cavitating flows around a Clark-Y hydrofoil. Based on the evaluations of existing models, we identified the differences between these two vaporization and condensation processes in the affected region, and provided a modified density based cavitation model (Model-3). The numerical results of the cavity shapes, velocity distributions and dynamics of the cavity oscillations were compared to existing experimental data. Compared with the other cavitation models, a significant improvement for the numerical results of unsteady cavitating flows has been obtained with the new model. Our study provides the information for further modeling development.