Direct numerical simulation of separated flow in a three-dimensional diffuser

2010 ◽  
Vol 650 ◽  
pp. 307-318 ◽  
Author(s):  
JOHAN OHLSSON ◽  
PHILIPP SCHLATTER ◽  
PAUL F. FISCHER ◽  
DAN S. HENNINGSON
Keyword(s):  
Numerical Simulation ◽  
Direct Numerical Simulation ◽  
Turbulent Flows ◽  
Three Dimensional ◽  
Turbulence Models ◽  
Separated Flow ◽  
Spectral Element ◽  
Mean Flow ◽  
Development Section ◽  
Inflow Condition

A direct numerical simulation (DNS) of turbulent flow in a three-dimensional diffuser at Re = 10000 (based on bulk velocity and inflow-duct height) was performed with a massively parallel high-order spectral element method running on up to 32768 processors. Accurate inflow condition is ensured through unsteady trip forcing and a long development section. Mean flow results are in good agreement with experimental data by Cherry et al. (Intl J. Heat Fluid Flow, vol. 29, 2008, pp. 803–811), in particular the separated region starting from one corner and gradually spreading to the top expanding diffuser wall. It is found that the corner vortices induced by the secondary flow in the duct persist into the diffuser, where they give rise to a dominant low-speed streak, due to a similar mechanism as the ‘lift-up effect’ in transitional shear flows, thus governing the separation behaviour. Well-resolved simulations of complex turbulent flows are thus possible even at realistic Reynolds numbers, providing accurate and detailed information about the flow physics. The available Reynolds stress budgets provide valuable references for future development of turbulence models.

scholarly journals Characterization of Model-Based Uncertainties in Incompressible Turbulent Flows by Machine Learning

2018 ◽  
Author(s):  
Mustafa Usta ◽  
Ali Tosyali
Keyword(s):  
Machine Learning ◽  
Turbulent Flows ◽  
Three Dimensional ◽  
Turbulence Models ◽  
Machine Learning Algorithms ◽  
Mean Flow ◽  
Flow Properties ◽  
Significant Discrepancy ◽  
Model Based ◽  
Flow Features

This work determines the inaccuracy of using Reynolds averaged Navier Stokes (RANS) turbulence models in transition to turbulent flow regimes by predicting the model-based discrepancies between RANS and large eddy simulation (LES) models. Then, it incorporates the capabilities of machine learning algorithms to characterize the discrepancies which are defined as a function of mean flow properties of RANS simulations. First, three-dimensional CFD simulations using k-omega Shear Stress Transport (SST) and dynamic one-equation subgrid-scale models are conducted in a wall-bounded channel containing a cylinder for RANS and LES, respectively, to identify the turbulent kinetic energy discrepancy. Second, several flow features such as viscosity ratio, wall-distance based Reynolds number, and vortex stretching are calculated from the mean flow properties of RANS. Then the discrepancy is regressed on these flow features using the Random Forests regression algorithm. Finally, the discrepancy of the test flow is predicted using the trained algorithm. The results reveal that a significant discrepancy exists between RANS and LES simulations, and ML algorithm successfully predicts the increased model uncertainties caused by the employment of k-omega SST turbulence model for transitional fluid flows.

An explicit algebraic Reynolds stress model for incompressible and compressible turbulent flows

2000 ◽  
Vol 403 ◽  
pp. 89-132 ◽  
Author(s):  
STEFAN WALLIN ◽  
ARNE V. JOHANSSON
Keyword(s):  
Turbulent Flows ◽  
Reynolds Stress ◽  
Compressible Flows ◽  
Three Dimensional ◽  
Turbulence Models ◽  
Reynolds Stress Model ◽  
Reynolds Stresses ◽  
Mean Flow ◽  
New Developments ◽  
The Individual

Some new developments of explicit algebraic Reynolds stress turbulence models (EARSM) are presented. The new developments include a new near-wall treatment ensuring realizability for the individual stress components, a formulation for compressible flows, and a suggestion for a possible approximation of diffusion terms in the anisotropy transport equation. Recent developments in this area are assessed and collected into a model for both incompressible and compressible three-dimensional wall-bounded turbulent flows. This model represents a solution of the implicit ARSM equations, where the production to dissipation ratio is obtained as a solution to a nonlinear algebraic relation. Three-dimensionality is fully accounted for in the mean flow description of the stress anisotropy. The resulting EARSM has been found to be well suited to integration to the wall and all individual Reynolds stresses can be well predicted by introducing wall damping functions derived from the van Driest damping function. The platform for the model consists of the transport equations for the kinetic energy and an auxiliary quantity. The proposed model can be used with any such platform, and examples are shown for two different choices of the auxiliary quantity.

scholarly journals Numerical modeling of turbulent flow in a plane channel on the basis of the Cabaret scheme

2019 ◽  
pp. 356-362
Author(s):  
Д.Г. Асфандияров
Keyword(s):  
Numerical Simulation ◽  
Direct Numerical Simulation ◽  
Turbulent Flows ◽  
Plane Channel ◽  
Classical Problem ◽  
Flow Characteristics ◽  
Pseudospectral Method ◽  
Numerical Results ◽  
Mean Flow ◽  
Cabaret Scheme

Представлены результаты моделирования классической задачи течения вязкой несжимаемой жидкости в плоском канале по схеме Кабаре при числах Рейнольдса, равных 5600, 13750 и 21900. Расчеты выполнены как при полном (прямое численное моделирование DNS Direct Numerical Simulation), так и неполном разрешении спектра турбулентных пульсаций. Во втором случае для расчетов используются сетки, характерные для моделирования пристенных течений методом крупных вихрей. Для более точного моделирования потока импульса на стенки при грубом разрешении пристенной области вводятся специальные искусcтвенные граничные условия. Это позволяет повысить точность определения средних характеристик течения. Проведено сравнение полученных результатов по схеме Кабаре с результатами прямого численного моделирования по псевдоспектральному методу. Some results of modeling the classical problem of flow of a viscous incompressible fluid in a plane channel at the Reynolds numbers equal to 5600, 13750, and 21900 using the Cabaret scheme are discussed. The computations are performed for the complete turbulence spectrum resolution (direct numerical simulation) and for the incomplete resolution. In the latter case, the grids typical for the large eddy simulation of nearwall turbulent flows are used. In order to obtain a more accurate representation of the momentum transfer toward the wall, some artificial boundary conditions are introduced. This allows us to model the mean flow characteristics with a higher accuracy. The numerical results obtained by the Cabaret scheme are compared with the numerical results obtained by the pseudospectral method.

DIRECT NUMERICAL SIMULATION OF TURBULENT FLOWS IN A VERTICAL ROTATING OPEN-CHANNEL

2005 ◽  
Vol 19 (28n29) ◽  
pp. 1443-1446
Author(s):  
BU-YANG LI ◽  
NAN-SHENG LIU ◽  
XI-YUN LU
Keyword(s):  
Numerical Simulation ◽  
Direct Numerical Simulation ◽  
Turbulent Flows ◽  
Coriolis Force ◽  
Friction Velocity ◽  
Open Channel ◽  
Dominant Role ◽  
Wall Friction ◽  
Mean Flow ◽  
Turbulence Statistics

Direct numerical simulation (DNS) is carried out to study turbulence characteristics in a vertical rotating open-channel with the rotation number N τ = 0-0.12 and the Reynolds number Re τ = 180 based on the wall friction velocity of non-rotating case and the channel depth. Here, two typical rotation regimes are identified. As 0 < N τ < 0.06, the turbulence statistics correlated with the spanwise velocity fluctuation are enhanced since the shear rate of spanwise mean flow induced by Coriolis force increases; however, the other statistics are suppressed. As N τ > 0.06, the turbulence statistics are suppressed significantly because the effect of Coriolis force plays as a dominant role.

scholarly journals Direct numerical simulation of ‘short’ laminar separation bubbles with turbulent reattachment

2000 ◽  
Vol 403 ◽  
pp. 223-250 ◽  
Author(s):  
M. ALAM ◽  
N. D. SANDHAM
Keyword(s):  
Numerical Simulation ◽  
Boundary Layer ◽  
Direct Numerical Simulation ◽  
Laminar Boundary Layer ◽  
Stokes Equations ◽  
Separation Bubble ◽  
Three Dimensional ◽  
Wall Turbulence ◽  
Mean Flow ◽  
Laminar Separation

Direct numerical simulation of the incompressible Navier-Stokes equations is used to study flows where laminar boundary-layer separation is followed by turbulent reattachment forming a closed region known as a laminar separation bubble. In the simulations a laminar boundary layer is forced to separate by the action of a suction profile applied as the upper boundary condition. The separated shear layer undergoes transition via oblique modes and Λ-vortex-induced breakdown and reattaches as turbulent flow, slowly recovering to an equilibrium turbulent boundary layer. Compared with classical experiments the computed bubbles may be classified as ‘short’, as the external potential flow is only affected in the immediate vicinity of the bubble. Near reattachment budgets of turbulence kinetic energy are dominated by turbulence events away from the wall. Characteristics of near-wall turbulence only develop several bubble lengths downstream of reattachment. Comparisons are made with two-dimensional simulations which fail to capture many of the detailed features of the full three-dimensional simulations. Stability characteristics of mean flow profiles are computed in the separated flow region for a family of velocity profiles generated using simulation data. Absolute instability is shown to require reverse flows of the order of 15–20%. The three-dimensional bubbles with turbulent reattachment have maximum reverse flows of less than 8% and it is concluded that for these bubbles the basic instability is convective in nature.

Numerical Simulation of Turbulent Flows in Centrifugal Compressor Stages With Different Radial Gaps

2007 ◽  
Author(s):  
Pavel E. Smirnov ◽  
Thorsten Hansen ◽  
Florian R. Menter
Keyword(s):  
Numerical Simulation ◽  
Steady State ◽  
Turbulent Flows ◽  
Centrifugal Compressor ◽  
Three Dimensional ◽  
Flow Characteristics ◽  
Turbulence Models ◽  
Detailed Comparison ◽  
Operating Point ◽  
Vaned Diffuser

Numerical simulation of three-dimensional flow in a one-stage centrifugal compressor with a diffuser of variable geometry has been performed using the ANSYS CFX 10 code. The computations were conducted using steady and unsteady flow formulations and employing the RANS two-equation turbulence models. Steady-state flow simulations in the compressor were done for two vaned diffuser geometries with different radial gaps. A detailed comparison with the experimental data reported in the literature for different operating points of the “Radiver” test case compressor is presented and discussed. Good agreement of the computed velocity field with the measurements data is obtained at the impeller exit. Downstream of the diffuser vane, prediction quality depends on the operating point. Transient simulations performed for the best operating point of the compressor did not improve considerably predictions of flow characteristics in the diffuser as compared to the steady-state approach.

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