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- Basara, Branislav, 1964, et al.
(författare)
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Calculations of turbulent flow through a staggered tube bank
- 2017
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Ingår i: Proceedings of the Thermal and Fluids Engineering Summer Conference. - 2379-1748. ; 2017-April, s. 1563-1566
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Konferensbidrag (refereegranskat)abstract
- The primary aim of this paper is to numerically investigate the crossflow in a staggered tube bank by using a variable-resolution method. Experimental data of Simonin and Barcoude (1988) is available in the ERCOFTAC database. There are also few ERCOFTAC workshops, e.g. 1993, 19999, which were considering this test case primarily for checking the performance of the Reynolds-Averaged Navier-Stokes (RANS) models. Therefore, there are number of results with very different models which can be found in the literature. The work presented here aims to add one more set of results but this time with recently advanced variable resolution method, namely the Partially-Averaged Navier-Stokes (PANS). This method (Girimaji, 2006) belongs to so called bridging or seamless methods. The PANS approach adjusts seamlessly from the Reynolds-Averaged Navier-Stokes (RANS) to the Direct Numerical Solution (DNS) of the Navier-Stokes equation. The results are largely improved by using the PANS as for example shown in Basara (2015). This turbulence bridging method is derived from the RANS model equations. It inevitably improves results when compared with its corresponding RANS model if more scales of motions are resolved. This is done by varying the unresolved-to-total ratios of kinetic energy and dissipation. In the practice, the parameter which determines the unresolved-to-total kinetic energy ratio is defined by using the grid spacing and calculated integral length scale of turbulence. When the grid size is smaller, then more of the turbulent kinetic energy can be resolved. Usually, the integral scale of turbulence is obtained by summing up resolved turbulence, calculated as difference between instantaneous filtered velocity and the averaged velocity field, and unresolved turbulence obtained from its own equation. The turbulence model adopted in the present PANS variant is the four-equation ζ - f formulation (Hanjalic et al., 2004) which is the variant of more known v2-f model based on the elliptic relaxation concept.). As this model represents a practical and accurate RANS choice for a wide range of industrial applications, especially when used in conjunction with the universal wall approach (Popovac and Hanjalic, 2007, Basara, 2006), its PANS variant therefore guarantees that the proper near-wall model is used when fkis of a higher value. Therefore, the near-wall PANS variant of Basara et al. (2011) was used in the present study. The PANS model is implemented into the commercial CFD code AVL FIRE (AVL FIRE Manual, 2011).
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- Basara, Branislav, 1964, et al.
(författare)
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Effects of convection schemes on hybrid RANS-LES calculations
- 2018
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Ingår i: Notes on Numerical Fluid Mechanics and Multidisciplinary Design. - Cham : Springer International Publishing. - 1612-2909 .- 1860-0824. ; 137, s. 145-155
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Tidskriftsartikel (refereegranskat)abstract
- Nowadays it is commonly accepted to report on convections schemes in the case of Large Eddy Simulations. However, this is still not mandatory for the hybrid Reynolds-Averaged Navier-Stokes (RANS)—LES calculations. Therefore, this paper intends to show that the effects of convection scheme is equally important for hybrid RANS-LES calculations as well as for LES runs. We choose here the Partially-Averaged Navier–Stokes (PANS) model as the representative hybrid RANS-LES model but the conclusions derived in this work are equally applicable to other methods. The aim of the paper is to provide a proper criterion from the employment of higher order differencing schemes. The paper will assess two approaches, namely step functions where central-differencing scheme is used in the regions with a lower unresolved turbulent kinetic energy and a continuous function which depends on the ratio between unresolved and total kinetic energy. The results will be presented for the flow around the square cylinder. Direct numerical simulation (DNS) data and measurements are available for comparisons.
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| 6. |
- Basara, B., et al.
(författare)
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Near-Wall Formulation of the Partially Averaged Navier-Stokes Turbulence Model
- 2011
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Ingår i: AIAA Journal. - : American Institute of Aeronautics and Astronautics (AIAA). - 1533-385X .- 0001-1452. ; 49:12, s. 2627-2636
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Tidskriftsartikel (refereegranskat)abstract
- The variable-resolution partially averaged Navier-Stokes bridging strategy is applied to the four-equation k-epsilon-zeta-f turbulence model. In this approach, the popular two-equation model is enhanced with an additional transport equation for the velocity scale ratio zeta and an equation for the elliptic relaxation function f for the purpose of improved near-wall behavior. By using the elliptic relaxation technique to model the wall blocking effect, the new four-equation partially averaged Navier-Stokes model retains the simplicity of the previous two-equation partially averaged Navier-Stokes versions but significantly improves predictions in the near-wall region. The proposed partially averaged Navier-Stokes k-epsilon-zeta-f model is evaluated in a turbulent channel flow and flow around a three-dimensional circular cylinder mounted vertically on a flat plate. The results clearly show benefits of the improved near-wall modeling and extend partially averaged Navier-Stokes applicability to a broader range of smooth bluff-body separated flows.
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| 7. |
- Basara, Branislav, 1964, et al.
(författare)
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PANS methodology applied to elliptic-relaxation based eddy viscosity transport model
- 2010
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Ingår i: Notes on Numerical Fluid Mechanics and Multidisciplinary Design. - Berlin, Heidelberg : Springer Berlin Heidelberg. - 1612-2909 .- 1860-0824. - 9783642141386 ; 110, s. 63-69
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Konferensbidrag (refereegranskat)abstract
- The Partially-Averaged Navier-Stokes (PANS) approach is a recently proposed method which changes seamlessly from the Reynolds-Averaged Navier-Stokes (RANS) model equations to the direct numerical solution (DNS) of the Navier-Stokes equations as the unresolved-to-total ratios of kinetic energy and dissipation are varied. Two variants of the PANS model are derived up to now, one based on the k-epsilon formulation and the other based on the k-omega formulation. We introduce here another variant which is based on four equation eddy viscosity transport model, namely zeta-f turbulence model. Benefits of using such near wall model inside the PANS concept are clearly presented in this paper.
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| 8. |
- Basara, Branislav, 1964, et al.
(författare)
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PANS VS. LES FOR COMPUTATIONS OF THE FLOW AROUND A 3D BLUFF BODY
- 2008
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Ingår i: “7th International ERCOFTAC Symposium on “Engineering Turbulence Modelling and Measurements”, June 4-6, 2008, Limassol, Cyprus..
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Konferensbidrag (refereegranskat)abstract
- This paper presents comparisons between recentlyproposed Partially-Averaged Navier-Stokes(PANS) method with the Large Eddy Simulation(LES) for the flow around the Ahmed bluff body.PANS model changes seamlessly from Reynolds-Averaged Navier-Stokes (RANS) to the direct numericalsolution of the Navier-Stokes equations(DNS) as the unresolved-to-total ratios of kinetic energyand dissipation are varied. The parameter whichdetermines the unresolved-to-total kinetic energy ratiois defined based on the grid spacing and it is dynamicallyadjusted at each point at the end of everytime step. The main target of this paper is to show ifPANS obtains comparable results with LES calculationson finer meshes and improves results on coarsermeshes. In other words, the question to be answeredis whether PANS can provide the optimum solutionfor every computational mesh. This will be achievedby comparing PANS calculations on two differentcomputational meshes: 3.4 mil.(grid A), and 8.0 mil.(grid B), with LES calculations on grid A and additionaltwo grids consisting of 9.5 mil. cells (grid C)and 16.5 mil. cells (grid D).
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| 9. |
- Basara, B., et al.
(författare)
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Performance analysis of partially-averaged navier-stokes method for complex turbulent flows
- 2011
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Ingår i: 6th AIAA Theoretical Fluid Mechanics Conference, Honolulu, 27 - 30 June 2011. - Reston, Virigina : American Institute of Aeronautics and Astronautics.
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Konferensbidrag (refereegranskat)abstract
- The paper evaluates the performance of the Partially-Averaged Navier-Stokes (PANS) method on complex industrial Computational Fluid Dynamics (CFD) applications. The PANS method changes seamlessly from Reynolds-Averaged Navier-Stokes (RANS) to the Direct Numerical Solution (DNS) of the Navier-Stokes equations and supports any filter width or scale resolution and therefore, it has potential to be efficient tool to solve large and complex applications on available computational resources. The PANS variant, which is used in the present study, is based on the RANS k-ε-ζ-f model (v2/k-f model). In the work presented here, the filter width is controlled by specifying one control parameter: unresolved-to-total ratios of kinetic energy fk. Furthermore, this parameter is defined based on the grid spacing. A dynamic update of fk at each computational point and at the end of every time step makes the computational procedure very simple and attractive for industrial CFD. The model's performance is shown for complex CFD cases: an external car aerodynamics, a train aerodynamics and engine intake ports.
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| 10. |
- Basara, Branislav, 1964, et al.
(författare)
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Scale-resolving simulations of the flow in internal combustion engines
- 2018
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Ingår i: Proceedings of the Thermal and Fluids Engineering Summer Conference. - 2379-1748. ; 2018-March, s. 245-248
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Konferensbidrag (refereegranskat)abstract
- The common practice in predicting engine flows is to use the Reynolds-averaged Navier-Stokes (RANS) models. However, the RANS models are single point closures relying on the assumption of self-similarity of the turbulence spectrum, the fact leading to only one characteristic turbulence length scale, defining the entire spectrum. Consequently, the complex physics of the flow in engines could not be captured well in such a way. In the case of running RANS model for multi-cycle engine calculations, calculation results will converge to one cycle results (the first 3-4 cycles are different due to different cycle starting fields, but then the results of the each next cycle will be the same) without predicting cycle-to cycle variations (CCV). Some of reported CCV results with RANS models, are probably due to numerical artefacts rather than physical background of the RANS models. In order to correctly capture all engine related flow phenomena, Large-Eddy Simulation (LES) has been recently more often used, but due to high computational costs, mainly as a research but not as a productive tool. An alternative approach could be found in the use of hybrid LES/RANS (HLR) models. Therefore, we use here a seamless HLR method denoted as Partial-Averaged Navier-Stokes (PANS) in conjunction with the universal wall treatment (Basara, 2006) in order to provide the optimum regarding the accuracy and computational costs. This turbulence bridging method, which supports any filter width or scale resolution, is derived from the Reynolds-Averaged Navier-Stokes (RANS) model equations. It inevitably improves results when compared with its corresponding RANS model if more scales of motions are resolved. The PANS (Girimaji, 2006) variant derived from the four equation near-wall eddy viscosity transport model (see Basara et al. 2011), namely k-ε-ζ-f turbulence model is used here (commercial software AVL FIRE®). This is done by varying the unresolved-to-total ratios of kinetic energy and dissipation given as (euqacation presented) This procedure is applied here on the full engine case. First, Figure 1 shows the resolution parameter fk which is based on RANS results and Eq. (2). This roughly shows that the mesh is in general coarse, for example for LES calculations, but fine enough to try PANS model (note that fk = 1 is in the 'RANS region' and fk = 0.4 is the minimum value). One could propose such mesh analysis prior to LES calculations in order to avoid long calculations, an assessment of the resolved energy and then again new meshing etc. Following the SSV approach applied on the same mesh, an instantaneous fk, which is different than one based on the RANS calculations, is shown in Figure 2, but overall, there are a lot of similarities. This is of course also due to different turbulence level predicted by two approaches. Calculations are performed here also by using spray and combustion modules. Predicted flame front propagation is shown in Figure 3, so the variations are visible for two neighbour cycles N and N+1. One can see that at the top most position of the piston towards head side of position (top dead center - TDC) and few degrees after TDC, the flame front is very different. This can be also seen at different cross-section as shown in Figure 4. This of course causes the pressure variations. It should be also reported that for this particular mesh, cycle to cycle variations obtained by LES are double larger than obtained with PANS calculations (20% vs. 10%) and measurements provided the value of 30%. However, having in mind Figures (1) and (2), and knowing that for proper LES calculations fk should be at least less than 0.2, those variations of 20% could be more addressed to the calculation error. The results presented here show the advantage of the variable resolution PANS method when compared to RANS models. With the help of the computational mesh and the resolution parameters incorporated used in PANS, cycle-to-cycle variations could be predicted. These results approve the PANS basic idea of providing the optimum fidelity on the given numerical mesh. It is also clear that in order to get the most accurate results, meshes should be further refine. However, many engine cases can be calculated with coarser meshes and the then last investigations could be made with refined meshes.
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