1,721,007 research outputs found
Numerical solutions of turbulent flows: industrial applications
No full textLa Computational Fluid-Dynamics (CFD) si trova sempre maggiormente coinvolta
nello studio di sistemi energetici innovativi. Quindi è logico pensare a filoni di ricerca
in cui si sviluppano metodi numerici efficienti, robusti ed accurati per la soluzione di
tali problemi.
In questa tesi si affronta la soluzione numerica di alcuni problemi di interesse industriale
sia con tecniche risolutive standard che innovative. In particolare sono stati
sviluppati solutori ad elavato ordine di accuratezza per flussi incomprimibili basati sul
metodo ad elementi finiti discontinui di Galerkin (DG). Il metodo DG è basato su approssimazioni
polinomiali all’interno del singolo elemento computazionale senza richiesta
di continuità globale della soluzione. Recentemente tale metodo sta ricevendo
particolare interesse per l’applicazione a problemi di CFD. Partendo da un codice presistente
2D viscoso, basato su un flusso a comprimibilità artificiale, è stata sviluppata
una versione 3D che si sta dimostrando capace di essere utilizata come solutore DNS.
Quindi è stato aggiunto il modello di turbolenza di Spalart-Allmaras (SA) sia sulla
versione 2D che 3D. Vale la pena notare che la soluzione DG delle equazioni RANS
è molto complessa per via dell’enorme rigidezza numerica del problema. In questo
lavoro viene proposta un’implementazione innovativa del modello SA che modifica
opportunamente i termini sorgenti e diffusivi dell’equazione di evoluzione SA quando
la variabile di lavoro, o una della funzioni di chiusura del modello, diventano negative.
E’ importante notare che ad oggi in letteratura non sono presenti lavori che trattano
della soluzione DG del sistema di equazioni RANS-SA incomprimibili. L’approccio
proposto è stato quindi testato su un’ampia gamma di problemi. Parallelamente è stata
studiata sia l’Aerodinamica instazionaria dei rotori eolici di tipo Savonius che il
campo di moto interno al tubo vortice ad effetto Ranque-Hilsch (RHVT) attraverso
solutori standard a volumi finiti. Nonostante durante questo lavoro di tesi siano stati
sviluppati solutori DG per un ampio range di numeri di Reynolds, ad oggi essi non
sono stati ancora applicati a problemi come il Savonius o RHVT in quanto al tempo
di quelle analisi non si disponeva dei codici allo stato di sviluppo attuale
Spalart–Allmaras model apparent transition and RANS simulations of laminar separation bubbles on airfoils
No full textThe present paper deals with the Reynolds Averaged Navier–Stokes (RANS) simulation of Laminar Separation Bubble (LSB). This phenomenon is of large interest in several engineering fields, such as the study of wind turbines, unmanned aerial vehicles (UAV) and micro-air vehicles (MAV) characterized by a low operating Reynolds number. In such contexts a laminar boundary layer separation followed by a turbulent transition and afterwards by a turbulent reattachment may appear in the flow-field.
The main novelty of this work is that an almost standard Spalart–Allmaras (SA) model, without additional equations for transition modeling, was successfully employed. The result achieved is very surprising being the model not developed for this purpose, but for fully-turbulent flows or for cases with imposed transition location. This result is of large interest, since the SA model is widely used in commercial, open-source and research codes. However, our approach cannot be advocated to predict natural transition within an attached boundary layer, indeed it is only able to deal with transitions triggered by a separated flow.
The reliability and accuracy of our approach are here proved computing, by means of a high-order Discontinuous Galerkin (DG) incompressible solver, the flow-field over two airfoils at different flow regimes showing the formation of a LSB
High-order discontinuous Galerkin solutions of three-dimensional incompressible RANS equations
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Detached–eddy simulations of the flow over a cylinder at Re = 3900 using OpenFOAM
Full text linkIn this paper we present Detached–Eddy Simulations (DES) of the flow field past a cylinder at Reynolds number 3900 obtained by means of the open–source code OpenFOAM. A first aim of the work was to implement a complex DES model based on the v2 ̄–f approach in order to take advantage of its good performance in the near-wall as well as in the LES region. The model was tested successfully against the selected benchmark case using a Dirichlet type wall boundary condition for the ε equation. The role of non–linear constitutive relations in unsolved term modelling was also investigated, but only for the standard Spalart–Allmaras DES approach. A second aim of this study was to validate the DES models available in the OpenFOAM official release. Supercomputing facilities are essential for DES computations to be performed within an acceptable wall–clock time, so we also conducted a detailed code scalability assessment on latest-generation supercomputing equipment. These results are also extensively discussed in this paper. © 2016 Elsevier Lt
High-order discontinuous Galerkin RANS solutions of the incompressible flow over a delta wing
Full text linkIn this paper, high-order Discontinuous Galerkin (DG) Reynolds Averaged Navier Stokes (RANS) computations of the turbulent incompressible flow over a 65° deg sweep delta wing are presented. The flow regimes and the wing geometric configuration refer to those adopted in the international Vortex Flow Experiment 2 (VFE-2) project (Hummel, 2009). Here we consider the incompressible case, at Reynolds numbers 10^6 and 2 × 10^6, which has not yet been analyzed in-depth in literature from the numerical point of view. An extensive comparison of our results with experimental data, mainly those obtained by Furman and Breitsamter (2006, 2008, 2009, 2013) is here supplied.
As regards the turbulence modeling approach, recent researches have revealed that fully-turbulent RANS computations, at Reynolds number greater than 3 × 10^6 and at Mach numbers above 0.4, predict the main flow field features correctly.
The same behavior was not observed in the incompressible case, however we have found that it is sufficient to fix a turbulent transition, at approximately the 25% of the delta wing root chord, to greatly improve the results quality.
The employed transition model is quite simplistic and the resulting simulation approach is not in general completely satisfactory, however the results accuracy establishes the major role of the turbulent transition in predicting the delta wing flow-field. Moreover our solutions fully confirm the interpretation of this flow given by Furman and Breitsamter (2006, 2008, 2009, 2013). Finally, these computations put in evidence the robustness and the effectiveness of our high-order DG solver in dealing with large and complex problems on massively parallel architectures, using up to 4096 CPU cores
Assessment of a high-order discontinuous Galerkin method for incompressible three-dimensional Navier–Stokes equations: Benchmark results for the flow past a sphere up to Re=500
No full textThis paper deals with the implementation of the high-order Discontinuous Galerkin (DG) artificial compressibility flux method into a three-dimensional incompressible Navier–Stokes (INS) solver. The method is fully implicit in time and its distinguishing feature is the formulation of the inviscid interface flux, which is based on the solution of the Riemann problem associated with a local artificial compressibility-like perturbation of the equations.
The code has been tested on a wide range of flow regimes considering the flow past a sphere at moderate Reynolds numbers. In order to asses the code reliability and its accuracy in space, up to the sixth order polynomial approximation, and in time, up to the fourth order, both steady (Re = 20, 200, 250) and unsteady (Re = 300, 500) problems have been approached.
With the largest Reynolds number here considered the flow exhibits a complex behavior, even if it still laminar, since undergoes the transition from a regular to an almost chaotic system. For this problem, for which the flow field characteristics are not completely well established and no many data are available in the present literature, detailed results are reported in this paper
A Spalart-Allmaras turbulence model implementation in a discontinuous Galerkin solver for incompressible flows
Full text linkIn this paper the artificial compressibility flux Discontinuous Galerkin (DG) method for the solution of the incompressible Navier–Stokes equations has been extended to deal with the Reynolds-Averaged Navier–Stokes (RANS) equations coupled with the Spalart–Allmaras (SA) turbulence model. DG implementations of the RANS and SA equations for compressible flows have already been reported in the literature, including the description of limiting or stabilization techniques adopted in order to prevent the turbulent viscosity View the MathML sourceν˜ from becoming negative. In this paper we introduce an SA model implementation that deals with negative View the MathML sourceν˜ values by modifying the source and diffusion terms in the SA model equation only when the working variable or one of the model closure functions become negative. This results in an efficient high-order implementation where either stabilization terms or even additional equations are avoided. We remark that the proposed implementation is not DG specific and it is well suited for any numerical discretization of the RANS-SA governing equations. The reliability, robustness and accuracy of the proposed implementation have been assessed by computing several high Reynolds number turbulent test cases: the flow over a flat plate (Re=107Re=107), the flow past a backward-facing step (Re=37400Re=37400) and the flow around a NACA 0012 airfoil at different angles of attack (View the MathML sourceα=0°,10°,15°) and Reynolds numbers (Re=2.88×106,6×106Re=2.88×106,6×106)
A Spalart-Allmaras local correlation based transition model for Thermal-Fluid Dynamics applications
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