1,721,015 research outputs found
Algebraic modifications of the k-ω̃ and Spalart–Allmaras turbulence models to predict bypass and separation-induced transition
Full text linkMany reliable and robust turbulence models are nowadays available for the Reynolds-Averaged Navier-Stokes (RANS) equations to accurately simulate a wide range of engineering flows. However, turbulence models are not suited to correctly described flows with low to moderate Reynolds numbers, which are characterized by strong transitional phenomena. Therefore, numerical models able to accurately predict transitional flows are mandatory to overcome the limits of turbulence models for the efficient design of many industrial applications. The only ways to describe transition are Direct Numerical Simulation (DNS), Large Eddy Simulation (LES), and transition models, where the computational cost of DNS and LES is still too high for their routine use in industry. A modified version of the k-(omega) over tilde and Spalart-Allmaras turbulence models is here proposed to predict transition due to the bypass and separation-induced modes. The modifications are based on the gamma k-(omega) over tilde and the SA-BCM models and avoid complex formulations of transport equations ad-hoc defined for transition. Both the transition models are correlation-based algebraic models that rely only on local flow information and an intermittency function, which damps the turbulent production according to some transition onset requirements. The proposed transition models are implemented in a high-order discontinuous Galerkin (dG) solver and validated on benchmark cases from the ERCOFTAC suite to the Eppler 387 airfoil, with different transition mode, freestream Reynolds number and turbulent intensity, and pressure gradient
Assessment of a Wall Distance Free Transition Model Based on the Laminar Kinetic Energy in a Discontinuous Galerkin Solver
No full text
Assessment of an Improved Delayed X-LES Hybrid Model for the Study of Off-Design Conditions in Centrifugal Pumps
No full textCentrifugal pumps work in a wide range of conditions, often far from the design condition. The flow field can be characterized by large separations, vortex dynamics, and, in general, unsteady turbulent phenomena. Strongly off-design conditions are characterized by large separations that lead to efficiency loss, vibrations, and even to fatigue failure. Therefore, the capability to predict the flow field in these conditions is of great interest and computational fluid dynamics (CFD) can represent a viable solution, which can also complement or substitute experimental measurements. In this context, the Reynolds-averaged Navier-Stokes (RANS) approach allows to accurately simulate attached turbulent flows around complex geometries but it fails the prediction of massively separated flows, crucial for the off-design performance. To overcome this limitation, scale-resolving simulations based on the large eddy simulation (LES) can be used. However, their computational cost is too large for a routine use in industry. In centrifugal pumps, where the typical Reynolds number is in the range 105-106, the use of a hybrid RANS-LES model or a wall modeled LES approach seems mandatory to improve the RANS accuracy and reduce the LES computational cost. In this work, an improved version of the extra-large eddy simulation (X-LES) model, the delayed X-LES or DX-LES model, is implemented in the open-source tool-box openfoam v.1812 and is assessed in the computation of the flow field through a centrifugal pump impeller, both at the design and one-quarter loads. The results are compared with experimental data and LES results available in literature
Transition model based on the laminar kinetic energy concept for the prediction of all transition modes
No full textSeparation-induced transition showed to be the weakness of the transition models based on the laminar kinetic energy concept. In fact, these models contemplate only the Tolmien-Schlichting waves, for the natural mode, and the Klebanoff streaks, for the bypass mode. Literature is very poor about the use of these models to cases with the separation-induced mode and in all these works no proofs of the phenomenological agreement between the models and the physics of the flow are spotlighted. A further improvement for the Reynolds-Averaged Navier-Stokes equations in separated and transitional shear layers needs more accurate models to describe the physics behind the phenomena, e.g., the introduction of ad-hoc designed terms for the Kelvin-Helmholtz instability in the transport equations. The objective of this work is to assess a phenomenological and local transition model based on the laminar kinetic energy concept, implemented in a high-order discontinuous Galerkin solver, for the simulation of transitional flows. The prediction capabilities of the model are proved with the simulations of the flow over the ERCOFTAC and UNIGE flat plates, characterized by the bypass and separation-induced mode of transition. The education of the model is not only based on integral coefficients and first-order statistics, but also on the turbulence intensity, laminar and turbulent kinetic energy distributions extracted from finely processed experimental data
ON THE ALGEBRAIC MODIFICATIONS OF TRADITIONAL TURBULENCE MODELS TO PREDICT BYPASS AND SEPARATION-INDUCED TRANSITION
No full textp -Multigrid High-Order Discontinuous Galerkin Solution of Compressible Flows
No full textDiscontinuous finite element methods are finding widespread use in a wide range of scientific and technical applications since they are among the few available methods for the approximation of partial differential problems that combines high-order accuracy, geometric flexibility, and robustness. The price to pay for the robustness, accuracy, and flexibility of these methods is their high computational cost and storage requirement. However, the computational efficiency of discontinuous finite element methods can be substantially improved by resorting to multilevel solution techniques. This chapter presents the application of a p-multigrid high-order accurate discontinuous finite element method to the numerical solution of compressible laminar viscous flows (compressible Navier–Stokes equations) and to compressible turbulent flows modeled with the Reynolds-Averaged Navier–Stokes equations coupled with the k- ω turbulence model
Steam boiler mixing channel optimization with a surrogate based multi objective genetic algorithm
No full textNowadays, thanks to ever-increasing computational resources, a viable path to a robust and fast design strategy for both thermal machines and turbomachinery is the coupling of Computational Fluid Dynamics (CFD) and shape optimization algorithms. In general, numerical optimization approaches require less time than the trial-and-error procedure traditionally employed, where the designer produces only a tentative initial geometry. This work assesses the capability of a shape optimization algorithm to enhance the design of a steam boiler mixing channel to guarantee negligible NOx production, avoid combustion instabilities especially at lower thermal powers, due to a bad mixing quality of the mixture, and thermal deformation on the burner surface mesh, due to a non uniform distribution of the flame. In particular, the effect of the mixing quality, flow uniformity and the pressure losses at the outlet section of the mixing channel are investigated. The shape optimization approach is here based on a Surrogate Based Optimization (SBO) with the Multi Objective Genetic Algorithm (MOGA), where response surfaces based on the Kriging meta-model are adopted to decrease the computational cost of the proposed approach
ON THE ALGEBRAIC MODIFICATIONS OF TRADITIONAL TURBULENCE MODELS TO PREDICT BYPASS AND SEPARATION-INDUCED TRANSITION
No full textMany reliable and robust turbulence models are nowadays available for the Reynolds-Averaged NavierStokes (RANS) equations to accurately simulate a wide range of engineering flows. However, turbulence models are not able to correctly predict flow phenomena with low to moderate Reynolds numbers, which are characterized by strong transitions. Laminar to turbulent transition is common in aerospace, turbomachinery, maritime, and automotive. Therefore, numerical models able to accurately predict transitional flows are mandatory to overcome the limits of turbulence models for the efficient design of many industrial applications. A modified version of the k-u and Spalart-Allmaras turbulence models is proposed in order to predict transition due to the bypass and separation-induced modes. The modifications here proposed are based on the yk-u and the SA-BCM transition models. Both the models are correlation-based algebraic transition models that relies on local flow information and include an intermittency function instead of an intermittency equation. The basic idea behind the models is that, instead of writing a transport equation for intermittency, an intermittency function multiplies the production terms of the turbulent working variables of the formulation of the turbulence models. In particular, the turbulence production is damped until it satisfies some transition onset re-quirements. The proposed models are implemented in a highorder discontinuous Galerkin (dG) solver and validated on different transitional benchmark cases from the ERCOFTAC T3 suite, with bypass (T3A, T3A- and T3B) and separation-induced (T3L1 and T3L3) transition
- …
