1,721,031 research outputs found
Accuracy of numerically evaluated flutter derivatives of bridge deck sections using RANS: Effects on the flutter onset velocity
No full textIn this article, the accuracy of standard computational fluid dynamics techniques and turbulence models in predicting the critical flutter speed of streamlined and bluff deck sections is investigated. It is well known that such numerical tools can provide inaccurate prediction of the flow field around bluff bodies which reflects in biases in their aeroelastic behavior. Aiming at systematically study the effects of such inaccuracies, in this article, six deck sections are analyzed by numerically extracting flutter derivatives and calculating the critical flutter speed for a set of representative test structures. It is shown that the flutter onset velocity is mainly underestimated but cases showing opposite behavior are identified and discussed. Two simulation strategies are adopted: the differences in the flutter mechanism between such two approaches are taken as an indicator of possible strong inaccuracies
On the generation of synthetic divergence-free homogeneous anisotropic turbulence
No full textIt is well known that the generation of appropriate unsteady boundary conditions represents an important component of successful Large Eddy Simulations in turbulent flows. In particular, when Computational Wind Engineering applications are considered, a recurrent problem consists in imposing turbulent fluctuations characterized by given spectra and length scales at the inlet boundary. In the present contribution, firstly, currently available techniques for the generation of synthetic turbulence are revised with a focus on their mathematical formulation. Then, two new approaches for the generation of homogeneous turbulence are proposed. The first one can be seen as a correction over existing techniques which allows to control the obtained length scales. The second method, conceived to generate anisotropic turbulence characterized by arbitrary harmonic content in both time and space, is designed to be computationally efficient, to guarantee the divergence-free condition and to ensure a good approximation of the resulting turbulence integral scales. Finally, the procedure is validated by synthesizing a homogeneous turbulent field characterized by time and length scales typical of the atmospheric boundary layer showing good results
A systematic approach to the generation of synthetic turbulence using spectral methods
No full textIn this paper, a systematic discussion on the generation of synthetic turbulence using spectral methods is presented. After a brief introduction which reviews existing methodologies, the role played by the fulfilment of the divergence-free condition and Taylor assumption is investigated. Special attention is given to the case in which such random fields are applied as inflow condition for Computational Fluid Dynamics simulations. Subsequently, a new methodology of general applicability for the generation of synthetic turbulence is proposed. The strength of the new approach lies in its generality and conceptual simplicity. The obtained random field fulfils the divergence-free condition as well as Taylor assumption and it is approximately characterised by preselected spectral content in each spatial direction, so also providing direct control over all turbulence integral scales. Synthetic turbulent fields characterised by different spectral content are generated confirming the soundness of the proposed approach and showing its ability to target strongly anisotropic fields. Finally, some remarks on the generation of inhomogeneous fields, obtained by combination of homogeneous ones, are provided so generalising the proposed procedure
Unsteady inflow conditions: A variationally based solution to the insurgence of pressure fluctuations
Full text linkThe application of unsteady inflow conditions represents an important aspect when scale resolving turbulence models are adopted in Computational Fluid Dynamics (CFD) analyses. In such context, with reference to Wind Engineering applications, the main concern is often represented by the generation of a synthetic velocity field representative of the turbulent fluctuations impinging on the studied body. Once such synthetic field has been generated, it is applied at the inflow patch. Unfortunately, such operation is not trivial: undesired pressure fluctuations are often generated due to the incompatibility between the applied inflow condition and the flow field found inside the computational domain. In this paper, a procedure able to guarantee a correct application of synthetically generated velocity fields to CFD simulations is described. The procedure, which relies on a variational background, is simple and can be readily implemented in existing codes. Numerical results confirm that, by adopting the proposed corrections, pressure fluctuations are strongly reduced, so confirming the soundness of the proposed approach. An OpenFoam implementation of the method is available for download at https://site.unibo.it/cwe-lamc/en
A pressure–velocity jump approach for the CFD modelling of permeable surfaces
No full textPermeable surfaces are extremely common in applications, ranging from wind shields installed on bridge decks to the outer layer of permeable double skin facades. However, due to the large scale separation between the overall dimensions of the structure and the size of the pores, their modelling in Computational Fluid Dynamics, CFD, simulations are still extremely problematic. In particular, explicitly modelling the pores geometry leads to prohibitive computational costs, while homogenized models based on the use of so-called pressure-jumps are often very crude simplifications of their aerodynamic behaviour. In this paper, a novel approach based on the use of pressure–velocity jumps, PVJ, is proposed. Firstly, the approach is deduced in a general form, based on mass and momentum conservation across the permeable surface. Then, some limit cases for which an analytical evaluation of the coefficients characterizing the model can be obtained are discussed. Finally, a ground mounted barrier is modelled, considering permeable surfaces of widely different aerodynamic behaviour. Results obtained modelling the barrier geometrical details and using the proposed PVJ approach are compared, confirming the soundness of the proposed approach. An OpenFOAM boundary condition implementing the proposed method is available at https://site.unibo.it/cwe-lamc/en
An equilibrium-based stress recovery procedure for the VEM
Full text linkWithin the framework of the displacement-based virtual element method (VEM), namely, for plane elasticity, an important topic is the development of optimal techniques for the evaluation of the stress field. In fact, in the classical VEM formulation, the same projection operator used to approximate the strain field (and then evaluate the stiffness matrix) is employed to recover, via constitutive law, the stress field. Considering a first-order formulation, strains are locally mapped onto constant functions, and stresses are piecewise constant. However, the virtual displacements might engender more complex strain fields for polygons, which are not triangles. This leads to an undesirable loss of information with respect to the underlying virtual stress field. The recovery by compatibility in patches, originally proposed for finite element schemes, is here extended to VEM, aiming at mitigating such an effect. Stresses are recovered by minimizing the complementary energy of patches of elements over an assumed set of equilibrated stress modes. The procedure is simple, efficient, and can be readily implemented in existing codes. Numerical tests confirm the good performance of the proposed technique in terms of accuracy and indicate an increase of convergence rate with respect to the classical approach in many cases
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