Documents published in Scipedia

• Investigation of the PANS Method for the Prediction of Aerodynamic Noise Around a Circular Cylinder

  A. Moosavifard, E. Kolb, M. Schäfer, S. Jakirlic

  eccomas2022.

  
  

  Abstract

  Aeroacoustics is the field of studying flow-induced sound, which results from the interaction of unsteady flow with solid structures, such as aircraft and automobiles. Different methods are available to achieve this, including theoretical, experimental, and computational methods. Due to the high costs of experiments, the concentration on computational methods has increased. Computational aeroacoustics (CAA), based on computational fluid dynamics (CFD), has received special attention from researchers because of its outstanding capability to get acceptable results with reasonable computational costs. The partially-Averaged Navier Stokes (PANS) method is a hybrid LES/RANS method based on dynamic resolution parameters. The SSVPANS method is a k---f based PANS method with an additional modeled equation for the resolved kinetic energy. This method has been implemented in FASTEST, an in-house finitevolume solver to compute the flows in complex applications. This study aims to investigate the aeroacoustic performance of the SSV-PANS method compared to a reference Large-eddy Simulation [1] regarding the computational accuracy and costs. To do this, hybrid method based on decomposing the fluid variables into incompressible hydrodynamics and compressible perturbation equations is used to study the aerodynamic noise. The aeroacoustic sources are computed from the incompressible flow field using the SSV-PANS method. In addition, the Kirchhoff wave extrapolation method is used to have an efficient evaluation of the far-field noise.

  Abstract
  Aeroacoustics is the field of studying flow-induced sound, which results from the interaction of unsteady flow with solid structures, such as aircraft and automobiles. Different [...]

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• Development of a Framework for Internal Combustion Engine Simulations in OpenFOAM

  C. Gößnitzer, S. Posch, G. Pirker

  eccomas2022.

  
  

  Abstract

  High-accuracy simulations of internal combustion engines (ICE) allow deep insight into the physical processes of the different phases of the engine cycle: gas exchange, mixture formation, compression, combustion and emission formation. The commercial solvers for ICE simulations provide a full package which covers these areas. However, the user of such software is unable to look into the source code, making it impossible to implement new models or investigate possible implementation errors in the code, and costs arise due to licensing requirements for commercial solvers. Although the open source framework OpenFOAM already includes multiple classes and two solvers dedicated to internal combustion engine simulations, there is no way to move engine valves and piston simultaneously with its standard tools. Thus, this paper presents a new engine library for ICE simulations written for OpenFOAM. The new framework is capable of simulating a complete fired engine cycle. The piston and the valves are moved simultaneously. To address large deformations in the mesh, a methodology to avoid insufficient mesh quality was developed. Ignition and combustion is modeled with standard tools from OpenFOAM. To validate the method, the simulation results for the averaged in-cylinder quantities pressure, temperature and mass are compared with experimental data.

  Abstract
  High-accuracy simulations of internal combustion engines (ICE) allow deep insight into the physical processes of the different phases of the engine cycle: gas exchange, mixture [...]

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• An energy-preserving unconditionally stable fractional step method on collocated grids

  D. Santos Serrano, F. Trias Miquel, G. Colomer Rey, C. Pérez Segarra

  eccomas2022.

  
  

  Abstract

  Preservation of energy is fundamental in order to avoid the introduction of unphysical energy that can lead to unstable simulations. In this work, an energy-preserving unconditionally stable fractional step method on collocated grids is presented as a method which guarantees both preservation of energy and stability of our simulation. Using an algebraic (matrix-vector) representation of the classical incompressible Navier-Stokes equations mimicking the continuous properties of the differential operators, conservation of energy is formally proven. Furthermore, the appearence of unphysical velocities in highly distorted meshes is also adressed. This problem comes from the interpolation of the pressure gradient from faces to cells in the velocity correction equation, and can be corrected by using a proper interpolation.

  Abstract
  Preservation of energy is fundamental in order to avoid the introduction of unphysical energy that can lead to unstable simulations. In this work, an energy-preserving unconditionally [...]

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• On the conservation of primary and secondary properties in the simulation of multiphase flows

  N. Valle, F. Trias, R. Verstappen

  eccomas2022.

  
  

  Abstract

  The formulation of multiphase flows emanates from basic conservation laws: mass, momentum and energy. While these are embedded in the celebrated Navier-Stokes equations, none of these properties do necessarily hold when constructing a computational model, unless special care is taken in discretizing the different terms of the governing equations. The conservation of both primary (mass, momentum) and secondary (energy) quantities is not only relevant to mimic the dynamics of the system, but also computationally beneficial. Conservation of such quantities produce an enhanced physical reliability, removing most of the need for stabilization artifacts. In addition, discrete conservation implies numerical stability as well, producing inherently stable problems. Focusing on the capillary force, which is one of the most distinguishable features of multiphase flows, we present here our most recent developments in the quest for conservation. Departing from an inherently mass conservative method, in this work we sketch our previous developments to obtain an energy conservation and next we present our attempt at momentum. By carefully assessing the continuum formulation, we delve into the mathematical properties responsible for the conservation of linear momentum, which we then mimic in the regularized and discrete formulations.

  Abstract
  The formulation of multiphase flows emanates from basic conservation laws: mass, momentum and energy. While these are embedded in the celebrated Navier-Stokes equations, none [...]

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• An assessment of various discretizations of the energy equation in compressible flows

  C. De Michele, G. Coppola

  eccomas2022.

  
  

  Abstract

  Nonlinear instabilities are one of the major problems in turbulence simulations. One reason behind this problem is the accumulation of aliasing errors produced by the discrete evaluation of the convective term. This can be improved by preserving the quadratic invariants in a discrete sense. However, another source of instabilities is the error due to an incorrect evolution of thermodynamic variables, such as entropy. An appropriate discretization of the energy equation is needed to address this issue. An analysis of the preservation properties of various discretizations of the compressible Euler equations is reported, which includes some of the most common approaches used in the literature, together with some new formulations. Two main factors have been identified and studied: one is the choice of the energy equation to be directly discretized; the other is the particular splitting of the convective terms, chosen among the Kinetic Energy Preserving (KEP) forms. The energy equations analyzed in this paper are total and internal energy, entropy, and speed of sound. All the cases examined are locally conservative and KEP, since this is considered an essential condition for a robust simulation. The differences among the formulations have been theoretically investigated through the study of the discrete evolution equations induced by the chosen energy variable, showing which quantities may be preserved. Both one-dimensional and two-dimensional tests have been performed to assess the advantages and disadvantages of the various options in different cases.

  Abstract
  Nonlinear instabilities are one of the major problems in turbulence simulations. One reason behind this problem is the accumulation of aliasing errors produced by the discrete [...]

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• Development of a discontinuous Galerkin solver for the simulation of turbine stages

  A. Colombo, A. Ghidoni, E. Mantecca, G. Noventa, S. Rebay, D. Pasquale

  eccomas2022.

  
  

  Abstract

  A high-order Discontinuous Galerkin (DG) solver is assessed in the computation of the flow through an Organic Rankine Cycle turbine nozzle and stage. The flow features are predicted with a RANS (Reynolds averaged Navier­Stoke) approach and the k-log() turbulence model in a multi reference frame, where interfaces between fixed and rotating zones are treated with a mixing plane approach, and non reflecting boundary conditions are used. Primitive variables based on pressure and temperature logarithms are adopted to ensure non-negative thermodynamic variables at a discrete level. The fluid can be modeled with the polytropic ideal gas law and the Peng-Robinson equation of state.

  Abstract
  A high-order Discontinuous Galerkin (DG) solver is assessed in the computation of the flow through an Organic Rankine Cycle turbine nozzle and stage. The flow features are [...]

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• Modal analysis of a 3D gravitational liquid sheet

  A. Colanera, A. Della Pia, M. Acquaviva, M. Chiatto, L. de Luca

  eccomas2022.

  
  

  Abstract

  Modal analysis of three-dimensional gravitational thin liquid sheet flows, interacting with unconfined gaseous environments located on both sides of the liquid phase, is performed in the present work. Numerical data of this relevant two-phase flow configuration are obtained through the single-phase formulation and the Volume-of-Fluid (VOF) technique implemented in the flow solver Basilisk. This class of flows exhibits a variety of spatial and temporal relevant structures, both in free and forced configurations, that are investigated through the Spectral Proper Orthogonal Decomposition (SPOD). By means of such methodology, we explore the effect of two main governing parameters on the flow dynamics, namely the liquid sheet aspect ratio, AR = W/H, where H and W are the sheet inlet thickness and width, and the Weber number, We = lU2H/(2), in which U is the inlet liquid velocity, lthe liquid density, and the surface tension coefficient. Finally, for the highest aspect ratio value considered (AR = 40), we investigate the forced dynamics of the system excited by a harmonic perturbation in transverse velocity component applied at the inlet section, comparing results with ones arising from a purely two-dimensional analysis of the flow. The obtained results highlight the low rank behavior exhibited by the flow, suggesting that reduced order modeling could be particularly appealing to reduce complexity and computational effort in numerical simulation of this class of flows.

  Abstract
  Modal analysis of three-dimensional gravitational thin liquid sheet flows, interacting with unconfined gaseous environments located on both sides of the liquid phase, is performed [...]

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• A fully-discrete entropy conserving/stable discretization for inviscid unsteady flows

  A. Colombo, A. Crivellini, A. Nigro

  eccomas2022.

  
  

  Abstract

  The aim of this work is to contribute to the development of a high-order accurate discretization that is entropy conserving and entropy stable both in space and in time. To do this, the general framework is based on a high-order accurate discontinuous Galerkin (dG) method in space with entropy working variables, several entropy conservative and stable numerical fluxes and an entropy conserving modified Crank-Nicolson method. We present the first results, obtained with the discretizations here proposed, for two bi-dimensional unsteady viscous test-case: the Taylor-Green vortex and the double shear layer.

  Abstract
  The aim of this work is to contribute to the development of a high-order accurate discretization that is entropy conserving and entropy stable both in space and in time. To [...]

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• Influence of the turbulent wake downstream offshore wind turbines on larval dispersal: development of a new Lagrangian-Eulerian model

  S. Ajmi, M. Boutet, A. Bennis, J. Dauvin

  eccomas2022.

  
  

  Abstract

  In the context of future offshore wind farms along the French coasts of the English Channel, the impacts of foundations on larval dispersal from bentho-pelagic species colonizing the hard substratum of artificial structures are studied in order to assess how the species connectivity could be modified by the farms. In particular, the effects of turbulent wake and horseshoe vortices are investigated. To this end, a new numerical approach is developed that combines the Eulerian model, OpenFoam, solving the 3D Navier-Stokes equations to compute the hydrodynamics, and the Lagrangian model, Ichthyop, based on an advection-diffusion equation to compute the larval trajectories. Firstly, some simple test cases are performed to validate the numerical coupling between OpenFoam and Ichthyop, such as the dispersion of larvae downstream a 2D cylinder in water. Secondly, the ability of OpenFoam turbulence models to simulate turbulent structures around monopile and gravity type foundations is evaluated. The RANS (Reynolds Averaged Navier-Stokes) k-omega SST turbulence model is chosen for the realistic application because it can reproduce the horseshoe vortices and turbulent wake with less computing time than the Smagorinsky LES (Large Eddy Simulation) model. Lastly, larval dispersal simulations for four benthic species and for a set of monopile and gravity foundations are performed.

  Abstract
  In the context of future offshore wind farms along the French coasts of the English Channel, the impacts of foundations on larval dispersal from bentho-pelagic species colonizing [...]

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• Surrogate modeling of unsteady aerodynamic loads acting on a plunging airfoil

  R. Sundar, V. KUMAR, D. Majumdar, C. Shah, S. Sarkar

  eccomas2022.

  
  

  Abstract

  Cost-effective parameteric surrogate models of unsteady aerodynamic loads acting on a flapping wing are highly desirable. They would enable real time aerodynamic load prediction, multiobjective optimisation and optimal control of intelligent flapping wing flight devices. In the present work, a parametric surrogate modeling framework for unsteady aerodynamic loads based on a non-intrusive reduced order modeling approach is presented. The unsteady flow past a plunging 2D flat plate is considered where the aerodynamic load time histories are obtained for different plunging frequencies and amplitudes using a potential flow solver. The parametric non-intrusive reduced order model (p-NIROM) for the obtained loads is constructed using a combination of snapshot proper orthogonal decomposition (POD) for dimensionality reduction and a fully connected feed forward neural network (FCNN) for modeling the input parametric dependency. Both, linear and non-linear FCNN based p-NIROM are explored and compared on the basis of load time history reconstruction accuracy. The non-linear FCNN regression for the p-NIROM is observed to generalise well for unseen parametric instances as compared to the linear approach when a systematic data sampling strategy is adopted.

  Abstract
  Cost-effective parameteric surrogate models of unsteady aerodynamic loads acting on a flapping wing are highly desirable. They would enable real time aerodynamic load prediction, [...]

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