Julien Favier

Immersed-boundary methods for general finite-difference and finite-volume Navier-Stokes solvers

We present an immersed-boundary algorithm for incompressible flows with complex boundaries, suitable for Cartesian or curvilinear grid system. The key stages of any immersed-boundary technique are the interpolation of a velocity field given on a mesh onto a general boundary (a line in 2D, a surface in 3D), and the spreading of a force field from the immersed boundary to the neighboring mesh points, to enforce the desired boundary conditions on the immersed-boundary points. We propose a technique that uses the Reproducing Kernel Particle Method [W.K. Liu, S. Jun, Y.F. Zhang, Reproducing kernel particle methods, Int. J. Numer. Methods Fluids 20(8) (1995) 1081-1106] for the interpolation and spreading. Unlike other methods presented in the literature, the one proposed here has the property that the integrals of the force field and of its moment on the grid are conserved, independent of the grid topology (uniform or non-uniform, Cartesian or curvilinear). The technique is easy to implement, and is able to maintain the order of the original underlying spatial discretization. Applications to two- and three-dimensional flows in Cartesian and non-Cartesian grid system, with uniform and non-uniform meshes are presented.

Flow over a Wing with Leading-Edge Undulations

The stall-delaying properties of the humpback whale flipper have been observed and quantified in recent years, through both experimental and numerical studies. In the present work, numerical simulations of an infinite-span wing with an idealized representation of this geometry are reported at a Reynolds number of 1.2×10(to the power of 5). Using large-eddy simulation, an adequate spatial resolution is first established before also examining the spanwise extent of the domain. These results are then analyzed to provide an explanation of the conditions that drive the lift observed beyond the conventional stall angle. The undulating leading-edge geometry gives rise to a spanwise pressure gradient that drives a secondary flow toward the regions of minimum chord. In turn, this leads to the entrainment of higher-momentum fluid into the region behind the maximum chord, which energizes the boundary layer and delays stall. Aside from demonstrating a significant poststall lift, the undulations also have the added benefit of substantially reducing lift fluctuations.

An immersed boundary-lattice Boltzmann method for single- and multi-component fluid flows

The paper presents a numerical method to simulate single- and multi-component fluid flows around moving/deformable solid boundaries, based on the coupling of Immersed Boundary (IB) and Lattice Boltzmann (LB) methods. The fluid domain is simulated with LB method using the single relaxation time BGK model, in which an interparticle potential model is applied for multi-component fluid flows. The IB-related force is directly calculated with the interpolated definition of the fluid macroscopic velocity on the Lagrangian points that define the immersed solid boundary. The present IB-LB method can better ensure the no-slip solid boundary condition, thanks to an improved spreading operator. The proposed method is validated through several 2D/3D single- and multi-component fluid test cases with a particular emphasis on wetting conditions on solid wall. Finally, a 3D two-fluid application case is given to show the feasibility of modeling the fluid transport via a cluster of beating cilia.

Accelerating fluid–solid simulations (Lattice-Boltzmann & Immersed-Boundary) on heterogeneous architectures

We propose a numerical approach based on the Lattice-Boltzmann (LBM) and Immersed Boundary (IB) methods to tackle the problem of the interaction of solids with an incompressible fluid flow, and its implementation on heterogeneous platforms based on data-parallel accelerators such as NVIDIA GPUs and the Intel Xeon Phi. We explain in detail the parallelization of these methods and describe a number of optimizations, mainly focusing on improving memory management and reducing the cost of host-accelerator communication. As previous research has consistently shown, pure LBM simulations are able to achieve good performance results on heterogeneous systems thanks to the high parallel efficiency of this method. Unfortunately, when coupling LBM and IB methods, the overheads of IB degrade the overall performance. As an alternative, we have explored different hybrid implementations that effectively hide such overheads and allow us to exploit both the multi-core and the hardware accelerator in a cooperative way, with excellent performance results.

Mechanisms of non-modal energy amplification in channel flow between compliant walls

The mechanisms leading to large transient growth of disturbances for the flow in a channel with compliant walls are investigated. The walls are modelled as thin spring-backed plates, and the flow dynamics is modelled using the Navier–Stokes equations linearized round the Poiseuille profile. Analysis for streamwise invariant perturbations show that this fluid-structure system can sustain oscillatory energy evolution of large amplitude, in the form of spanwise standing waves. Such waves are related to the travelling waves which a free wall can support, modified to account for an ‘added mass’ effect. Simple scaling arguments are found to provide results in excellent agreement with computations of optimal disturbances, for low-to-moderate values of the stiffness parameter characterizing the compliant surface.

Block Tridiagonal Solvers on Heterogeneous Architectures

Modern multi-core and many-core systems offer a very impressive cost/performance ratio. In this paper a set of new parallel implementations for the solution of linear systems with block-tridiagonal coefficient matrix on current parallel architectures is proposed and evaluated: one of them on multi-core, others on many-core and finally, a new heterogeneous implementation on both architectures. The results show a speedup higher than 6 on certain parts of the problem, being the heterogeneous implementation the fastest.

A coupled Immersed Boundary-Lattice Boltzmann method for incompressible flows through moving porous media

In this work, we propose a numerical framework to simulate fluid flows in interaction with moving porous media of complex geometry. It is based on the Lattice Boltzmann method including porous effects via a Brinkman-Forchheimer-Darcy force model coupled to the Immersed Boundary method to handle complex geometries and moving structures. The coupling algorithm is described in detail and it is validated on well-established literature test cases for both stationary and moving porous configurations. The proposed method is easy to implement and efficient in terms of CPU cost and memory management compared to alternative methods which can be used to deal with moving immersed porous media, e.g. re-meshing at each time step or use of a moving/chimera mesh. An overall good agreement was obtained with reference results, opening the way to the numerical simulation of moving porous media for flow control applications.

Improved Implicit Immersed Boundary Method via Operator Splitting

We present an implicit immersed boundary method via operator splitting technique for simulating fluid flow over moving solid with complex shape. An additional moving force equation is derived in order to impose the interface velocity condition exactly on the immersed surface. The moving force matrix is formulated to be symmetric and positive definite, thus its calculation is computational inexpensive by using the conjugate gradient method. Moreover, the proposed immersed boundary method is incorporated into the rotational incremental projection method as a plug-in. No numerical boundary layers will be generated towards the velocity and pressure during the calculation. The method is validated through various benchmark tests.

A QUANTITATIVE COMPARISON BETWEEN A LOW ORDER MODEL AND A 3D FEM CODE FOR THE STUDY OF THERMOACOUSTIC COMBUSTION INSTABILITIES

The study of thermoacoustic combustion instabilities has an important role for safety operation in modern gas turbines equipped with lean premixed dry low emission combustion systems. Gas turbine manufacturers often adopt simulation tools based on low order models for predicting the phenomenon of humming. These simulation codes provide fast responses and good physical insight, but only one-dimensional or two-dimensional simplified schemes can be generally examined. The finite element method can overcome such limitations, because it allows to examine three-dimensional geometries and to search the complex eigenfrequencies of the system. Large Eddy Simulation (LES) techniques are proposed in order to investigate the instability phenomenon, matching pressure fluctuations with turbulent combustion phenomena to study thermoacoustic combustion oscillations, even if they require large numerical resources. The finite element approach solves numerically the Helmholtz equation problem converted in a complex eigenvalue problem in the frequency domain. Complex eigenvalues of the system allow us to identify the complex eigenfrequencies of the combustion system analyzed, so that we can have a valid indication of the frequencies at which thermoacoustic instabilities are expected and of the growth rate of the pressure oscillations at the onset of instability. Through the collaboration among Ansaldo Energia, University of Genoa and Polytechnic University of Bari, a quantitative comparison between a low order model, called LOMTI, and the three-dimensional finite element method has been examined, in order to exploit the advantages of both the methodologies.Copyright

Transport efficiency of metachronal waves in 3D cilium arrays immersed in a two-phase flow

The present work reports the formation and the characterization of antipleptic and symplectic metachronal waves in 3D cilia arrays immersed in a two-fluid environment, with a viscosity ratio of 20. A coupled lattice-Boltzmann-Immersed-Boundary solver is used. The periciliary layer is confined between the epithelial surface and the mucus. Its thickness is chosen such that the tips of the cilia can penetrate the mucus. A purely hydrodynamical feedback of the fluid is taken into account and a coupling parameter