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a b s t r a c tThe numerical simulation of the interaction between a free surface flow and a moving obstacle is considered for the analysis of hydroplaning flows. A new augmented Lagrangian method, coupled to fictitious domains and penalty methods, is proposed for the simulation of multi-phase flows. The augmented Lagrangian parameter is estimated by an automatic analysis of the discretization matrix resulting from the approximation of the momentum equations. The algebraic automatic augmented Lagrangian 3AL approach is validated on the natural convection in a differentially heated cavity, a two-dimensional collapse of a water column, the three-dimensional settling of a particle in a tank and the falling of a dense cylinder in air. Finally, the 3AL method is utilized to simulate the hydroplaning of a tire under various pattern shape conditions.

SUMMARYOur purpose is to develop an efficient coupling between incompressible multiphase flows and fixed or moving obstacles of complex shape. The flow is solved on a fixed Cartesian grid and the solid objects are represented by surface elements. Our strategy is based on two main originalities: the generation and management of the objects are ensured by computer graphics software and front-tracking methods, while the coupling between the flow and the obstacle grids is ensured by a fictitious domain approach and new high-order penalty techniques. Several validation problems are presented to demonstrate the interest and accuracy of the method.

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International audienceA global methodology dealing with fictitious domains of all kinds on orthogonal curvilinear grids is presented. The main idea is to transform the curvilinear workframe and its associated elements (velocity, immersed interfaces...) into a Cartesian grid. On such a grid, many operations can be performed much faster than on curvilinear grids. The method is coupled with a Thread Ray-casting algorithm which work on Cartesian grids only. This algorithm computes quickly the Heaviside function related to the interior of an object on an Eulerian grid. The approach is also coupled with an immersed boundary method ($L^2$-penalty method) or with phase advection with VOF-PLIC, VOF-TVD, Front-tracking or Level-set methods. Applications, convergence and speed tests are performed for shape initializations, immersed boundary methods, and interface tracking

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In the case of a gyroscope including a cylindrical fluid-filled cavity, the classic Poinsot's coning motion can become unstable. For certain values of the solid inertia ratio, the coning angle opens under the effect of the hydrodynamic torque. The coupled dynamics of such a non-solid system is ruled by four dimensionless numbers: the small viscous parameter ε = Re−1/2 (where Re denotes the Reynolds number), the fluid–solid inertia ratio κ which quantifies the proportion of liquid relative to the total mass of the gyroscope, the solid inertia ratio σ and the aspect ratio h of the cylindrical cavity. The calculation of the hydrodynamic torque on the solid part of the gyroscope requires the preliminary evaluation of the possibly resonant flow inside the cavity. The hydrodynamic scaling used to derive such a flow essentially depends on the relative values of κ and ε. For small values of the ratio /ε (compared to 1), Gans derived an expression of the growth rate of the coning angle. The principles of Gans' approach (Gans, AIAA J., vol. 22, 1984, pp. 1465–1471) are briefly recalled but the details of the whole calculation are not given. At the opposite limit, that is for large values of /ε, the dominating flow is given by a linear inviscid theory. In order to take account of viscous effects, we propose a direct method involving an exhaustive calculation of the flow at order ε. We show that the deviations from Stewartson's inviscid theory (Stewartson, J. Fluid Mech., vol. 5, 1958, p. 577) do not originate from the viscous shear at the walls but rather from the bulk pressure at order ε related to the Ekman suction. Physical contents of Wedemeyer's heuristic theory (Wedemeyer, BRL Report N 1325, 1966) are analysed in the view of our analytical results. The latter are tested numerically in a large range of parameters. Complete Navier–Stokes (NS) equations are solved in the cavity. The hydrodynamic torque obtained by numerical integration of the stress is used as a forcing term in the coupled fluid–solid equations. Numerical results and analytical predictions show a fairly good quantitative agreement.

A new simple fictitious domain method, the algebraic immersed interface and boundary (AIIB) method, is presented for elliptic equations with immersed interface conditions. This method allows jump conditions on immersed interfaces to be discretized with a good accuracy on a compact stencil. Auxiliary unknowns are created at existing grid locations to increase the degrees of freedom of the initial problem. These auxiliary unknowns allow imposing various constraints to the system on interfaces of complex shapes. For instance, the method is able to deal with immersed interfaces for elliptic equations with jump conditions on the solution or discontinuous coefficients with a second order of spatial accuracy. As the AIIB method acts on an algebraic level and only changes the problem matrix, no particular attention to the initial discretization is required. The method can be easily implemented in any structured grid code and can deal with immersed boundary problems too. Several validation problems are presented to demonstrate the interest and accuracy of the method.