The advantages of using the generalised-alpha scheme for first-order systems for computing the numerical solutions of second-order equations encountered in structural dynamics are presented. The governing equations are rewritten so that the second-order equations can be solved directly without having to convert them into state-space. The stability, accuracy, dissipation and dispersion characteristics of the scheme are discussed. It is proved through spectral analysis that the proposed scheme has improved dissipation properties when compared with the standard generalised-alpha scheme for second-order equations. It is also proved that the proposed scheme does not suffer from overshoot. Towards demonstrating the application to practical problems, proposed scheme is applied to the benchmark example of three degrees of freedom stiff-flexible spring-mass system, two-dimensional Howe truss model, and elastic pendulum problem discretised with non-linear truss finite elements.

We present a numerical scheme for fluid-structure interaction based on hierarchical B-Spline grids and fictitious domain/distributed Lagrange multipliers. The incompressible Navier-Stokes equations are solved over a Cartesian grid discretised with B-Splines. The fluid grid near the immersed solids is refined locally using hierarchical B-Splines. The immersed solid is modelled as geometrically-exact beam discretised with standard linear Lagrange shape functions. The kinematic constraint at the fluid-solid interface are enforced with distributed Lagrange multipliers. The unconditionally-stable and second-order accurate generalised-α method is used for integration in time for both the fluid and solid domains. A fully-implicit and fully-coupled solution scheme is developed by using Newton-Raphson method to solve the non-linear system of equations obtained with Galerkin weak formulation. First, the spatial and temporal convergence of the proposed scheme is assessed by studying steady and unsteady flow past a fixed cylinder. Then, the scheme is applied to several benchmark problems to demonstrate the efficiency and robustness of the proposed scheme. The results obtained with the present scheme are compared with the reference values.

An accurate, efficient and robust numerical scheme is presented for the simulation of the interaction between flexibly-supported rigid bodies and incompressible fluid flow with topology changes and solid-solid contact. The solution of the incompressible Navier-Stokes equations is approximated by employing a stabilised formulation on Cartesian grids discretised with hierarchical b-splines. The solid is modelled as a rigid body and represented by linear segments along its boundary. Kinematic conditions along the fluid-rigid body interface are enforced weakly using Nitsche's method, while ghost penalty operators are employed to avoid excessive ill-conditioning of the system matrix arising from small cut cells. A staggered scheme is used for resolving the coupled fluid-rigid body interaction. The contact between moving or moving and fixed solid bodies is modelled with Lagrange multipliers. The excellent performance and wide range of applicability of the proposed scheme are demonstrated in a number of benchmark tests as well as industrially relevant model problems. The examples cover the galloping phenomena, particulate flow, hydraulic check valves and a model turbine.

A stabilised immersed framework on hierarchical b-spline grids for fluidflexible structure interaction with solid-solid contact. Computer Methods in Applied Mechanics and Engineering

In this work, an immersed boundary finite element method is proposed which is based on a hierarchically refined cartesian b-spline grid and employs the non-symmetric and penalty-free version of Nitsche's method to enforce the boundary conditions. The strategy allows for h-and p-refinement and employs a so-called ghost penalty term to stabilise the cut cells. An effective procedure based on hierarchical subdivision and sub-cell merging, which avoids excessive numbers of quadrature points, is used for the integration of the cut cells. A basic Laplace problem is used to demonstrate the effectiveness of the cut cell stabilisation and of the penalty-free Nitsche method as well as their impact on accuracy. The methodology is also applied to the incompressible Navier-Stokes equations, where the SUPG/PSPG stabilisation is employed. Simulations of the lid-driven cavity flow and the flow around a cylinder at low Reynolds number show the good performance of the methodology. Excessive ill-conditioning of the system matrix is robustly avoided without jeopardising the accuracy at the immersed boundaries or in the field.