Evaluation of a new implicit coupling algorithm for the partitioned fluid-structure interaction simulation of bileaflet mechanical heart valves

The movement of the leaflets of Bileaflet Mechanical Heart Valves (BMHVs) strongly interacts with the surrounding fluid motion and therefore it needs to be modeled through a Fluid-Structure Interaction (FSI) scheme with implicit coupling. Therefore, when using partitioned solvers, a subiteration loop within each time step is needed. The stability of such a scheme depends on the value of the under-relaxation factor. For the simulation of a BMHV, several methods can be used to find such an appropriate under-relaxation factor, like fixed under-relaxation or the dynamically changing Aitken Δ2 under-relaxation. Also, a stable scheme can be achieved with a newly developed algorithm which uses the Jacobian with the derivatives of the pressure and viscous moments acting on the leaflets with resprect to the angular accelerations of the leaflets. In this paper, this new algorithm is presented and compared to existing coupling schemes. It is shown through numerical experiments that our newly developed algorithm outperforms these existing coupling schemes

Comparative study of transition models for high-angle-of-attack behavior

This paper considers transition modeling for the flow over small unmanned aerial vehicles with a span of around 1 m. Such flows are characterized by very low values of turbulence intensity, and the main cause for transition corresponds to flow separation. Four different turbulence models for low-Reynolds-number flow are compared with the experimental data for a NACA 0018 airfoil over a range of two-dimensional as well as three-dimensional (3-D) conditions. The turbulence models under consideration are the k-omega shear-stress transport (SST) model with low-Reynolds-number modification, (k-omega SST) gamma-Re theta model along with its simplified version in the form of the (k-omega SST) gamma model, and k-kl-omega model. The NACA 0018 profile is rotated in a flow with a chord-based Reynolds number of 3x105 at three different rotational speeds between an angle of attack of 0 and 25 deg. Using a curve fitting methodology, an estimate of the results at an infinitesimally slow rotation can be made. Both clockwise and counterclockwise rotations are considered to allow an assessment of the model for predicting steady hysteresis. Furthermore, 3-D computations for an infinite wing are performed to examine the appearance of coherent structures at high angle of attack, namely, stall cells or low-frequency fluctuations

A multi-solver quasi-Newton method for the partitioned simulation of fluid-structure interaction

In partitioned fluid-structure interaction simulations, the flow equations and the structural equations are solved separately. Consequently, the stresses and displacements on both sides of the fluid-structure interface are not automatically in equilibrium. Coupling techniques like Aitken relaxation and the Interface Block Quasi-Newton method with approximate Jacobians from Least-Squares models (IBQN-LS) enforce this equilibrium, even with black-box solvers. However, all existing coupling techniques use only one flow solver and one structural solver. To benefit from the large number of multi-core processors in modern clusters, a new Multi-Solver Interface Block Quasi-Newton (MS-IBQN-LS) algorithm has been developed. This algorithm uses more than one flow solver and structural solver, each running in parallel on a number of cores. One-dimensional and three-dimensional numerical experiments demonstrate that the run time of a simulation decreases as the number of solvers increases, albeit at a slower pace. Hence, the presented multi-solver algorithm accelerates fluid-structure interaction calculations by increasing the number of solvers, especially when the run time does not decrease further if more cores are used per solver

3D CFD analysis of an oil injected twin screw expander

Small scale Organic Rankine Cycle (ORC) systems have a big potential for waste heat recovery in the market. Due to the smaller volume flows inside these systems, non-conventional expansion technologies such as screw expanders become more interesting. Recent economic studies have shown the important role of screw machines in such cycles. However, in order to get a better understanding of the expansion behaviour in an ORC, appropriate simulation models of screw expanders are necessary. The flow inside an oil-injected twin screw expander is modeled in detail with 3D CFD (Computational Fluid Dynamics) calculations. These simulations are challenging because of the deforming domain and the narrow gaps between the screws or between a screw and the casing. The deforming mesh motion is handled by an in-house code which generates a block-structured grid with the help of the solutions of the Laplace problem. The oil-phase was modeled with an Eulerian multiphase model and the working fluid is treated compressible. The performance of the screw expander is strongly affected by the oil-injection which provides lubrication and a better sealing of the gaps. Therefore, the different types of leakages inside the screw expander are studied and monitored. As the result of the simulations, knowledge about the flow process and the losses inside the oil-injected screw expander is built up

Stability analysis of different combinations of time-integration schemes in fluid-structure interaction simulations

Partitioned fluid-structure interaction simulations often use different time-integration schemes to discretize the different sub-problems. As such, the flow and structural equations can be solved with schemes that are particularly suited for each individual problem. However, using incompatible schemes, these simulations can encounter stability problems. In this research an analytical stability analysis is performed for a model of blood flow in an artery. The backward Euler scheme is used for the time discretization of the flow equations. For the structure two schemes are used: the BE scheme and the Hilber-Hughes-Taylor operator in which the numerical damping is controlled by a single parameter alpha. The influence of this parameter and some physiological parameters on the stability and the damping of the spurious modes is studied. According to this analysis, the combination of the BE and HHT scheme is stable, but the wave number, the numerical damping and the flow and structural density can affect the damping of the spurious modes considerably. To verify the analytical results, a numerical study is performed using nonlinear two-dimensional axisymmetric FSI simulations