Andreas Lintermann

Fluid mechanics based classification of the respiratory efficiency of several nasal cavities

The flow in the human nasal cavity is of great importance to understand rhinologic pathologies like impaired respiration or heating capabilities, a diminished sense of taste and smell, and the presence of dry mucous membranes. To numerically analyze this flow problem a highly efficient and scalable Thermal Lattice-BGK (TLBGK) solver is used, which is very well suited for flows in intricate geometries. The generation of the computational mesh is completely automatic and highly parallelized such that it can be executed efficiently on High Performance Computers (HPCs). An evaluation of the functionality of nasal cavities is based on an analysis of pressure drop, secondary flow structures, wall-shear stress distributions, and temperature variations from the nostrils to the pharynx. The results of the flow fields of three completely different nasal cavities allow their classification into ability groups and support the a priori decision process on surgical interventions.

Numerical Simulation of Nasal Cavity Flow Based on a Lattice-Boltzmann Method

The flow in a real human nose is numerically simulated at steady inspiration and expiration. The analysis uses a Lattice Boltzmann method (LBM) which is particularly suited for flows in extremely intricate geometries. The nasal geometry is extracted from computer tomography (CT) data using a so-called reconstruction pipeline. Thus, for any nose the surface geometry can be defined and a numerical mesh can be generated. The focus of this investigation is on the analysis of the flow field at steady inspiration and expiration with respect to secondary flow structures. It is evidenced that strong vortical structures appear near the throat at inspiration forming a pair of counter-rotating vortices which disappear at expiration. Overall, at exhalation less vorticity is generated in the flow than at inhalation.

Lattice-Boltzmann Solutions with Local Grid Refinement for Nasal Cavity Flows

To analyze rhinological complaints like a reduction of breathing capabilities, performance reduction of the olfactory organ, and complaints inherited by diminished heating and moisturizing functions of the human nasal cavity, flow simulations with a Lattice − Boltzmann method are carried out using refined Cartesian meshes to resolve, e.g., the wall-bounded shear layers. This method is particularly suited for flows in highly intricate geometries. By using hierarchical grid refinement the overall number of cells is reduced in order to improve the computational efficency. Before applying the algorithm to real human nasal geometries obtained from Computer Tomography (CT) it is validated by simulating the flow over a cylinder and over a sphere at low Reynolds numbers.

Large-Scale Simulations of a Non-generic Helicopter Engine Nozzle and a Ducted Axial Fan

Large-eddy simulations (LESs) of a helicopter engine jet and an axial fan are performed by using locally refined Cartesian hierarchical meshes. For the computations a high-fidelity, massively parallelized solver for compressible flow is used. To verify the numerical method, a coaxial hot round jet is computed and the results are compared to reference data. The analysis is complemented by a grid convergence study for both applications, i.e., for the helicopter engine jet and the axial fan. For the helicopter engine jet, additional computations have been performed for two different nozzle geometries, i.e., a simplified nozzle geometry that is consisting of a center body and divergent outer annular channel, and a complete engine nozzle geometry with four additional struts were used. The presence of the struts results in a different potential core break-down and turbulence intensity. Furthermore, for the axial fan configuration, computations have been performed at two different volume flow rates. The reduction of the volume flow rate results in an interaction of the tip-gap vortex with the neighboring blade which leads to a higher turbulent kinetic energy near and inside the tip-gap region.

CFD/CAA Simulations on HPC Systems

In this paper, a highly scalable numerical method is presented that allows to compute the aerodynamic sound from a turbulent flow field on HPC systems. A hybrid CFD-CAA method is used to compute the flow and the acoustic field, in which the two solvers are running in parallel to avoid expensive I/O operations for the acoustic source terms. Herein, the acoustic perturbation equations are solved by a high-order discontinuous Galerkin scheme using the acoustic source terms obtained from an approximate solution of the Navier-Stokes equations. Both solvers run simultaneously and operate on differently refined hierarchical Cartesian grids. This direct-hybrid method is validated by monopole and pressure pulse simulations and is used for performance measurements on current HPC systems. The results highlight the limitations of classic hybrid methods and show that the new approach is suitable for highly parallel simulations.

EFFICIENT PARALLEL GEOMETRY DISTRIBUTION FOR THE SIMULATION OF COMPLEX FLOWS

Highly resolved intrinsic geometrical shapes used in three-dimensional parallel simulations of fluid flows consume a large portion of the available memory when loaded serially on every process. This demands for a memory efficient implementation of a distributed geometry which is however a non-trivial task when complex spatial domain decomposition methods for the flow domain are involved. To overcome this problem, an algorithm to generate a parallel geometry during the mesh generation is proposed that enables a low-memory subdivision of the geometry based on the decomposition of the flow field. The applied meshing method generates computational grids that can be used for simulations on a quasi-arbitrary number of cores on which the geometry is distributed in an efficient preprocessing step. This allows reducing the number of instances of the geometry in the global memory of the simulation to about one. The algorithm is used to generate a parallel geometry for a large shape consisting of 7 · 10 triangles, i.e., for a geometry representing the whole respiratory tract down to the 12 lung generation. For this case, performance and memory consumption measurements are given for simulations on 8,192 up to 131,072 cores and juxtaposed to results obtained from simulations using non-parallel geometries. The findings show that with the new method not only the memory usage could be reduced by the factors of 1,802 and 19,936 for core numbers of 8,192 and 131,072 but also a large speed-up factor of about 51 is obtained in the geometry I/O and preprocessing. Furthermore, the parallel geometry allows using the sweet spot with respect to a combination of distributed and shared memory parallelization leading to an increase of the computational speed of about 1.43.

Assessment of changes in the nasal airway after nonsurgical miniscrew-assisted rapid maxillary expansion in young adults

OBJECTIVES To evaluate changes in the volume and cross-sectional area of the nasal airway before and 1 year after nonsurgical miniscrew-assisted rapid maxillary expansion (MARME) in young adults. MATERIALS AND METHODS Fourteen patients (mean age, 22.7 years; 10 women, four men) with a transverse discrepancy who underwent cone beam computed tomography before (T0), immediately after (T1), and 1 year after (T2) expansion were retrospectively included in this study. The volume of the nasal cavity and nasopharynx and the cross-sectional area of the anterior, middle, and posterior segments of the nasal airway were measured and compared among the three timepoints using paired t-tests. RESULTS The volume of the nasal cavity showed a significant increase at T1 and T2 ( P < .05), while that of the nasopharynx increased only at T2 ( P < .05). The anterior and middle cross-sectional areas significantly increased at T1 and T2 ( P < .05), while the posterior cross-sectional area showed no significant change throughout the observation period ( P > .05). CONCLUSIONS The results demonstrate that the volume and cross-sectional area of the nasal cavity increased after MARME and were maintained at 1 year after expansion. Therefore, MARME may be helpful in expanding the nasal airway.

The Direct-Hybrid Method for Computational Aeroacoustics on HPC Systems

Classic hybrid methods for computational aeroacoustics use different solvers and methods to predict the flow field and the acoustic pressure field in two separate steps, which involves data exchange via disk I/O between the solvers. This limits the efficiency of the approach, as parallel I/O usually does not scale well to large numbers of cores. In this work, a highly scalable direct-hybrid scheme is presented, in which both the flow and the acoustics simulations run simultaneously. That is, all data between the two solvers is transferred in-memory, avoiding the restrictions of the I/O subsystem. Results for the simulation of a pair of co-rotating vortices show that the method is able to correctly predict the acoustic pressure field and that it is suitable for highly parallel simulations.

Towards Large Multi-scale Particle Simulations with Conjugate Heat Transfer on Heterogeneous Super Computers

We present numerical methods based on hierarchical Cartesian grids for the simulation of particle flows of different length scales. These include Eulerian-Lagrangian approaches for fully resolved moving particles with conjugate heat-transfer as well as one-way coupled Lagrangian particle models for large-scale particle simulations. The domain decomposition of all phases involved is performed on a joint hierarchical Cartesian grid where the individual cells can belong to one or more sub-grids discretizing different physics, such that numerical methods can operate independently on these sub-sets of the joint mesh to solve, e.g., the Navier-Stokes equations, the heat equation, or the particle motion. Due to the wide range of length scales involved, we first demonstrate the scalability of our automatic mesh generation approach. We then proceed to detail the method for fully-resolved particle simulation and the first steps towards its porting to heterogeneous supercomputers. Finally, we detail the parallelization strategy for the particle motion used by large scale one-way Lagrangian particle simulations.