A linear-elasticity-based mesh moving method with no cycle-to-cycle accumulated distortion

Good mesh moving methods are always part of what makes moving-mesh methods good in computation of flow problems with moving boundaries and interfaces, including fluid–structure interaction. Moving-mesh methods, such as the space–time (ST) and arbitrary Lagrangian–Eulerian (ALE) methods, enable mesh-resolution control near solid surfaces and thus high-resolution representation of the boundary layers. Mesh moving based on linear elasticity and mesh-Jacobian-based stiffening (MJBS) has been in use with the ST and ALE methods since 1992. In the MJBS, the objective is to stiffen the smaller elements, which are typically placed near solid surfaces, more than the larger ones, and this is accomplished by altering the way we account for the Jacobian of the transformation from the element domain to the physical domain. In computing the mesh motion between time levels $$t_n$$ t n and $$t_{n+1}$$ t n + 1 with the linear-elasticity equations, the most common option is to compute the displacement from the configuration at $$t_n$$ t n . While this option works well for most problems, because the method is path-dependent, it involves cycle-to-cycle accumulated mesh distortion. The back-cycle-based mesh moving (BCBMM) method, introduced recently with two versions, can remedy that. In the BCBMM, there is no cycle-to-cycle accumulated distortion. In this article, for the first time, we present mesh moving test computations with the BCBMM. We also introduce a version we call “half-cycle-based mesh moving” (HCBMM) method, and that is for computations where the boundary or interface motion in the second half of the cycle consists of just reversing the steps in the first half and we want the mesh to behave the same way. We present detailed 2D and 3D test computations with finite element meshes, using as the test case the mesh motion associated with wing pitching. The computations show that all versions of the BCBMM perform well, with no cycle-to-cycle accumulated distortion, and with the HCBMM, as the wing in the second half of the cycle just reverses its motion steps in the first half, the mesh behaves the same way.

Numerical Analysis of Fluid Flow in 2D Domains Containing Moving Objects

This study presented a two-dimensional (2D) numerical analysis of fluid flow in domains containing moving objects. The method falls into the general category of Arbitrary-Lagrangian-Eulerian (ALE) methods, which is based on a fixed mesh that is locally fitted at the moving objects. The moving objects are described using sets of marker points which can slide over the basic mesh. Once the moving object has gone through the stationary element, the element is restored to its original form. Therefore, the mesh adaptation is performed only in those elements intersected by an object and is local both in space and time. As a result, the method does not require interpolation and there are a fixed number of possible modifications to the intersected elements. As the global mesh is independent of object movement, therefore it eliminates the possibility of mesh entanglement. The mesh never becomes unsuitable due to its continuous deformation, thus eliminating the need for repeated re-meshing and interpolation. A validation is presented via a problem with an exact analytical solution to the case of 2D flow between two parallel plates separating with a prescribed velocity. The method’s capabilities and accuracy are illustrated through application in realistic geometrical settings which show the robustness and flexibility of the technique.

Impact Response of Sacrificial Cladding Structure with an Alporas Aluminum Foam Core Under Blast Loading

This paper presents numerical and analytical studies of the response of a sacrificial cladding structure with an Alporas aluminum foam core and a thick mild steel cover and rear plates under blast loading. A suitable numerical model in LS-DYNA based on the coupled Load Blast Enhanced/Multi-Material Arbitrary Lagrangian–Eulerian (LBE/MM-ALE) methods is selected and validated using the experimental data available in the literature. The shock front propagation and micro-inertia effects are responsible for the strength enhancement predicted in the virtual blast test. Two models with different decaying blast loading functions are examined to study the fluid–structure interaction (FSI) effect. The simulation results show that the FSI effect is negligible if the foam core is strain-rate insensitive. Further investigations should be conducted with analytical models if the core material of the sacrificial cladding structures exhibits a strong strain-rate effect (for example, Alporas foam).

Validation of an Improved Contact Method for Multi-Material Eulerian Hydrocodes in Three-Dimensions

Realistic and accurate modeling of contact for problems involving large deformations and severe distortions presents a host of computational challenges. Due to their natural description of surfaces, Lagrangian finite element methods are traditionally used for problems involving sliding contact. However, problems such as those involving ballistic penetrations, blast-structure interactions, and vehicular crash dynamics, can result in elements developing large aspect ratios, twisting, or even inverting. For this reason, Eulerian, and by extension Arbitrary Lagrangian-Eulerian (ALE), methods have become popular. However, additional complexities arise when these methods permit multiple materials to occupy a single finite element.

Factors Associated with Axial Length Elongation in High Myopia

Background: To investigate the impact of axial length (AL) and ocular factors on axial length elongation (ALE). Methods: A retrospective chart review of patients who underwent more than two axial length examinations using a single instrument Results: The mean age of the participants was 47.21 ± 7.79 years. Eyes were classified into four groups according to their initial AL measurement. AL remained almost unchanged in the groups with AL < 26mm. On the contrary, AL increased by 0.011mm/year in the group with 26 ≤ AL < 28mm and 0.035mm/year in the group with AL ≥ 28mm (P < .001). In high myopia, ALE increased in eyes with longer axial lengths (r = 0.003, P = .024), females (r = 0.014, P = .019), eyes with larger peripapillary chorioretinal atrophic areas (r = 0.002, P = .019), and smaller vascular arcade angles (r = -0.004, P = .006). The risk of elongation 0.03mm/year in high myopia was increased in females (OR = 2.265, P = .040), and gradually increased in eyes with large peripapillary chorioretinal atrophy area (OR = 5.604, 6.939, and 7.470, respectively; P = .001, < .001, and .008, respectively) Conclusions: AL remained almost unchanged in the group with AL < 26mm. On the contrary, ALE was observed in the group with AL ≥ 26mm. AL elongated significantly in eyes with longer AL, female, and eyes with larger atrophic areas and smaller arcade angles on fundus photographs.

An Accurate SPH Scheme for Dynamic Fragmentation modelling

We focus on the use of a meshless numerical method called Smooth Particle Hydrodynamics (SPH), to solve fragmentation issues as Hyper Velocity Impact (HVI) cases. Firstly applied to fluid flow simulations, this method can be extended to the solid dynamics framework. However it suffers from a lack of accuracy when evaluating state variables as the pressure field. And such inaccuracy generally generates non-physical processes (as numerical fragmentation). In the hydrodynamic context, SPH-ALE methods based on Riemann solvers significantly improve this evaluation, but increase the scheme complexity and low-Mach issues are difficult to prevent. We propose an alternative scheme called γ-SPH-ALE, firstly implemented to solve multi-regime barotropic flows, and secondly extended to solid dynamic cases. It relies on the combination of the SPH-ALE formalism and a finite volume stabilizing low-Mach scheme. Its characteristics are detailed and evaluated through a nonlinear stability analysis, highlighting CFL-like conditions on the scheme parameters. Finally, its implementation on several test cases reveals that the proposed scheme actually increases both stability and accuracy, in reduced computation time, with respect to classical solvers.

Arbitrary Lagrangian–Eulerian simulations of highly electrically charged micro-droplet Coulomb explosion deformation pathways

Numerical simulation results of the Coulomb explosion pathways of cooled water and heated glycol droplets electrically charged to the critical Rayleigh limit are presented, calculated using an axi-symmetric finite element scheme previously used for the same problem [Radcliffe A. J., Non-conforming finite elements for axisymmetric charged droplet deformation dynamics and Coulomb explosions, Int. J. Num. Meth. Fluids 71:249–268, (2013), doi:10.1002/fld.3667.] which has been adapted to use arbitrary Lagrangian–Eulerian (ALE) methods and a novel tip reconstruction technique in order to greatly improve its accuracy in matching available experimental data.

Numerical Investigation of Flow Interference Effects in Fixed and Vibrating Tandem Square Columns Using Hybrid RANS-LES Models

Investigation of flow past tandem and side-by-side circular and square columns is of interest in offshore engineering. Flow past fixed and vibrating circular columns has received a lot of focus in the literature. However, the studies focused on square columns, especially at high Reynolds numbers are very limited due to the computational cost of large eddy simulation (LES). Unsteady Reynolds-averaged Navier-Stokes (URANS) methods are limited by their accuracy, especially for tandem columns in the wake interference regime (spacing to diameter ratio: L=D ∼ 3:0). Hybrid URANS-LES models (URANS near the solid-wall and LES away from the wall) can overcome the drawbacks of the traditional URANS methods and can provide a reasonable prediction of the flow physics at a fraction of the cost of LES without significantly sacrificing numerical accuracy. Arbitrary Lagrangian-Eulerian (ALE) methods fails when vibrating tandem bodies are in close proximity to each other or vibrate at high reduced velocities. Remeshing the domain can be expensive, especially at high Reynolds numbers (Re). Alternate strategies are necessary to efficiently simulation this problems. This study proposes the use of a non-linear URANS-LES model, coupled with an overset mesh method (for vibrating columns), for studying flow past tandem square columns. Simulations are performed at sub-critical Re to match the experimental Re. The initial results are encouraging for further investigation of fixed and vibrating tandem square column flow interference at high Reynolds numbers.