Non-invasive in silico determination of ventricular wall pre-straining and characteristic cavity pressures

The clinical application of patient-specific modelling of the heart can provide valuable insights in supplementing and advancing methods of diagnosis as well as helping to devise the best possible therapeutic approach for each individual pathological heart condition. The potential of computational cardiac mechanics, however, has not yet been fully leveraged due to the heart's complex physiology and limitations in the non-invasive in vivo characterisation of heart properties necessary required for accurate patient-specific modelling such as the heart anatomy in an unloaded state, ventricular pressure, the elastic constitutive parameters and the myocardial muscle fibre orientation distribution. From a solid mechanics point of view without prior knowledge of the unloaded heart configuration and the cavity pressure-volume evolution, in particular, the constitutive parameters cannot be accurately estimated to describe the highly nonlinear elastic material behaviour of myocardial tissue. Here, knowledge of the volume-normalized end-diastolic pressure relation for larger mammals is exploited in combination with a novel iterative inverse parameter optimisation framework to determine end-systolic and end diastolic pressures, ventricular wall pre-straining and pre-stressing due the residual end-systolic cavity pressure as well as myocardial tissue stiffness parameters for biventricular heart models

The pathophysiology of RHD and outstanding gaps

Rheumatic heart disease (RHD) is the major cause of cardiovascular morbidity and mortality in children and young adults in low- and middle-income countries. Acute rheumatic fever (ARF) is characterised by multiorgan inflammatory symptoms initiated through cross reaction of immune responses (IRs) to group A streptococcus (GAS) proteins to host proteins. Recurrence of these IRs targeting the heart valves may lead to permanent damage, a sequela which is termed RHD. Preliminary studies suggested genetic associations in RF reactions, but that other host factors are also involved, leaving the determinants of RHD progression incompletely understood. Previous clinical and recent epidemiological studies support differential clinical phenotypes, with varying history from different settings. This review summarises the protein-centric biomolecular changes in RHD and highlights outstanding molecular gaps where urgent focus is required to improve our understanding RHD pathophysiology. Numerous studies have confirmed alterations in the expression of structural and immune response proteins, but the modifications giving rise to neo-epitopes and their involvement in RHD have not been established. As RHD is associated with poor living conditions, identification of other factors driving inflammation to enhance RHD progression is necessary to advance our knowledge and improve patient management. Furthermore, biomarkers for early identification, disease stratification, and alternative therapeutic strategies are necessary to improve treatment and prevention strategies in order to reduce the burden of RHD. Relevance: Despite the explosion of scientific innovation over the last few decades, fundamental scientific studies to understand the pathophysiological mechanisms of RHD remain in their infancy and the determinants of RHD progression thus remain uncertain. Moreover, inconsistency in natural history and phenotypic presentations are seen between Africans and other cohorts in which preliminary studies were conducted, implying that differences in genetic complexity and environmental factors may be responsible for the differential disease progression rates. SAHeart 2022;19:38-4

A computational fluid dynamics model for the small-scale dynamics of wave, ice floe and interstitial grease ice interaction

The marginal ice zone is a highly dynamical region where sea ice and ocean waves interact. Large-scale sea ice models only compute domain-averaged responses. As the majority of the marginal ice zone consists of mobile ice floes surrounded by grease ice, finer-scale modelling is needed to resolve variations of its mechanical properties, wave-induced pressure gradients and drag forces acting on the ice floes. A novel computational fluid dynamics approach is presented that considers the heterogeneous sea ice material composition and accounts for the wave-ice interaction dynamics. Results show, after comparing three realistic sea ice layouts with similar concentration and floe diameter, that the discrepancy between the domain-averaged temporal stress and strain rate evolutions increases for decreasing wave period. Furthermore, strain rate and viscosity are mostly affected by the variability of ice floe shape and diameter

Frazil Ice in the Antarctic Marginal Ice Zone

Frazil ice, consisting of loose disc-shaped ice crystals, is the first ice that forms in the annual cycle in the marginal ice zone (MIZ) of the Antarctic. A sufficient number of frazil ice crystals form the surface “grease ice” layer, playing a fundamental role in the freezing processes in the MIZ. As soon as the ocean waves are sufficiently damped by a frazil ice cover, a closed ice cover can form. In this article, we investigate the rheological properties of frazil ice, which has a crucial influence on the growth of sea ice in the MIZ. An in situ test setup for measuring temperature and rheological properties was developed. Frazil ice shows shear thinning flow behavior. The presented measurements enable real-data-founded modelling of the annual ice cycle in the MIZ

Effects of an explosive polar cyclone crossing the Antarctic marginal ice zone

Antarctic sea ice shows a large degree of regional variability, which is partly driven by severe weather events. Here we bring a new perspective on synoptic sea ice changes by presenting the first in situ observations of an explosive extratropical cyclone crossing the winter Antarctic marginal ice zone (MIZ) in the South Atlantic. This is complemented by the analysis of subsequent cyclones and highlights the rapid variations that ice-landing cyclones cause on sea ice: Midlatitude warm oceanic air is advected onto the ice, and storm waves generated close to the ice edge contribute to the maintenance of an unconsolidated surface through which waves propagate far into the ice. MIZ features may thus extend further poleward in the Southern Ocean than currently estimated. A concentration-based MIZ definition is inadequate, since it fails to describe a sea ice configuration which is deeply rearranged by synoptic weather

Computational aspects of generalized continua based on moving least square approximations.

In recent years, current engineering technology lead to a renewed interest in generalized continuum theories. In particular, generalized continua are able to address fundamental physical phenomena which are related to the underlying microstructure of the material. Specifically scale-effects are of special interest. In this work a generalized deformation formulation is developed which allows to incorporate material information from the microscopic and the macroscopic space into an unified constitutive model. The approach is based on a theory developed by Sansour (1998) which was originated in theoretical considerations of Ericksen and Truesdell (1957) and later on Eringen and his co-workers (Eringen 1999). The basic idea is to construct a generalized continuum consisting of macro- and micro-continuum and subsequently to compose the generalized deformation by a macro- and micro-component. This procedure results in a generalized problem formulation. Furthermore, new strain measures as well as corresponding field equations can be identified. Here, it is assumed that the deformation field can only be varied within the macro-continuum so that the balance equations are established for the macro-space. The constitutive law is defined at the microscopic level and the geometrical specification of the micro-continuum is the only material input which goes beyond those needed in a classical description. A special detail of this approach is that it involves first order strain gradients which are expressed by second order derivatives of the field. It allows to address relative motion between micro- and macro-space without adding extra degrees of freedom. In order to model this formulation this work makes use of a meshfree method based on moving least squares (MLS) which is able to provide the required higher order continuity (Lancaster and Salkauskas 1981). Examples of meshfree methods are the diffuse element method (Nayroles, Touzot, and Villon 1992), the element-free Galerkin method (Belytschko, Lu and Gu 1994), the reproducing kernel particle method (Liu and Chen, 1995), the partition of unity method (Melenk and Babuska 1997) and the hp-cloud method (Duarte and Oden 1996), just to name a few. It was demonstrated that these kind of methods can deal especially well with problems which are characterized by large deformation or changing domain geometry. The potential in modelling formulations involving higher order derivatives has not been widely recognized yet, with the exception of a few one- respectively two-dimensional case studies (Tang et al., 2003). This work now aims to illustrate the excellent applicability of the proposed generalized deformation formulation in combination with MLS by modelling elastic and plastic problems which are proven to exhibit size-scale effects (Yang and Lakes 1981 ; Fleck et al. 1994 ; Aifantis 1999 ; Lam et al. 2003). Furthermore, a large-scale case study on underground excavation design reveals the potential and adaptivity of this theory with respect to heterogeneous material such as rock.Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 2006

A higher gradient formulation and meshfree-based computations for elastic rock

In this work a non-classical continuum is derived from a unified formulation based on the generalized deformation field. This approach involves new strain measures which allow for the formulation of a generalized variational principle. It is demonstrated how to model micro-structure related material behaviour by a continuum mechanical approach in a very straight forward way. As an illustration excavations in dry rock conglomerate with two different geometrical configuration are modelled with a meshfree method based on moving least squares (Lancaster and Salkauskas, 1981). Thereby, the resulting displacement and stress field using a classical Green strain tensor based formulation (Skatulla and Sansour, 2005) is compared with the solution of the non-classical formulation. The use of meshfree methods in particular, proves to be advantageous, because it makes it possible to approximate the displacement field, and also its first derivatives.C. Sansour and S. Skatull

A non-linear Cosserat continuum-based formulation and moving least square approximations in computations of size-scale effects in elasticity

Copyright © 2007 Elsevier B.V. All rights reserved.Carlo Sansour and Sebastian Skatullahttp://www.elsevier.com/wps/find/journaldescription.cws_home/523412/description#descriptio