Jon Summers

The Importance of Fluid-Structure Interaction in Spinal Trauma Models

While recent studies have demonstrated the importance of the initial mechanical insult in the severity of spinal cord injury, there is a lack of information on the detailed cord-column interaction during such events. In vitro models have demonstrated the protective properties of the cerebrospinal fluid, but visualization of the impact is difficult. In this study a computational model was developed in order to clarify the role of the cerebrospinal fluid and provide a more detailed picture of the cord-column interaction. The study was validated against a parallel in vitro study on bovine tissue. Previous assumptions about complete subdural collapse before any cord deformation were found to be incorrect. Both the presence of the dura mater and the cerebrospinal fluid led to a reduction in the longitudinal strains within the cord. The division of the spinal cord into white and grey matter perturbed the bone fragment trajectory only marginally. In conclusion, the cerebrospinal fluid had a significant effect on the deformation pattern of the cord during impact and should be included in future models. The type of material models used for the spinal cord and the dura mater were found to be important to the stress and strain values within the components, but less important to the fragment trajectory.

The effect of bone fragment size and cerebrospinal fluid on spinal cord deformation during trauma: an ex vivo study

OBJECT The purpose of the study was to assess the effect of CSF and the size of the impacting bone fragment area on spinal cord deformation during trauma. METHODS A transverse impact rig was used to produce repeated impacts on bovine and surrogate cord models. Tests were recorded with high-speed video and performed on specimens with and without CSF and/or dura mater and with 3 different impactor areas. RESULTS The CSF layer was found to reduce the maximum cord deformation significantly. A 50% reduction in impact area significantly increased the maximum cord deformation by 20-30%. The surrogate model showed similar trends to the bovine model but with lower absolute deformation values. CONCLUSIONS Cerebrospinal fluid protects the cord during impact by reducing its deformation. A smaller bone fragment impact area increases the deformation of the cord, in agreement with clinical results, where a higher impact energy-possibly giving rise to smaller fragments-results in a worse neurological deficit.

Multiphysics simulation of the effect of leaflet thickness inhomogeneity and material anisotropy on the stress–strain distribution on the aortic valve

This study developed a realistic 3D FSI computational model of the aortic valve using the fixed-grid method, which was eventually employed to investigate the effect of the leaflet thickness inhomogeneity and leaflet mechanical nonlinearity and anisotropy on the simulation results. The leaflet anisotropy and thickness inhomogeneity were found to significantly affect the valve stress-strain distribution. However, their effect on valve dynamics and fluid flow through the valve were minor. Comparison of the simulation results against in-vivo and in-vitro data indicated good agreement between the computational models and experimental data. The study highlighted the importance of simulating multi-physics phenomena (such as fluid flow and structural deformation), regional leaflet thickness inhomogeneity and anisotropic nonlinear mechanical properties, to accurately predict the stress-strain distribution on the natural aortic valve.

A unified model for holistic power usage in cloud datacenter servers

Cloud datacenters are compute facilities formed by hundreds and thousands of heterogeneous servers requiring significant power requirements to operate effectively. Servers are composed by multiple interacting sub-systems including applications, microelectronic processors, and cooling which reflect their respective power profiles via different parameters. What is presently unknown is how to accurately model the holistic power usage of the entire server when including all these sub-systems together. This becomes increasingly challenging when considering diverse utilization patterns, server hardware characteristics, air and liquid cooling techniques, and importantly quantifying the non-electrical energy cost imposed by cooling operation. Such a challenge arises due to the need for multi-disciplinary expertise required to study server operation holistically. This work provides a unified model for capturing holistic power usage within Cloud datacenter servers. Constructed through controlled laboratory experiments, the model captures the relationship of server power usage between software, hardware, and cooling agnostic of architecture and cooling type (air and liquid). An exciting prospect is the ability to quantify the amount of non-electrical power consumed through cooling, allowing for more realistic and accurate server power profiles. This work represents the first empirically supported analysis and modeling of holistic power usage for Cloud datacenter servers, and bridges a significant gap between computer science and mechanical engineering research. Model validation through experiments demonstrates an average standard error of 3% for server power usage within both air and liquid cooled environments.

Modelling of Spinal Cord Biomechanics: In Vitro and Computational Approaches

In vivo testing of spinal cord biomechanics is surrounded with difficulties. This is due to methodological issues such as reproducibility and visualization problems as well as ethical concerns and the possible lack of correlation between animal models and the human tissue response. Therefore, a number of in vitro and computational studies have been performed in order to increase the understanding of the spinal cord’s mechanical behaviour during trauma. Due to its well defined nature, the burst fracture and its effect on the spinal cord have been frequently studied, but investigations on other injuries such as distraction and dislocation are also cited in the literature. In vitro models have utilised a variety of materials including polymers and biological tissue from different animals. Similarly a variety of models have been used in computational studies to represent the cord which range in terms of both geometric complexity and material models. This chapter aims to provide a review of the methods and materials used in vitro and computational models of the spinal cord injury.

Holistic data centres: Next generation data and thermal energy infrastructures

Digital infrastructure is becoming more distributed and requiring more power for operation. At the same time, many countries are working to de-carbonise their energy, which will require electrical generation of heat for populated areas. What if this heat generation was combined with digital processing?

A Framework and Task Allocation Analysis for Infrastructure Independent Energy-Efficient Scheduling in Cloud Data Centers

Cloud computing represents a paradigm shift in provisioning on-demand computational resources underpinned by data center infrastructure, which now constitutes 1.5% of worldwide energy consumption. Such consumption is not merely limited to operating IT devices, but encompasses cooling systems representing 40% total data center energy usage. Given the substantive complexity and heterogeneity of data center operation spanning both computing and cooling components, obtaining analytical models for optimizing data center energy-efficiency is an inherently difficult challenge. Specifically, difficulties arise pertaining to the non-intuitive relationship between computing and cooling energy in the data center, computationally complex energy modeling, as well as cooling models restricted to a specific class of data center facility geometry - all of which arise from the interdisciplinary nature of this research domain.In this paper we propose a framework for energy-efficient scheduling to alleviate these challenges. It is applicable to any type of data center infrastructure and does not require complex modeling of energy.Instead, the concept of a target workload distribution is proposed. If the workload is assigned to nodes according to the target workload distribution, then the energy consumption is minimized. The exact target workload distribution is unknown, but an approximated distribution is delivered by the framework. The scheduling objective is to assign workload to nodes such that the workload distribution becomes as similar as possible to the target distribution in order to reduce energy consumption.Several mathematically sound algorithms have been designed to address this novel type of scheduling problem. Simulation results demonstrate that our algorithms reduce the relative deviation by at least 16.9% and the relative variance by at least 22.67% in comparison to (asymmetric) load balancing algorithms.