City Research Online

Abstract

Bubbles and ultrasound in biomedical applications often involve large to extreme deformations of the surrounding medium which conventional models fail to handle. The present work introduces a novel unified numerical model for compressible multi-material flows with block-structured adaptive mesh refinement. The five-equation diffuse interface model is expanded to include Eulerian hyperelasticity. This is done by tracking deformations of all solid materials with a conservation law for the elastic stretch tensor. Thus, the model is applicable to any arbitrary number of interacting fluid and solid materials. Subsequently, the capabilities of the model are showcased by investigating the potential for mechanical, and later by incorporating complex thermodynamics, thermal damage in ultrasound-induced collapse of air bubbles near soft materials. Firstly, the results reveal that soft materials primarily experience tensile forces during these interactions, suggesting potential tensile-driven injuries that may occur in relevant treatments. The bubble radius was found to play a crucial role in dictating the stresses experienced by the tissue, underscoring its significance in medical applications. It is documented that while early bubble dynamics remain relatively unaffected by changes in shear modulus of the soft material, at later stages the penetration processes and the deformation shapes, exhibit notable variations. Secondly, the lack of accuracy of commonly used equation of states (EoS) such as the stiffened-gas (SG) EoS is depicted through a compression case and spherical bubble collapse. It was found that the SG EoS can lead to up to 800% error in the predicted temperature at a 10GPa compression compared to the IAPWS EoS. Moreover, the fake temperature front observed in the spherical bubble collapse when using SG is alleviated by using IAPWS, the Modified Tait or the MNASG EoSs. Then, the rigid-wall heating in lithotripter pulse induced collapse of an air bubble is investigated. A significant temperature increase of 25K was observed at the lowest selected standoff distance which could lead to thermal damage. Thirdly, motived by the latest work on temperature predictions by real-fluid EoSs, the multi-material model was extended to complex thermodynamics. The temperatures induced in ultrasound-driven bubble collapses near soft materials was examined utilizing the RKPR EoS in tabulated format and the MNASG in parametric form. Three primary heating mechanisms were identified, with the highest temperature resulting from post-collapse shock. Findings reveal smaller bubbles induce stronger shocks and higher temperatures, while increased standoff distances diminish maximum temperatures due to spherical shock propagation and rapid gas cooling on approaching soft materials.

Publication Type:	Thesis (Doctoral)
Subjects:	Q Science > Q Science (General) T Technology > T Technology (General) T Technology > TJ Mechanical engineering and machinery T Technology > TL Motor vehicles. Aeronautics. Astronautics
Departments:	School of Science & Technology > Engineering > Mechanical Engineering & Aeronautics School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses

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