Mean exit time in irregularly-shaped annular and composite disc domains

Calculating the mean exit time (MET) for models of diffusion is a classical problem in statistical physics, with various applications in biophysics, economics and heat and mass transfer. While many exact results for MET are known for diffusion in simple geometries involving homogeneous materials, calculating MET for diffusion in realistic geometries involving heterogeneous materials is typically limited to repeated stochastic simulations or numerical solutions of the associated boundary value problem (BVP). In this work we derive exact solutions for the MET in irregular annular domains, including some applications where diffusion occurs in heterogenous media. These solutions are obtained by taking the exact results for MET in an annulus, and then constructing various perturbation solutions to account for the irregular geometries involved. These solutions, with a range of boundary conditions, are implemented symbolically and compare very well with averaged data from repeated stochastic simulations and with numerical solutions of the associated BVP. Software to implement the exact solutions is available on \href{https://github.com/ProfMJSimpson/Exit_time}{GitHub}.

Specifications of Resonant Acoustic Rotating Waves in Circular and Annular Domains

Circular and annular domains of hydroacoustic vibration are very common in modern technology due to their simplicity. On the other hand it turns out that such a shape possesses remarkable vibration properties. It is determined that there are two classes of resonant rotating waves, predominantly tangential and predominantly radial, in terms of prevalence of tangential or radial components of the vectors of vibrational velocities and displacements. The complete map of resonant angular velocities shows that all predominantly tangential angular velocities for all values of ring thickness are assembled into the self-isolating unique single low-frequency branch, whereas predominantly radial ones fill the entire high-frequency region very densely.