In aerospace Micro-ElectroMechanical Systems (MEMS), the characteristic length scale of the flow approaches the molecular mean free path, thus invalidating the continuum description and enforcing the use of particle methods, like the Direct Simulation Monte Carlo (DSMC), to deal with the non-equilibrium regions. Within the slip-regime (0.01

Micro-rockets for propulsion of small spacecrafts exhibit significant differences with regard to their macroscale counterparts, mainly caused by the role of the viscous dissipation and heat transfer processes in the micron-sized scale. The goal of this work is to simulate the transient operation of a micro-rocket to investigate the effects of viscous heating on the flow and performance for four configurations of the expanding gas and wafer material. The modelling follows a multiphysics approach that solves the fluid and solid regions fully coupled. A continuum-based description that incorporates the effects of gas rarefaction through the micro-nozzle, viscous dissipation and heat transfer at the solid-gas interface is presented. Non-equilibrium is addressed with the implementation of a 2nd-order slip-model for the velocity and temperature at the walls. The results stress that solid-fluid coupling exerts a strong influence on the flowfield and performance as well as the effect of the wafer during the first instants of the transient in micro-rockets made of low and high thermal conductivity materials.

Helical strakes are the most employed devices to mitigate or suppress vortex shedding behind circular cylinders. Although several investigations have been performed in order to predict the performance of these devices, proving its efficiency in specific configurations, little is understood regarding the physical mechanisms leading to the efficiency of these devices. The present work addresses this question from a global linear instability analysis point of view. Direct Numerical Simulation, three-dimensional global (TriGlobal) and Floquet stability analysis of the flow around a cylinder fitted with helical strakes is performed at low and moderate Reynolds number in order to understand more deeply the flow instabilities and physical mechanisms that mitigate and suppress the vortex-shedding.

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