Space Medicine and cardiovascular mathematical-computational simulation: Applications of Navier-Stokes Solutions to Astronaut Cardiovascular Systems

Prolonged human exposure to microgravity precipitates severe cardiovascular deconditioning driven by the elimination of hydrostatic pressure gradients. Gravitational unloading triggers an immediate cephalad fluid shift, progressive myocardial atrophy, spaceflight-associated neuro-ocular syndrome (SANS), and venous stasis predisposing astronauts to internal jugular vein (IJV) thrombosis. Concurrently, microgravity-induced hemolysis (``space anemia'') reduces hematocrit and macroscopic blood viscosity, directly altering endothelial wall shear stress (WSS) and accelerating inward vascular remodeling. Given the logistical and physiological constraints of invasive in-orbit clinical measurements, computational hemodynamics serves as an essential investigative tool. This work synthesizes multiscale computational frameworks based on numerical solutions of the incompressible Navier-Stokes equations across terrestrial ($1g$), microgravity ($0g$), Martian ($0.38g$), and hyper-gravity ($>1g$) regimes. We examine non-Newtonian Carreau-Yasuda rheological modeling, monolithic Arbitrary Lagrangian-Eulerian (ALE) fluid-structure interaction (FSI) solvers that overcome density-matched added-mass instabilities, and multiscale fractional-order Windkessel boundary couplings. Finally, we demonstrate how high-fidelity computational fluid dynamics uncovers the fluid mechanics driving venous flow reversal, assesses microvascular hemodynamic pulsatility in ocular beds, and optimizes physiological countermeasure protocols, including lower body negative pressure (LBNP) and short-arm human centrifuges.