Deborah Number

Thixotropy and rheopexy describe time-dependent changes in viscosity. A thixotropic fluid's viscosity decreases over time under constant shear stress (e.g., yogurt becomes runnier when stirred). A rheopectic (or anti-thixotropic) fluid's viscosity increases over time under constant shear (e.g., some lubricants). These effects are reversible; the fluid returns to its original state after a period of rest.

A non-Newtonian fluid is one whose viscosity changes under an applied shear stress. Unlike a Newtonian fluid where viscosity is constant, its flow properties are not described by a linear relationship between shear stress (\(\tau\)) and shear rate (\(\dot{gamma}\)). This dependency can manifest as shear-thinning (viscosity decreases with stress) or shear-thickening (viscosity increases with stress).

The Weissenberg effect is a phenomenon in viscoelastic fluids where the fluid climbs up a rotating rod inserted into it. This is contrary to the behavior of Newtonian fluids, which are pushed outwards by centrifugal force, forming a vortex. The effect is caused by normal stress differences that develop in the fluid under shear.

The Deborah number is a dimensionless quantity in rheology, used to characterize the fluidity of materials. It is the ratio of the relaxation time, which is an intrinsic property of the material, to the characteristic time scale of the experiment or observation. The formula is \(De = \frac{t_c}{t_p}\), where \(t_c\) is the relaxation time and \(t_p\) is the observation time.

General relativity predicts that accelerating masses generate ripples in the fabric of spacetime called gravitational waves. These waves propagate outwards at the speed of light, carrying energy as gravitational radiation. They are created by cataclysmic cosmic events like the merger of black holes or neutron stars, causing spacetime to stretch and squeeze as they pass.

Shear-thinning, or pseudoplasticity, is a non-Newtonian behavior where a fluid's viscosity decreases with an increasing rate of shear stress. Many common substances, like ketchup or paint, exhibit this property. At rest, they are thick, but they flow more easily when shaken, stirred, or spread. This is often modeled by the Ostwald–de Waele power-law relationship.

The Courant–Friedrichs–Lewy (CFL) condition is a necessary stability criterion for numerical solutions of hyperbolic partial differential equations using explicit time-integration schemes. It dictates that the time step size must be small enough that information does not travel further than one spatial grid cell per time step. For a 1D case, \(C = u \frac{\Delta t}{\Delta x} \le C_{max}\), ensuring numerical stability.

The Finite Element Method (FEM) is a powerful numerical technique for solving complex engineering and physics problems described by partial differential equations. It works by discretizing a continuous domain into a set of smaller, simpler subdomains called 'finite elements'. This allows for the approximate numerical solution of problems in structural analysis, heat transfer, fluid flow, and electromagnetism.

Plasticity is the theory describing the deformation of a solid material undergoing non-reversible changes of shape in response to applied forces. Unlike elasticity, where deformation is recovered upon unloading, plastic deformation is permanent. The theory involves a yield criterion to define the onset of plasticity, a flow rule to describe the evolution of plastic strain, and a hardening rule.

The Finite Volume Method (FVM) is a dominant numerical technique in CFD for solving partial differential equations. It discretizes the domain into a mesh of control volumes and applies the governing equations in their integral form to each volume. By converting volume integrals to surface integrals using the divergence theorem, it focuses on calculating the flux of conserved properties across cell faces.