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[FAQ]

In a structural transient analysis, when we have damping, we see a phase lag due to the damping term (being proportional to velocity rather than displacement). In a transient FSI analysis, the situation is different – Fluent does not give Mechanical a damping matrix. Instead, Mechanical passes displacements to Fluent, which returns forces (pressures). but the fluid flow is usually considered as incompressible, so any changes upstream are immediately seen downstream. (Compressible flow is typically used when we have density variation as function of temperature, so we need to solve the energy equation in conjunction with flow – such as with external flow at high Mach numbers.) Because of this, viscosity does not affects FSI simulations the way it would in purely Mechanical simulations – a displacement sent to Fluent will immediately return a force and not show a phase lag.