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Subsection 8.7.4: Dynamic versus Steady-State Analyses Up Section 8.7: Guidelines for Fluid Analyses Subsection 8.7.6: Fluid-Structure Interactions

### 8.7.5 Isothermal Compressible Flow versus Acoustics

The governing equations for fluid flow in FEBio represent isothermal conditions, thus eliminating temperature as a variable in the numerical analysis. Since the fluid solver accommodates some measure of fluid compressibility (via the fluid dilatation variable ), it is possible to solve for pressure wave propagation in fluids using this formulation. However, from a theoretical perspective, it should be recognized that the speed of sound (i.e., the wave speed) in isothermal (constant temperature) flow is different from the speed of sound in isentropic (constant entropy) flow. The field of acoustics typically examines pressure wave propagations under isentropic conditions, which are found to be more consistent with experimental measurements of the speed of sound in air. Therefore, the FEBio fluid solver may not produce realistic wave speeds when simulating acoustics. A fluid solver for isentropic flow may be developed for FEBio in the future.
Subsection 8.7.4: Dynamic versus Steady-State Analyses Up Section 8.7: Guidelines for Fluid Analyses Subsection 8.7.6: Fluid-Structure Interactions