# /CAA

Block Format Keyword Describes the computation Aero-Acoustic formulation for Eulerian or quasi-Eulerian simulations. Internal forces are computed with finite volume formulation. It improves results for under integrated elements. By default artificial viscosity is canceled, (qa,qb) default values are zero.

## Format

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
/CAA

1. To use "Computation Aero-Acoustic" formulation and specific options, it is necessary to use the option /CAA in the input deck.
2. /CAA is fully consistent only for Eulerian formulation. /CAA is recommended for Eulerian formulation, but not for ALE (except if ALE is quasi-Eulerian).
3. This option has no influence with fully-integrated elements. It is recommended only with one integration point elements.
4. This option uses a finite volume formulation to compute internal forces.
 ${F}_{k}^{{N}_{i}}={\iiint }_{\Omega }\sigma \cdot \nabla {\Phi }_{{N}_{i}}\cdot {e}_{k}d\Omega$ ${F}_{k}^{{N}_{i}}=p{\iiint }_{{S}_{{N}_{i}}}n\cdot {e}_{k}dS+{\iiint }_{\Omega }{\sigma }_{dev}\cdot \nabla {\Phi }_{{N}_{i}}\cdot {e}_{k}d\Omega$
Correct with:
• Parallelepipedic shape, or
• Fully integrated elements
Improve internal forces calculation, in general for:
• One integration point elements
• Trapezoidal shapes
5. Finite volume formulation implies better internal forces estimations.
In the case of a homogeneous fluid pressure, equilibrium on nodes will be verified even with non-trapezoidal shapes like truncated pyramid. This may not be the case with volume integration, due to hypothesis on shape functions for one integration point element. (Refer to Reduced Integration Method in the Theory Manual).
6. Numerical viscosities default values (qa,qb) are set to 0.

This is suitable for low Mach number problem, otherwise these parameters should be non-zero.

7. For old CFD input (input deck V43 or V46 format), this option will be automatically activated running Radioss.
8. For other Block Format input (V41 and V44), it is allowed to add this option in the input deck file to enable this feature running Radioss.