# Modeling Methodology

The process of defining a numerical model for a given CAA application unfolds in four consecutive phases. The quality of the results is directly tight to the quality of the numerical model developed in the points 1 and 2 below. It is, therefore, critical to define precisely the questions the model is supposed to answer to before starting any development:
1. Mesh definition: Targeting specific answers to engineering questions and constrained by the available computer resources.
2. Numerical model construction, including material, boundary conditions and desired output.
3. Run monitoring.
4. Post-processing of the time domain data. Including 3D visualization, time history and frequency content analysis.

## Mesh Definition

The mesh building process is the art of building models able to solve the CFD and the CAA problem for a given set of boundary conditions and range of frequencies while keeping the model as small as possible to minimize the compute time. To do so, a set of practical rules have been developed. These rules should be used as initial guidelines to get started on a given problem.

However each class of problems has its own requirements and subtleties and a good knowledge of the problem physics through experimental data and/or numerical simulations will be necessary to refine these rules and get the best possible results.

## Post-processing

Post-processing of the numerical simulation is very similar to the post-processing of a detailed experimental study of the same problem. The analysis will be carried out by using:
• FFT's of recorded time domain signal to access the frequency domain content at any given location of the computational domain.
• Visualization and analysis of intensities on the structures
• Propagation in the far field (if and when needed) of the pressure signal. Typically, this is required for simulations where the measurement locations are not located in the computational domain. In most of the internal flow problems for instance the limit of the domain is the structure and a boundary elements layer to represent the outside air impedance (exhaust, HVAC, and so on). The noise is often measured at a given distance outside the ducts in still air where there is no reason to have an expensive CAA solution. This propagation can be performed by a simple monopolar approximation that returns satisfactory results in the free conditions or by more sophisticated tools such as BEM methods.