## 3 Proposed Approach for the LG Far-Field Noise Prediction
### The flow domain setup:  
1. Divide the 3D LG model into multiple slices with different configurations and dimensions, similar to Figure 3. For long aspect ratio (i.e., length-to-diameter ratio, AR \(>\) 3) bluff bodies, mainly located at the upper part of the LG, a single C-S for each is sufficient to capture the flow behaviours. However, the lower part of the LG consists of bulky components with short aspect ratio; thus, at least three horizontal planar cuts should be taken. For example, one of the C-Ss is placed at the middle of the axle-wheel region, and the other two are located symmetrically at the upper and lower portion of the wheels. The latter two C-Ss should be placed away from the upper and lower wheels' circumference zones as well as the inner cavities' rim to avoid the flow separation points.
2. Model and size the 2D computational domain for each C-S separately based on the corresponding \(\mathrm{L_{a}}\).
3. Discretize each domain using a hybrid mesh, where an unstructured mesh is used for the entire domain, and a structured mesh is used around the body to capture the BL.
4. Simulate each 2D domain individually using the CFD solver.
5. Extract and save all the necessary near-field flow data such as the unsteady surface pressure after the statistically steady state is established to be used as an input for the acoustic calculations in the next step.
1. Specify the outer surface of each resulting C-S as an acoustic source.
2. Locate the receiver for each individual domain right above the body and at a distance relative to the microphone location in the experiment.
3. Run the simulation for many time steps to acquire enough samples for a recorded acoustic pressure signal.
4. In order to calculate the far-field noise from the 2D flow data, the SCL is assumed based on the object geometry, its associated flow characteristics, and the available experimental data or empirical formulas.
5. The overall far-field noise of the LG is calculated by summing up all the noise contributions from each C-S.  
Figure 3a,b show the 3D model of the LAGOON NLG with the locations of the C-S's at different heights along the Z-axis and the resulting C-Ss, respectively. Below is a description of the C-Ss used in the CFD computations.  
* The first plane is located along the main strut to capture the flow structures and noise generated around the vertical circular cylinder.
* The second plane is taken at the next sub-set strut component, which also has a cylindrical geometry but with a smaller diameter. Thus, this C-S would capture the changes in flow behaviour.
* The third plane is placed at the upper rim of the outer cavities, as shown in Figure 3c,d, so that the essential flow characteristics of the upper part of the two wheels can be addressed.
* The fourth plane represents a more complex C-S at the middle of the axle-wheels components.  
Figure 3: The 3D Landing Gear nOise database and CAA validatiON (LAGOON) nose landing gear (NLG) model with: (**a**) locations of section planes; (**b**) 2D cross-sections (C-S); (**c,d**) illustrations for the location of Sec.3.  
Finally, the fifth plane is located at the lower part of the two wheels along the lower rim of the outer cavities. It has a shape similar to the third C-S but without the circular component in between the two wheels.  
These five primary C-Ss are defined to capture different flow behaviours and examine their associated noise, named as: Sec.1, Sec.2, Sec.3, Sec.4, and Sec.5, respectively.