Add aeroacoustic pressure spectrum analysis

Docs/Aeroacoustics.md

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+# Aeroacoustic analysis of unsteady pressure
+
+`aeroacoustics.py` converts pressure histories from an unsteady SU2
+simulation into frequency-domain acoustic quantities. It is intended for
+pressure probes from LES, DES, DDES, ZDES, or EDDES cases, but it can analyze
+any uniformly sampled pressure history.
+
+The tool estimates the one-sided power spectral density (PSD) with Welch's
+method and reports:
+
+- pressure PSD in Pa²/Hz;
+- PSD level in dB/Hz;
+- bin-integrated narrowband sound pressure level (SPL) in dB;
+- overall sound pressure level (OASPL) in dB; and
+- the dominant non-zero frequency.
+
+Only NumPy is required.
+
+## Recording pressure probes
+
+SU2 custom outputs can record pressure at one or more points. For example:
+
+```ini
+CUSTOM_OUTPUTS= 'mic1 : Probe{PRESSURE}[1.0, 0.0, 0.0]; \
+                 mic2 : Probe{PRESSURE}[0.0, 1.0, 0.0]'
+HISTORY_OUTPUT= TIME_ITER, CUR_TIME, mic1, mic2
+```
+
+Use a fixed physical time step and write every desired acoustic sample. The
+history column containing physical time is commonly named `Cur_Time`; inspect
+the CSV header and pass its exact name with `--time`.
+
+## Usage
+
+Analyze two pressure columns that are already dimensional:
+
+```bash
+python SU2_PY/aeroacoustics.py history.csv \
+  --pressure mic1 \
+  --pressure mic2 \
+  --skip-samples 10000 \
+  --segment-length 2048 \
+  --summary acoustic_summary.json \
+  --output acoustic_spectrum.csv
+```
+
+If the file contains an iteration rather than physical time, provide the
+sampling rate explicitly:
+
+```bash
+python SU2_PY/aeroacoustics.py history.csv \
+  --sample-rate 50000 \
+  --pressure mic1
+```
+
+Pressure must be dimensional in pascals for physical SPL. If an SU2 case uses
+nondimensional pressure, apply its dimensionalization factor in pascals per
+input unit:
+
+```bash
+python SU2_PY/aeroacoustics.py history.csv \
+  --time Cur_Time \
+  --pressure mic1 \
+  --pressure-scale 101325
+```
+
+The default acoustic reference is 20 µPa, appropriate for airborne sound. Use
+`--reference-pressure` for another medium or convention.
+
+## Sampling guidance
+
+- Resolve the highest frequency of interest with adequate samples per period;
+  the absolute Nyquist limit is half the sample rate.
+- Start collecting samples after initial transients have left the domain, or
+  remove them with `--skip-samples`.
+- Increase `--segment-length` for finer frequency resolution. Frequency-bin
+  width is the sample rate divided by segment length.
+- Longer signals give more Welch segments and a smoother PSD estimate.
+- The default Hann window, 50% overlap, and mean removal are suitable general
+  choices. `--detrend linear` can remove slow numerical drift.
+- Avoid interpreting probe pressure as a far-field acoustic result when the
+  probe is inside a vortical or hydrodynamic near field. Permeable-surface
+  Ffowcs Williams–Hawkings propagation is a larger, separate capability.
+
+The spectrum CSV contains one frequency column followed by PSD, PSD-level, and
+narrowband-SPL columns for every requested pressure signal. The optional JSON
+summary contains scalar metrics suitable for automated comparisons.