Study on the flow mechanism and frequency characteristics of rales in lower respiratory tract

The frequency characteristics of lung sounds have great significance for noninvasive diagnosis of respiratory diseases. The rales in the lower respiratory tract region that can provide rich information about symptoms of respiratory diseases are not clear. In this paper, a three-dimensional idealized bifurcated lower respiratory tract geometric model, which contains 3(rd) to 13th generation (G(3)-G(13)) bronchi is constructed, where Re similar to 10(1)-10(3), and then the large eddy simulation and volume of fluid are used to study the fluid flow characteristics. Ffowcs Williams and Hawkings model are subsequently used to study the frequency characteristics of rale of different generations of bronchi. The results showed that bronchial blockage and sputum movement will enhance the turbulence intensity and vortex shedding intensity of flow. The dominant frequency and highest value of sound pressure level (SPL) of rhonchi/moist crackles decrease with the increase of bronchial generation. The change rates of dominant frequency of rhonchi / moist crackles in adjacent generations were 5.0 +/- 0.1 similar to 9.1 +/- 0.2% and 3.1 +/- 0.1 similar to 11.9 +/- 0.3%, respectively, which is concentrated in 290 similar to 420 Hz and 200 similar to 300 Hz, respectively. The change rates of SPL of rhonchi/moist crackles were 8.8 +/- 0.1 similar to 15.7 +/- 0.1% and 7.1 +/- 0.1 similar to 19.5 +/- 0.2%, respectively, which is concentrated in 28 similar to 50 dB and 16 similar to 32 dB, respectively. In the same generation of bronchus (e.g., G(8), G(9)) with the same degree of initial blockage, the dominant frequency and SPL of moist crackles can be 3.7 +/- 0.2% and 4.5 +/- 0.3% slightly higher than that of rhonchi, respectively. This research is conducive to the establishment of a rapid and accurate noninvasive diagnosis system for respiratory diseases.