Numerical study of the instantaneous flow fields by large eddy simulation and stability analysis in a single aisle cabin model

Airflow movement is one of the most important physical parameter greatly influencing the contaminants transmission and passengers' thermal perception in aircraft cabin environment. Although many measured and computational results showed that the airflows in aircraft cabins were highly unsteady or even unstable, most of the existing studies focused on time-averaged air distributions in aircraft cabins. The instantaneous characteristics of flow fields, such as airflow velocity fluctuation, turbulence intensity and fluctuation frequency, were seldom reported. In this paper, a refined large eddy simulation (LES) was conducted to investigate the complex airflow characteristics in a simplified Boeing 737-200 cabin model. Simulated results indicated that the flow fields in this cabin model, especially around the aisle region, displayed obvious quasi-periodic large scale unsteadiness and continuing instabilities, as seen in our PIV measurements, even though the boundary conditions were fixed and symmetrical with the middle plane and various perturbations were as much removed as possible. A triple decomposition was used to extract the highly unsteady characteristics of the instantaneous flow fields and the large scale unsteadiness term was used to interpret why the actual kinetic energies of turbulence in LES was amplified. The structural stability analysis approach was used to investigate the mechanism of such unstable flow fields. Saddle points and heteroclinic orbits were found to exist in cross sections of the cabin model. The Peixoto theorem was used to interpret the reason for this unstable instantaneous flow fields in this type of flow regime and geometry.