Vortical flow structure of thermoacoustic oscillations in a closed tube

Spontaneous thermoacoustic oscillations of a gas in a closed cylindrical tube are studied. Numerical simulations of the flow field in the tube on which a temperature gradient along the axis is imposed are performed by solving the axisymmetric compressible Navier-Stokes equations. The wall temperature of the hot part near both ends (300 K) and that of the cold part near the center (20 K) are fixed. The computations are done for various values of the length ratio of the hot part to the cold part between 0.3 and 1.0, and steady oscillatory states are obtained. These states are divided into three groups according to the magnitude of the pressure amplitude. The state in each group has distinguished features of the flow field. We analyze the effect of vortices on the structure of the temperature distribution. The difference of the boundary layer thickness between the hot part and the cold part is shown to play an important role.