A Numerical Study For The Interaction Of T-S Wave And The Trailing Edge Of Boundary Layer

The mechanism of generation of noises due to the fluctuation in the flows is one of the key problems in studying the noises generated by aircraft in aeroacoustics. Modern jet plane would generate strong noises when it flies off or lands fall. One of the main causes of the noises is the strong fluctuation generated by the flows passing throng the trailing edge of the airfoil and the outlet of jet engine. The probable strong disperse generated by the disturbance in boundary layer flows at the trailing edge, the instable waves in the wake flows generated by the upstream disturbance and the interactions between the disturbance in the boundary layer flows and instable waves in the wake flows might be main sources of the cause. Whether or not the sound field would be generated by the interactions among the T-S wave introduced at the entrance of one side of the plate, the trailing edge and wake flows is studied by direct numerical simulation,when the same 2-D subsonic incoming flows pass through a infinitely thin flat plate. To eliminate the error caused by computer, we adopt a restrictional condition that if the absolute value v is not greater than 10^-12, we give zero to v to gain a stable basic flow. Four different instances are considered and the numerical results are listed below: (1) The restrictional condition is used for the symmetrical line from the trailing edge of the plate to the outlet of the computational domain. The trailing edge has almost no influence on the T-S wave after the T-S wave passes through it. There are no changes in the frequency and wavelength of the T-S wave and no sound wave emerges. (2) The restrictional condition is used for the symmetrical line where the x values from x_z to x_l. The control of the disturbance in the flows generated a strong artificial disturbance at x_z. A sound field that is similarly like the single source would be generated due to the presence of the artificial disturbance. And it is not the T-S waves but the instable waves that have the relations with the characters of sound field. (3) Under the conditions that T-S wave is introduced initially on the upper side of the plate and then the T-S wave is introduced at inlet all the time without the restrictional condition, the disturbances lead to a sound wave like the one generated by single source because of the discontinuation at the rear edge of the T-S wave existed in all the field on the upper side of the plate. But this sound source can't last. (4) No restrictional conditions are used and the T-S wave is introduced only at the inlet of the computational domain. The Lighthill stress tensors in the wake flows could generate sound field in the far field. However, the sound source is not a single source, but locates in a region. The characteristics of the sound field mainly rely on the disturbance generated by the instability of the volicity's distribution of the steady wake flows.