Study On The Stability Of 3-D Plane Boundary Layer With Local Distributed Roughness And On The Cause Of Coherent Structures In Shear Flow

A method for direct numerical simulation of 3-D, unsteady, incompressible N-S equations has been established with Fourier spectral expansion in the spanwise direction, higher accuray compact finite difference is constructed here on non-uniform meshes in the x,y direction. The stability of 3-D plane boundary layer with local distributed roughness and the cause of coherent structures in shear flow are studied in detail.1．Localized roughness wall was simulated by distributing localized suction and blowing at the wall, and new basic flows of the boundary layer were numerically calculated. Nonlinear evolutions of the 2-D and 3-D disturbance T-S waves were anylized in virtue of the theory of hydrodynamic stability. Results suggest that, all the 3-D localized rough walls, as well as 2-D localized rough wall of single blowing, can accelerate disturbance waves' fast growth , 2-D localized rough wall of single suction has been tested better hydrodynamical stability than the slippery wall. Direction of the disturbance waves' propagation and phase of the disturbance waves have been shifted. Because of the nonlinear interaction's gradual increase, high-frequency harmonics emerge, 2-D disturbance wave triggers the 3-D disturbance wave whose streamwise wave number and frequency are the same as 2-D disturbance wave's ones in a 3-D boundary layer induced by localized roughness.,2．A pair of vortex is used to be the initial theory model of coherent structures, and its nonlinear evolution in boundary layer with distributed localized wall roughness was bumerical simulated, more plumper streamwise velocity section is, the slower coherent structure amplitude grow rate is, on the contrary, less plumper streamwise velocity section is, the faster coherent structure amplitude grow rate is. Intial structure and amplitude play an important role in the process of coherent structures evolution.3．Local impulse disturbance at the wall was designed as an initial theoritical model to induce coherent structures in boundary layer, many factors are of importance; such as impulse strength, influence area, loading time and distributing type etc. to coherent structures. If local impulse strength and area is small enough, coherent structures are uneasy to be excited in boundary layer.4．Coherent structures are easily to be excited in the adverse pressure gradient boundary layer, and growth of amplitudes, form of vortices, affecting area, as well as 3-D properties are much greater than those in the favourable pressure gradient boundary layer, though propagation speed is slower in the adverse pressure gradient boundary than in he favourable pressure gradient boundary layer.