The instability and breakdown of three-dimensional shear layer associated with a near-wall steak is investigated experimentally. A single low-speed streak is produced in a laminar boundary layer by using a small piece of screen set normal to the wall. Symmetric and anti-symmetric instability modes are excited separately by means of orifice-loudspeaker system. The symmetric mode excited is essentially governed by the Kelvin-Helmholtz instability of inflectional velocity profile in the normal-to-wall direction and its growth rate is reduce with decreasing the lateral scale of the uplifted high-shear layer associated with the streak. Whereas, the grown of the anti-symmetric mode is caused by the instability of inflectional velocity profile in the spanwise direction. Importantly, this mode does not strongly depend on the lateral scale of the low speed streak. So, the anti-symmetric mode may continue to grow exponentially, not being affected by the viscous diffusion of streak shear layer.
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