Numerical investigation on mechanism of multiple vortex rings formation in late boundary-layer transition

Mechanism of multiple vortex ring formation is one of key issues of late flow transition in a boundary layer. In this paper, a new mechanism is presented that the multiple ring formation is a result of shear layer instability. According to our DNS observation, a momentum deficit zone is formed above the original hairpin vortex tube due to the ejection of the streamwise hairpin vortex tube, which brings the low speed flow from the lower level to the higher level of the boundary layer through vortex rotation. Then the momentum deficit zone generates a sharp olive-shaped shear layer surrounded by the high speed flow. As the shear layer moves up, inviscid instability is developed due to the existence of inflection points (inflection surface in 3-D). Consequently, the new vortex rings are formed one by one. During the new spanwise vortex ring formation process, the spanwise vorticity is increased; however, the original streamwise hairpin vortex tube is weakened with decreasing of the streamwise vorticity. This process follows the Helmoholtz vorticity conservation law.

► Shear layer instability is the mechanism of the multiple ring formation in boundary layer transition.
► The shear layer is generated by momentum deficit zone which is located above the original hairpin vortex.
► The momentum deficit zone is formed by the ejection of the original hairpin vortex rotation.
► The streamwise vorticity is transferred to spanwise vorticity during the multiple vortex ring formation.
► The visualization of the λ2λ2 method matches the method by showing three components of the vorticity.