Separation of a Laminar Boundary Layer Subjected to Pressure Gradients with Spanwise Variations

Abstract

The characteristics of three-dimensional laminar separation bubbles (LSBs), compared with their quasi-two-dimensional counterparts, are studied using direct numerical simulation. A three-dimensional suction-blowing distribution is applied on the top boundary to induce flow separation. Three levels of suction strength (i.e., adverse pressure gradient (APG) strength) are tested. For each suction level, the spanwise extent of the suction is varied. Two non-uniform suction distributions are compared with uniformly-applied suction across the full span (implying a two-dimensional LSB), totaling nine cases. All cases are performed at $Re_δ= U_{\infty}δ/ν= 1000$ based on the inflow boundary layer thickness ($δ$) and free-stream velocity ($U_{\infty}$). Initial results indicate stark contrast between three-dimensional and two-dimensional LSBs. Most notably, the role of suction width is determining the topology of the LSB and its reattachment mechanism. Meanwhile, APG is responsible for determining the size of the LSB and level of turbulence at reattachment. Streamwise-oriented vortices are identified along the crest of the three-dimensional LSBs.