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

TL;DR

This study uses direct numerical simulation to analyze three-dimensional laminar separation bubbles under pressure gradients and spanwise variations, revealing how suction width and adverse pressure gradients influence their topology and reattachment mechanisms.

Contribution

It provides new insights into the effects of spanwise suction distribution and pressure gradients on 3D laminar separation bubbles, highlighting differences from 2D cases.

Findings

Spanwise suction width determines LSB topology and reattachment.

Adverse pressure gradient influences LSB size and turbulence levels.

Streamwise vortices are observed along the LSB crest.

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 $Re_\delta = U_{\infty}\delta/\nu = 1000$ based on the inflow boundary layer thickness ($\delta$) 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.