Acoustic Control of Separated Shear Layers at Low Reynolds Numbers

At low Reynolds numbers, the flow over an airfoil can separate from the surface before transitioning to turbulence. This laminar separation significantly degrades aerodynamic performance by reducing lift and increasing drag. Under the right conditions, the separated shear layer reattaches downstream, forming a laminar separation bubble (LSB) — a small recirculating region that promotes the transition to turbulence and allows the flow to recover, restoring lift. Understanding and controlling this reattachment process is critical for applications operating in low-speed, low-Reynolds-number regimes.

This project investigates the formation and control of laminar separation bubbles over a NACA 0018 airfoil at an angle of attack of 6° using time-resolved Particle Image Velocimetry (TR-PIV). A key focus is the use of acoustic excitation to accelerate the reattachment of the separated shear layer and promote earlier LSB formation, thereby recovering lift.

Experiment

The video below shows the transient evolution of the flow field over the NACA 0018 airfoil. The flow moves from left to right in Cartesian coordinates. The flow accelerated from Re ≈ 15,000 up to Re ≈ 70,000. As the laminar shear layer transitioned to a turbulent shear layer, the separated shear layer reattached and the laminar separation bubble formed. In this movie, reattachment occurred at Re ≈ 65,000.

Key Findings

• At sufficiently high Reynolds numbers, the separated shear layer reattaches spontaneously, forming a laminar separation bubble and recovering the lift coefficient.
• Acoustic excitation accelerates the reattachment process, triggering LSB formation at lower Reynolds numbers than in the unforced case.
• Sound forcing provides a non-intrusive, energy-efficient mechanism to control flow separation — without physical modifications to the airfoil surface.

Potential Applications

This research has direct relevance for engineering systems that operate at low Reynolds numbers, where laminar separation is a persistent challenge:

• Small unmanned aerial vehicles (UAVs) and drones — which routinely operate at the Re numbers
• Wind turbine blades at low wind speeds and near the blade tips of large turbines
• Flow control systems — acoustic excitation as a practical, low-power alternative to traditional active flow control methods

Publications

Manuscript in preparation.