Design Considerations for Using Indented Surface Treatments to Control Boundary Layer Separation

Flow-Control Effectiveness of Convergent Surface Indentations on an Aerofoil at Low Reynolds Numbers

“…In another study, the effectiveness of a spanwise groove in controlling a LSB on a NACA-0015 aerofoil at 3° AoA and a similar Re of 2.38 x 10 5 was examined in Ref. 5. Here, although the spanwise groove reduced the length of the separation bubble by ~73.0%, c l and c d were degraded by ~11.0% and ~27.7%, respectively.…”

“…The indentations have been located around the maximum-thickness location, i.e., 0.30c, of the aerofoil's upper surface. In order to size the depth of the indentations, initial boundary-layer measurements were performed in a natural-transition (baseline) condition at this location and a chord-based blockage-corrected Re (Re corr ) of ~2.70 x 10 5 . The boundary-layer thickness at 0.30c of the baseline aerofoil at 3° AoA, b,3AoA , was found to be ~0.63 mm.…”

“…3,4 Boundary-layer separation is especially common at low Reynolds numbers (Re) at which laminar flow is prevalent. 5 Several aerofoil applications operate in the low Re regime, including micro air vehicles, high-altitude aircraft, compressor and low-pressure-turbine blades, and wind turbines. Typically, many of these aerofoils operate at a chord-based Re of less than 1.0 x l0 6 and experience a laminar separation bubble (LSB) for angles of attack (AoA) less than the stall angle.…”

Passive control of laminar separation bubble with spanwise groove on a low-speed highly loaded low-pressure turbine blade

“…Based on the literature, several rules of thumb for achieving the most effective separation control by use of indented surface treatments, have been summarized in Ref. [8], and also in Ref. [8], a recessed groove placed spanwise across a NACA0015 airfoil has been assessed to be effective for laminar separation control by experiment and the loss reduction was ascribed to the flow inside the groove.…”

“…[8], a recessed groove placed spanwise across a NACA0015 airfoil has been assessed to be effective for laminar separation control by experiment and the loss reduction was ascribed to the flow inside the groove. Specifically, two-dimensional vortex structure inside the groove energizing the boundary layer, and the highly time depended flow inside the groove causing the boundary layer to begin transition to turbulence, were thought to be responsible for laminar separation control with the span-wise groove [8] .…”