Experimental investigation on the effect of forward-facing steps and gaps combined with wall suction on boundary layer transition

Wall suction is known to be an effective Laminar Flow Control (LFC) technique to delay laminar-turbulent transition, but its commercial implementation is still limited due to design and integration issues. In particular, current design tools that model wall suction do not account for potential surface discontinuities that can arise during installation. However, these defects should be accounted for since they generally move transition further upstream, which could cancel drag reduction benefits from wall suction. Given this context, the present investigation aims at experimentally characterizing the combined effect of wall suction and two types of surface defects that can be found on aerodynamic surfaces, i.e., forward-facing steps (FFS) and gaps. Critical relative dimensions (at which transition occurs at the defect location) were different for each type of defect but remained unchanged regardless of whether wall suction was applied or not. For subcritical defects (defects where transition occurs further downstream of their location), wall suction could still delay transition, with reduced effectiveness. Spectral analysis inside the boundary layer revealed that the transition mechanism, governed by Tollmien–Schlichting instabilities, was unchanged in the presence of either critical or subcritical defects. The resulting increase in amplification of the existing instabilities due to either defect warranted the use of the Δ N model to capture this effect. Wall suction is therefore a robust LFC technique that can compete with the destabilizing effects of subcritical defects, albeit less effectively than in a smooth configuration. However, given the chosen suction flow rates, this technique could not delay or prevent the critical dimensions determined for cases without suction Graphical abstract