Mapping the flow field induced by a HLFC perforation using a high resolution LDV

The successful application of hybrid laminar flow control, using surface suction and favourable pressure gradients to control the boundary layer, promises significant improvement in aircraft performance. One of the more encouraging design approaches for wing and tail surface HLFC is to apply suction in the leading edge region through micro-perforations in the surface (typical hole diameter is 50 × 10^-3mm). To enable HLFC design criteria to be established with confidence, a full understanding of how suction holes influence the surrounding flow field is required. A fundamental experimental investigation has been performed to examine, in detail, the flow field of a super-scale HLFC suction hole. The experiments were conducted in a low-speed, low-turbulence wind tunnel. Non-intrusive, three-dimensional, high resolution measurements were obtained using a three-component laser Deppler velocimetry system. Suction holes of 0.8mm, 1.0mm and 1.3mm diameter were investigated. Critical suction ratios for each hole, over a range of freestream speeds, were determined. These critical suction ratios are compatible with previous experimental results.The suction hole flow fields are mapped for a range of sub-critical and critical suction rates. These measurements confirm that the flow field of a HLFC suction hole is of a highly three-dimensional nature. A rich variety of flow field patterns and features are observed; including two contra-rotating longitudinal vortices, multiple co-rotating longitudinal vortices, large crossflow Reynolds numbers and inherently unstable boundary layer profiles. For super-critical suction the transition appears to commence in the longitudinal vortices. This experimental investigation examines, in detail, and for the first time, the influence of flow through a HLFC suction perforation on the local and downstream flow field of a laminar boundary layer.