Passive flow control technique associated with leading-edge tubercles has recently attracted research attention owing to their applications in numerous engineering fields, including aerodynamics and propeller flows. For example, the extreme maneuverability observed in humpback whales has been linked to the deployment of the distributed tubercles along the pectoral flipper leading-edges. As such, the possibility of using tubercles in improving marine-lifting surfaces has been considered. Also, the generation of counter-rotating vortex pairs by the leading-edge tubercles has been of great benefit in improving momentum exchange and reenergizing the boundary layer to mitigate flow separations at higher pitch angles. However, the evolution of the streamwise aligned counter-rotating vortex pairs and their impact on the flow separations downstream of the leading-edge tubercles have not been fully explored.
With that in mind, Dr. Zhaoyu Wei of Shanghai Jiao Tong University (SJTU) worked with Dr. T.H. New from Nanyang Technological University (NTU), with contributions from Prof. Lian Lian of SJTU and Yanni Zhang from Northwestern Polytechnical University (NWPU), by investigating the effects of the leading-edge tubercles on flow separation and coherent flow structures of tapered swept-back wings. The experiments were conducted in a low velocity water tunnel at NTU and was based on two-dimensional time-resolved particle image `velocimetry. Their research work is currently published in the journal, Ocean Engineering.
Briefly, the work considered two different wing types: one with straight leading-edge and the other with tubercled leading-edge all based on SD7032 aerofoiled profile at a Reynolds number Re= 1.4×104, close to that of the working velocity of common underwater gliders. The leading-edge tubercles design was efficient enough to permit a linear decrease in the amplitude from the wing root to the wing top at a constant wavelength. The differences in the flow separation behaviour between the inboard and outboard regions were determined by aligning consecutive measurement planes in the streamwise experimental direction.
The authors observed mild separated flows over the baseline wing surface in both the inboard and outboard regions at pitch angles of 10° and 20°, while the wake vortex shed in both streamwise and spanwise attributed to the presence of sweep. On the other hand, the relatively larger local Reynolds number kept the flow attached in the inboard region. It was worth noting that the implementation of the leading-edge tubercles effectively mitigated the flow separation downstream of both troughs and peaks. For instance, at higher pitch angles, most of the baseline wing surface was covered by the separated flow while the flow remained attached to the downstream for most of the tubercle peaks.
Streamwise aligned counter-rotating vortex pairs were formed over the tubercles. However, they were significantly tilted and asymmetrical due to the amplitude difference and sweep between the two tubercle sides. As such, weaker vortices in the counter-rotating vortex pairs close to the wing root were rapidly dissipated allowing the counter-rotating vortex pairs to evolve into a series of co-rotating vortices. This significantly impacted on the flow separation characteristics downstream of the tubercles. Based on the findings, Dr. Wei, the corresponding author indicated that their study will be of great benefit in advancing applications that seek flow separation mitigations.
Wei, Zhaoyu, New, T.H., Lian, L., & Zhang, Y.N. (2019). Leading-edge tubercles delay flow separation for a tapered swept-back wing at very low Reynolds number. Ocean Engineering, 181, 173-184.