Experimental and numerical investigations on the aerodynamics of a simplified helicopter fuselage

• Main Author: Cheawchan, Atcha-uea
• Other Authors: New Tze How, Daniel
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Engineering::Aeronautical engineering::Aerodynamics
• Online Access: https://hdl.handle.net/10356/162553
• Institution: Nanyang Technological University

Depending on helicopter weight class, fuselage parasite drag contributes up to 70 percent of total helicopter drag. One source of the parasite drag is the flow separation behind a helicopter fuselage. During the long cruising flight operation, this could lead to higher fuel consumption, more air pollution, and shorter flight distance. Thus, it is necessary to find a solution to eliminate such flow behavior. Vortex generators (VGs) are a type of flow control devices chosen for this purpose due to its reliability, implementation simplicity, and low manufacturing cost. In this thesis, the rectangular and wishbone VGs with the optimal configurations suggested by previous studies have been tested out experimentally and numerically in the normal operating condition as well as off-design conditions where the crosswind occurs. The thesis novelties lie firstly, in the investigation of the flow separation mitigation effectiveness of the VGs on a backward-facing ramp with a sharp transition. Most of the studies are based on the arbitrary ramp geometry with smooth transition or curvature connecting between the horizontal plate and incline plate. Secondly, to the best of the author’s knowledge, no study has been reported on the impact of the VGs skewness angle on a backward-facing ramp, at least at the time the thesis was written. The primary differences between the thesis and previous research can be distinguished into twofold. Firstly, details on the resulting flow topologies and vortical interactions of VGs used in conjunction with a backwards facing ramp are investigated experimentally and numerically in order to provide information averaged flow behaviour. Secondly, vortex core characteristics and unsteadiness behaviour are analyzed based on the instantaneous experimental results. The impact of skew angle on the effectiveness of VG and the behaviour of vortex cores on flow separation behind a backward-facing ramp (BFR) was experimentally investigated. In order to imitate the crosswind condition, the VGs have been skewed toward the starboard side ahead of the flow separation region with a local Reynold number Rex ≈ 3×106. Counter-rotating vortices were generated by a set of boundary layer height rectangular and wishbone VGs on a 30° BFR with different skewness of 10°, 20°, and 30°. It was observed that VG that were positioned five times its height ahead of the ramp can successfully shorten the reattachment length by approximately 45% for rectangular VG and 80% for wishbone VG with more localized effect. In addition, skew angle played an important role in the stability of vortex cores and behaviours that include vortex meandering, vorticity, circulation, among others. When rectangular VG was skewed, the behaviour of the vortex core generated by the windward vane displayed strong dependence on skew angle, which amplified and became unstable as the skew angle was increased. On the other hand, the leeward vane experienced interference from the windward vane resulting in adverse effects, especially at the extreme skew angle of 30°. Meanwhile, the wishbone VG vortices were close to the floor and substantially weaker than rectangular VG vortices. Therefore, the core instability analysis could not be performed on the wishbone VG. Numerical results showed that single VGs produce counter-rotating streamwise vortices with increasingly different vortex-core strengths and vortical interactions when skewness angle increases. At 30° skewness however, the rectangular VG formed co-rotating vortices instead with significantly heightened vortical interaction levels, leading to asymmetric flow separation and reattachment behaviour. On the other hand, the wishbone VG produced only one primary vortex with multiple horseshoe vortices ahead of the VG and a massive flow separation occurred on the VG. The use of multiple rectangular VGs under the same condition further accentuate these behaviour and results in significant changes to the wall shear stress distribution. Clarifications on how the flow separation region is distorted by the symmetric/asymmetric streamwise vortices based on velocity component analysis are also provided. Lastly, trajectories of the streamwise vortices and vortex-core characteristics support the notion that the streamwise vortices behave significantly more non-linearly at 30° skewness angle here, and that skewing the present VGs such that they produce co-rotating vortices instead of counter-rotating ones leads to very different flow separation control characteristics. More details on the aforementioned streamwise vortices behaviour, their interaction between each vortex, and their effect on the reattachment behaviour will be elaborated in the thesis.