Bluff body aerodynamics is an intriguing research field from both fundamental studies and engineering applications. Reducing drag and attenuating noise is desired and interesting for many practical applications, i.e., cycling and train and landing gear noise. In particular, cycling aerodynamics has drawn a lot of attentions from academia and industry. Even small drag reductions can decide wining or losing in a race.

This thesis research was conducted from the basic circular cylinder flow to the practical cycling aerodynamics. In the first part of this thesis research, wind tunnel tests were conducted to investigate the effects of the surface pattern fabric on the drag and noise reductions from a circular cylinder. The benchmark case, flow around a smooth cylinder, was firstly investigat...[ Read more ]

Bluff body aerodynamics is an intriguing research field from both fundamental studies and engineering applications. Reducing drag and attenuating noise is desired and interesting for many practical applications, i.e., cycling and train and landing gear noise. In particular, cycling aerodynamics has drawn a lot of attentions from academia and industry. Even small drag reductions can decide wining or losing in a race.

This thesis research was conducted from the basic circular cylinder flow to the practical cycling aerodynamics. In the first part of this thesis research, wind tunnel tests were conducted to investigate the effects of the surface pattern fabric on the drag and noise reductions from a circular cylinder. The benchmark case, flow around a smooth cylinder, was firstly investigated, on which force, noise and wake measurements were evaluated. Based on the results, LES simulation on predicting turbulence wake and noise emission was validated. Secondly, different surface pattern fabrics can simultaneously reduce drag and noise from the circular cylinder at different Reynolds numbers. The control effects share the common features corresponding to the cylinder flow in the sub-critical, critical and super-critical flow regimes. Based on the theoretical formula of the Aeolian tone, a detailed analysis was conducted on the cylinder with the longitudinal grooves to explore the noise reduction mechanism. It is found that the longitudinal grooves on the circular cylinder can achieve a maximum drag reduction of 50% and maximum noise attenuation of 35 dB at the end of the critical regime. The critical Reynolds number can also be controlled by selecting the total number of the longitudinal grooves distributed on the cylinder’s surface. Through hotwire measurement, it is observed that noise reduction is associated with suppressed vortex shedding in the critical regime. Further theoretical relations reveal that the lift force fluctuation is mainly responsible for the controlled noise reduction. The control effects are also influenced by the changes in pressure coherent length in the critical and super-critical regimes. Based on the vortex sound theory, another comprehensive study was conducted to investigate the effects of the fabric with dimples on drag and noise reduction. Time-resolved Particle Image Velocimetry measurement revealed the dominant sound sources were concentrated near the cylinder’s surface, corresponding to the changes in bound vortices. The reduced sound sources can further explain the noise reduction from the dimple cylinder started at the critical regime.

In the second part of this thesis research, wind tunnel tests and numerical simulation were conducted on a track cyclist mannequin to investigate the flow around a track cyclist. It was found that the streamwise vortices originating from each body part are the dominant flow structures, and the significant flow separations mainly caused the drag production. The development of streamwise vortices involved strong vortex interactions around the upper thigh, and the formation of the vortex structures is also associated with drag production. Moreover, the leg position significantly affected the lateral displacement between the elbow vortex and upper thigh, leading to distinct behaviours of vortex interactions. Based on the understanding of the flow around the cyclist, the longitudinal groove fabric on the upper arms was found to be able to reduce drag force up to 7% at flow speeds larger than 17 m/s. The corresponding flow mechanism was also revealed by the large-scale Particle Image Velocimetry measurement around the mannequin. The results revealed that the drag reduction from the longitudinal groove fabric relies on the happening of the drag crisis. More importantly, the control effects varied at different arm’s spanwise locations, suggesting the influences of the strong wake interactions. Finally, from a perspective of the bluff body aerodynamics, the connections between the cylinder flow and the flow over a cyclist were discussed, enhancing the understanding and benefiting future works.