Applications are invited for a PhD studentship (3 years) in the Fluids and Thermal Engineering Research Group within the Faculty of Engineering at the University of Nottingham.
A study of artificially initiated turbulent spots was recently carried out using a low-turbulence wind tunnel (Wang et al. 2021, doi:10.1017/jfm.2021.152). Here, an excellent reproducibility of the experimental results allowed us to observe fine structural details in the early development of turbulent spots that have never been previously observed. It was demonstrated that the turbulent spots are composed of low-speed pillars that are anchored at the wall, which stretch out to the edge of the boundary layer. These pillars represent the low-speed regions that are pumped up within each hairpin vortex of the turbulent spot. Opposition control using wall-normal jets was then applied to these turbulent spots to delay transition to turbulence.
The successful student will develop further understanding of the mechanism and process of the transition to turbulence by carrying out a detained wind tunnel investigation into the laminar boundary layer. We are particularly interested in understanding the role of turbulent spots in the transition to turbulence. For example, does the flow instability need to be involved in the development of turbulent spots if the flow disturbance is above the critical intensity? Are the hairpin-like structures essential elements in the boundary layer transition both in the initiation and the development of turbulent spots? Can we find turbulent spots even after the transition to turbulence is completed? Are the hairpin structures observed in the turbulent boundary layer similar to those in the turbulent spots?
We can answer some of these questions by treating this study as a “black box” problem, by carefully investigating the response to flow control applied to the transitional boundary layer. If the response is as expected, our understanding of flow physics should be correct. If not, we can carry out further or different flow control until we are satisfied with the response. This is an approach that is often used in the investigation of turbulence -- “the oldest unsolved problem of classical physics.” Many different control devices are available at the Flow Control and Drag Reduction Laboratory at the University of Nottingham, from mechanical and flow actuators to plasma actuators, which may be used for the PhD study.
The ideal candidate for this PhD studentship will have a good knowledge and/or research experience in advanced fluid mechanics, and will be interested in experimental investigation into boundary layer control and drag reduction. He/she must have good communication skills at all levels. Proficiency in programming, data analysis and presentation using Matlab is highly desirable. The successful student will have access to excellent experimental facility and equipment at the University of Nottingham, including low-speed wind tunnels, particle image velocimetry (PIV) and hot-wire anemometry together with a dedicated data acquisition system. During the PhD study, there will be an opportunity to access to the low-turbulence wind tunnel at City, University of London, which is a part of the National Wind Tunnel Facility.