About the Project
Turbulence and secondary currents in open-channel flows such as rivers and canals play key roles in transport of momentum, energy, heat, sediments, nutrients, and other substances. Thus, the knowledge on the secondary currents and turbulence structure is required for making accurate predictions and assessments relevant to water systems management, their maintenance and restoration.
Although the mixing mechanisms due to randomness in turbulence alone have been extensively studied theoretically and in the experiments, the effects of coherent (quasi-organised) structures on mixing remains unclear. This is particularly relevant when considering the so called Very-Large-Scale-Motions (VLSMs), which are instantaneous rotational streamwise motions, the diameter of which is comparable to flow depth while their length may reach up to 50H or more. Similarly, the presence of secondary currents may significantly modify vertical, transverse, and longitudinal mixing in both straight and curved channels. Depending on the specific flow configuration, secondary currents may either enhance or dampen mixing rates in all directions or selectively. Progress in understanding the mixing processes when secondary currents are present depends on the depth of understanding of the overall and specific hydrodynamics of secondary currents and of their inter-relations with multi-scale turbulence. E.g., some data suggest the existence of an interesting phenomenon – the suppression of transverse mixing in the narrow regions between the helical near-wall currents and the central flow. A qualitative confirmation of the suggested phenomenon can be found in aerial photographs from tracer experiments depicted on the cover of Rutherford’s (1994) book. This phenomenon could be explained by the near-bank helical currents, which suppress mixing between the near-bank flow region and the central flow region. However, this effect requires further investigation and explanation for a wider range of conditions.
The objective of this PhD study is thus to advance the current understanding of mixing processes in straight channels paying particular attention to the effects of VLSMs and secondary currents in flows over streamwise ridges on the bed. The key project methodology is experimental, involving experiments in a large flume and Particle Image Velocimetry. The data interpretation will be based on appropriate transport equations including double-averaged transport equations where effects of secondary currents are explicitly accounted for and quantified with so called dispersive fluxes. This study is associated with a current EPSRC project ‘Secondary currents in turbulent flows over rough walls’.
Selection will be made on the basis of academic merit. The successful candidate should have, or expect to obtain, a UK Honours degree at 2.1 or above (or equivalent) in Mechanical Engineering or Civil Engineering or Aerospace Engineering or Physics or equivalent.
Essential background and Knowledge: Fluid Mechanics, Open-Channel Hydraulics, Turbulence, Mixing processes, Roughness effects along with knowledge of: Engineering Mathematics, Fluid Mechanics (with focus on turbulence), Hydraulics, Statistical methods, Programming, Water engineering, Numerical methods
APPLICATION PROCEDURE:
Formal applications can be completed online: https://www.abdn.ac.uk/pgap/login.php
- Apply for the Degree of Doctor of Philosophy in Engineering
- State the name of the lead supervisor as the Name of Proposed Supervisor
- State the exact project title on the application form
- All Degree Certificates/Academic Transcripts (officially translated into English and original)
- 2 Academic References on official headed paper and signed or sent from referees official email address
- Intended source of funding to meet the difference between UK and International Tuition fees (if applicable)
- Motivation Letter/Personal Statement
- Detailed CV
The start date of the project is as soon as possible and to be agreed with the lead supervisor.
We reserve the right to remove the advert, if a suitable candidate is found before the closing date of 12 noon on 30 November 2021.
Funding Notes
Tuition fees will be paid at UK rates (for 2021/2022 this is £4,500). A stipend will also be provided at a rate of £15,609 for the 2021/2022 academic year. This will be paid monthly in arrears. Applications can be accepted from International students (including EU citizens who do not have pre-settled (applied for before 30 June 2021)/settled status) providing they can meet the difference in costs between UK and International tuition fees from their own resources. For 2021/2022 this will be £17,000 per annum.
References
Adrian, R., Marusic, I. Coherent structures in flow over hydraulic engineering surfaces. Journal of Hydraulic Research, 2012, https://doi.org/10.1080/00221686.2012.729540.
Cameron, S.M., Nikora, V., Stewart, M.T. Very-large-scale motions in rough-bed open-channel flow. Journal of Fluid Mechanics, 2017, 814, 416-429.
Nikora, V., Roy, A.G. Secondary flows in rivers: theoretical framework, recent advances, and current challenges. In Gravel Bed Rivers: Processes, Tools, Environments, edited by M. Church, P.M. Biron, and A.G. Roy, London, Wiley and Sons, 2012, 3-22.
Nikora, V., Stoesser, T., Cameron, S., Stewart, M., Papadopoulos, K., Ouro, P., McSherry, R., Zampiron, A., Marusic, I., Falconer, R. Friction factor decomposition for rough-wall flows: theoretical background and application to open-channel flows. Journal of Fluid Mechanics, 2019, 872, 626-664.
Proust, S., Nikora, V. Compound open-channel flows: effects of transverse currents on the flow structure. Journal of Fluid Mechanics, 2020, 885, A24.
Rutherford, J.C. River Mixing. Wiley, 1994.
Zampiron, A., Cameron, S., Nikora, V. Momentum and energy transfer in open-channel flow over streamwise ridges. Journal of Fluid Mechanics 2021, 915, A42, doi:10.1017/jfm.2021.44.
Zampiron, A., Cameron, S., Nikora, V. Secondary currents and very-large-scale motions in open channel flow over streamwise ridges. Journal of Fluid Mechanics, 2020, 887, A17.
