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Effect of secondary currents and very large scale motions on water surface dynamics in open-channel flows


   School of Engineering

  ,  Thursday, July 14, 2022  Funded PhD Project (Students Worldwide)

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. They are also influential in the exchange of gases between water bodies and atmosphere. The rate of this exchange is defined by the water surface dynamics which in turn reflect the structure of turbulence and secondary currents. Thus, the knowledge on the interactions between water surface, turbulence and secondary currents is required for making accurate predictions and assessments relevant to water systems management as well as for stream ecosystems maintenance and restoration.

Although the mechanisms affecting water surface via turbulent eddies alone have been recently studied theoretically and in the experiments, they mainly concern the eddies up to 3-4 flow depths in length. The role of much larger turbulent motions, known as Very-Large-Scale-Motions (VLSMs), remain unclear. VLSMs are instantaneous rotational streamwise motions, the diameter of which is comparable to flow depth while their length may reach up to 50H or more. These flow structures have been discovered only recently and it was shown that they significantly contribute to the total turbulent energy and turbulent stresses, and thus may strongly affect the water surface dynamics. Similarly, the presence of secondary currents may considerably modify the interactions between turbulent flow and water surface in both straight and curved channels. Depending on the specific flow configuration and bed heterogeneity, secondary currents may either enhance or dampen water surface fluctuations.

The objective of this PhD study is thus to advance the current understanding for the case of straight channels paying particular attention to the effects of VLSMs and secondary currents in flows over streamwise ridges on the bed. Specifically, this study will (1) advance the theoretical framework for studying VLSMs, secondary flows, and water surface dynamics in straight channels, (2) clarify interrelations between turbulence, secondary flows and water surface, and (3) identify and quantify specific interaction regimes depending on the secondary currents topology and strength. 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 stresses. 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

  1. Apply for the Degree of Doctor of Philosophy in Engineering
  2. State the name of the lead supervisor as the Name of Proposed Supervisor
  3. State the exact project title on the application form
  4. All Degree Certificates/Academic Transcripts (officially translated into English and original)
  5. 2 Academic References on official headed paper and signed or sent from referees official email address
  6. Intended source of funding to meet the difference between UK and International Tuition fees (if applicable)
  7. Motivation Letter/Personal Statement
  8. Detailed CV

The start date of the project is as soon as possible, but no later than October 2022.

We reserve the right to remove the advert, if a suitable candidate is found before the closing date of 12 noon on 14 July 2022.


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.
Dolcetti, G., & Nava, H. G. (2019). Wavelet spectral analysis of the free surface of turbulent flows. Journal of Hydraulic Research, 57(4):211-226.
Horoshenkov, K. V., Nichols, A., Tait, S. J., & Maximov, G. A. (2013). The pattern of surface waves in a shallow free surface flow. Journal of Geophysical Research, 118(3):1864-1876.
Nichols, A., Tait, S., Horoshenkov, K. V., & Sheperd, S. J. (2016). A model of the free surface dynamics of shallow turbulent flows. Journal of Hydraulic Research, 54(5):516-526.
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.
Patella, W. Hydraulic Resistance, Secondary Currents and Free Surface Fluctuations in Open-Channel Flows with Streamwise Ridges. PhD Thesis, University of Aberdeen, 2021.
Stoesser, T., McSherry, R., & Fraga, B. (2015). Secondary currents and turbulence over a non-uniform roughened open-channel bed. Water, 7(9):4896-4913.
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.

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