Very large scale motions (VLSM) in rough-bed open-channel flows and their effects on hydraulic resistance

Turbulence in open-channel flows such as rivers and canals plays a key role in transport of momentum, sediments, nutrients, and other substances. Thus, the knowledge of the turbulence structure is required for making predictions and assessments relevant to water management and maintenance of water ecosystems.

The structural approach in turbulence research and its concept of coherent structures have emerged from the recognition of some order in turbulent flows that may exhibit highly organized motions. A coherent structure (or motion) can be broadly defined as a persistent three-dimensional flow region over which at least one fundamental flow variable exhibits significant correlation with itself or with another variable over a range of space and/or time.

Based on extensive experimental studies, the following classification of coherent structures in hydraulically-smooth wall-bounded flows has been proposed (e.g., Marusic & Adrian, 2013): (1) quasi-streamwise vortices, residing within the viscous and buffer sublayers; (2) hairpin vortices, ‘growing’ from the solid surface and scaled with the distance from the bed; (3) large-scale motions which are epitomized by hairpin packets reaching 2 to 4 flow depths; and (4) very-large scale motions (or superstructures), the diameter of which is comparable to flow depth H while their length may reach up to 50H. A similar classification should also be applicable for rough-bed flows, with the exception of the smallest structures which are associated with wake eddies behind roughness elements rather than with quasi-streamwise vortices as in smooth-bed flows.

The objective of this PhD is to advance the classification of turbulent structures for rough-bed open-channel flows particularly focusing on very-large scale motions VLSM (or superstructures) which are the least studied motions in open-channel flows. The project focus will be on the identification of VLSMs, their statistics, and dynamics, and finally on the relations between VLSMs and the overall hydraulic resistance in open-channel flows. The key project methodology is experimental, involving experiments in a large flume and advanced Particle Image Velocimetry. The data analyses will involve a wide range of statistical methods such as spectral analysis as well as time-averaged and double-averaged Navier-Stokes equations and decomposition of the friction factor in its components due to turbulence (VLSMs), secondary currents, bed roughness and flow heterogeneity.

The successful candidate should have (or expect to achieve) a minimum of a UK Honours degree at 2.1 or above (or equivalent) in Mechanical Engineering or Civil Engineering or Aerospace Engineering.

Essential background: Physics, Mathematics, Programming, Fluid Mechanics.

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: http://www.abdn.ac.uk/postgraduate/apply. You should apply for Degree of Doctor of Philosophy in Engineering, to ensure that your application is passed to the correct person for processing.

NOTE CLEARLY THE NAME OF THE SUPERVISOR AND EXACT PROJECT TITLE YOU WISH TO BE CONSIDERED FOR ON THE APPLICATION FORM.

Informal inquiries can be made to Professor V Nikora ([Email Address Removed]) with a copy of your curriculum vitae and cover letter. All general enquiries should be directed to the Postgraduate Research School ([Email Address Removed]).