Dr Arthur Hajaali

Job: Research Fellow

Employer: University College London

PhD Thesis: Flow separation characteristics within rectangular and conical diffusers

Supervisors: Professor Shunqi Pan and Dr Zhihua Xie

Arthur was a WISE CDT doctoral student between 2015-2019 and was based at Cardiff University.  He successfully defended his PhD thesis on 7th January 2021 and is currently a Research Fellow at University College London within the Civil, Environmental, and Geomatic Engineering Department (CEGE).  This role includes research into the human aerosols and droplets propagation in different public transports using CFD software including Hydro3D and OpenFoam.

Arthur’s background prior to joining WISE was a BEng in Civil Engineering from Cardiff University.  He graduated in June 2015. Arthur’s BEng programme enabled him to discover the various fields of civil engineering such as soil mechanics, solid mechanics and material sciences. He had the chance to put his theoretical knowledge into practice completing various design projects such as a hotel, car park and wastewater treatment network, where he also enhanced his team working and organisation skills.  Of particular interest to Arthur was fluids mechanics and its multiple application in water engineering.  A growing curiosity for marine renewable energies led him to take part into the development work of a vertical tidal turbine with the hydro-environmental research team.  Arthur relished this involvement in research work and took great satisfaction in executing various laboratory test on the turbine adequately pre-defined to observe and perfect the design.  Arthur’s BEng experience and research interests led him to apply to the WISE CDT programme.

Aside from university and his work, Arthur is a sports enthusiast and finds great satisfaction in trying and discovering new ones, including half marathons and climbing.

Abstract:

The concept of a diffuser relies on transforming the dynamics or velocity pressure into static pressure. This simple concept found numerous applications including in the jet engine, sewage, optics, automotive, and ventilation systems. During the first half of the 20th century, a multitude of diffuser designs was investigated to provide valuable information and guideline for optimising their efficiency. The performance of diffusers is extremely sensitive to their geometry and inlet condition, which can lead to the flow separation of the turbulent boundary layer and the development of reversal flow. This reversal flow is the main contributor to the pressure losses leading to a drop in the diffuser’s efficiency. The three-dimensional nature and the unsteadiness of the reversal flow make it experimentally and numerically challenging to study and quantify its characteristics and underlying mechanisms. This research operates and analyses high-resolution Large Eddy Simulation (LES) to further the understanding of the flow mechanisms and interactions with the coherent structures responsible for the emergence of instantaneous reversal flow pockets. The flow separation geometrical sensitivity is assessed through the investigation of two different asymmetric rectangular diffusers. Moreover, the flow separation temporal behaviour and the arrangement of the energy-containing motions are evaluated within a conical diffuser when subjected to a fully developed pipe inlet and a swirling boundary condition. Prior to these parametric studies, the hydrodynamics of each diffuser were calibrated and validated extensively with experimental data

Publications:

Hajaali, A., Stoesser, T. Flow Separation Dynamics in Three-Dimensional Asymmetric Diffusers. Flow Turbulence Combust 108, 973–999 (2022). https://doi.org/10.1007/s10494-021-00307-5