ENGINEERING AND CFD

The problem of generating significant net energy saving using wall-normal blowing could be seen as an optimization of an unknown black-box function (the optimal balance between drag reduction and power needed for the wall-normal blowing) and invoke algorithms developed for such problems. A good choice is Bayesian optimization, which has been shown to outperform other state of the art global optimization algorithms on a number of challenging optimization benchmark functions. Bayesian optimization typically works by assuming the unknown function was sampled from a Gaussian process and maintains a posterior distribution for this function as observations are made, in the present case, as the results of running simulations with different parameters (frequency and intensity of blowing for example). For this project, the Bayesian Optimization Algorithm already existing in MATLAB was combined with Xcompact3d, in order to maximise net energy saving in a turbulent boundary layer using wall-normal blowing.

First, some experimental data are used in order to evaluate the potential net-power saving associated with an experimental   blowing solution through a permeable wall with microholes. Up to   60% of drag-reduction is achieved and, after 18 DNS, corresponding to 18 iterates of the Bayesian optimisation scheme, the optimum strategy achieves a net-power saving of 5% via uniform blowing with moderate intensity (0.29% of the freestream velocity). A Fukagata-Iwamoto-Kasagi (FIK) analysis shows two different mechanisms responsible for the drag reduction over and downstream of the blowing region. The reduction of the friction coefficient is associated with the FIK convection term over the blowing region, and the FIK spatial development term downstream of  the blowing region.  However, the mechanical power to supply the air for the blowing solution used in this first Bayesian Optimisation study was not included and therefore a second Bayesian optimisation study is carried out with an original blowing solution based on micro-speakers to determine if net-power savings can really be achieved using vertical wall blowing. For this second study, taking into account the full power required for blowing, a net-power saving of 0.5% is achieved, giving a lot of hope for future practical applications.

This work demonstrates the potential of Bayesian Optimisation as an efficient tool to find optimal parameters to  achieve a specific target (either net-power saving or high levels of drag reduction) for a given flow configuration.

Future work in the area will significantly advance and exploit our Bayesian optimisation framework to locate new, innovative, adaptive and autonomous pathways to reduce the skin-friction drag of turbulent boundary-layer flows with focus on achieving net-energy savings. This advanced Bayesian optimisation framework will be informed by flow physics to locate new drag reduction strategies using either low-amplitude wall-normal blowing or spanwise-wall blowing control techniques. These two forcing techniques are well-known to reduce skin-friction drag and are therefore ideal candidates for innovative optimisation and exploitation

References and relevant publications associated with the project:

• Bartholomew, P., Deskos, G., Frantz, R. A., Schuch, F. N., Lamballais, E., & Laizet, S. (2020). Xcompact3D: An open-source framework for solving turbulence problems on a Cartesian mesh. SoftwareX, 12, 100550.
• Mahfoze O., Moody A., Wynn A., Whalley R.D. and Laizet S. (2019). Reducing the skin-friction drag of a turbulent boundary-layer flow with low-amplitude wall-normal blowing within a Bayesian optimisation framework. Physical Review Fluids, 4, 094601, 2019
• Mahfoze O., Wynn A.,  Laizet S. and Whalley R.D., A Bayesian optimisation framework for drag reduction and net-energy saving in a turbulent boundary layers using wall-normal blowing, 2021 AIAA AVIATION Forum (online), 2-6/08/2021
• Mahfoze O., Wynn A., Whalley R.D.  and Laizet S., A Bayesian Optimisation Framework for Drag Reduction and Net-Energy Saving in a TurbulentBoundary Layer Using Wall Blowing, The 3AF International Conference on Applied Aerodynamics (online), 12-14/04/2021
• Mahfoze O., Moody A., A. Wynn A., Whalley R. and Laizet S. , Bayesian optimisation of intermittent wall blowing in a turbulent boundary layer for net power saving, UK Fluid Networks SIG meeting, City University of London, UK,  13-14/01/2020.
• Mahfoze O., Moody A., A. Wynn A., Whalley R. and Laizet S. , Bayesian optimisation of intermittent wall blowing in a turbulent boundary layer for net power saving, UK Fluid Networks SIG meeting, Cambridge, UK,  21-23/08/ 2019.
• Mahfoze O., Moody A., A. Wynn A., Whalley R. and Laizet S. , Bayesian optimisation of intermittent wall blowing in a turbulent boundary layer for net power saving, 11th International Symposium on Turbulence and Shear Flow Phenomena, Southampton, UK, 30/07-02/08 /2019.
• Mahfoze O., Moody A., A. Wynn A., Whalley R. and Laizet S. , Bayesian Optimisation of wall blowing to control energy and skin-friction in a turbulent boundary layer, European Drag Reduction and Flow Control Meeting, Germany, 25-29/03/ 2019.

Scientific Contact:

Dr. Sylvain Laizet
Department of Aeronautics
Imperial College London
London SW7 2AZ, UK
e-mail: s.laizet [@] imperial.ac.uk

https://www.turbulencesimulation.com/
https://www.imperial.ac.uk/people/s.laizet

NOTE: The simulation project was made possible by PRACE (Partnership for Advanced Computing in Europe) allocating a computing time grant on GCS HPC system Hazel Hen of the High-Performance Computing Center (HLRS) Stuttgart. GCS is a hosting member of PRACE.

Local project ID: PRACE4381

April 2021