Thermal Power and Fluid Engineering
Example dissertation project
Wall functions for oscillating turbulent flow.
Supervisor Dr A P Watkins
Internal combustion engines work over a wide range of engine speeds because the time scales of the turbulence in the fuel/air mixture approximately vary linearly with engine speed. Accurate calculation of the turbulence structure in the pre-ignition flow in engines is thus essential. This includes the near wall regions, as here combustion may not be complete due to quenching.
Because of the complexity of the three-dimensional time-dependent flows in engines, the use of low-Reynolds number turbulence models to resolve the near wall regions is computationally very expensive. Thus wall functions are applied in these regions, coupled to a high Reynolds number turbulence model for the bulk flow. The functions used apply the log-law for velocity variation across the near wall regions derived from simple wall shear flows. However, because of the oscillating nature of the flow in engines due to the piston motion, it is not clear that such wall functions are appropriate, as the near-wall flow will also vary in an unsteady fashion.
The recently-developed UMIST-N sub-grid model promises to be much more general in its applications than traditional wall functions. A computer program has been developed in an MPhil project to investigate oscillating flows, using this new approach, and applied to oscillating channel flows, imposed on top of a steady through-flow. The results indicate that the sub-grid approach provides considerably more accurate near-wall variations in mean velocity and turbulence parameters than does the log-law.
The objective of this project is to apply the model to fully-reversing oscillating flows as in i.c. engines. In the first instance the channel flow code will be used for this investigation, examining fully-developed (in space) flows. Later the sub-grid model will be inserted into a two-dimensional i.c. engine flow code that will allow calculations to be made throughout the time- and space-dependent flows.