We report new laboratory experiments of a flow accelerating from an initially turbulent state following the opening of a valve, together with large eddy simulations of the experiments and extended Stokes first problem solutions for the early stages of the flow. The results show that the transient flow closely resembles an accelerating laminar flow superimposed on the original steady turbulent flow. The primary consequence of the acceleration is the temporal growth of a boundary layer from the wall, gradually leading to a strong instability causing transition. This extends the findings of previous DNS simulations of transient flow following a near-step increase in flow rate. In this interpretation, the initial turbulence is not the primary characteristic of the resulting transient flow, but can be regarded as noise the evolution of which is strongly influenced by the development of the boundary layer. We observe the spontaneous appearance of turbulent spots and discontinuities in the velocity signals in time and space, revealing rich detail of the transition process, including a striking contrast between streamwise and wall-normal fluctuating velocities.
Bibliographical noteWe gratefully acknowledge the contribution of Dr C. Ariyaratne at the early stage of the research and the advice provided by Professor P. Orlandi on numerical methods. Mr B.S. Oluwadare assisted with the experiments. The research was principally funded by EPSRC through grant EP/G068925/1. This work made use of computing facilities of the N8 HPC and ARCHER, funded by EPSRC through the N8 consortium (EP/K000225/1) and the UK Turbulence Consortium (EP/L000261/1), respectively. S.H., A.E.V., T.OD. & D.P. initiated the research. S.G. designed the test rig with contributions from all other authors, and conducted preliminary experiments. M.S. wrote the DNS code. A.M. together with M.S. implemented LES in the code. A.M. conducted the experiments and LES simulations. S.H., A.M., M.S. & S.G. analysed the results. S.H. led the writing of the manuscript with contributions from all other authors. A.M. & S.G. made equal contributions.
- Pipe flow boundary layer < Boundary Layers
- Turbulent transition
- turbulence theory