Eda Dogan, Bharath Ganapathisubramani

Effects of Large-Scale Free-Stream Turbulence on a Zero-Pressure-Gradient Turbulent Boundary Layer

Bharath Ganapathisubramani's

Experimental Fluid Mechanics

Research Laboratory

Figure: Schematic of setup used in this project (published in http://doi.org/10.1098/rsta.2016.0091).

The focus of this study is to investigate the characteristics of a zero-pressure-gradient turbulent boundary layer in the presence of large-scale free-stream turbulence. Particular attention is given to scale interactions occurring within the turbulent boundary layer. The free-stream turbulence was generated by an active grid. The investigation was conducted as an experimental work using hot-wire anemometry and Particle Image Velocimetry. Large-scale structures occurring in the free-stream are shown to penetrate the boundary layer and increase the streamwise velocity fluctuations throughout. The near-wall peak amplitude of the streamwise velocity fluctuations are observed to increase with increasing turbulence level in the free-stream. This trend has been found analogous to high Reynolds number flows for increasing Re. Free-stream turbulence imposes an outer energy peak in the boundary layer and the energy level of this peak is observed to increase with increasing turbulence level. It is also shown that the large-scales dominating the outer region of the boundary layer have a modulating effect on the small-scales in the near wall region; this effect becomes more significant with increasing turbulence level in the free-stream. These added up to the analogy between the high Reynolds number flows and present study cases. The latest efforts in the field of high Reynolds number flow investigations are aimed towards predicting the near-wall turbulence using only the large-scale information input. Therefore, this analogy has encouraging implications towards generalising large-scale influences on the near-wall small scales. The study also presents results regarding the structural organisation inside the boundary layer using the PIV data. The coherent structures found inside the boundary layer are observed to have inclined features as consistent with the previous studies for canonical flows. The fact that the external disturbance, such as FST in this study, does not alter the organisation of the structures inside the boundary layer could potentially provide an evidence for a universal structure for all wall-bounded flows as also proposed from previous studies in the literature.

Parts of this project have been published in Journal of Fluid Mechanics (see https://doi.org/10.1017/jfm.2016.435) and Philosophical Transactions of the Royal Society A (see http://doi.org/10.1098/rsta.2016.0091).