| dc.description.abstract | Laminar to turbulent transition is an important unsolved problem. In addition to being of fundamental interest as a fluid mechanics problem, it has practical applications dealing with aircraft drag reduction and similar areas. At high enough disturbance amplitudes, discrete roughness induced transition will manifest with the creation of a turbulent wedge. The present work deals exclusively with incompressible, zero pressure gradient turbulent wedges. Turbulent Wedges have been the focus of many studies, the literature reveals the following characteristics: Turbulent wedges are composed of a fully turbulent core with a half angle of 6.4 ◦ surrounded by an intermittent region of 10.6 ◦ . While these observations are highly consistent throughout the literature, there is no explanation behind why turbulent wedges spread at this particular rate. Roughness induced transition is predictable using the roughness height-based Reynolds number Rek. Multiple spreading mechanisms are at play, in addition to turbulent mixing, there is a destabilization effect of the turbulent wedge acting on the surrounding laminar flow. Finally, DNS studies have revealed the presence of high- and low-speed structures on the edges of the turbulent wedges called “dog-teeth”. From these conclusions, the following hypothesis are considered in this work. The roughness disturbs the incoming laminar boundary layer by creating high velocity gradient regions and an initial pair of high- and low-speed streaks. These streaks breakdown through a Kelvin-Helmholtz like instability in the shear layers. A self-regenerating process then acts to create new streaks on the sides of the wedge. This last process is no longer influenced by the roughness and is responsible for turbulent wedge spreading. The objective of this work is to investigate the roles of high- and low-speed streaks and the hypothesized self-regenerating mechanism in the fundamental spreading mechanism of turbulent wedges. Experimental work has been performed at the Klebanoff Saric Wind Tunnel at Texas A&M University using combined hotwire measurements and naphthalene flow visualization. Naphthalene was first used to observe many turbulent wedges for different values of Rek and Rex in order to study their combined influence of the geometry of the subsequent wedges. This study offers no insight into the evolution of the spreading angle with respect to Reynolds number. However, while the emergence of primary structures is scaled strongly by Rek and weakly by Rex, the secondary structures seem to only scale with flow parameter Rex. This indicates that the self-regenerating mechanism is independent of the roughness element. Guided by the naphthalene study, detailed hotwire measurements were performed on three wedges at each Rek of interest. The overall topology of the wedge was studied using mean velocity and fluctuations intensity u ′ rms. Three distinct high- and low-speed sets of streaks are observed and their emergence documented. The data are consistent with similar computational studies describing this problem from a vorticity dynamics point of view. This good experimental agreement adds to this descriptions credibility. In addition, spectra are observed in selected locations of the flow. The near field amplification of specific frequency bands is observed in the high shear layers as well as the breakdown of a single low-speed streak. However secondary structures remain difficult to observe due to supposed spanwise streak meandering. Evidence for this phenomenon is provided which explains the difficulty of observing secondary structures in the present, as well as past experimental studies. This work provides significant experimental evidence that the high- and low-speed structures play a significant role in maintaining a self-regenerating destabilization process responsible for turbulent wedge spreading. | en |
