Abstract
The development of secondary instabilities in a boundary layer over a backward-facing step is investigated numerically. Two step heights are considered, h/δo*=0.5 and 1.0 (where δo* is the displacement thickness at the step location), in addition to a reference flat-plate case. A case with a realistic freestream-velocity distribution is also examined. A controlled K-type transition is initiated using a narrow ribbon upstream of the step, which generates small and monochromatic perturbations by periodic blowing and suction. A well-resolved direct numerical simulation is performed. The step height and the imposed freestream-velocity distribution exert a significant influence on the transition process. The results for the h/δo*=1.0 case exhibit a rapid transition primarily due to the Kelvin–Helmholtz instability downstream of step; non-linear interactions already occur within the recirculation region, and the initial symmetry and periodicity of the flow are lost by the middle stage of transition. In contrast, case h/δo*=0.5 presents a transition road map in which transition occurs far downstream of the step, and the flow remains spatially symmetric and temporally periodic until the late stage of transition. A realistic freestream-velocity distribution (which induces an adverse pressure gradient) advances the onset of transition to turbulence, but does not fundamentally modify the flow features observed in the zero-pressure gradient case. Considering the budgets of the perturbation kinetic energy, both the step and the induced pressure-gradient increase, rather than modify, the energy transfer.
Highlights
Laminar-turbulent transition is a complex, multi-stage process, through which a laminar flow becomes turbulent
The large step destabilizes the flow the most. It reduces the size of the recirculation region and modifies the overall behavior of C f, which no longer resembles that of the flat-plate case
We considered a freestream-velocity profile of the type encountered on the engine-nacelle lip; this induced a favorable and adverse pressure gradient in the domain
Summary
Laminar-turbulent transition is a complex, multi-stage process, through which a laminar flow becomes turbulent. The second type is associated with direct breakdown of the laminar flow, and normally arises when high levels of perturbations (O(10−2 ) of U∞ ) are present; common sources of such perturbations include surface roughness or freestream turbulence The evolution of this type of transition typically involves rapid and transient amplification of perturbations and bypasses the slowly growing phases of T-S waves, and is known as bypass transition [1,2]. One important source of perturbation that can induce transition (following either of the paths mentioned before) are the surface imperfections that are inevitable in flows in engineering and the natural sciences They may be due to the manufacturing process itself, to damage to the surface (pitting, cavitation, ice accumulation ...) or to the geometry itself (terrain topology, vegetation).
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
