MTech(Res): Characterization of Turbulent Spots near the onset of Transition in a Flat-plate Boundary Layer

Boundary layer (BL) transition to turbulence occurs through the inception, growth, and merging of turbulent spots. Understanding these spots is vital for predicting aerodynamic performance in disturbance-dominated environments like gas turbines.

However, conventional spot identification has been limited by subjective, manual thresholding. The first phase of this work introduces a novel, threshold-free demarcation scheme combining a Pre-Multiplied Wavelet Energy (PMWE) detector—scaled with BL thickness and log-transformed to establish an objective criterion for detection— along with a Gaussian Mixture Model (GMM) as a classifier. The framework generates wall-normal and spanwise variations of intermittency to a good accuracy and is validated across Direct Numerical Simulation (DNS) and wind-tunnel datasets for multiple transition scenarios.

​Using this detection framework, we track the precursors of spots (velocity spikes) upstream. Analysis of the instantaneous energetics of the pre-transitional region reveals that while stable streaks exhibit positive production within their cores and weak dissipation along their interfaces, localized streak breakdown is consistently preceded by a distinct patch of negative turbulence production (a counter-gradient energy transfer back to the mean flow) co-located with intense dissipation. Crucially, velocity spikes lacking this negative production signature fail to convert into downstream turbulent spots. Thus the presence of negative production appears to be a necessary precursor to spot inception.

​Furthermore, we evaluate the statistics near the transition front using a new intermittency-based conditioning method. While conventional conditional averaging at fixed streamwise locations obscures the physics by blending spots at different developmental stages, conditioning along surfaces of constant local intermittency aligns spots at equivalent evolutionary stages much better. This approach reveals that as the transition onset is approached from downstream, local turbulence production and dissipation increase manyfold. The sharp rise in these quantities signifies an abrupt, highly localized energy transfer during the inception of spots, occurring even while the corresponding skin friction coefficient remains close to laminar values.