Towards modelling scale interactions in wake-induced laminar-turbulent transition in separating boundary layers
University of Calgary
Abstract
Wake-induced laminar-to-turbulent bypass transition in a separated laminar boundary layer (SLB) is investigated downstream of a cylinder of diameter D mounted in a constant free stream near a smooth flat plate. Direct numerical simulation (DNS) is conducted for a moderate gap of 0.9D with ReD = 3900 and Reθ = 150, based on the momentum thickness. The transition process is driven by coherent vortex dynamics along the wall synchronized with Kármán shedding behind the cylinder. The transition process is identified from the mean velocity and Reynolds stress fields and then characterized from the spatial evolution of the velocity field spectra. Turbulence emerges at such remarkably low Reθ because the transition process follows a hybrid pathway combining SLB instability with wake-driven near-wall Λ-vortex formation and their interaction with periodically shed vortices. The transition unfolds in distinct stages: linear disturbance amplification; nonlinear saturation via super-harmonic resonance; and turbulent breakdown. Unlike classical SLB transitions, the wake’s periodicity imposes unique spectral signatures on the transition dynamics. An approach based on weakly nonlinear theory in proposed to investigate the spectral energy transfer between scales. A combination of orthogonal wavelet decomposition and proper orthogonal decomposition (WPOD) is introduced for separating scales. The approach is effective in separating linear amplification of instabilities due to linear processes and isolating triadic interactions responsible for spectral redistribution. The findings characterize wake-boundary interactions inherent to turbomachinery cascades or slotted-wing aerodynamic systems, offering insights for potential flow control strategies.
About the speaker
Dr.-Ing. Robert Martinuzzi is a Fellow of the ASME, a Pratt & Whitney Canada Research Fellow and Professor of Fluid Mechanics at the University of Calgary (Canada). He is an expert in experimental fluid mechanics, optical diagnostics, bluff-body aerodynamics and separated, turbulent flows. His fundamental work addresses rapidly evolving turbulence in wakes and separated flows. His research addresses reduced-order dynamical models, low-dimensional system representations, nonlinear dynamics, data-driven sensor-based estimation and flow control. Over the last 30 years, industry applications of his research include: high-speed compressor aerodynamics, flow-induced vibrations in exposed pipelines and on aircraft components, and propulsion systems. Dr. Martinuzzi is ditor-in-Chief of the International Journal of Heat & Fluid Flow and Associate Editor of the Journal of Turbulence.
