Stability and Transition Analysis of Shear Flows
Time: Fri 2025-10-24 09.00
Location: Kollegiesalen, Brinellvägen 8, Stockholm
Video link: https://kth-se.zoom.us/j/61597830603
Language: English
Subject area: Engineering Mechanics
Doctoral student: Mohammad Moniripiri , Teknisk mekanik, FLOW
Opponent: Dr. Ing. Markus Kloker, University of Stuttgart
Supervisor: Docent Ardeshir Hanifi, SeRC - Swedish e-Science Research Centre, Strömningsmekanik; Professor Dan S. Henningson, SeRC - Swedish e-Science Research Centre, Strömningsmekanik
QC 250929
Abstract
The transition from laminar to turbulent flow is a fundamental phenomenon that significantly affects aerodynamic performance and energy efficiency across a wide range of engineering systems. A deeper understanding of the mechanisms driving instability growth and transition is therefore essential for improving flow control strategies and optimising system design. In this thesis, Direct Numerical Simulation (DNS) and linear stability analysis are employed to investigate flow instabilities and transition mechanisms in several shear flows.
First, adjoint methods are used to analyse the sensitivity of instability growth to smooth surface waviness in a two-dimensional subsonic compressible boundary layer. An optimisation framework is then developed to identify worst--case waviness profiles and quantify the permissible surface deformations before instabilities reach critical amplification levels.
Next, self-excited instability mechanisms in laminar separation bubbles induced by wall waviness are investigated using global stability analysis in combination with DNS. These studies examine how variations in waviness geometry influence the formation, growth, and nonlinear evolution of instabilities within waviness-induced separation bubbles, providing a deeper understanding of self-sustained transition mechanisms in such flow configurations.
The influence of smooth surface humps on laminar--turbulent transition in three-dimensional, crossflow-dominated swept-wing boundary layers is then explored. First, DNS is used to gain a deep understanding of the hump--driven mechanisms responsible for delaying or promoting transition. Then, the applicability of linear stability theory for predicting the growth of secondary instabilities in these configurations is evaluated through quantitative comparisons with DNS results. Finally, the effects of hump geometry as well as the simultaneous presence of two humps on instability growth is analysed. To this end, DNS is employed to analyse the evolution of primary crossflow instabilities, while linear theory is used to characterise the growth of secondary instabilities, offering a comprehensive framework for understanding hump--induced effects on crossflow-dominated transition to turbulence.
Finally, the onset of transition downstream of fluttering and non-fluttering bioprosthetic aortic valves is investigated using transient growth analysis and optimal perturbation theory, providing new insights into the complex vortex system shed from the bioprosthetic valves.