Modal analysis of roughness-induced crossflow vortices in a Falkner-Skan-Cooke boundary layer

• A three-dimensional global stability analysis using high-order direct numerical simulations is performed to investigate the effect of surface roughness with Reynolds number (based on roughness height) Rek above and below the critical value for transition, on the eigenmodes of a Falkner-Skan-Cooke boundary layer. The surface roughness is introduced with the immersed boundary method and the eigenvalues and eigenfunctions are solved using an iterative time-stepper method. The study reveals a global instability for the case with higher Reynolds number that causes the flow in the non-linear simulations to break down to turbulence shortly downstream of the roughness. Examination of the unstable linear global modes show that these are the same modes that are observed in experiments immediately before breakdown due to secondary instability, which emphasizes the importance of these modes in transition.

Ämnesord

TEKNIK OCH TEKNOLOGIER  -- Maskinteknik -- Strömningsmekanik och akustik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Mechanical Engineering -- Fluid Mechanics and Acoustics (hsv//eng)

Nyckelord

Atmospheric thermodynamics

Boundary layers

Computational fluid dynamics

Eigenvalues and eigenfunctions

Iterative methods

Modal analysis

Reynolds equation

Reynolds number

Surface roughness

Turbulence

Turbulent flow

Vortex flow

Crossflow vortices

Falkner-Skan-Cooke boundary layers

Global instability

Global stability analysis

Immersed boundary methods

Nonlinear simulations

Roughness height

Secondary instability

Shear flow

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