This thesis considers laminar-turbulent transition in wall bounded shear flows, with a focus on transition emanating from a pair of oblique disturbance waves. The oblique waves interact nonlinearly and transfers disturbance energy into streamwise vortices, which generate streamwise streaks through a linear mechanism that operates also at conditions where stability theory predicts decay of all eigenmodes. If the strength of this transient growth is sufficient to produce streaks with an amplitude exceeding a threshold value, the streaks break down through what is describecl as a secondary instability operating on the streaks, which is not to be confused with the traditional secondary instability operating on finite amplitude waves.
A survey of transition in plane Poiseuille and Blasius flows is presented together with a description of different methods for generating disturbances in a flow experiment. Details about the specific methods for disturbance generation used in the present in vestigations are also given.
Experimental investigations of oblique transition in plane Poiseuille and Blasius flow have been made using hot-wire measurements and flow visualisations. The main effort in the experimental work has been in describing the structure of the flows, but also to determine the amplitude of individual frequency-spanwise wavenumber modes and their development during the transition process. During an initial stage of each experiment, messurements on the stability characteristics of single waves were compared to results from linear stability calculations.
Spatial direct numerical simulations (DNS) together with numerical modelling of the vibrating ribbons used in the plane Poiseuille flow experiments helped to clarify the reason for symmetry properties observed in the measurements.
A model experiment is also reported which wes designed to study the last stage in the oblique transition scenario. Stationary streamwise streaks of large arnplitudes were generated and the breakdown of the streaks was investigated both at unforced conditions and by forting with phase controlled time-dependent disturbances. Most of the experimental results were found to be in agreement with previous theoretical and numerical work.
Stockholm: Mekanik , 1998. , 39 p.