Instability and transition of boundary layer flows disturbed by steps and bumps
An investigation of the influence of various wall perturbations on the
development of instability waves has been carried out through a series of
experiments to study different aspects of the stability/transition problem.
The first experiment was devoted to an investigation of the effects of an isolated
step on the transition of the boundary layer over a flat plate. Steps of various heights,
both positive and negative, produced an upstream movement of the transition point,
which was interpreted as arising from an increase in the amplification of the
instability waves. The transition Reynolds number and the resulting change in the
so-called 'N-factor', used for transition prediction, has been correlated with the step
height to provide a useful empirical engineering design tool.
The second experiment concerned the effect of a two-dimensional (2-D) sharpedged
short bump on instability waves (Tollmien-Schlichting (TS) waves). The
experiment was designed to study the mechanisms by which a sharp-edged surface
roughness element, in the form of a 2-D bump, modified the evolution of TS wave
and hence affected the process of transition. This investigation demonstrated that the
interaction of a 2-D TS wave with a 2-D bump was a many-sided complex problem.
The bump played multiple roles as a disturbance generator, an amplifier and a
promoter of three-dimensionality. The crucial parameter was the height. Nonlinearity
was the main factor that caused the enhancement of boundary layer transition and
manifested itself in the appearance of higher harmonics in the spectra.
In the last experiment the bump was three-dimensional (3-D) and in the form of a
cylinder. The experimental results revealed that the effect of a 3-D bump was
confined to a wedge-shaped region downstream of the bump. The mean velocity
distortion had a distinct spanwise distribution from which a pair of streamwise vortex
structures could be inferred. The interaction of a 2-D TS wave with a 3-D bump
generated oblique waves with a dominant band of spanwise wavenumbers as
predicted by theoretical analysis. Three thresholds were found that defined bump
height ranges where there was no-effect, a linear behaviour and a nonlinear one.