Effect of environmental disturbances on crossflow instability

• Published in: Experiments in fluids Vol. 64; no. 2
• Main Authors: Placidi, Marco, Ashworth, Richard, Atkin, Chris J., Rolston, Stephen
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01-02-2023; Springer Nature B.V
• Subjects: Amplitudes; Cross flow; Disturbances; Engineering; Engineering Fluid Dynamics; Engineering Thermodynamics; Fluid- and Aerodynamics; Heat and Mass Transfer; Research Article; Surface roughness; Three dimensional boundary layer; Turbulence; Wind tunnel testing; Wind tunnels
• ISSN: 0723-4864; 1432-1114
• DOI: 10.1007/s00348-023-03579-x

Wind tunnel experiments on the receptivity of three-dimensional boundary layers were performed in a range of freestream turbulence intensities,
Tu
, from 0.01%—the lowest level ever achieved in this type of work—up to 0.41%. This work confirms that for
T
u
=
0.01
%
, and presumably below this level, the transition process is dominated by stationary modes. These are receptive to surface roughness and generate Type-I and Type-II secondary instabilities that eventually cause the transition to turbulence. The saturation amplitude of these stationary waves is highly sensitive to the level of environmental disturbances; the former is here recorded to be the highest in the literature, with the latter being the lowest. Travelling modes are still present; however, their influence on the transition process is marginal. At matched surface roughness levels, when the level of environmental disturbance is enhanced to
T
u
≥
0.33
%
, the travelling modes acquire more importance, strongly influencing the laminar/turbulent transition process, whilst the initial amplitude and growth of the stationary modes are hindered. For this level of
Tu
, is the interaction of steady and unsteady disturbances that produces highly amplified waves (Type-III), that quickly lead to nonlinear growth and anticipated turbulence. Finally, a simple rule of thumb is proposed, where the transition front was found to move forward by roughly
10
%
chord for an increase in one order of magnitude in the
Tu
levels.
Graphical abstract