Laminar Turbulent Transition in a Boundary Layer Subjected to Weak Free Stream Turbulence

Abstract

For revealing the transition process in a flat plate boundary layer subjected to a weak free stream turbulence, flow visualization and hot-wire measurements were performed. A weak free stream turbulence was generated by a turbulence grid mounted upstream of the contraction. The flow visualization clearly displayed a transition scenario in which a local two-dimensional wave packet rapidly forms a Λ shape structure and then breaks down to turbulence, resulting in the generation of a turbulent spot. Quantitative measurements performed by using a hot-wire anemometer also confirmed the existence of local Tollmien-Schlichting waves that agreed with the parallel linear theory in terms of their frequency, phase velocity, and the wall-normal distribution of band-pass-filtered fluctuations. For comparison, a boundary layer subjected to a moderate-intensity free stream turbulence was investigated. This investigation showed that streaky structures play an important role in the boundary layer transition, as shown by Matsubara et al. [ J. Fluid Mech., 430, (2001), 149-168. ] A drastic change occurred in the transition process and this change could be sensitively determined by employing the intensity and/or spectra of the free stream turbulence.

Aerofoil wake-induced transition characteristics on a flat-plate boundary layer

“…It seems evident from the characteristics in figures 12( a – c ), 13, 14 and 15( a ), that the amplification seen at mid-range frequencies and its exponential energy growth is due to the T-S waves and that both modal and non-modal disturbances coexist and are amplified in the present experiment. This coexistence can be attributed to the wake–boundary layer interaction occurring at the moderate level of turbulence: Kosorygin & Polyakov (1990) and Kenchi, Matsubara & Ikeda (2008) have shown, for a moderate level of FST-induced transition, that a T-S wave coexists with the non-modal distribution. However, other characteristics such as maximum energy growth and the spacing of streaky structures (discussed later) differ from the present results.…”