Free-stream turbulence and the development of cross-flow disturbances

Abstract

The cross-flow instability that arises in swept-wing boundary layers has resisted attempts to describe the path from disturbance initiation to transition. Following concerted research efforts, surface roughness and free-stream turbulence have been identified as the leading providers of initial disturbances for cross-flow instability growth. Although a significant body of work examines the role of free-stream turbulence in the cross-flow problem, the data more relevant to the flight environment (turbulence intensities less than 0.07 %) are sparse. A series of recent experiments indicates that variations within this range may affect the initiation or growth of cross-flow instability amplitudes, hindering comparison among results obtained in different disturbance environments. To address this problem, a series of wind tunnel experiments is performed in which the free-stream turbulence intensity is varied between 0.02 % and 0.2 % of free-stream velocity, U ∞ . Measurements of the stationary and travelling mode amplitudes are made in the boundary layer of a 1.83 m chord, 45 • swept-wing model. These results are compared to those of similar experiments at higher turbulence levels to broaden the current knowledge of this portion of the crossflow problem. It is observed that both free-stream turbulence and surface roughness contribute to the initiation of unsteady disturbances, and that free-stream turbulence affects the development of both stationary and unsteady cross-flow disturbances. For the range tested, enhanced free-stream turbulence advances the transition location except when a subcritically spaced roughness array is employed.

Three-dimensional organisation of primary and secondary crossflow instability

“…Similar results were recently shown through the extensive measurements performed by Downs & White (2013), by carefully modifying and measuring the turbulence intensity of the KSWT facility at TAMU by means of turbulence screens. Their results confirm the effectiveness of surface roughness in forcing the primary stationary modes and the capability of free stream turbulence to enhance primary travelling instabilities adding that the latter are also highly sensitive to the surface roughness.…”

“…The experiment of Saric et al (1998), done at the same flow conditions as the one of Reibert et al (1996), led to the remarkable conclusion that subcritical forcing can delay transition to turbulence for stationary crossflow instability boundary layers. This transition control strategy has been followed by several other experimental (Downs & White 2013;Serpieri & Kotsonis 2015b), theoretical (Malik et al 1999) and numerical (Wassermann & Kloker 2002;Hosseini et al 2013) studies. Transition delay by this technique has not been always observed (Downs & White 2013) and would need further investigation.…”

“…This transition control strategy has been followed by several other experimental (Downs & White 2013;Serpieri & Kotsonis 2015b), theoretical (Malik et al 1999) and numerical (Wassermann & Kloker 2002;Hosseini et al 2013) studies. Transition delay by this technique has not been always observed (Downs & White 2013) and would need further investigation. Radeztsky et al (1999) demonstrated the effect of discrete surface roughness on transition location.…”

“…One of the main outcomes from these studies is the investigation of the low-frequency type-III mode. This mode refers to the interactions between primary stationary and primary travelling modes and therefore its relevance in the transition pattern depends on the free stream turbulence level (Högberg & Henningson 1998;Downs & White 2013). White & Saric (2005) dedicated a detailed experimental investigation to the secondary instability mechanisms and sensitivity to different base flow cases.…”

“…For more complete reviews the reader is instead referred to Bippes (1999); Arnal & Casalis (2000); Saric et al (2003) and to some more recent studies such as Wassermann & Kloker (2002; White & Saric (2005); Bonfigli & Kloker (2007); Downs & White (2013) and Hosseini et al (2013).…”