Experiments on localized secondary instability in bypass boundary layer transition

Abstract

An experimental study on localized secondary instability of unsteady streamwise streaks in bypass boundary layer transition under an elevated level of free-stream turbulence has been carried out mainly using the particle image velocimetry (PIV) technique. Simultaneous orthogonal dual-plane PIV measurements were performed for a concurrent examination of the transitional flow features in both wall-normal and spanwise planes. These quantitative and simultaneous visualizations clearly show the wall-normal view of a low-speed streak undergoing sinuous/varicose motion in the spanwise plane. An oscillating shear layer in the wall-normal plane is found to be associated with the sinuous/varicose streak oscillation in the spanwise plane. Further, these measurements indicate that a localized secondary instability wavepacket can originate near the boundary layer edge. The time-resolved PIV measurements in the wall-normal plane clearly show how an instability develops on a lifted-up inclined shear layer and leads to flow breakdown. The estimated wavelength and convection velocity of such instabilities are found to compare well with those calculated from the one-dimensional linear stability analysis of the spatially averaged velocity profiles associated with the lifted-up shear layers. The time-resolved PIV measurements in the spanwise plane also facilitate quantitative visualizations of sinuous and varicose instabilities. These measurements experimentally confirm that a varicose instability at the juncture of an incoming high-speed streak and a downstream low-speed streak can eventually lead to the formation of lambda structures. The estimated convection velocity, wavelength and growth rate of these instabilities are found to be consistent with the numerical results reported in the literature. Moreover, the streak secondary instability is found to be apparent in the velocity contours, while the estimated streak amplitude is approximately 30 % of the free-stream velocity.

Characteristics of the turbulent non-turbulent interface in a spatially evolving turbulent mixing layer

“…The uncertainty in the velocity is found to be around 1.5 % of the free stream velocity difference (U d ) in the mixing layer. Similar uncertainty levels in velocity were also reported in our previous work (Balamurugan & Mandal 2017). Similarly, uncertainty in the total kinetic energy and the Reynolds stress, calculated following Sciacchitano (2019), are found to be 2.3 % and 4 %, respectively.…”

“…The triple bladed tunnel fan was powered by a Siemens made AC motor (14.5 kW), driven by a speed controller. The streamwise turbulent intensity at the test section, measured following the work of Mandal, Venkatakrishnan & Dey (2010), was found to be 0.1 % of the free stream velocity, U 0 , as was also reported in our previous work in this tunnel (Balamurugan & Mandal 2017).…”

“…These particles were generated using a SAFEX fog generator (Dantec Dynamics, Denmark) placed at the tunnel entrance. The fog was distributed uniformly across the tunnel area with help of a fan, similar to the previous works (Mandal et al 2010;Phani Kumar et al 2015;Balamurugan & Mandal 2017). The cameras were mounted adjacent to each other to capture the entire region of interest.…”

Low-frequency selectivity in flat-plate boundary layer with elliptic leading edge

“…Matsubara & Alfredsson (2001) captured the progression of streaks from amplification through instability and breakdown to turbulence in their experiments, and Mans, de Lange & van Steenhoven (2007) provided detailed measurements of the streak sinuous instability. Very recently, Balamurugan & Mandal (2017) studied the localized secondary instability by simultaneous orthogonal dual-plane particle image velocimetry (PIV) measurements. They found an oscillating shear layer in the wall-normal plane is associated with the sinuous or varicose streak oscillation in the spanwise plane, and they also identified a localized secondary instability wavepacket which can originate near the boundary-layer edge.…”

Bypass transition in flow over a vibrating flat plate

“…Swearingen & Blackwelder (1987) identified two types of secondary instabilities of streaks in Görtler flow: sinuous mode and varicose mode, referring to spanwisely symmetric and asymmetric forms of the unstable modes, respectively. These two types of instability are confirmed in the experiments using dye visualization (Matsubara & Alfredsson 2001;Asai, Minagawa & Nishioka 2002;Mans et al 2005) and a simultaneous dual plane particle image velocimetry measurements technique (Balamurugan & Mandal 2017). Sinuous instability occurs at the top of the boundary layer and is related to the velocity inflection point in the spanwise direction.…”