Three-dimensional organisation of primary and secondary crossflow instability

Abstract

An experimental investigation of primary and secondary crossflow instability developing in the boundary layer of a 45 • swept wing, at a chord Reynolds number of 2.17·10 6 is presented. Linear stability theory is applied for preliminary estimation of the flow stability while surface flow visualisation using fluorescent oil is employed to inspect the topological features of the transition region. Hot-wire anemometry is extensively used for the investigation of the developing boundary layer and identification of the statistical and spectral characteristics of the instability modes. Primary stationary as well as unsteady type-I (z-mode), type-II (y-mode) and type-III modes are detected and quantified. Finally, three-component, three-dimensional measurements of the transitional boundary layer are performed using tomographic particle image velocimetry. This research presents the first application of an optical experimental technique for this type of flow. Among the optical techniques tomographic velocimetry represents, to date, the most advanced approach allowing the investigation of spatially correlated flow structures in threedimensional fields. Proper orthogonal decomposition (POD) analysis of the captured flow fields is applied to this goal. The first POD mode features a newly reported structure related to low-frequency oscillatory motion of the stationary vortices along the spanwise direction. The cause of this phenomenon is only conjectured. Its effect on transition is considered negligible but, given the related high energy level, it needs to be accounted for in experimental investigations. Secondary instability mechanisms are captured as well. The type-III mode corresponds to low frequency primary travelling crossflow waves interacting with the stationary ones. It appears in the inner upwelling region of the stationary crossflow vortices and is characterised by elongated structures approximately aligned with the axis of the stationary waves. The type-I secondary instability consists instead of significantly inclined structures located at the outer upwelling region of the stationary vortices. The much narrower wavelength and higher advection velocity of these structures correlate with the higher-frequency content of this mode. The results of the investigation of both primary and secondary instability from the exploited techniques agree with and complement each other and are in line with existing literature. Finally, they present the first experimental observation of the secondary instability structures under natural flow conditions.

Conditioning of cross-flow instability modes using dielectric barrier discharge plasma actuators

“…The latter was measured with a single hot-wire sensor to be T u/U ∞ = 0.07% at the free stream velocity of U ∞ =24m s −1 (bandpass filtered between 2Hz and 5000Hz). The T u/U ∞ value is low enough to observe stationary cross-flow waves (see Bippes (1999)) and indeed this was the case for the test case at U ∞ =25.6m s −1 and free steam Reynolds number Re=2.17·10 6 (see also Serpieri & Kotsonis (2016)). The experiment was performed at incidence of α=3…”

“…In a recent study, Serpieri & Kotsonis (2016) followed the traditional approach of Reibert et al (1996) of using micron-sized roughness elements to condition the fundamental mode. The development of the primary and secondary instability modes on a swept wing model at chord Reynolds number of Re=2.17 · 10 6 was thoroughly investigated with oil flow visualisation, linear stability theory, hot-wire anemometry (HWA) and tomographic-PIV measurements.…”

“…The airfoil used is a modified version of the NACA 66018 shape that was named 66018M3J (more details can be found in Serpieri & Kotsonis (2016), the wing and the airfoil are presented in figures 1b and 5).…”

“…The spacing between the stems defines the spanwise wavelength of the plasma forcing, and as such the spanwise wavelength of the forced CF modes. Three different modes were forced in this study, namely λ z =9mm, which corresponds to critical mode (Serpieri & Kotsonis 2016), and λ z =7mm and λ z =12mm, corresponding to sub-critical and super-critical modes respectively. A detail of the DBD plasma actuators used in this study is presented in figure 2.…”

“…Some more recent studies (e.g. White & Saric (2005); Bonfigli & Kloker (2007); Serpieri & Kotsonis (2016)) further focus on the secondary cross-flow instability.…”