Enhancement of disturbance wave amplification due to the intrinsic three-dimensionalisation of laminar separation bubbles

Abstract

Previous studies demonstrated that laminar separation bubbles (LSBs) in the absence of external disturbances or forcing are intrinsically unstable with respect to a three-dimensional instability of centrifugal nature. This instability produces topological modifications of the recirculation region with the introduction of streamwise vorticity in an otherwise purely two-dimensional time-averaged flows. Concurrently, the existence of spanwise inhomogeneities in LSBs have been reported in experiments in which the amplification of convective instability waves dominates the physics. The co-existence of the two instability mechanisms is investigated herein by means of three-dimensional parabolised stability equations. The spanwise waviness of the LSB on account of the primary instability is found to modify the amplification of incoming disturbance waves in the linear regime, resulting in a remarkable enhancement of the amplitude growth and a three-dimensional arrangement of the disturbance waves in the aft portion of the bubble. Present findings suggest that the oblique transition scenario should be expected in LSBs dominated by the convective instability, unless high-amplitude disturbances are imposed.

Structure and dynamics of a laminar separation bubble near a wing root: towards reconstructing the complete LSB topology on a finite wing

“…Therefore, the primary instability in the junction and midspan regions is expected to be convective in nature. Although the identification of global instability modes is outside the scope of this work, the reverse flow does exceed the threshold required for the stationary global instability identified by Rodríguez & Theofilis (2010), which may be responsible for the spanwise waviness seen in the LSB, similar to that reported by Rodríguez & Gennaro (2019).

Figure 8.Maximum displacement thickness, maximum height of contour and minimum streamwise velocity at each side-view measurement plane.

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“…Gaster 1967; Brendel & Mueller 1988;Watmuff 1999;Simoni et al 2017) and numerical investigations (e.g. Pauley, Moin & Reynolds 1990;Marxen, Lang & Rist 2013;Rodríguez & Gennaro 2019) on two-dimensional flow geometries. However, in many practical devices where LSBs occur, such as aircraft wings and low pressure gas turbine stages (e.g.…”

Self-excited primary and secondary instability of laminar separation bubbles

“…Further work is required to shed light on the role of the primary and secondary self-excited instabilities discussed herein when environmental disturbances and/or explicit forcing are present. The plethora of possibilities span from the constructive interaction of disturbance waves existing in the pre-separated boundary layer with the inherent spanwise distortion of the LSB (as studied in Rodríguez & Gennaro 2019), to the formation of strongly coherent two-dimensional vortices by high-amplitude forcing (e.g. Embacher & Fasel 2014; Hosseinverdi & Fasel 2018), completely precluding the self-excited scenario.…”

“…Herein, inherent dynamics not only refers to that corresponding to ideal, unforced LSBs, but also to that which may exist in practical externally disturbed flow, underlying the dominant inflectional instability and yet having an impact on the evolution and amplification of the external disturbances (e.g. Marquillie & Ehrenstein 2003; Rodríguez & Gennaro 2019).…”

The effect of compressibility on the primary global instability of unforced laminar separation bubbles

“…Consequently, LSBs at very low-noise environments should be expected to become three-dimensional and give rise to spanwise-periodic flow topologies as described by Rodríguez and Theofilis [35], prior to the appearance of time-periodic fluctuations. The implications of this discovery have been explored further: First, the topology modifications exerted by the centrifugal instability have been shown to enhance remarkably the convective amplification of external disturbances [32]. Additionally, if the spanwise inhomogeneity is strong enough, a new self-excited instability can emerge, leading to turbulent flow [34].…”