A large-eddy simulation of laminar transonic buffet on an airfoil at a Mach number $M=0.735$, an angle of attack $\unicode[STIX]{x1D6FC}=4^{\circ }$, a Reynolds number $Re_{c}=3\times 10^{6}$ has been carried out. The boundary layer is laminar up to the shock foot and laminar/turbulent transition occurs in the separation bubble at the shock foot. Contrary to the turbulent case for which wall pressure spectra are characterised by well-marked peaks at low frequencies ($St=f\cdot c/U_{\infty }\simeq 0.06{-}0.07$, where $St$ is the Strouhal number, $f$ the shock oscillation frequency, $c$ the chord length and $U_{\infty }$ the free-stream velocity), in the laminar case, there are also well-marked peaks but at a much higher frequency ($St=1.2$). The shock oscillation amplitude is also lower: 6 % of chord and limited to the shock foot area in the laminar case instead of 20 % with a whole shock oscillation and intermittent boundary layer separation and reattachment in the turbulent case. The analysis of the phase-averaged fields allowed linking of the frequency of the laminar transonic buffet to a separation bubble breathing phenomenon associated with a vortex shedding mechanism. These vortices are convected at $U_{c}/U_{\infty }\simeq 0.4$ (where $U_{c}$ is the convection velocity). The main finding of the present paper is that the higher frequency of the shock oscillation in the laminar regime is due to a different mechanism than in the turbulent one: laminar transonic buffet is due to a separation bubble breathing phenomenon occurring at the shock foot.
A mechanism for promoting the Crow instability in a counter-rotating vortex pair is presented within the framework of linear dynamics. It consists of ͑i͒ the creation of a periodic array of vortex rings along the length of the vortices by stretching of vorticity at the leading hyperbolic point of the dipole, and ͑ii͒ the deformation of the vortices by the vortex rings leading to the Crow instability. A reduction of the characteristic time of the Crow instability by a factor of roughly 2 can be obtained by this mechanism.Vortex hazard caused by aircraft trailing vortices has gained much attention during the past decades with the advent of jumbo jets. In certain situations, vortex wakes collapse through a chain process including deformation of columnar vortices by the Crow instability, vortex linking, and turbulence. 1 One way to alleviate vortex hazard is to accelerate the process by exciting the intrinsic instabilities of the wake. This is the idea developed in this article, in which we optimize the energy of the Crow perturbation by means of an appropriate initial perturbation. The growth rate of the Crow instability was first derived by Crow. 2 Vortices of the pair deform by mutual induction and oscillate in a plane inclined at approximately 45°about the horizontal. The oscillations grow exponentially in amplitude until the point when the two vortices touch, leading to final collapse. Several studies such as that of Crow and Bate 3 showed that exciting the vortex pair at the wavelength of the mutual induction instability could be efficient in accelerating the chain process. Other studies such as those of Crouch 4 and Fabre, Jacquin, and Loof 5 showed by a vortex filament method that systems of four vortices exhibit much larger amplification rates than the Crow instability. While these previous studies arbitrarily specify the structure of the perturbation as vortex filaments, in this article we use a global stability method based on a finite-element discretization with a high number of degrees of freedom that presupposes no particular shape for the initial perturbation. In the case of a single Lamb-Oseen vortex, Antkowiak and Brancher 6 and Pradeep and Hussain 7 have already reported that the optimal perturbation takes the form of spirals of vorticity outside the vortex core, which suggests that a similar mechanism of amplification can be expected in the case of the dipole as far as the two vortices are not too close to each other. Yet this suggestion is partially hindered by the presence of two hyperbolic stagnation points ͑hereaf-ter simply referred to as hyperbolic points, "hyperbolic" standing for the hyperbolicity of the streamlines in the vicinity of these points, see Fig. 1͒ in the flow that are also known 8 to behave as energy amplifiers. The main objective of the study is to understand the roles that these two dynamics ͑that of the vortex and that of the hyperbolic point͒ play in the optimal amplification of the Crow instability.Base flow. The basic flow is a two-dimensional pair of counter-rotating vor...
An experimental investigation of the transonic flow past the laminar OALT25 airfoil has been conducted to analyze the impact of laminar flow upon the shock wave dynamics and the existence of a laminar buffet like phenomenon. Tests have been carried out at freestream Mach numbers varying in the range of 0.7–0.8, angle of attack from 0.5° to 4°, and with two tripping configurations at the upper surface of the wing. The (airfoil) chord based Reynolds number is about three million. Results obtained from pressure taps and sensors measurements, as well as Schlieren visualizations of the flow reveal the presence of a laminar buffet phenomenon in sharp contrast with the turbulent phenomenon, as it features a freestream- and chord-based normalized frequency of about unity while turbulent buffet occurs for a frequency close to 0.07 (Jacquin et al., AIAA J 2009; 47). A low-frequency mode, at a frequency of about 0.05 is also present in the laminar situation, notably lower than the high-frequency component. The latter exhibits strong oscillations of the shock foot and vertical wavelike deformations of the shock wave and the former moves the shock back and forth over a small portion of chord, quite similar to the turbulent phenomenon. The mean flow past the laminar wing is characterized by a laminar separation bubble under the shock foot, which likely contributes much to the novel dynamics revealed by the present experiments. Two control strategies of the unsteady shock wave are implemented, one consisting of three-dimensional bumps and one consisting of steady jets blowing transversely to the freestream. It is found that bumps provide a significant reduction of the buffet intensity in the laminar situation. The jets are able to completely remove the flow unsteadiness in both laminar and turbulent conditions.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.