Large-eddy simulation of boundary-layer separation and transition at a change of surface curvature

By Zhiyin Yang, P. R. Voke. Journal of Fluid Mechanics, 2001

298 citations recorded by the source corpus.

Transition arising from a separated region of flow is quite common and plays an important role in engineering. It is difficult to predict using conventional models and the transition mechanism is still not fully understood. We report the results of a numerical simulation to study the physics of separated boundary-layer transition induced by a change of curvature of the surface. The geometry is a flat plate with a semicircular leading edge. The Reynolds number based on the uniform inlet velocity and the leading-edge diameter is 3450. The simulated mean and turbulence quantities compare well with the available experimental data. The numerical data have been comprehensively analysed to elucidate the entire transition process leading to breakdown to turbulence. It is evident from the simulation that the primary two-dimensional instability originates from the free shear in the bubble as the free shear layer is inviscidly unstable via the Kelvin–Helmholtz mechanism. These initial two-dimensional instability waves grow downstream with a amplification rate usually larger than that of Tollmien–Schlichting waves. Three-dimensional motions start to develop slowly under any small spanwise disturbance via a secondary instability mechanism associated with distortion of two-dimensional spanwise vortices and the formation of a spanwise peak–valley wave structure. Further downstream the distorted spanwise two-dimensional vortices roll up, leading to streamwise vorticity formation. Significant growth of three-dimensional motions occurs at about half the mean bubble length with hairpin vortices appearing at this stage, leading eventually to full breakdown to turbulence around the mean reattachment point. Vortex shedding from the separated shear layer is also observed and the ‘instantaneous reattachment’ position moves over a distance up to 50% of the mean reattachment length. Following reattachment, a turbulent boundary layer is established very quickly, but it is different from an equilibrium boundary layer.

Subfields: Computational Mechanics, Earth-Surface Processes, Ocean Engineering

View Paper: https://www.semanticscholar.org/paper/449643e8cf602775f154189200e29ae596803e5c

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Citations in the corpus

A numerical method for solving the 3D unsteady incompressible Navier-Stokes equations in curvilinear domains with complex immersed boundaries (2007, methodology): “FT-1 formulations have been proposed and applied to solve the incompressible Navier-Stokes equations, see for example [17,18], but because of the aforementioned difficulties such methods have not been widely used in the literature.”
On vortex shedding from an airfoil in low-Reynolds-number flows (2009, result): “Similar to this approach, the fundamental frequency in separated shear layers is commonly scaled with the momentum thickness and free-stream velocity at separation (e.g. Lin & Pauley 1996; Watmuff 1999; Yang & Voke 2001; McAuliffe & Yaras 2007; Burgmann et al. 2008).”
Investigation of the vortex induced unsteadiness of a separation bubble via time-resolved and scanning PIV measurements (2008, background): “These phenomena have been analyzed numerically and experimentally, e.g., in Watmuff (1999), Yang and Voke (2001) , Lang et al. (2004), Hain and Ka ¨hler (2005).”
Numerical simulation of active separation control by a synthetic jet (2007, unknown): “Also, a low-frequency flapping phenomenon has been reported in the experiments of Eaton & Johnston (1981), Cherry et al. (1984), Kiya & Sasaki (1985) and in the simulations by Yang & Voke (2001).”
A Combined Experimental/Numerical Study of Unsteady Phenomena in a Laminar Separation Bubble (2003, methodology): “instability is discussed in detail in [13], concluding that the separation bubble is likely to become inviscidly unstable via the Kelvin-Helmholtz mechanism.”
Laminar-Turbulent Transition of a Low Reynolds Number Rigid or Flexible Airfoil (2006, methodology): “For example, Yang and Voke [39] investigated boundarylayer separation and transition employing LES.”
On the onset of nonlinear oscillations in a separating boundary-layer flow (2003, background): “Two-dimensional fluctuations have, for instance, been reported for separated flow induced by adverse pressure gradients (Pauley et al. 1990; Häggmark et al. 2000), but also for separation bubbles triggered by leading-edge geometries (Cherry et al. 1984; Yang & Voke 2001).”
Numerical study of passive and active flow separation control over a NACA0012 airfoil (2008, background): “In this case, transition takes place due to nonlinear breakdown of spatially growing traveling waves in the separated free shear layer [37].”
Stability and receptivity characteristics of a laminar separation bubble on an aerofoil (2010, background): “Such vortex shedding is characteristic of both two- and three-dimensional separation bubble flows (Pauley, Moin & Reynolds 1990; Yang & Voke 2001), and for three-dimensional flows vortices have been observed to become more coherent with decreasing Reynolds number (Zhang, Hain & Kähler 2008).”
Time-resolved and volumetric PIV measurements of a transitional separation bubble on an SD7003 airfoil (2008, methodology): “Furthermore, it has been observed that the length of the separation bubble is not constant, i.e., the reattachment point moves up- and downstream (Watmuff 1999, Yang and Voke 2001, McAuliffe and Yaras 2005, Burgmann et al. 2006a).”