Large-eddy simulations of the flow on an aerofoil with leading-edge imperfections

Abstract

We performed large-eddy simulations of the flow over an airfoil to understand the effects of leading-edge roughness designed to mimic ice accretion. The roughness elements protrude outside the boundary layer, which, near the leading edge, is very thin; thus, the configuration does not represent a classical rough-wall boundary layer, but rather the flow over macroscopic obstacles. A grid convergence study is conducted and results are validated by comparison to numerical and experimental studies in the literature. The main effect of the obstacles is to accelerate transition to turbulence. Significant variations in structure generation are observed for different roughness shapes. The three-dimensionality of the irregularities has a strong impact on the flow: it creates alternating regions of high-speed ("peaks") and lowspeed ("valleys") regions, a phenomenon termed "channelling". The valley regions resemble a decelerating boundary layer: they exhibit considerable wake and higher levels of Reynolds stresses. The peak regions, on the other hand, are more similar to an accelerating one. Implications of the channelling phenomenon on turbulence modelling are discussed.

Flow Separation in Airfoils with Rough Leading Edges

“…and Kafyeke [3]. A discussion of the research relevant to the present investigation can also be found in the recent paper by Kumar et al [4]; here we summarize the main results of these studies.…”

“…In the former cases, the ensuing heterogeneities were called either "streaks" or "merged low-speed blobs" (MLSs), in the latter "low-" and "high-momentum pathways" (LMP, HMP). Compared to the the LMPs and HMPs present in turbulent boundary-layer flow, however, the scale of the channelling phenomenon described in [4] is much larger. Note also that most of these studies are performed on flat-plate boundary layer configurations with zero-or-moderate pressure gradients, where flow separation does not occur [4].…”

“…Compared to the the LMPs and HMPs present in turbulent boundary-layer flow, however, the scale of the channelling phenomenon described in [4] is much larger. Note also that most of these studies are performed on flat-plate boundary layer configurations with zero-or-moderate pressure gradients, where flow separation does not occur [4].…”

“…Kumar et al [4] performed large-eddy-simulations (LES) of flow over smooth and rough NACA4412 airfoils at Reynolds number = / = 200, 000 (where is the chord length, the free-stream velocity, and the kinematic viscosity) and angle of attack = 5 . To the best of our knowledge, this was the first eddy-resolving simulation of the flow over large, spatially heterogeneous leading-edge obstacles mimicking aircraft icing on an airfoil.…”

“…The high-and low-speed channels had widths of the order of the roughness size. Kumar et al [4] also found that, while the regions of fast-moving fluid (which occur behind roughness elements and were called "peak regions") resemble an accelerating boundary layer, the regions of slower fluid (which are found between the roughness peaks, and were termed "valley regions") have characteristics that are similar to a decelerating boundary-layer.…”