F. H. Abernathy

Bio: F. H. Abernathy is an academic researcher from Harvard University. The author has contributed to research in topics: Vortex & Vortex shedding. The author has an hindex of 1, co-authored 1 publications receiving 64 citations.

Detached-eddy simulation of an airfoil at high angle of attack

Abstract: We present the first true applications of Detached-Eddy Simulation (DES), in the sense of being three-dimensional. DES was defined in 1997 with hopes of combining the strengths of Reynolds-averaged methods and of Large-Eddy Simulations, in a non-zonal manner, to treat separated flows at high Reynolds numbers. We first simulate isotropic turbulence, to check the concept in LES mode and set its adjustable constant. Smooth inertial ranges are obtained up to the cutoff in the spectra. We then treat an airfoil in the challenging regime of massive separation and do so very successfully, in that lift and drag are within 10% of the experimental results at all angles of attack, to 90°. Such an accuracy is not achieved with traditional modelling, even unsteady, which gives up to 40% error. Cost puts a pure LES of the same flow (at Reynolds number 105 and beyond) out of reach on any computer, yet we use personal computers for the DES, and about 200,000 grid points. On the other hand, grid refinement, domain-size and Reynolds-number studies have not been completed yet. Hysteresis in the 15 - 25° range has not been addressed.

Physical and Numerical Upgrades in the Detached-Eddy Simulation of Complex Turbulent Flows

Abstract: A new formulation of Detached-Eddy Simulation (DES) based on the k-ω RANS model of Menter (M-SST model) is presented, the goal being an improvement in separation prediction over the S-A model. A new numerical scheme adjusted to the hybrid nature of the DES approach and the demands of complex flows is also presented. The scheme functions as a fourth-order centered differentiation in the LES regions of DES and as an upwind-biased (fifth or third order) differentiation in the RANS and outer irrotational flow regions. The capabilities of both suggested upgrades in DES are evaluated on a set of complex separated flows.

An inviscid model of two-dimensional vortex shedding for transient and asymptotically steady separated flow over an inclined plate

Abstract: A potential flow model of two-dimensional vortex shedding behind an inclined plate is developed. The free shear layers which emanate from the sides of the plate are represented by discrete vortices through the use of the appropriate complex-velocity potential, the Kutta condition and the Joukowsky transformation between a circle and the plate cross-section. The analysis is then applied to predict the kinematic and dynamic characteristics of the flow for various angles of attack. The results compare favourably with the available experimental data as far as the form of vortex shedding and the Strouhal number are concerned. The calculated normal-force coefficients are 20−25% yo larger than those measured by Fage & Johansen (1927).

Unsteady fluid-structure interactions of membrane airfoils at low Reynolds numbers

Abstract: Membrane wings are used both in nature and small aircraft as lifting surfaces. Separated flows are common at low Reynolds numbers and are the main sources of unsteadiness. Yet, the unsteady aspects of the fluid-structure interactions of membrane airfoils are largely unknown. An experimental study of unsteady aerodynamics of two-dimensional membrane airfoils at low Reynolds numbers has been conducted. Measurements of membrane shape with a high-speed camera were complemented with the simultaneous measurements of unsteady velocity field with a high frame-rate particle image velocimetry system and flow visualization. Vibrations of the membrane and mode shapes were investigated as a function of angle of attack and free stream velocity. While the mean membrane shape is not very sensitive to angle of attack, the amplitude and mode of the vibrations of the membrane depend on the relative location and the magnitude of the unsteadiness of the separated shear layer. The results indicate strong coupling of unsteady flow with the membrane oscillations. There is evidence of coupling of membrane oscillations with the vortex shedding in the wake, in particular, for the post-stall incidences. Comparison of rigid (but cambered) and flexible membrane airfoils shows that the flexibility might delay the stall. Hence this is a potential passive flow control method using flexibility in nature and engineering applications.