Daniel J. Garmann

Three-dimensional flow structure and aerodynamic loading on a revolving wing

A numerical study is conducted to examine the vortex structure and aerodynamic loading on a revolving wing in quiescent flow. A high-fidelity, implicit large eddy simulation technique is employed to simulate a revolving wing configuration consisting of a single, aspect-ratio-one rectangular plate extended out a distance of half a chord from the rotational axis at a fixed angle relative to the axis. Shortly after the onset of the motion, the rotating wing generates a coherent vortex system along the leading-edge. This vortex system remains attached throughout the motion for the range of Reynolds numbers explored, despite the unsteadiness and vortex breakdown observed at higher Reynolds numbers. The average and instantaneous wing loading also increases with Reynolds number. At a fixed Reynolds number, the attachment of the leading-edge vortex is also shown to be insensitive to the geometric angle of the wing. Additionally, the flow structure and forcing generated by a purely translating wing is investigated...

Numerical investigation of transitional flow over a rapidly pitching plate

A computational study of a plate undergoing high amplitude, pitch, hold, and return motions is presented. An implicit large eddy simulation (ILES) technique is employed to capture the laminar-to-turbulent transition process as the plate is pitched to high angles of attack. Simulations are performed for Reynolds numbers between 5000 and 40 000 with motion profiles of varying accelerations and hold times. The solutions show extremely favorable flow field comparisons of span-averaged stream-wise velocity and out-of-plane vorticity with available experimental Particle Image Velocimetry (PIV) measurements. At a given Reynolds number, the span-averaged flow fields and aerodynamic loading show little sensitivity to the acceleration of the plate for the motions examined. The three-dimensional flow field structure reveals a very rapid transition process that occurs almost at the onset of motion for the higher Reynolds number cases. In spite of this, the aerodynamic loads for those cases compare closely with the sa...

Experiments and Computations on Abstractions of Perching

The flight maneuver of perching is abstracted as a linear pitch ramp, with and without a deceleration in the free-stream direction. We consider, first, experimental-computational comparison for flowfield and aerodynamic force coefficients for an SD7003 airfoil pitching from α = 0o to 45o; and second, an experimental survey of reduced frequency and pivot point for a range of flat plate pitching cases from 0o to 90o. The computational approach is 3D Large Eddy Simulation, and the experimental approach is by three degree of freedom electric motion-rig in a water tunnel. Accurate flowfield resolution in deep-stall is seen to require a large spanwise extent of computational domain. Meanwhile, experiment can be plagued with blockage, and dynamic blockage was seen to behave differently than static blockage. Even very low reduced frequencies of motion give lift overshoot beyond static stall, but comparatively large frequencies are necessary before the lift curve slope changes, either due to rate effects or acceleration effects. Moving the pitch pivot point further aft tends to attenuate both lift and drag production, and pitching about the three-quarter chord point cancels the rate-effect, in agreement with quasi-steady linear airfoil theory. Surprisingly, the aerodynamic coefficient history differs little between cases with and without streamwise deceleration, except towards the very end of the motion. The implication is that perching-type of ground tests or computations can be adequately conducted in a steady free-stream.

Analysis of Dynamic Stall on a Pitching Airfoil Using High-Fidelity Large-Eddy Simulations

The onset of unsteady separation and dynamic stall vortex formation over a constant-rate pitching airfoil is analyzed by means of high-fidelity large-eddy simulations. The flowfields are computed b...

A numerical study of hovering wings undergoing revolving or translating motions

This paper presents simulations of revolving and translating wings undergoing reciprocating motions that are representative of hovering flight. A high-fidelity, implicit large-eddy simulation (ILES) approach is utilized to examine the vortex structure and unsteady loading generated on each wing to compare and contrast the mechanisms of lift production. For each stroke of the motion, the revolving wing produces a coherent and attached vortex system across the leading edge that remains in close proximity to the surface until the wing decelerates and flips at the end of the stroke. As the vortex collides with the wing, it breaks up rapidly before becoming entrained by the newly forming vortex loop on the reversed side. The back stroke produces effectively the same structure as the wing passes through its previously shed wake. The flow structure each stroke closely resembles that of a simple, unidirectionally revolving wing indicating that the wake capture effect is minimal. Alternatively, a translating hovering wing undergoing a rectilinear motion produces a leading edge vortex that develops into an arch-type vortex by lifting off at the wing mid-span and unpinning from the front edge of the wing to form legs that are anchored on the surface. The translating wing also generates about 32% less cycle-averaged lift as the revolving wing as the mechanism of lift is inherently different in the two cases. During rotation, the lift is attributed to the close proximity of the spanwise-oriented leading edge section of the vortex loop above the surface that creates persistent suction throughout the stroke. The majority of the lift on the translating wing, on the other hand, is due to the counter-rotating feet of the arch-type vortex that are anchored on the surface and produce diminishing suction through the stroke as the arch vortex weakens.

Flow Structure Above Stationary and Oscillating Low-Aspect-Ratio Wing

Computations have been carried out in order to describe the complex unsteady flow structure over a stationary and plunging aspect-ratio-two wing under low Reynolds number conditions (Rec = 104). The flow fields are computed employing a high-fidelity implicit large-eddy simulation (ILES) approach found to be effective for moderate Reynolds number flows exhibiting mixed laminar, transitional and turbulent regions. The evolution of the flow structure and aerodynamic loading as a function of increasing angle of attack is presented. Lift and pressure fluctuations are found to be primarily dominated by the large scale circulatory pattern established above the wing due to separation from the leading edge, and by the inherent three dimensionality of the flow induced by the finite aspect ratio. The spanwise distribution of the sectional lift coefficient revealed only a minor direct contribution to the loading exherted by the tip vortex. High-frequency, small-amplitude oscillations are shown to have a significant effect on the separation process and accompanying loads suggesting potential flow control through either suitable actuation or aero-elastic tailoring.© 2012 ASME

Numerical Investigation of the Three-Dimensional Flow Structure About a Revolving Wing

A numerical study is conducted to examine the vortex structure about a revolving wing in quiescent flow employing a high-fidelity, implicit large eddy simulation (ILES) technique found to be effective in simulating flows that exhibit interspersed regions of laminar, transitional, and turbulent flows. The revolving wing configuration consists of a single, aspect ratio one rectangular plate extended out a distance of 0.5 chords from the origin. Shortly after the onset of the motion, the rotating wing generates a stable and coherent vortex system across the leading edge and wing root that remains throughout the motion. The aerodynamic loads are also analyzed and found to remain mostly constant during the maneuver. Transitional effects on the vortex system are investigated over a range of Reynolds numbers (3,000 < Re < 15,000). It is found that higher Reynolds numbers promote more breakdown of the leading edge and root vortices, but do not alter the stability of the vortex system. The aerodynamic loads also show little sensitivity to Reynolds number with the higher Reynolds numbers producing only moderately higher forces. Comparisons with recent experimental PIV measurements using a PIV-like data reduction technique applied to the computational solution show very favorable agreement with the mid-span velocity and vorticity contours.Copyright

Analysis of Tip Vortex Near-Wake Evolution for Stationary and Oscillating Wings

A numerical study is conducted to characterize the tip vortex unsteady evolution on a rounded-tip NACA0012 wing of aspect ratio operating at a Reynolds number of Re=2.0×105 and an incidence of α=8 ...

Comparative Study of Implicit and Subgrid-Scale Model LES Techniques for Low-Reynolds Number Airfoil Applications

A computational study of the standard large eddy simulation (LES) technique employing the dynamic Smagorinsky subgrid scale (SGS) model is compared to results using a highorder, implicit LES (ILES) technique with no SGS modeling. It has been shown in the past that ILES presents a viable substitute to the standard LES technique on canonical turbulent problems. The purpose of this paper is to evaluate the use of implicit LES with and without explicit subgrid scale models applied to problems of transitional ow over the SD7003 airfoil for various grid resolutions and Reynolds numbers. It will be shown that the addition of the SGS model does not oer a signicant advantage on coarser meshes when compared to the ILES approach. The ILES technique is shown to be an attractive alternative to SGS modeling in solution quality and computational cost and robustness for the ows examined.

High-Fidelity Computations for Flexible Micro Air Vehicle Applications

Implicit large-eddy simulation (ILES) computations have been performed for canonical problems associated with flexible, flapping-wing micro air vehicles (MAVs). This computationally-intensive approach, which is able to directly model laminar/transitional/turbulent flow fields, requires the use of the best high performance computational platforms available. Results for the direct numerical simulation of the deep dynamic stall phenomenon over a rigid plunging airfoil section at transitional Reynolds numbers relevant to MAV systems are presented. Next, computations for two different flexible-wing geometries, a membrane-wing section and a three-dimensional, flexible-wing with an NACA0012 cross-section, are discussed. Finally, to investigate the relevant physics associated with a perching maneuver, computational results for a pitch, hold and return motion are examined. All computed results show good correlation with corresponding experimental measurements.