Aaron Altman

Résumé of the AIAA FDTC Low Reynolds Number Discussion Group's Canonical Cases

The AIAA Fluid Dynamics Technical Committee’s Low Reynolds Number Discussion Group has introduced several “canonical” pitch motions, with objectives of (1) experimental-numerical comparison, (2) assessment of closed-form models for aerodynamic force coefficient time history, and (3) exploration of the vast and rather amorphous parameter space of the possible kinematics. The baseline geometry is a flat plate of nominally 2.5% thickness and round edges, wall-to-wall in ground test facilities and spanwise-periodic or 2D in computations. Motions are various smoothings of a linear pitch ramp, hold and return, of 40 and 45 amplitude. In an attempt to discern acceleration effects, sinusoidal and linear-ramp motions are compared, where the latter have short runs of high acceleration and thus high noncirculatory lift and pitch. Parameter variations include comparison of the flat plate with an airfoil and ellipse, variation of reduced frequency, pitch pivot point location and comparison of pitch to quasi-steady equivalent plunge. All motions involve strong leading edge vortices, whose growth history depends on pitch pivot point location and reduced frequency, and which can persist over the model suction-side for well after motion completion. Noncirculatory loads were indeed found to be localized to phases of motion where acceleration was large. To the extent discernable so far, closed-form models of lift coefficient on the pitch upstroke are relatively straightforward, but not so on the downstroke, where motion history effects complicate the return from stall. Broad Reynolds number independency, in flowfield evolution and lift coefficient, was found in the 10 to 10 range.

Wake Vorticity Measurements for Low Aspect Ratio Wings at Low Reynolds Number

Trailing vortex structure of low aspect ratio wings was studied in a water tunnel at Reynolds numbers of 8000 and 24,000 using dye injection and digital particle image velocimetry in cross-flow planes in the near wake, for rectangular, semi-elliptical, and delta-wing planforms. The velocity data were used to calculate lift via circulation and effective span, and the results were compared with force balance measurements and classical inviscid theory. The objectives of the study were to assess how low-Reynolds number effects might affect the measurement of lift coefficient from tip-vortex circulation, how well the measurements fit the various theoretical models of lift curve slope for low aspect ratio wings, and the extent to which planform shape affects lift coefficient while aspect ratio and planform area are kept constant. All models were thin flat plates with square edges

Lift from Spanwise Flow in Simple Flapping Wings

Spanwise flow contributes to lift in thin flat-plate zero-pitch-angle flapping wings in quiescent air. It is reasonable to maintain only the kinematics and mechanical complexity absolutely necessary in developing flapping-wing micro air vehicles. This study continues the quantification of the lift generated from a flapping motion of absolute minimum complexity thought to be capable of generating lift. A flapping-wing micro air vehicle with rectangular planform wings fabricated in-house (semispan aspect ratios from 1.5 to 4.0) was used to quantify the contributions to lift from flow along the span of wings at numerous points throughout the flapping cycle under a variety of operating conditions (3-6 Hz and Reynolds numbers of 6000-15,000). These experiments were performed for several aspect ratios for flat-plate and spanwise-cambered wings. The lift force was quantified experimentally using a force transducer and a high-speed camera. Digital particle image velocimetry was used to determine the lift contributions of spanwise flow to the total measured lift. Additionally, the presence of spanwise camber was shown to affect the transient lift behavior.

Streamwise vorticity in simple mechanical flapping wings

The presence of streamwise vorticity in the vicinity of the wing tip contributes to lift in thin flat plate zero pitch angle flapping wings in quiescent air. In creating flapping wing micro air vehicles it is desirable to maintain only the mechanical and kinematic complexity absolutely necessary to artificially duplicate flapping wing flight. This study quantifies the lift generated from a flapping motion of absolute minimum complexity thought to be capable of generating lift Using a flapping wing micro air vehicle with wings fabricated in-house, streamwise vortices were identified along the span of wings of various aspect ratios and at numerous different points throughout the flapping cycle under a variety of operating conditions. The lift generated by the flapping mechanism was quantified experimentally using a force transducer and a high speed camera. Digital particle image velocimetry was used to determine the contributions of streamwise vorticity to the total measured lift. Further evidence was found of the importance of the relationship between wing span and flapping frequency in the nature of the formation and shedding of vortices.

Leading Edge Serrations on Flat Plates at Low Reynolds Number

This study determines the utility of abstractions of wing leading edge features observed across the natural world when operated at low Reynolds numbers and at low aspect ratio. Flat plates are some of the most well understood aerodynamic shapes and thus provide a good baseline case from which to build an understanding of the effects of leading edge serrations. Thus an aspect ratio 2 thin flat plate is tested first in pure rectangular planform and subsequently across a range of leading edge serration densities. Forces are recorded across a range of Reynolds number from 140,000 to 210,000 and smoke flow visualization is also performed. Ultimately, the serrations were found to present little benefit at low angles of attack, however showed potential promise at higher Re numbers. The small serrations at low angles of attack showed the greatest increase in lift coefficient. The medium serrations showed little difference from the flat plate at all angles of attack. The large serrations showed a decrease in lift and increase in drag coefficients at all angles of attack, yet still demonstrated the best stall angle performance. Pre-stall lift departures on the lift curve observed by others were also observed for the large serrations. Trailing edge serrations resulted in promoting much earlier stall implying that the same mechanism responsible for extending stall when serrations are placed on the leading edge, have the opposite effect on the trailing edge.

Post-Stall Performance Improvement through Bio-inspired Passive Covert Feathers

Inspired by birds’ covert feathers, previous research has shown that artificial self-deployable flaps placed on the suction surface of wings can act as a passive boundary layer flow control mechanism improving post-stall wing lift performance by as much as 15%. Parametric variations are performed in this study, to investigate the range of effectiveness of these artificial flaps. Experiments were performed at the University of Dayton Low Speed Wind Tunnel at a Reynolds number of 2.0x10 5 . A NACA-0012, an intermediate camber USA-28 and a highly cambered Eppler-423 were studied. Flap settings (flap length and chordwise placement) informed by previous research by the authors at another facility did not result in similar improvement in the present study. Subsequent experiments and complementary XFoil investigations revealed that each wing will respond differently to the placement of the artificial feathers depending on the wing’s stall characteristics, boundary layer thickness, and pressure distribution. No universally effective flap configuration was discovered, however, an optimum flap configuration was ascertained for each wing resulting in post-stall lift performance improvements on the order of 5% to 30%. This was true even for the NACA 0012 which experienced a leading edge stall at the Reynolds number tested. Segmenting the flap in the spanwise direction was found to be beneficial in the case of trailing edge stall. Finally, some insight into the physics underlying the effectiveness of these flaps can be gained through careful evaluation of the pressure gradients along the upper surface. This insight can ultimately inform optimal flap placement.

Wing Tip Vortices from an Exergy-Based Perspective

The lens of exergy is used to investigate a wingtip vortex in the near wake over a range of angles of attack. Exergy is the measure of thermodynamically “available” energy as determined through the more discriminating second law of thermodynamics. Experiments were conducted in a water tunnel at Institute of Aerospace Systems at Aachen. The data were taken three chord lengths downstream in the Trefftz plane of an aspect ratio 5 Clark-Y wing with a square-edged wing tip using particle image velocimetry. Intuitively, the minimum available energy state is expected to correspond to the maximum lift-to-drag ratio angle of attack. This, however, is not the case here. Most interestingly, although only two-dimensional Trefftz plane data were used to obtain the exergy distribution across the individual wing-tip vortices, the crossover point for the out-of-plane change from wakelike to jetlike wing-tip vortex core axial flow (indicating the peak lift-to-drag ratio) is identified by the in-plane exergy distribution. ...

Experimental and Computational Analysis of High Angle of Attack Perching Maneuvers

Experimental and numerical investigations are performed in an effort to elucidate the necessity of inclusion of three-dimensional effects in the determination of lift and drag forces during rapid pitching and impulsively started maneuvers to/at high angle of attack. Following the guidelines of the NATO RTO AVT 202 test case, wind tunnel data was acquired for pure pitching cases from 0 to 45 degrees for an AR 4 flat plate using the University of Dayton Low Speed Wind Tunnel (UD-LWST) and compared to a 2D Discrete Vortex Method (DVM). Results show that the DVM compares well to the wind tunnel data in the pre-stall region before the deformation of the wake at the trailing edge by the trailing edge vortex and show similar trends in the post-stall region. It is believed that these differences in post-stall occur due to 3D effects and discussion is provided to support this assertion. The DVM is also used to compare results of a 45 degree linearly accelerated impulsive start case to data obtained in an experiment at the AFRL Horizontal Freesurface Water Tunnel under conditions identical to the simulation. Comparing these results again demonstrates the that DVM matches well to the water tunnel data except in a region where it is believed 3D instabilities are responsible for effects in the flowfield that are not adequately modeled by the 2D DVM.

A Discrete Vortex Method Investigation of Canonical Pitch Ramp-Hold Case

olinearly ramping pitch to hold case was analyzed. Parametric studies were performed to gage sensitivity to various parameters (such as angle of attack at which separation occurs, leading edge separation factor, and number of vortices) to the flow field generated. The FDTC-LRWG Pitch-Ramp-Hold Canonical cases executed by others in the group are used for the purpose of refining the accuracy of the code and are also used in an effort to define the bounds of applicability of this lower order method. After some tuning, the method ultimately proves to be quite versatile and accurate within the constraints of low Reynolds number unsteady flow fields to which is applied here. Nomenclature

Aerodynamics of Vertical-Axis Wind Turbines: Assessment of Accepted Wind Tunnel Blockage Practice

An ongoing investigation into wake and solid blockage effects of vertical axis wind turbines (VAWTs) in closed test-section wind tunnel testing is described. Static wall pressures have been used to derive velocity increments along a wind tunnel test-section which in-turn are applied to provide evidence of wake interference characteristics of rotating bodies interacting within this spatially restricted domain. Vertical-axis wind turbines present a unique aerodynamic obstruction in wind tunnel testing whose blockage effects have not yet been extensively investigated. The flowfield surrounding these wind turbines is asymmetric, periodic, unsteady, separated and highly turbulent. Static pressure measurements are taken along a test-section sidewall to provide a pressure signature of the test models under varying rotor tip-speed ratios (freestream conditions and model RPMs). Wake characteristics and VAWT performance produced by the same vertical-axis wind turbine concept tested at different physical scales and in two different wind tunnels are investigated in an attempt to provide some guidance on the scaling of the combined effects on blockage. This investigation provides evidence of the effects of large wall interactions and wake propagation caused by these models at well below generally accepted standard blockage figures. Nomenclature