Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session L23: Low Re Number Aerodynamics |
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Chair: Geoffrey Spedding, University of Southern California Room: 326 |
Monday, November 21, 2011 3:35PM - 3:48PM |
L23.00001: Spanwise drag variation on low Re wings -- revisited Shanling Yang, Geoffrey Spedding Aerodynamic performance measurement and prediction of airfoils and wings at chord Reynolds numbers below $10^5$ is both difficult and increasingly important in application to small-scale aircraft. Not only are the aerodynamics strongly affected by the dynamics of the unstable laminar boundary layer but the flow is decreasingly likely to be two-dimensional as Re decreases. The spanwise variation of the flow along a two-dimensional geometry is often held to be responsible for the large variations in measured profile drag coefficient. Here we measure local two-dimensional drag coefficients along a finite wing using non-intrusive PIV methods. Variations in $C_{d} (y)$ can be related to local flow variations on the wing itself. Integrated values can be compared with force balance data, and the proper description of drag components at low Re will be discussed. [Preview Abstract] |
Monday, November 21, 2011 3:48PM - 4:01PM |
L23.00002: Exploration of the Relationship Between Wake Vortex Parameters and Thrust Force on Oscillating Airfoils Using a Vortex Array Model Ahmed Naguib, Manoochehr Koochesfahani Recently, we demonstrated the ability of a simple model, based on an array of finite-core Gaussian vortices, to accurately reproduce the unsteady velocity field in the wake of, and drag/thrust force acting on harmonically/non-harmonically pitching airfoils. In the present work, this model is employed to explore how the thrust force varies with wake vortex parameters; i.e. circulation, core radius and streamwise/cross-flow spacing of the vortices. Insight from this investigation will be helpful to draw links between trailing-edge flexibility and the detailed process of generation of wake vortices. Such links may have the potential for providing a path towards a rational, yet efficient, approach for tailoring trailing-edge flexibility to obtain desirable force characteristics for flapping-wings Micro Air Vehicles. [Preview Abstract] |
Monday, November 21, 2011 4:01PM - 4:14PM |
L23.00003: Vortex arrangement in the wake of rigid and flexible rapidly pitching airfoils at low Reynolds number Bruno Monnier, Ahmed Naguib, Manoochehr Koochesfahani An experimental investigation of the wake of an airfoil undergoing rapid pitch oscillation is conducted in a water tunnel at a chord Reynolds number of about 2000. ~Flow visualization is utilized to characterize the vortical patterns in the wake of the airfoil, which is constructed from a NACA 0036 profile fitted with an extended trailing edge with controllable flexibility. The spatial configuration of the vortices is extracted in terms of streamwise and cross-flow spacing over a range of pitching frequencies and amplitudes. We discuss how different levels of flexibility alter the vortex spacing parameters and the conditions under which the traditional Karman vortex pattern, corresponding to a wake profile, changes to the reverse Karman pattern associated with a jet profile. [Preview Abstract] |
Monday, November 21, 2011 4:14PM - 4:27PM |
L23.00004: Effects of fluid behavior around low aspect ratio, low Reynolds number wings on aerodynamic stability Matthew Shields, Kamran Mohseni The innovation of micro aerial vehicles (MAVs) has brought to attention the unique flow regime associated with low aspect ratio (LAR), low Reynolds number fliers. The dominant effects of developing tip vortices and leading edge vortices create a fundamentally different flow regime than that of conventional aircraft. An improved knowledge of low aspect ratio, low Reynolds number aerodynamics can be greatly beneficial for future MAV design. A little investigated but vital aspect of LAR aerodynamics is the behavior of the fluid as the wing yaws. Flow visualization experiments undertaken in the group for the canonical case of varying AR flat plates indicate that the propagation of the tip vortex keeps the flow attached over the upstream portion of the wing, while the downstream vortex is convected away from the wing. This induces asymmetric, destabilizing loading on the wing which has been observed to adversely affect MAV flight. In addition, experimental load measurements indicate significant nonlinearities in forces and moments which can be attributed to the development and propagation of these vortical structures. A non-dimensional analysis of the rigid body equations of motion indicates that these nonlinearities create dependencies which dramatically change the conventional linearization process. These flow phenomena are investigated with intent to apply to future MAV design. [Preview Abstract] |
Monday, November 21, 2011 4:27PM - 4:40PM |
L23.00005: The Effects of AR on Membrane Wing Performance in Low Re Flight Alex Jordan, James Hubner There is increased interest in the design of micro air vehicles (MAVs) due to their military reconnaissance and surveying capabilities. Research has shown that the use of membrane wings in low Reynolds number flight results in performance characteristics that, when compared to rigid wing counterparts of similar geometry, are beneficial. An experimental study was performed to determine if the benefits of membrane wings change when AR is decreased. The membrane wings used silicon rubber affixed to aluminum frames of repeated cell geometry. The wings tested employed 1, 3, 5 and 9 cells and had ARs of 0.9, 2.6, 4.1, and 4.33 respectively. Measurements of lift and drag at a Reynolds number of 49,000 were acquired over a range of angles of attack. Vibration frequencies of the membranes were obtained via high-speed imagery. Comparisons of lift and drag data for the flat plates and membrane wings showed that the membrane wings with ARs of 0.9 and 2.6 did not show the same performance benefits as the higher AR membrane wings. [Preview Abstract] |
Monday, November 21, 2011 4:40PM - 4:53PM |
L23.00006: Volumetric Three-Component Measurements of the Flowfield around Periodically Cambered Plates Redha Wahidi, Zheng Zhang, James Hubner, Amy Lang The small size and low speed of the micro aerial vehicles (MAVs) place them in the low Reynolds number (Re) regime. The performance of conventional airfoils severely deteriorates at low Reynolds numbers. Therefore, unconventional wing designs are necessary to meet the operational requirements of the MAVs. Periodically cambered plates with an aspect ratio of five (AR = 5) were constructed for flowfield and performance measurements. Three-dimensional three-component (3D3C) flowfield measurements were carried out at a Reynolds number of 28,000 at different angles of attack. The locations of separation, transition and reattachment are estimated based on the mean velocities and Reynolds shear and normal stresses in the streamwise, spanwise and wall-normal directions. The effects of the tip vortices on the three-dimensionality of the reattachment line are resolved by the 3C3C measurement technique. Additionally, the effects of the cell size on the separation and reattachment locations are investigated. The results also include details about the vorticity about the X, Y and Z axes. [Preview Abstract] |
Monday, November 21, 2011 4:53PM - 5:06PM |
L23.00007: Dynamics of Spanwise Vorticity on a Rotating Flat Plate Craig Wojcik, James Buchholz Leading-edge vortex (LEV) structures were examined using phase-locking digital particle image velocimetry for two rotating flat plates with aspect ratios of 2 and 4. The plates were accelerated uniformly for approximately 45\r{ } to a constant rotational speed. Reynolds numbers of 4,000, 8,000, and 16,000 were investigated. The flow field was measured in chordwise planes at two spanwise positions, while varying angle of attack and azimuthal position of the plate. The results show a concentrated, stationary vortex structure at the leading-edge of the plate. The strength of the leading edge vortex (LEV) was found to vary with azimuthal position, Reynolds number, and angle of attack. The time variation in LEV strength is studied using a control volume analysis taking into account in-plane and out-of-plane vorticity fluxes as well as interactions with opposite-sign vorticity generated beneath the LEV on the plate surface. [Preview Abstract] |
Monday, November 21, 2011 5:06PM - 5:19PM |
L23.00008: Vortex Shedding vs. Thrust Production for Free-to-Pivot Flat Plates Michael Ol, Kenneth Granlund As an abstraction of flapping-wings, we consider flat plates in rectilinear motion, with the leading edge undergoing periodic oscillation, and the plate left free to pivot about its leading edge, between incidence angle limits of $\pm $45\r{ }. Measurements include thrust production and resistive force, with leading edge and trailing edge vortices visualized by dye injection, conducted in a water tunnel operated here as a towing tank. Imposed acceleration of the plate's leading edge produces a rotational motion followed by a translational phase at constant incidence angle, and a reverse rotation at the semi-stroke extremum. Varying aspect ratio from 3.4 to nominally 2D, neither thrust nor resistive force evince an aspect ratio dependency. Reynolds number does not effect flow development or force production across 5000 $<$ Re $<$ 25000. Investigating the conjecture that imposed acceleration stabilizes the leading edge vortex, we find no difference across a range of acceleration profiles. The dominant parameter affecting thrust production is the plate stroke to chord ratio, with values of $\sim $6 and above being most favorable. Further, as a simplification of aeroelastic effects conducted with otherwise rigid plates, we consider a plate sliced spanwise and thus forming two hinged plates. This produces both lower thrust and resistive force than in the single-plate case, resulting in no improvement in hovering figure of merit, which amongst all cases peaks at $\sim $ 0.3. [Preview Abstract] |
Monday, November 21, 2011 5:19PM - 5:32PM |
L23.00009: Fluid Structure Interactions on an Electro-Elastomer Membrane Wing Lawrence Ukeiley, Adam Hart, Michael Hays, William Oates, Benjamin Dickinson Wing flexibility is an important aspect of many natural flyers that has been demonstrated to have several specific aerodynamic benefits. An engineering abstraction of flexibility in wings is to use an extensible membrane stretched over a rigid frame. A key aspect of a membrane wing's performance is its tension as that will dictate how far it can stretch and its natural vibrational frequency. In this work we use a dielectric elastomer membrane, whose ability to stretch is a function of a voltage applied across it. The membrane is adhered to an elliptical planform wing which was placed in the freestream of an open jet wind tunnel. Measurements of the aerodynamic performance, membrane shape and flow field over the wing have been acquired as a function of angle of attack and voltage across the elastomer independently as well as synchronously. These measurements demonstrate how the membrane's characteristics alter the flow over it and translate to the generation of aerodynamic forces. These characteristics are studied in terms of both the static deflection increasing the wings camber as well as the time dependent excitation through the membrane's vibrations. [Preview Abstract] |
Monday, November 21, 2011 5:32PM - 5:45PM |
L23.00010: Fluid and Structure Dynamics of Flow over a Membrane Wing Amory Timpe, Lawrence Ukeiley The coupled effect of flow induced membrane deformations and their return influence on the flow are investigated. Multi-cell wings are made by adhering heated, thin Silicone membranes to 2.7 percent thick, rectangular aluminum frames with rigid leading edge and battens. Time-resolved flow and structure deformations are measured by synchronized acquisition of high-speed, two-component Particle Image Velocimetry (PIV) and stereoscopic Digital Image Correlation (DIC). Mean and instantaneous effects are studied at a chord based Reynolds number of 48,000, while metrics are compared for membrane cell size of aspect ratio 1 and 0.5. Two PIV measurement fields of view are employed to study flow over the membrane surface as well as that of the near wake. Power spectral density and correlation techniques will be utilized, along with analysis of membrane mean deformation and rms fluctuation behavior to better understand the fluid-structure interactions. The effects of membrane behavior on flow separation, shear layer size and location, along with vorticity will be analyzed in comparison to flow over a similar geometry flat plate. [Preview Abstract] |
Monday, November 21, 2011 5:45PM - 5:58PM |
L23.00011: Falling Maple Seed: The mechanism for its spin Z. Jane Wang, Kapil Varshney, Song Chang A maple seed falls in a characteristic downward spiral. One explanation of its spinning motion is the auto-rotational effect. As the seed falls, the aerodynamic torque on the feather will cause it to spin like a wind-mill. However, the film of a falling seed shows that its spinning motion precedes the steady descent. And surprisingly, the seed with little feather can also spin. We experimentally quantify the kinematics of falling seeds with different shapes. The analysis of the coupling between the rigid body dynamics and the aerodynamics offers a new explanation for the cause of the spin. [Preview Abstract] |
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