Yeon Sik Baik

Experimental Investigation of Pitching and Plunging Airfoils at Reynolds Number between 1x10^4 and 6x10^4

An experimental investigation was performed on a nominally two-dimensional pitching and plunging SD7003 and flat plate at Reynolds number 1 × 10, 3 × 10, and 6 × 10. The experiment was conducted at the University of Michigan water channel facility using phaseaveraged particle image velocimetry (PIV) technique to quantify the flow field. Two sets of airfoil kinematics were used in this study; a combined pitching and plunging motion, and a pure plunging motion. The flow topology and wall velocity profiles from the PIV measurements showed a Re dependence on a pitching and plunging SD7003 where the extent of flow separation is reduced at a relatively high Re. On the contrary, flat plate displayed a large leading edge separation flow characteristic that was independent of Re. For both airfoil cross-sections used in the experiment, turbulence statistics indicated laminar to turbulent transition phenomena at low Re. The study shows the leading edge shape effect on the flow transition and separation characteristics. A pure plunging motion of SD7003 and flat plate at Re = 60,000 showed the formation of the leading and trailing edge vortices. In addition, a quantitative analysis showed an apparent phase lag present on SD7003 relative to the flat plate. In order to validate the experimental data, a flow comparison between the University of Michigan and AFRL was performed.

Fluid Dynamics of Pitching and Plunging Flat Plate at Intermediate Reynolds Numbers

A combined numerical and experimental study of two- and three-dimensional pitching and plunging flat plates at Reynolds numbers of O(104) is presented. The focus of this paper is the interplay between the geometry, kinematics, Reynolds numbers, and three-dimensional effects with the resulting aerodynamic forces and flow structures. A shallow-stall and a deep-stall motion with higher maximum effective angles of attacks are considered. Under both kinematic motions, massive leading-edge separation is observed at the sharp leading edge of the flat plate. This geometric effect is seen to dominate over other viscosity effects, and the Reynolds number dependence is limited. Compared with the blunter SD7003 airfoil, where the flow is mostly attached for the shallow-stall motion at Re=6×104, the sharper leading edge of the flat plate leads to earlier and stronger leading-edge vortex formation and greater lift and drag. Finally, the presence of a tip vortex significantly reduces lift generation during the downstrok...

Fluid Dynamics of Pitching and Plunging Airfoils of Reynolds Number between 1×10 4 and 6×10 4

We consider a combined experimental (two-dimensional particle image velocimetry in a water tunnel) and computational (two-dimensional Reynoldsaveraged Navier-Stokes) investigation to examine the effects of chord Reynolds number on the dynamics of rigid SD7003 airfoil undergoing pitching and plunging motion in nominally two-dimensional conditions. Appreciable qualitative distinction in a moderately dynamically-stalled case in going from Re = 1×10 4 to Re = 6×10 4 was observed, suggesting nontrivial impact of viscosity even in conditions of strong forcing by motion kinematics. Additionally, computed lift coefficient time history is compared with Theodorsen’s unsteady linear airfoil theory. The velocity and vorticity fields were in excellent agreement between experiment and computation for those phases of motion where the flow was attached; moderate agreement was achieved when the flow was separated. The small disagreements were consistent with the expected inaccuracies due to the turbulence model used. Similarly, Theodorsen’s theory was able to predict the computed lift coefficient quite well when the flow was attached, and moderately acceptable otherwise.

Experimental study of governing parameters in pitching and plunging airfoil at low reynolds number

2. Graduate Research Assistant, University of Michigan, Department of Aerospace Engineering, rauschjm@umich.edu 3. Associate Professor, University of Michigan, Department of Aerospace Engineering, lpb@umich.edu 4. Clarence “Kelly” Johnson Collegiate Professor and Chair, University of Michigan, Department of Aerospace Engineering, weishyy@umich.edu 5. Aerospace Engineer, Air Vehicles Directorate, Wright-Patterson AFB, Michael.Ol@wpafb.af.mil Experimental Study of Governing Parameters in Pitching and Plunging Airfoil at Low Reynolds Number

Unsteady Force Generation and Vortex Dynamics of Pitching and Plunging Flat Plates at Low Reynolds Number

An experimental investigation of pitching and plunging flat plate at a prescribed effective angle of attack in the St range of 0.16 to 0.32 and k range of 0.5 to 1.0 is presented. Force measurements and PIV technique were used to analyze and quantify the unsteady flow field created by the wing kinematics. A momentumbalance based non-intrusive force measurement technique and a systematic vortex detection algorithm were successfully implemented as a post-processing for PIV data to gain insight on the interplay between the unsteady force generation and vortex dynamics. It was determined that the inertial effects, which are proportional to the St, dominate the unsteady force generation while the differences in the force generation for the same St kinematics are contributed by the circulatory effects such as the size and location of the leading edge vortex. The normal force with respect the flat plate was the primary contributor to the lift and drag production, and the projection of the normal force due to high pitch amplitude at higher St prompted higher thrust generation.

Aerodynamics of a Successful Perching Maneuver

An experimental study of a perching maneuver performed by a flat plate at Re – 35,000 is presented in this paper. The autonomous perching maneuver of a UAV that consists of a pitch-up motion at moderate pitching rate is used in the study. Experiments were conducted at the University of Michigan’s low-turbulence water channel facility in two different phases. The first phase was flow visualization by dye injection. The second phase was flow field calculation using 2D phase-averaged particle image velocimetry (PIV). The behavior of the flow field was studied at 50% and 75% span-wise locations during each phase. The formation of a separation bubble followed by the shedding of the trailing edge vortex (TEV) is observed. Vorticity is introduced at the leading edge of the flat plate, and can be traced downstream and observed in the recirculation zone.

Effect of Aspect Ratio on Rigid Lifting Flat Plates in Pitch- Plunge Motion at Low Reynolds Numbers

Flowfield behavior of wall-to-wall and low aspect ratio (AR=2) rigid flat plates in pi tching and plunging motion is studied in a water tunnel at low Reynolds numbers. All models have a 152mm chord and t/c = 2.25% . Two sets of kinematics already wellestablished in the recent literature are used: combined pi tch and plunge, where pitch angle of attack partially cancels plunge-induced angle of attack, and pure plunge. Three Reynolds numbers (1x10, 3x10, and 6x10) are considered, using dye flow visualization and particle image velocimetry (PIV). Broadly, sectional vorticity contour plots reveal similarity between the nominally 2D and AR=2 plates, but there are several interesting exceptions. The AR=2 plate exhibited a marked axial flow when the effective angle of attack was near its maximum. For the pi tching and plunging motion the AR=2 plate showed open separation at phase of 180°, while the nominally two-dimensional model has closed separation throughout the downstroke. For validation of the PIV results, a comparison is made between Reynolds number 3x10 and 4x10 three-dimensional models at the University of Michigan and Air Force Research Laboratory (AFRL), respectively.

Fluid Dynamics of Rigid and Spanwise-Flexible Elliptical Flat Plates at Low Reynolds Numbers

Presented here is an experimental investigation of sinusoidal plunging rigid and flexible elliptical flat plates at Reynolds number 5. 3 × 103. The sinusoidal plunging motion is characterized by the normalized plunge ampli tude 0.175, and the Strouhal number 0.203. The investigation varies flexibility by varying the Π1-parameter by varying the thickness of the models while maintaining the planform shape, the root chord 79 mm, and the s pan 241 mm. A range of Π1 was used from 30 to 90,240. Dye flow visualization was performed using a 7-pronged rake oriented in the plane of the chord. Two camera views are used for dye flow visualization, one is normal to the plane of the chord and the other is perpendicular to the plane’s normal. Structural deformations were performed with Laser Doppler vibrometry (LDV). Deformations were sampled at the quarter and three-quarter chord location at 0% , 25% , 50% and 75% span. Particle image velocimetry (PIV) measurements have been performed at 50% and 75% span at 30° phase increments of the motion. Results show that a flexibility effect is only observed at Π1 = 30. The flexibility effect is marked by bending (40% of motion’s root amplitude at 75% span) and a phase lag (60° phase at 75% span).

Modeling of pitching and plunging airfoils at Reynolds number between 1×10 4 and 6×10 4

Fluid physics associated with a pitching and plunging airfoil, while critical to the development of flapping wing air vehicles, is not adequately understood. To help assess the state-of-the-art of engineering predictive tools, we utilize recently obtained experimental information based on particle image velocimetry (PIV) in a water tunnel from two different facilities to examine the effects of chord Reynolds number, and the airfoil shape on the associated flow structures. Two rigid airfoils, SD7003 and flat plate, undergoing pitching and plunging motion in nominally two-dimensional conditions are investigated with the aid of the original Menter’s Shear Stress Transport (SST) turbulence model and a modified version which limits the production of turbulence kinetic energy to reduce the build-up of turbulence in stagnation regions. We consider two kinematic schemes, a pitching and plunging, and a pure plunging motion. For the SD7003 airfoil under pitching and plunging motion, the original SST model offers consistently favorable agreement with both PIV measurements. For the pure plunging SD7003 airfoil case, depending on the turbulence characteristics including those caused the motion of the wing, and the implied eddy viscosity level, qualitatively different flow structures are observed experimentally and computationally. The flat plate creates flow fields insensitive to the Reynolds number, and quite different from those around the SD7003 airfoil, due to the leading edge effect.