Unsteady airfoil experiments

International Journal of Micro Air Vehicles, 2010

This paper presents recent progress in a continuing investigation of the aeromechanical aspects of unsteady flapping wings for micro air vehicles (MAV). Numerical simulations were performed for two-dimensional (2D) pitching-plunging airfoils and three-dimensional (3D) flapping wings, mainly at hover conditions, using an in-house code called INSflow. The results were compared with available experimental data obtained in the water tunnel at the NRC-IAR. The investigation revealed that, at hover conditions, the vortices formed during the airfoil plunging motion may remain near the airfoil and affect new vortex formations, and thus the integral aerodynamic performance. In addition, the flow around the 3D insect-like wing is fully three-dimensional. The tip flow affects the flow separation, reducing the separation intensity. Two-dimensional calculations may over-predict the separation and the shedding vortices, thus affecting the generation of aerodynamic forces.

EPJ Web of Conferences, 2013

In order to study the forces acting on a flapping wing, an experimental investigation is performed in steady water flow. In this study, a SD7003 airfoil undergoes combined pitching and plunging motion which simulates the forward flight of small birds. The frequency of pitching motion is equal to the frequency of plunging motion and pitch leads the plunge by a phase angle of 90 degrees. The experiments are conducted at Reynolds numbers of 2500 ≤ Re ≤ 13700 and the vortex formation is recorded using the digital particle image velocimetry (DPIV) technique. A prediction of thrust force and efficiency is calculated from the average wake deficit of DPIV data, the near-wake vorticity patterns and time dependent velocity vectors are determined to comment on the thrust and drag indication. Direct force measurements are attempted using a Force/Torque sensor which is capable of measuring forces and moments in three axial directions.

2005

2-D and 3-D flows over flapping airfoils are computed, and thrust generation and propulsive efficiencies are compared. The flapping motion of the airfoils is described by a combined harmonic plunge and pitch motion. The flows are computed for flapping motions which produce maximum thrust and propulsive efficiency, predicted by a gradient based optimization algorithm using a 2-D flow solver. 2-D and 3-D flow computations are both performed in parallel. PVM and MPI message passing libraries are used. 3-D simulations resulted in slightly higher thrust and efficiency values than 2-D predictions. Both 2-D and 3-D numerical simulations show that high thrust may be obtained at the expense of reduced efficiency. For high propulsive efficiency, the large scale formations at the leading edge are prevented.

Journal of Fluids Engineering

The effect of nonsinusoidal trajectory on the propulsive performances and the vortex shedding process behind a flapping airfoil is investigated in this study. A movement of a rigid NACA0012 airfoil undergoing a combined heaving and pitching motions at low Reynolds number (Re = 11,000) is considered. An elliptic function with an adjustable parameter S (flattening parameter) is used to realize various nonsinusoidal trajectories of both motions. The two-dimensional (2D) unsteady and incompressible Navier–Stokes equation governing the flow over the flapping airfoil are resolved using the commercial software starccm+. It is shown that the nonsinusoidal flapping motion has a major effect on the propulsive performances of the flapping airfoil. Although the maximum propulsive efficiency is always achievable with sinusoidal trajectories, nonsinusoidal trajectories are found to considerably improve performance: a 110% increase of the thrust force was obtained in the best studied case. This im...

25th AIAA Aerodynamic Measurement Technology and Ground Testing Conference, 2006

The unsteady flow field of a flapping-and-pitching flat plate wing of elliptic planform at a nominal Reynolds number, Re c = 4100 and reduced frequency, k = 0.138, was studied using flow visualization. Hydrogen bubbles and dye were employed as tracers. The study reveals many vortical flow features. The trailing edge vortex shows very interesting dynamics immediately following pitching at stroke reversal. A leeward surface spanwise flow towards the wing tip immediately following stroke reversal is observed. A vortex trapping model is proposed to explain enhanced lift observed in insects. The study has applications in flow control and micro air vehicle design and development.

Animal Locomotion, 2010

We consider a combined experimental (based on flow visualization, direct force measurement and phaseaveraged 2D particle image velocimetry in a water tunnel), computational (2D Reynolds-averaged Navier-Stokes) and theoretical (Theodorsen's formula) approach to study the fluid physics of rigid-airfoil pitch-plunge in nominally two-dimensional conditions. Shallow-stall (combined pitch-plunge) and deep-stall (pure-plunge) are compared at a reduced frequency commensurate with flapping-flight in cruise in nature. Objectives include assessment of how well attached-flow theory can predict lift coefficient even in the presence of significant separation, and how well 2D velocimetry and 2D computation can mutually validate one another. The shallow-stall case shows promising agreement between computation and experiment, while in the deepstall case, the computation's prediction of flow separation lags that of the experiment, but eventually evinces qualitatively similar leading edge vortex size. Dye injection was found to give good qualitative match with particle image velocimetry in describing leading edge vortex formation and return to flow reattachment, and also gave evidence of

Journal of Fluids and Structures, 2011

The flow field of a flapping airfoil in Low Reynolds Number (LRN) flow regime is associated with complex nonlinear vortex shedding and viscous phenomena. The respective fluid dynamics of such a flow is investigated here through Computational Fluid Dynamics (CFD) based on the Finite Volume Method (FVM). The governing equations are the unsteady, incompressible two-dimensional Navier-Stokes (N-S) equations. The airfoil is a thin ellipsoidal geometry performing a modified figure-of-eight-like flapping pattern. The flow field and vortical patterns around the airfoil are examined in detail, and the effects of several unsteady flow and system parameters on the flow characteristics are explored. The investigated parameters are the amplitude of pitching oscillations, phase angle between pitching and plunging motions, mean angle of attack, Reynolds number (Re), Strouhal number (St) based on the translational amplitudes of oscillations, and the pitching axis location (x/c). It is shown that these parameters change the instantaneous force coefficients quantitatively and qualitatively. It is also observed that the strength, interaction, and convection of the vortical structures surrounding the airfoil are significantly affected by the variations of these parameters.

43rd AIAA Aerospace Sciences Meeting and Exhibit, 2005

To investigate the mechanism of thrust generation by flapping airfoils, the inviscid version of a three-dimensional unsteady compressible Euler/Navier Stokes flow solver is used to simulate the flow field around flapping airfoil NACA 0012 at low speeds. Sinusoidally plunging or/and pitching oscillations are studied. The compressible Euler code with a low free-stream Mach number of 0.1 or 0.05 can simulate the incompressible flows without leading-edge separation. The wake vortex structures are visualized by numerical methods: vorticity filled-contours, perturbation-velocity vector plots, and streamlines. The computed wake-vortex structures almost coincide with known low-speed test results. The time-averaged thrust coefficients, input-power coefficients, and the efficiency over a period of the oscillation versus the Strouhal number based on the total excursion of the trailing edge of the airfoil for a number of combined plunging and pitching cases agree well with the linear and non-linear incompressible potential-flow solutions in the literature. In comparison with known test data, the accuracy of the computed forces decreases when the leading-edge vortices appear and become strong enough to interfere with the trailing-edge vortices.

2015

The wake of a NACA 0012 airfoil oscillating sinusoidal about its aerodynamic center in a general co-flow generates a well-known characteristic vortex street. The introduction of vorticity into the flow in a controlled way could be used as a means to either enhance or dampen any large-scale motion present. The oscillating aerofoil can then be seen as the actuation component of an active control system. To investigate the potential of using a flapping wing as an actuation device the flow field that it generates first needs to be characterized. This can then be used to affect a fluctuating flow field, which in this case is the vortex field generated by an identical oscillating airfoil downstream from the first. Flow visualization coupled with particle image velocimetry is used to investigate and characterize the flow field from both a single oscillating airfoil and the interacting flow field between two tandem airfoils. The relative phase between the two wings is adjusted to investigat...

Notes on Numerical Fluid Mechanics and Multidisciplinary Design (NNFM), 2005

The laminar flow around a NACA 4402 airfoil is investigated at a Reynolds number of Re = 6000 using the RANS solver FLOWer. Significant flow separation occurs over almost the whole regime of angles of attack for steady onset flow conditions. Therefore, the calculated propulsive efficiencies of pure plunge motions are rather poor. This leads to the need of a combined plunge and pitch motion, which is investigated subsequently. For combined motions, cases with high propulsive efficiencies are found over a wide range of produced thrust.