Sidaard Gunasekaran

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.

Errors in Off-axis Loading of Off-the-shelf 6-Component Force Transducers: A Cautionary Tale

Significant, repeatable errors can result when loading off-the-shelf 6-component force transducers at relatively short distances off-axis. The availability of low-cost high-precision 6-component force transducers has resulted in their wide adoption in aerodynamic testing of small unmanned aircraft and their propulsion systems. Companies such as ATI Industrial Automation (www.ati-ia.com) offer a comprehensive range of force transducers. These transducers are robust, have excellent overload protection, provide good frequency response, weak temperature sensitivity (and integral compensation), are available with an array of different interface options (Ethernet, Differential Voltage), have NIST traceable calibrations, and many are available in waterproof form. For the most part, these sensors have evolved from sensing elements in robotic end-effectors and as such are extremely accurate when the applied loads are in close proximity to the sensor face and axially aligned with the transducer center. This paper will describe an instance where such axial alignment is not possible. The endeavor of loading the sensor off of the balance center resulted in error in force and significant error in torque. As a result, a number of permutations of off-axis loading were investigated to better elucidate the cause of the error. The magnitude of the measurement error provides substantial incentive to produce an independent sensor interaction matrix when test circumstances dictate similar off balance center loading.

Trends in Early Vortex Formation on a Wall-to-wall Plate in Pure Plunge

Discernible trends in early vortex formation have been in inter vortex-plate distance, circulation, and vortex maximum azimuthal velocity when plunging a wall-to-wall flat plate at fixed angles of attack ranging from 15 to 90 with two different accelerative profiles. Vortex formation and shedding continues to play an important role in unsteady aerodynamics with applications ranging from flapping wings to maneuvering flight to helicopter rotors. Above a relatively low angle of attack when a flat plate is plunged in a fluid a leading edge vortex (LEV) is formed close to the leading edge and a trailing edge vortex (TEV) is formed close to the trailing edge. The formation, growth and convection of the LEV and TEV strongly influence the pressure field surrounding the flat plate and ultimately the forces experienced by the plate. Experiments were performed at the United States Air Force Research Labs Horizontal Free Surface Water Tunnel (AFRL/HFWT) with linear and sinusoidal acceleration profiles. The formation of the LEV was investigated for both acceleration profiles using Particle Image Velocimetry (PIV). Trends were identified in both the LEV distance to the plate as a function of convective distance and with angle of attack. Similarly, trends were identified in maximum vortex azimuthal velocity. The LEV normalized azimuthal velocity profiles were compared with several vortex models in the literature. The existing models were unable to reproduce the asymmetric azimuthal velocity distributions resulting from vortex proximity to the plate. A new model based on experimental results is proposed for the LEV core azimuthal velocity distribution inclusive of plate proximity effects.

Wing Performance Insight from the Self-Preserved Turbulent Wake

The self-preserved turbulent wake behind an airfoil is explored to determine the extent to which wing performance information can be extracted. The turbulent wake 10 chord lengths downstream of an SD7003 wall to wall airfoil model is analyzed through the lens of the irreversibilities in the flow. The concept of Exergy is used to quantify these irreversibilities. The variation in the [surrogate] eddy viscosity was previously obtained from mathematical models derived for wake identification studies by the authors. These models are developed further here to determine the Exergy destruction rate in the turbulent wake. To validate the model, the velocity field experimentally obtained by Particle Image Velocimetry in the wake of a wall-to-wall SD7003 airfoil is used to calculate the exergy destruction rate through the finite difference technique. The Exergy destruction rate is mathematically modeled as a function of the coefficient of drag and other initial conditions of the wing (thickness, chord, etc.) through the momentum-deficit model. A relationship between the Reynolds stress and exergy destruction rate was derived based on the eddy viscosity model. The results from the exergy destruction rate model are validated against the exergy destruction rate obtained from experimental data and they show good agreement.