Erik Sällström

Properties of subsonic open cavity flow fields

Flow over an open cavity was studied for several different subsonic free stream Mach numbers ranging from 0.19 to nearly 0.73. Velocity field information was acquired through an application of particle image velocimetry, while the fluctuating surface pressure was acquired through a linear array of surface pressure sensors. These data were acquired on the centerline of the cavity which had a length to depth ratio of 6 and a turbulent boundary layer upstream of its leading edge. Over the range of free stream Mach numbers the fluctuating surface pressure spectra in the cavity exhibited different behavior ranging from no apparent resonance to resonance being dominated by the second or third Rossiter modes. The broadband levels of surface pressure spectra with strong resonant tones collapse with scaling by the flow dynamic pressure. Velocity measurements reveal that the center of circulation of the flow within the cavity moves from the aft wall towards the center of the cavity with increasing Mach number. The ...

Structural dynamics and aerodynamics measurements of biologically inspired flexible flapping wings

Flapping wing flight as seen in hummingbirds and insects poses an interesting unsteady aerodynamic problem: coupling of wing kinematics, structural dynamics and aerodynamics. There have been numerous studies on the kinematics and aerodynamics in both experimental and computational cases with both natural and artificial wings. These studies tend to ignore wing flexibility; however, observation in nature affirms that passive wing deformation is predominant and may be crucial to the aerodynamic performance. This paper presents a multidisciplinary experimental endeavor in correlating a flapping micro air vehicle wings aeroelasticity and thrust production, by quantifying and comparing overall thrust, structural deformation and airflow of six pairs of hummingbird-shaped membrane wings of different properties. The results show that for a specific spatial distribution of flexibility, there is an effective frequency range in thrust production. The wing deformation at the thrust-productive frequencies indicates the importance of flexibility: both bending and twisting motion can interact with aerodynamic loads to enhance wing performance under certain conditions, such as the deformation phase and amplitude. By measuring structural deformations under the same aerodynamic conditions, beneficial effects of passive wing deformation can be observed from the visualized airflow and averaged thrust. The measurements and their presentation enable observation and understanding of the required structural properties for a thrust effective flapping wing. The intended passive responses of the different wings follow a particular pattern in correlation to their aerodynamic performance. Consequently, both the experimental technique and data analysis method can lead to further studies to determine the design principles for micro air vehicle flapping wings.

Three-Dimensional Averaged Flow Around Rigid Flapping Wings

The flow over a Zimmerman wing flapping to approximately ±45 is investigated using stereoscopic particle image velocimetry. Velocity snapshots are taken at various phases of the wing for 12 different chordwise locations while flapping at 5 Hz. These snapshots are divided into 50 different phases and the flow at the same phase and location is averaged to create a three-dimensional average flow field. The phase-averaged velocity fields are used to calculate the phase-averaged vorticity of which iso-surfaces are presented. At the chordwise location of the wing tip and the two closest chordwise locations, velocity field measurements were also acquired at a flapping frequency of 10 Hz. The wing is then moved in the y-direction to show the flow further away from the wing, and snapshots are taken at both 5 and 10 Hz at the three chordwise locations just mentioned. The phaseaveraged results of the snapshots taken at both y-locations are then combined to show how vorticity is created by acceleration/deceleration of the wing and spread through diffusion and convection.

An Integrated Experimental and Computational Approach to Analyze Flexible Flapping Wings in Hover

Biological flyers exploit wing deformation during flapping flight. There is a substantial need to improve the understanding of the aeroelastic effects associated with the wing deformation to build flapping wing micro air vehicles. This paper presents an effort to develop an integrated approach involving both experimental and computational methods to realize this goal. As the first step, an isotropic flat plate aluminum wing is manufactured and actuated to perform a single degree-of-freedom flapping motion. The wing deformation and airflow around the wing are measured with digital image correlation (DIC) and particle image velocimetry (PIV), respectively. Computational analyses are performed on this wing configuration using a combined nonlinear structural dynamics and Navier-Stokes solution. Reasonable agreement obtained between experimental and computational data in this preliminary effort shows a potential to analyze more complicated flexible flapping wings in future.

Active Rotation and Vibration during the Flapping Stroke of a Micro Elastic Wing for Thrust Production

The goal of this study is to understand the effect of active wing rotation in the aerodynamic performance of micro air vehicle flapping wings. Mimicking biological flight, flapping wing micro (or nano) air vehicles have been successfully developed. However, the flapping wing aerodynamic mechanisms have not been fully unveiled; especially on the topic of wing structure optimization for flapping flight. Such structure optimization is intended to enhance aerodynamic performance by passive wing deformation that results from the coupled inertial and aerodynamic loads. On the other hand, active kinematics can be used to create lift and thrust with stiff wings. Therefore, flapping wing structure can also be tailored with kinematics to achieve optimized efficiency. It has been observed that birds fly with different gaits and insects fly with different stroke patterns at different speeds. Actively controlling the wing rotation and vibration can change the interaction between the wing surface and the surrounding air. Magnetic coil actuators are combined with a one degree-offreedom flapping mechanism to realize the two degree of freedom kinematics up to 15 Hz. Besides synchronizing the rotational actuation with the flapping motion, the actuators are also actuated at much higher frequencies (from 30 to 1000 Hz) to examine the effect of vibration (multiple rotations during one flapping stroke). This would also allow the wing to exhibit dynamic properties during flapping. The results show that magnetic coil actuators can be used to effectively induce wing rotation during flapping and affect aerodynamics; the phase difference between the flapping and rotation motion can increase or decrease aerodynamic performance; and the high frequency structural vibration has insignificant impact on aerodynamics.