The speed at which a slung load can be carried under an aircraft is often limited by the onset of divergent oscillations. Simulations using highly-resolved airloads maps are described, in an effort to determine divergence speed. Airloads are obtained using the Continuous Rotation technique, which converts the discrete-attitude static airload measurements problem to a periodic problem amenable to closed-form analytical description. Free-swing tests of scaled models are performed in a wind tunnel with and without initial perturbations, to capture quantitative and qualitative records from encoders and from video, also capturing the likely modes that amplify. Results are presented for a cuboid and a porous box, the latter with and without one side closed. The roll divergence mechanism, the coupling of roll and yaw frequency, seen in swing tests is clearly observed in simulations run on the cuboid, both when yaw is forced and when the model is free.
A continuous-rotation testing technique is applied to capture the variation of aerodynamic loads with attitude on objects of arbitrary shape. The technique converts the problem of measuring static air loads at various attitudes into a periodic problem. Phase-resolved ensemble-averaging is used to capture load variations with arbitrarily fine azimuthal resolution. The airload variations are obtained in closed form as discrete Fourier series. Experiments on a cylinder model of equal length and diameter were used to study the ability to capture asymmetries, and resolve support interference issues. A closed cuboid is used to correlate with prior work. A flat plate with a central cylindrical load, and a porous box are also studied. Free-swing tests using rigid tethers fixed to a pitch-yaw-roll gimbal mount are used to derive dynamic behavior in a free stream. The cylinder results showed the ability to resolve the effect of minor geometric asymmetries on airloads. The flat plate at 10 degrees pitch shows strong differences in dynamics between cases with a rounded versus squared-off edge facing the freestream. The porous box shows the differences between cases with and without one side blocked.
The Continuous Rotation method enables efficient definition of all aerodynamic load components on bodies of arbitrary shape for arbitrary attitudes. This is applied to several bluff body shapes including cylinders, a cuboid, a flat plate and a porous box. Rate effects and unsteadiness are shown to be negligible using a cylinder of aspect ratio 1. The genesis of the side force on the yawed cylinder, and the differences between rough and smooth cylinders, are derived from comparisons between experiments and diagnostic computations with an unsteady Navier-Stokes solver. Interpolating Fourier coefficients of the azimuthal load variation appears to be viable to generalize loads on cylinders of varying aspect ratio. A large variation is seen for aspect ratio 0.5 to 1, with a more gradual transition to ‘high aspect ratio’ features beyond aspect ratio 2.
Acoustic or electromagnetic shaping in resonators can form thin walled structures from pulverized materials. The technology is applicable to a wide variety of materials, particle shapes, and sizes. While radiation force models adequately capture the transport of particles towards the nodal surfaces where walls form, the actual wall formation process involves complex particle-field and interparticle forces. A finite element computation is used along with pulsed laser particle image velocimetry for air movement, and particle tracking velocimetry for particle movement, to close the gap between predictions and measurements. Non-intrusive force measurement is attempted by deriving the acceleration field of particles, from velocity field data. Results are compared with particle acceleration computed from velocity calculations using the finite element code. The predicted acoustic velocity field from the standing wave pattern in the resonator, is compared with measurements. The difference between the computed particle velocity and the acoustic velocity is used to assess the relative roles of fluid dynamic drag and radiation forces on the acceleration of the particles. The measured particle acceleration does differ substantially and consistently from the computed values, showing the effect of the unmodeled near-field particle-field interaction forces.
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