In order to progress in the area of aeroacoustics, experimental measurements are necessary. Not only are they required for engineering applications in acoustics and noise engineering, but also they are necessary for developing models of acoustic phenomenon around us. One measurement of particular importance is acoustic impedance. Acoustic Impedance is the measure of opposition of acoustical flow due to the acoustic pressure. It indicates how much sound pressure is generated by the vibration of molecules of a particular acoustic medium at a given frequency and can be a characteristic of the medium.The aim of the present paper is to give a synthetic overview of the literature on impedance measurements and to discuss the advantage and disadvantage of each measurement technique. In this work, we investigate the three main categories of impedance measurement techniques, namely reverberation chamber techniques, impedance tube techniques, and far-field techniques. Theoretical principles for each technique are provided along with a discussion on historical development and recent advancements for each technique.

A cylinder of aspect ratio 1 experiences a rich mixture of phenomena when yawed through 360 degrees. Understanding this variation is crucial to aerodynamic load definition for objects of practical shape. This paper uses several diagnostics to explain the detailed airload map of cylinders with length to diameter ratios 0.25 to 4 in steady incompressible flow, at Reynolds numbers from 50,000 to 450,000. A sharp linear variation of side force coefficient with yaw is bounded by apparent stall. Drag and pitching moment depend more on the curved surfaces, while side force is dominated by flow over the end plates. Tuft visualization and stereo Particle Image Velocimetry complement 6-DOF loads. Pressure distributions are obtained from velocimetry as well as from computations. The flow over the suction side shows a curved leading edge vortex followed by reattachment and an aft separation. The lift at low yaw is largely due to suction from the zones near the front corner, while at higher yaw it comes from the pressure difference between the two sides. The presence of helical vortices differentiates the lift generation from models based purely on separation.

Prior work showed high-resolution maps of aerodynamic load coefficients for various geometric shapes, as well as parametric variations on canonical models. The problem of obtaining predictions on new shapes using generalized interpolations and combinations of canonical shapes, is explored in this paper. Two cases are considered. The first is that of an engine container used as a helicopter slung load. Full scale flight data provide the dynamics, so that the effects of different uncertainties and simplifications are examined. The progression is from interpolated Fourier coefficient data on an approximate, canonical shape, to dynamic simulation at different levels of fidelity. The second case is one of predicting the aerodynamic load map of a complex practical shape — a road vehicle used as a slung load, starting by combining data on canonical shapes. In both cases, this paper presents predictions using approximations; the future work expects to present wind tunnel results on a scale model of the engine container, and on an actual combination of canonical shapes. The cylinder approximation gives a good prediction of the flight test dynamics history and divergence speed. Adding a drag estimate for the tethers achieves good correlation with the flight test trail angle behavior, with some error apparent at the highest speed. However, this error underpredicts drag, which may be the effect of support structure including flat-plate baffles, not yet modeled. In the second case, a weighted combination of Fourier coefficients for a cylinder and a rectangular prism, gives a good representation of the drag and side force coefficient maps for a HumVee model; correlation of the yawing moment is less accurate.

This paper describes the test methodology and results for a wind tunnel experiment featuring a blended wing aircraft in ground effect with built-in circulation control. A 82.55cm wingspan blended wing model was tested in a subsonic wind tunnel at velocities ranging from 18m/s – 49m/s and corresponding Reynolds numbers ranging from 130k – 350k. Pitch angle was held constant at 0 degrees and the height above the wind tunnel floor was modified to determine lift and drag modification due to ground effect. At a normalized height (y/bw) of 0.06, ground effect increased lift production by 24% and reduced drag by 22% when compared to a normalized height of 0.5. The addition of the circulation control significantly increased the lift production of the model at a cost of increased drag. At a normalized height of 0.031, the lift production increased by 200% at a blowing coefficient of 0.01, but the drag also increased by 72%, ultimately increasing L/D by 178%. Experimental results also suggest that ground effect and circulation control have a synergistic effect when used simultaneously. The effects of Reynolds number and circulation control slot height are also investigated.

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