A Laser-Interferometer Method for Determining the Forces on a Freely-Flying Model in a Shock-Tunnel

Abstract

SummaryConventional force-balances, “accelerometer balances” and photographic monitoring of freely-flying models are not suitable for use in the Q.M.C. shock tunnel where the force levels are of order 10N for flow durations of about 1 ms. An interferometric method has therefore been developed for following the trajectory of a weakly-restrained model. The prototype system consists of two simple Michelson interferometers, a single He-Ne laser being employed to provide two measurement and two reference beams so that the motion of two points on the model can be followed. The measurement beams are returned by corner-cube retroreflectors carried on the model which ensures that for each measurement arm the reference and measurement beams recombine at the surface of a photo-detector. As the model moves, interference fringes are produced at the detectors, the cycle dark-light-dark corresponding to a model displacement along the measurement beam of ½λ, about 0.3μm. The frequency-modulated wave-trains produced are recorded using two transient recorders, the data being subsequently played back to a two-channel pen recorder giving a record 500 mm in length corresponding to the test time. The fringe number as a function of time is read manually, and the data analysed by curve fitting to a parabola which yields the accelerations of the measurement points. A knowledge of the inertial characteristics of the model then gives the forces on it. By suitably aligning the beams, lift and pitching moment for a ridge-delta of aspect-ratio 1 were obtained. Two models of the same geometric size but of different inertia were tested. All the data were obtained for model displacements less than 1 mm and pitch rotations less than 0.1°.

An accelerometer balance system for measurement of aerodynamic force coefficients over blunt bodies in a hypersonic shock tunnel

“…But the complete measurement system is very cumbersome and moreover it is very difficult to achieve the necessary level of accuracy during measurements. Bernstein et al [7] have used an interferometric system with reflectometers mounted on the model to generate the fringe shifts corresponding to the model movement in the test section. Constructions of the model with reflectometers, difficulties in measurement of drag and model breakage during the run are some of the disadvantages of this force measurement technique.…”