Environmental aerodynamic phenomena including sonic boom and wake turbulence are usually high-speed phenomena expanding in large sight of view in the field. Researchers have investigated these phenomena by analog experiments in laboratories. Some of characteristics being strongly related with inertial have been well reproduced. However, some of them being strongly related with viscosity have still remained to be improved. The best way to solve these problems is to measure sub/real-scale phenomena in the field.
To investigate life time of wake turbulence induced by a heavy aircraft taking off and landing at an airport is significantly important to estimate minimum separation distance among aircrafts departing or arriving congested airport. Recent studies employ Laser-Detection and Ranging (LIDER) to observe structure, intense, and life-time of wake turbulence on a runway. The researchers using LIDER for wake turbulence detection need improvement of spatial resolution because maximum spatial resolution provided by LIDER is currently about 10 meters. At least, less than one meter resolution is required to determine detail structure of wake turbulence including wing-tip vortex.
Several flow visualization methods based on shadowgraph and schlieren have been developed for large-scale experiments including explosions or free flight of high-speed projectiles. Most visualization methods are able to visualize just shape of the shock front induced by interested phenomena. Theoretically, the quantitative visualization including interferometry would be possible to carry out in the field. But practically, it is difficult to obtain high quality images because of significantly strong environmental disturbances including weather.
Background-oriented schlieren (BOS) has been proposed by Raffel and been applied for fluid measurements for a decade. Technically, BOS needs the background with pattern printed and recording device with an imaging lens to visualize the phase object in front of the background. BOS has two step procedures; first step is to record reference image of the background before appearing the target phenomena, the second after appearing. After recording, distribution of displacement of the background due to phase object will be determined by digital image analysis based on cross-correlation function. In the case of two-dimensional or axisymmetric phenomena, the displacement of the background is directly related to the density profile of the phenomena in test section. Therefore, BOS has potential capability to detect density profile of large-scale aerodynamic phenomena propagating at high-speed in the field without optics which needs fine optical alignment.
Although BOS has advantages against ordinary quantitative flow visualization methods for application for field experiments, accuracy dependence of the angle of sight needs to be clarified because non parallel light is employed for field measurement. In this research, as the first step of applying BOS for measuring large-scale phase phenomena in the field, the authors intend to clarify the accuracy of BOS measurement for propagating high-speed phenomena with large angle of sight.
