Optical techniques are preferred for investigating flow phenomenon because of its non-intrusive and non-inertial properties. Such techniques can provide qualitative as well as quantitative information of flows in a large field of view with reasonable sensitivity. There exists optical techniques such as Schlieren and Shadowgraphy for qualitative investigation of flows whereas Interferometry, BOS, Shadow-casting can provide quantitative information. The optical data i.e. fringes in Interferometry, dots in BOS and Shadow-casting carries information of the flow through which light has traversed through. The optical information is recorded by using CCD/CMOS camera and is processed later for further investigations.
The phase (φ), of light wavefront (Aejφ) exiting from a phase object (flow in this case) changes. Camera image sensors are intensity detectors which can’t record the phase information, therefore experimental arrangements are made such that the phase variation is equivalently encoded as intensity distribution. The conversion of phase to intensity is usually achieved with the help of random pattern in BOS and Shadow-casting, fringes in Interferometry, knife edge/color filters in Schlieren etc.
In this manuscript a method is proposed which exhibits and utilizes the inherent property of the camera sensors (ability to capture intensity only). In this scheme light is allowed to traverse through the phase object and is directly recorded with the help of cameras without any knife edge or pattern. For investigation of flows, a set of cameras are used to capture the steady state flow images by focusing the cameras at different planes on the optical axis perpendicular to flow direction. The images are processed for complete recovery of phase variation due to presence of flow. The calculated phase is further used for recovery of density distribution.
Experiments are conducted in hypersonic shock tunnel facility (HST2) and results obtained from this method are included in the full manuscript.
