Accuracy assessment of optical 3D measurements in hydrodynamic tunnel

Abstract

Hydrodynamic tunnel is an effective mean for studying wing flow process in aerodynamics and hydrodynamics. It allows to study flow characteristics in controlled conditions and to model the conditions that could not be studied in real flight, such as aerodynamic characteristics at critical angles of attack, in icing conditions etc. Techniques for flow visualisation such as coloured jets or small particles allow to have a qualitative data about flow behaviour, being the valuable means for understanding flow behaviour. But it is more important to have quantitative characteristics of the flow allowing to predict the process evolution and to develop safety measures and recommendations. The presented study addresses to developing a system for optical 3D measurements in hydrodynamic tunnel basing on photogrammetric techniques. To provide accurate measurements in condition of two optical media interfaces (air-glass and air-liquid) the accurate model of image formation accounting refraction is developed. The developed photogrammetric system includes several high speed cameras (from 2 to 4 cameras) mounted in a fixed position relatively the working space and a structured light projector. Original technique is applied for the system calibration. Two metrics has been used as a measure of the accuracy of the calibration: the first one being based on the test field points measurements, and the second one using points-to points distance for the surfaces of a reference object. The key contributions of this paper are: (1) accurate model of image formation in case of several media interfaces (2) a technique for photogrammetric system calibration for 3D measurements in hydrodynamic tunnel (3) experimental evaluation of calibration accuracy for multi-media 3D measurements. The performed experimental evaluation of the developed photogrammetric system has proved high accuracy of system calibration and optical 3D measurements in multi-media optical environment. The developed technique for photogrammetric system calibration and 3D measurements demonstrated applicability for the task of 3D flow analysis in hydrodynamic tunnel.