A modification of the classical correction factor for digital background oriented Schlieren when applied in the near-field

Abstract

For the conversion of the measured apparent background shift into the density gradients of the flow a constant correction factor is generally used in the applied equations for digital background oriented Schlieren. In case the distance between the Schlieren object and the BOS background pattern is of the same order as the width of the Schlieren object (the near-field), these equations fail to provide reliable density fields. In this paper a modification of the constant correction factor is presented for BOS in order to be applicable in this near-field. It is shown that the variable factor depends solely on two parameters that are fixed by the setup and by the shape of the investigated flow field. The achieved accuracy of the modified BOS equations is shown by the simulation of a known two-dimensional density field using a glass wedge prism. A comparison of the measured characteristic parameters of the diamond pattern of a supersonic jet with the theoretical values shows the applicability of this technique to fluid dynamic related studies.