Experimental study of the optically transparent gas flow and temperature field using the background oriented Schlieren method

The article presents the results of an experimental study of the flow structure and temperature field in a plume formed above a low-power burner flame. The pulsation and spectral characteristics of the flow at key sampling points were analyzed, which allowed us to draw a conclusion about the nature of the flow at the main points of the jet. It is proposed to use time series of changes in the point displacement field to analyze the spectral characteristics of the flow. In this work, the Background Oriented Schlieren method was used to visualize the flow and determine temperatures followed by post-processing in the program developed during the study. The advantage of this approach compared to the traditional optical Schlieren method is that there is no need for parabolic mirrors as well as the ability to obtain results in digital form convenient for further processing. During the experiment, a special background with randomly located bright dots was placed behind the object of study which was filmed by a video camera. Fluctuations in the medium density caused changes in the refractive indices of the medium, as a result of which the points on the background of the video frames displaced, and the displacements of the points was proportional to the change in the refractive index which in turn is proportional to the density gradient and, accordingly, to the temperature gradient of the medium. The displacement of the points was determined by cross-correlation analysis of each frame in comparison with the frames in the absence of disturbances. Then the displacement field was filtered by a median filter in order to minimize noise and statistical outliers. The filtered displacement field was used to calculate the temperature field, while solving the Cauchy problem for temperature with a known derivative at a point and specified boundary conditions. A set of instantaneous point displacement fields, instantaneous and time-averaged temperature fields was obtained, which allowed us to draw conclusions about the flow structure. At characteristic points of the jet, oscillograms of the displacement value were obtained as well as pulsation spectra with an inertial interval corresponding to the “–5/3” law. The approach proposed in the work allows, in addition to contactless study of the temperature field, also studying turbulent flow pulsations in the case of close to two-dimensional or axisymmetric flows.

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