Application of POD analysis to concentration field of a jet flow

•A fan-shaped region of interest (ROI) is used to re-sample the LIF field of a jet.•The ROI uses a spatial array which conforms to the spreading of the jet.•The ROI enhances flow analysis with proper orthogonal decomposition (POD).•More accurate reconstruction of the turbulent concentration fields is achieved.•The technique is applied to analyse spreading of a round jet in a counterflow.

This paper proposes a fan-shaped region of interest (ROI) for the analysis of the spreading flow of a round jet with proper orthogonal decomposition (POD). Time sequences of scalar concentration field of a jet in stagnant environment and in a counterflowing main stream is obtained with the time-resolved laser-induced fluorescence technique. The fan-shaped ROI re-samples the concentration field on a spatial array with resolutions varying with axial locations from the jet exit. This conforms to the increases of jet width and length scales of the flow structures as the jet spreads downstream. The use of this ROI is shown to achieve better reconstruction of the turbulent concentration fields using POD modes with smaller reconstruction errors. The enhanced POD analysis is applied to study the effect of counterflow strength on the flow features of the jet. It is found that the presence of a counterflow, with the exception of a very weak counterflow, causes concentration variations of very large scales in the form of lateral jet flapping and oscillations in jet penetration.