Moving measurements

Abstract

At the turbulent/non-turbulent interface (TNTI) of a jet flow, momentum is transferred from the turbulent jet fluid to the fluid at rest. This transfer is governed by two mechanisms. One of them is related to the large scales of the flow and the other relates to the small scales of the flow. Which of the two is dominant is still a point of discussion. To analyse the TNTI the instantaneous information of the flow and the location of the interface is of great importance. However, the interface simultaneously develops and travels downstream from the nozzle. With stationary measurement techniques this limits the number of frames the interface can be seen developing as it travels in and out of the FOV. In this thesis the TNTI is measured using a camera system that moves along with the TNTI to get high resolution instantaneous measurements of the same part of the evolving interface. The measurement techniques that are used for this experiment are Particle Image Velocimetry (PIV) and Laser Induced Fluorescence (LIF). The cameras for these measurements are mounted to a motorised frame to keep the same part of the interface in view of the cameras as it develops.

Three Reynolds numbers are measured with this setup and the cameras move approximately 50Dn at a velocity close to the velocity of the interface along a diagonal path to follow the evolution of the TNTI.

One measurement with a Reynolds number of approximately 1.2 × 104 has been processed to show the quality of the results that can be obtained from such a measurement. The velocities are computed by an in-house interrogation analysis program in MATLAB to overcome the wide range of particle displacement found in this experiment, due to the presence of both the centreline and the TNTI of the jet. The TNTI is detected using the LIF data and a threshold detection method from literature that determines a threshold value. The results from the PIV and LIF processing is combined to compute the average conditional vorticity over the interface.

The results show that the PIV analysis is able to compute the velocities of almost the entire jet. Only showing a lot of spurious vectors close to the nozzle in the core of the jet. The LIF edge detection algorithm, on the other hand, does not perform as expected. In multiple instances, an internal interface is detected instead of the TNTI. The TNTI is also determined by identifying a threshold value through a visual inspection of the LIF images. This manually determined TNTI is used as a point of comparison for the average conditional vorticity profiles. The average conditional vorticity profiles support the conclusion that an interface internal to the jet is detected when comparing the interface from the algorithm to the interface determined manually.

Although not quantified in this thesis, there are moving measurements that show the same section of the TNTI evolving for many frames. This gives the co-moving measurements a clear advantage in measurement time compared to stationary measurements when trying to measure the moving TNTI. Refinements to the experimental setup, the behaviour of these internal interfaces and the detection of the TNTI can be of interest for future research.