Activity: Examinations and supervision › Examination
Non-equilibrium turbulence is by nature challenging to measure accurately, especially in the regions where shear and turbulence intensities are dominant. This technical difficulty has contributed in mak-ing turbulence one of the unsolved and undefined problem in the classical physic. The problem has been left underexplored due to practical limitations suffered by the existing commercial measure-ment systems that compromise the measurement accuracy and reliability. This has consequently impaired the ability to properly test the critical assumptions of the existing Kolmogorov theory dating more than 70 years back.
Substantial and recent challenges to this theory by the previous scholars have driven us to carry out an experimental investigation to map the non-equilibrium (developing) region of a turbulent round jet and obtain breakthrough findings that can potentially help in filling the knowledge gap. To counter the downsides of the commercial systems, a state-of-the-art and transparently functioning laser Doppler anemometer (LDA) system has been developed, which is optimized for spectral turbulence meas-urements. This self-developed LDA system comprises state-of-the-art, off-the-shelf hardware and a novel software for signal and data processing. Both the hardware and software have been devel-oped in a way for resulting in unbiased statistics and achieving the highest possible spatial resolution and dynamic range as well as the minimum noise and dead time effect.
Three different series of measurements have been done to thoroughly investigate the flow in differ-ent regions of the jet. The first measurements were done and proven to validate the functionality of the LDA system in the equilibrium (fully developed) region, which results agreed well with the existing Kolmogorov laws (at least on average). The second measurements were done more thoroughly in the outer part of the jet, covering both the developing and fully developed regions as well as near the laminar jet core. The results show that the cornerstone assumption of the Kolmogorov universal equilibrium theory cannot strictly hold in general (even on average) and the direct transfer of energy from large to smaller scales violates the well-known Richardson cascade. The final measurements along the jet centerline demonstrates that the second-order statistics takes longer to develop fully than the first-order ones. Different positions of virtual origin are also resulted from the linear fits of the mean velocity and the velocity variance.
All measurements, especially the second and the third ones, provides valuable results that can be useful for the development and/or validation of turbulence models in a classical flow that at least on average displays the same physics as the Kolmogorov theory of turbulence, upon which the majority of current turbulence models are built. This thesis also comes with the comprehensive hands-on guide for someone to operate the LDA system in the future for possibly different scope of turbulence measurements.