Flow Measurements in a Near Wall Flow Using Evanescent Wave Illumination (Effect of Polymer Additive on Flow in a Microchannel)

The understanding and accurate prediction of turbulent flow separation on smooth surfaces is still a challenging task because the separation and the reattachment locations are not fixed in space and time. Consequently, reliable experimental data are essential for the validation of numerical flow simulations and the characterization and analysis of the complex flow physics. However, the uncertainty of the existing near-wall flow measurements make a precise analysis of the near-wall flow features, such as separation/reattachment locations and other predicted near-wall flow features which are under debate, often impossible. Therefore, the periodic hill experiment at TU Munich (ERCOFTAC test case 81) was repeated using high resolution particle image velocimetry and particle tracking velocimetry. The results confirm the strong effect of the spatial resolution on the near-wall flow statistics. Furthermore, it is shown that statistically stable values of the turbulent flow variables can only be obtained for averaging times which are challenging to realize with highly resolved large eddy simulation and direct numerical simulation techniques. Additionally, the analysis implies that regions of correlated velocity fluctuations with rather uniform streamwise momentum exist in the flow. Their size in the mean flow direction can be larger than the hill spacing. The possible impact of the correlated turbulent motion on the wake region is discussed, as this interaction might be important for the understanding and control of the flow separation dynamics on smooth bodies.

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Evanescent Wave-Based Flow Diagnostics

Journal of Fluids Engineering ◽

10.1115/1.4023448 ◽

2013 ◽

Vol 135 (2) ◽

Cited By ~ 8

Author(s):

Yutaka Kazoe ◽

Minami Yoda

Keyword(s):

Evanescent Wave ◽

Diagnostic Techniques ◽

Epifluorescence Microscopy ◽

Flow Diagnostics ◽

Interfacial Transport ◽

Surface Areas ◽

Flow Systems ◽

Wall Flow ◽

Voltage Gradients ◽

Near Wall

Miniaturized flow systems have been developed for various applications, including integrated chemical analyses and thermal management of microelectronics. Understanding interfacial transport is important in designing and optimizing such flow systems, since surface effects become significant due to the large surface areas and small volumes at these length scales. Recently, various near-wall flow diagnostic techniques have been developed based on evanescent-wave illumination. Since evanescent waves only illuminate the fluid in the region over the first few hundred nanometers next to the wall, these techniques have much better spatial resolution than conventional methods based on epifluorescence microscopy. This paper presents recent advances in evanescent wave-based flow diagnostics using fluorescent tracers, including evanescent-wave particle velocimetry applied to flows driven by both pressure and voltage gradients and evanescent-wave fluorescence, which has been used to measure near-wall liquid temperature and pH fields, as well as the surface charge, or wall ζ-potential, distributions.

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