Abstract
There are two general concerns in the velocity measurements of turbulence. One is the temporal characteristics which governs the turbulent mixing process. Turbulence is rotational and is characterized by high levels of fluctuating vorticity. In order to obtain the information of vorticity dynamics, the spatial characteristics is the other concern. These varying needs can be satisfied by using a variety of diagnostic techniques such as invasive physical probes and non-invasive optical instruments. Probe techniques for the turbulent measurements are inherently simple and less expensive than optical methods. However, the presence of a physical probe may alter the flow field, and velocity measurements usually become questionable when probing recirculation zones. The non-invasive optical methods are mostly made of the foreign particles (or seeding) instead of the fluid flow and are, thus, of indirect method. The difference between the velocities of fluid and foreign particles is always an issue to be discussed particularly in the measurements of complicated turbulent flows. Velocity measurements of the turbulent wake flow over a circular cylinder will be made by using two invasive instruments, namely, a cross-type hot-wire anemometry (HWA) and a split-fiber hot-film anemometry (HFA), and a non-invasive optical instrument, namely, particle image velocimetry (PIV) in this study. Comparison results show that all three employed diagnostic techniques yield similar measurements in the mean velocity while somewhat deviated results in the root-mean-squared velocity, particularly for the PIV measurements. It is demonstrated that HFA possesses more capability than HWA in the flow measurements of wake flow. Wake width is determined in terms of either the flatness factor or shear-induced vorticity. It is demonstrated that flow data obtained with the three employed diagnostic techniques are capable of yielding accurate determination of wake width.
Highlights
IntroductionFlow over circular cylinder (namely, wake) is a simple and useful model for many applications concerning obstacles to flow
Flow over circular cylinder is a simple and useful model for many applications concerning obstacles to flow
Comparison results show that all three employed diagnostic techniques yield similar measurements in the mean velocity while somewhat deviated results in the root-mean-squared velocity, for the particle image velocimetry (PIV) measurements
Summary
Flow over circular cylinder (namely, wake) is a simple and useful model for many applications concerning obstacles to flow. The velocity measurements of turbulent flow over a long circular cylinder are made with a X-array hot-wire probe and a split-fiber hot-film probe, which are of the intrusive diagnostics and can provide the velocity data with good temporal resolution, as well as a PIV which is of the nonintrusive diagnostics and can provide the velocity data with good spatial resolution. Lee and Chang[4] determined the wake width by monitoring the sectional distributions of high-order turbulence statistics such as the flatness (Kurtosis) factor (Fu) of the turbulent stream-wise velocity component which is defined as. Determination of the flatness factor (i.e., Eq (1)) follows the calculation process used in the study of Lee and Chang[4] and is not repeated here for brevity Uncertainties of both HWA and HFA measurements are estimated within 2.5 % for both the mean and root-mean-squared fluctuating velocities. More detailed information on the PIV measurements in turbulent flow is referred to the study of Li et al[11]
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More From: International Journal of Modern Physics: Conference Series
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