The study of canonical flows, such as channels, pipes, or boundary layers, is essential for a deeper understanding of the physical mechanisms present in wall-bounded turbulence. Of particular importance in flows delimited by solid walls is the near-wall region where a large fraction of the drag stems from velocity fluctuations in a thin boundary layer adjacent to surfaces. In that context it is interesting to recognize that globally about 10% of all energy is used to overcome turbulent drag in one way or another. The goal of this study is to clarify our understanding in these areas by combining computations and experiments of turbulent duct flows and boundary layers. Oil film interferometry (OFI) and static pressure measurements were carried out over the range 200 < Reτ < 800 (where Re<sub>τ</sub> is the friction Reynolds number, based on duct half-height h and friction velocity u τ) in an adjustable-geometry duct flow facility. Three-dimensional effects were studied by considering different aspect ratio (AR) configurations. Contrary to the accepted understanding in the field of turbulence research, we found that an aspect ratio of at least 24 is required in order to obtain "high-AR duct conditions" , and a development length of around 200 duct full-heights (for low and intermediate Reynolds numbers) is necessary for appropriate flow development.
The three-dimensional effects present in the flow, i.e., side-wall boundary layers and secondary motions, are also studied by means of direct numerical simulations (DNSs). The spectral element code Nek5000, developed by Fischer et al. (2008), is used to compute turbulent duct flows with aspect ratios from 1 to 10 in streamwise-periodic boxes of length 25h (long enough to capture the longest streamwise structures) and Reynolds numbers Reτ,c = 180 and 330. While preparing the duct simulations, we also considered the necessary averaging times for converged statistics in simulations of wall turbulence; as a result, a set of guidelines regarding sampling times and intervals is also given. We find that the conditions often computed in z-periodic channels cannot be reproduced experimentally, even at very high aspect ratios such as 48, and therefore conclude that "computational channels" and "experimental high-AR ducts" are two different flows. The implications of these findings on wind tunnel experiments (with aspect ratios typically ranging from 3 to 16), and the large volume of available "two-dimensional" zero pressure gradient boundary layer data, are also assessed in this study. We therefore recommend the computational and experimental study of turbulent pipe flows, since this is the only case where matching canonical conditions can be obtained both in DNS computations and experimental facilities.
In addition, we re-analyze currently available Pitot tube corrections for ZPG turbulent boundary layer measurements, and propose new forms with coefficients dependent on inner-scaled Pitot tube diameter, [special characters omitted]. Reynolds number and probe size effects are both introduced in these coefficients, yielding excellent collapse of data over a much wider range of Pitot tube diameters (from 0.2 to 12.82 mm), and very good agreement with reference hot-wire and PIV data. We developed a new correcting scheme, called κ B—Musker, which is able to provide the highest possible accuracy in probe position when applied to profile measurements of wall-bounded flows.
|Advisor:||Nagib, Hassan M., Schlatter, Philipp, Fischer, Paul F.|
|School:||Illinois Institute of Technology|
|Department:||Mechanical and Aerospace Engineering|
|School Location:||United States -- Illinois|
|Source:||DAI-B 75/02(E), Dissertation Abstracts International|
|Subjects:||Aerospace engineering, Mechanical engineering, Computer science|
|Keywords:||Boundary layers, Pitot tubes, Turbulent flows, Wall-bounded flows|
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