

Article
On near wall measurements of wall bounded flows—The necessity of an accurate determination of the wall position
AbstractThe present review paper is an account on the experimental determination of the wall position relative to the probe in wallbounded turbulent flow studies. A thorough review on common measurement techniques as well as correction methods reveals, that there are a number of pitfalls, that—when not accounted for—can lead to wrong conclusions about the wall position and thereby also on the nearwall behaviour of mean and turbulence quantities. Employing the stateoftheart databases from direct numerical simulations of wallbounded turbulent flows various indirect methods have been tested and assessed in terms of their robustness and accuracy. It is also demonstrated that accurate measurements reaching the viscous sublayer are necessary in order to ensure a correctly deduced wall position, and dependent quantities as for instance the nearwall scaling of mean (e.g. Reynolds number dependence of the buffer region or the log law constants) and turbulence (e.g. the nearwall peak location of Reynolds stresses) quantities. In experiments using hotwires near the wall it is well known that heat conduction between the hotwire and the wall gives errors and mean velocity data from the viscous sublayer can usually not be used to determine the wall position. In this paper we introduce a new method which takes advantage of the similarity of the probability density distributions (pdf) in the near wall region. By using the high velocity data of the pdf, which is shown not to be affected by heat conduction, the heat conduction problem can be circumvented. Extensive appendices are included, describing the history and present knowledge about the scaling of the mean velocity in the near wall and overlap regions in wall bounded turbulent flows.
http://dx.doi.org/10.1016/j.paerosci.2010.04.002

