Abstract
Spanwise space–time correlations of the wall shear stress and the longitudinal velocity fluctuations in the low buffer layer of an unsteady channel flow are reported. The imposed amplitude is 20% of the centerline velocity and the imposed frequency covers a large range going from the quasi-steady limit to the bursting frequency of the corresponding steady flow. The unsteady spanwise correlation coefficient is investigated both through its own modulation characteristics (amplitude and phase shifts) and those of the resulting streak spacing. A good correspondence is found between the modulation of the streak spacing and that of the ejection period. The data is further analyzed by temporal filtering of the wall shear stress and streamwise velocity fluctuations. It is shown that the large outer-layer structures play a “passive” role in the unsteady response of the near wall turbulence. The inner wall eddies, in return, are amply responsible for the unsteady reaction of both the turbulent wall shear stress and the streamwise velocity intensities in the buffer layer.
Highlights
Turbulent shear flows with periodically forced freestream velocity have been intensively investigated during the last two decades
An attempt is made here to give a more insight view of the results discussed in this paper and to explain the differences observed in hki obtained here and the streak spacing inferred from the wall shear stress correlations of FH
The results reported in this study are reminiscent of the important role played by the inner structures in the unsteady response of the near wall turbulence
Summary
Turbulent shear flows with periodically forced freestream velocity have been intensively investigated during the last two decades. The pipe flow experiments of Tu and Ramaprian (1983) and of the Illinois team (Mao and Hanratty, 1986; Finnicum and Hanratty, 1988), the unsteady boundary layer investigation of Brereton et al (1990) and the channel flow experiments conducted by the Grenoble group (Tardu and Binder, 1993; Tardu et al, 1994) could be representative of yet an incomplete list of recent research on this topic. The practical importance of forced unsteady turbulent layers only partly explains the increasing interest in this subject. The forcing provides an effective way of determining the ‘‘frequency response’’ of the near wall turbulence. The oscillating shear acts on the near wall flow viewed as a system. The sensing of the state (outputs) through the modulation characteristics of the flow quantities, determines its transfer function
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
