Interactions of large-scale free-stream turbulence with turbulent boundary layers

Abstract

The scale interactions occurring within a turbulent boundary layer are investigated in the presence of free-stream turbulence. The free-stream turbulence is generated by an active grid. The free stream is monitored by a single-component hot-wire probe while a second probe is roved across the height of the boundary layer at the same streamwise location. Large-scale structures occurring in the free-stream are shown to penetrate the boundary layer and increase the streamwise velocity fluctuations throughout. It is speculated that depending on the extent of the penetration, i.e. based on the level of freestream turbulence, the near-wall turbulence production peaks at different wall-normal locations than the expected location of y + ≈15 for a canonical turbulent boundary layer.It is shown that the large-scales dominating the log region have a modulating effect on the small-scales in the near wall region; this effect becomes more significant with increasing turbulence in the free-stream, i.e. similarly increasing Re λ0 . This modulating interaction and its Reynolds number trend have similarities with canonical turbulent boundary layers at high Reynolds numbers where the interaction between the large-scales and the envelope of the small-scales exhibit a pure amplitude modulation (Hutchins & Marusic 2007a;Mathis et al. 2009). This similarity has encouraging implications toward generalising scale interactions in turbulent boundary layers.

Skin-friction measurements in a turbulent boundary layer under the influence of free-stream turbulence

“…5 as a solid line. Previous studies have reported an increase in TBL skin friction under FST (Blair 1983;Hancock and Bradshaw 1989;Stefes and Fernholz 2004;Dogan et al 2016). Figure 3 confirms this trend with increasing FST.…”

“…TBL transition was promoted by the addition of a trip wire at the leading edge of the flat plate where the TBL develops. The details of the experimental setup and motor schemes of the active grid can be found in Dogan et al (2016). The superscript + will denote quantities normalised with the friction velocity U , and the kinematic viscosity , as for instance in the wall-normal coordinate y + = yU ∕ , or the mean streamwise velocity U + = U∕U .…”

“…This study combines the merits of these two measurement techniques to compare the skin-friction coefficient (C f = 2U 2 ∕U 2 ∞ ) obtained from a fitting to the mean velocity profile with a direct and independent measurement technique such as OFI. Furthermore, results are compared with Preston tube measurements from Dogan et al (2016) conducted under similar flow conditions.…”

“…Contrarily, Rodríguez-López et al (2015) proposed to leave these constants free to adopt the value that best fits the data. In addition, this method does not require to prescribe the extent of the logarithmic layer (which can vary under FST conditions, see Dogan et al 2016) and allows a certain uncertainty in the wall-probe initial position.Abstract This experimental investigation deals with the influence of free-stream turbulence (FST) produced by an active grid on the skin friction of a zero-pressure-gradient turbulent boundary layer. Wall shear stress is obtained by oil-film interferometry.…”

“…Contrarily, Rodríguez-López et al (2015) proposed to leave these constants free to adopt the value that best fits the data. In addition, this method does not require to prescribe the extent of the logarithmic layer (which can vary under FST conditions, see Dogan et al 2016) and allows a certain uncertainty in the wall-probe initial position.…”