Turbulent boundary layers and channels at moderate Reynolds numbers

Abstract

The behaviour of the velocity and pressure fluctuations in the outer layers of wall-bounded turbulent flows is analysed by comparing a new simulation of the zero-pressure-gradient boundary layer with older simulations of channels. The 99 % boundary-layer thickness is used as a reasonable analogue of the channel half-width, but the two flows are found to be too different for the analogy to be complete. In agreement with previous results, it is found that the fluctuations of the transverse velocities and of the pressure are stronger in the boundary layer, and this is traced to the pressure fluctuations induced in the outer intermittent layer by the differences between the potential and rotational flow regions. The same effect is also shown to be responsible for the stronger wake component of the mean velocity profile in external flows, whose increased energy production is the ultimate reason for the stronger fluctuations. Contrary to some previous results by our group, and by others, the streamwise velocity fluctuations are also found to be higher in boundary layers, although the effect is weaker. Within the limitations of the non-parallel nature of the boundary layer, the wall-parallel scales of all the fluctuations are similar in both the flows, suggesting that the scale-selection mechanism resides just below the intermittent region, y/S =0.3-0.5. This is also the location of the largest differences in the intensities, although the limited Reynolds number of the boundary-layer simulation (Reg «2000) prevents firm conclusions on the scaling of this location. The statistics of the new boundary layer are available from http://torroja.dmt.upm.es/ftp/blayers/.

The streamwise drag-reduction response of a boundary layer subjected to a sudden imposition of transverse oscillatory wall motion

“…Over the length of the computational domain, the free-stream velocity varies by less than 0.5% of the reference (inflow) free-stream velocity U 0 . Furthermore, the predicted streamwise variation of the momentum thickness with Re τ for the baseline case was found to agree closely with data by Jiménez et al 22 , Schlatter et al 19 and Wu and Moin 23 . Second moments and the turbulence-energy budget derived from the database are shown in Fig.…”

How linear is wall-bounded turbulence?

“…Even so, it is probably relevant that the angle closest to the vertical is the one for the wall-normal velocity, ψ 2 = 92 • , while the other two velocity components are more inclined, ψ 1 = 99 • and ψ 3 = 103 • . Over longer distances, it is known that the correlations of u 1 tend to be long, those of u 2 tall, and those of u 3 wide, 54 in agreement with the behavior of the linear models above. The velocity correlation functions are energy-weighted averages that tend to represent the orientation of the strongest structures.…”

Wall shear stress fluctuations: Mixed scaling and their effects on velocity fluctuations in a turbulent boundary layer

“…The mean profiles of streamwise velocity and the root-mean squared fluctuations of streamwise velocity have also been validated against the simulation data of Schlatter and Orlü [37] and Jimenez et al [21] and are plotted in Figure 1(b).…”

“…The outer region scaling is based on the boundary layer thickness δ, defined as the wall-normal distance where the mean streamwise velocity reaches 99% of the free-stream value. from [37] and [21].…”