A direct numerical simulation (DNS) of a spatially developing turbulent boundary layer over a flat plate under zero pressure gradient (ZPG) has been carried out. The evolution of several passive scalars with both isoscalar and isoflux wall boundary condition are computed during the simulation. The Navier-Stokes equations as well as the scalar transport equation are solved using a fully spectral method. The highest Reynolds number based on the free-stream velocity U-infinity and momentum thickness 0 is Re-0 = 830, and the molecular Prandtl numbers are 0.2, 0.71 and 2. To the authors' knowledge, this Reynolds number is to date the highest with such a variety of scalars. A large number of turbulence statistics for both flow and scalar fields are obtained and compared when possible to existing experimental and numerical simulations at comparable Reynolds number. The main focus of the present paper is on the statistical behaviour of the scalars in the outer region of the boundary layer, distinctly different from the channel-flow simulations. Agreements as well as discrepancies are discussed while the influence of the molecular Prandtl number and wall boundary conditions is also highlighted. A Pr scaling for various quantities is proposed in outer scalings. In addition, spanwise two-point correlation and instantaneous fields are employed to investigate the near-wall streak spacing and the coherence between the velocity and the scalar fields. Probability density functions (PDF) and joint probability density functions (JPDF) are shown to identify the intermittency both near the wall and in the outer region of the boundary layer. The present simulation data will be available online for the research community.

The present study investigates the effects of ambient free-stream turbulence (FST) on the momentum and heat transfer in a spatially developing, turbulent flat-plate boundary layer via large-eddy simulations using the ADM-RT model. Due to a local turbulence intensity Tu of 7% in the free stream, the skin-friction coefficient c_f and Stanton number St are substantially elevated up to 25% and 32%, respectively, in the fully turbulent region (Re_τ=300). This observation is in qualitative agreement with earlier experimental studies. Moreover, the Reynolds analogy factor is found to increase with the FST intensity Tu. The depression of both mean velocity and temperature profiles in the wake region due to FST is observed. In addition, the pre-multiplied spanwise spectra show that the outer peak residing in the logarithmic region in the case without FST is replaced by a new peak located near the boundary layer edge with a spanwise scale of about 3-4δ₉₅. It is suggested that these large-scale events and their imprint throughout the boundary layer cause the elevation of both the skin friction and heat transfer on the solid surface.

Direct numerical simulations (DNSs) and experiments of a spatially developing zero-pressure-gradient turbulent boundary layer are presented up to Reynolds number Re-theta=2500, based on momentum thickness theta and free-stream velocity. For the first time direct comparisons of DNS and experiments of turbulent boundary layers at the same (computationally high and experimentally low) Re-theta are given, showing excellent agreement in skin friction, mean velocity, and turbulent fluctuations. These results allow for a substantial reduction of the uncertainty of boundary-layer data, and cross validate the numerical setup and experimental technique. The additional insight into the flow provided by DNS clearly shows large-scale turbulent structures, which scale in outer units growing with Re-theta, spanning the whole boundary-layer height.

A well-resolved large-eddy simulation (LES) of a spatially developing turbulent boundary layer under zero-pressure-gradient up to comparably high Reynolds numbers (Re-theta = 4300) is performed. The laminar inflow is located at Re-delta = 450 (Re-theta approximate to 1180), a position where natural transition to turbulence can be expected. The simulation is validated and compared extensively to both numerical data sets, i.e. a recent spatial direct numerical simulation (DNS) up to Re-theta = 2500 (Schlatter et al., 2009) and available experimental measurements, e.g. the ones obtained by Osterlund (1999). The goal is to provide the research community with reliable numerical data for high Reynolds-number wall-bounded turbulence, which can in turn be employed for further model development and validation, but also to contribute to the characterisation and understanding of various aspects of wall turbulence. The results obtained via LES show that good agreement with DNS data at lower Reynolds numbers and experimental data can be obtained for both mean and fluctuating quantities. In addition, turbulence spectra characterising large-scale organisation in the flow have been computed and compared to literature results with good agreement. In particular, the near-wall streaks scaling in inner units and the outer layer large-scale structures can clearly be identified in both spanwise and temporal spectra. (C) 2010 Elsevier Inc. All rights reserved.

Negative streamwise velocities, extreme wall-normal velocites and high flatness values for the wall-normal fluctuations near the wall are investigated for turbulent channel flow simulations at a series of Reynolds numbers up to Reτ = 1000 in this paper. Probability density functions of the wall-shear stress and velocity components are presented, as well as joint probability density functions of the velocity components and the pressure. Backflow occurs more often (0.06% at Reτ = 1000) and further away from the wall into the buffer layer for rising Reynolds number. An oblique vortex outside the viscous sublayer is found to cause this backflow. Extreme v events occur also more often for rising Rey nolds number. Positive and negative velocity spikes appear in pairs, located on the two edges of a strong streamwise vortex: the negative spike occurring in a high speed streak indicating a sweeping motion, while the positive spike is located between a high and low speed streak. These extreme v events cause high flatness values near the wall (F(v) = 43 at Reτ = 1000).

A recent data base from direct numerical simulation of a turbulent boundary layer up to Reθ = 4300 [Schlatter & Örlü, J. Fluid Mech. 659, 2010] has been analysed in an effort to educe the dominant flow structures populating the near-wall region. In particular, the question of whether hairpin vortices are indeed observable as a dominant building block of near-wall turbulence is addressed. It is shown that during the initial phase, dominanted by the specific laminar-turbulent transition induced via the tripping mechanism, hairpin vortices are very numerous, and can certainly be considered as the dominant structure. This is in agreement with previous experiments and low Reynolds number simulations such as Wu & Moin [J. Fluid Mech. 630, 2009]. At sufficient distance from transition, the flow is dominated by a staggered array of quasi-streamwise vortices which is the same situation as in previous channel flows. It turns out that even quantitatively, no major differences between boundary layers and channels can be detected; structures are about 200 viscous units in length, and inclined by about 9 degrees [Jeong et al., J. Fluid Mech. 332, 1997]. The present results clearly show that the regeneration process of turbulence does not involve the generation of (symmetric) hairpin vortices, and that their dominant appearance as instantaneous flow structures in the outer boundary-layer region is at least very unlikely.