Ramis Örlü

Assessment of direct numerical simulation data of turbulent boundary layers

Statistics obtained from seven different direct numerical simulations (DNSs) pertaining to a canonical turbulent boundary layer (TBL) under zero pressure gradient are compiled and compared. The considered data sets include a recent DNS of a TBL with the extended range of Reynolds numbers Re-theta = 500-4300. Although all the simulations relate to the same physical flow case, the approaches differ in the applied numerical method, grid resolution and distribution, inflow generation method, boundary conditions and box dimensions. The resulting comparison shows surprisingly large differences not only in both basic integral quantities such as the friction coefficient c(f) or the shape factor II12, but also in their predictions of mean and fluctuation profiles far into the sublayer. It is thus shown that the numerical simulation of TBLs is, mainly due to the spatial development of the flow, very sensitive to, e. g. proper inflow condition, sufficient settling length and appropriate box dimensions. Thus, a DNS has to be considered as a numerical experiment and should be the subject of the same scrutiny as experimental data. However, if a DNS is set up with the necessary care, it can provide a faithful tool to predict even such notoriously difficult flow cases with great accuracy.

Turbulent boundary layers up to Reθ=2500 studied through simulation and experiment

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 th ...

Quantifying the interaction between large and small scales in wall-bounded turbulent flows: A note of caution

Turbulent flow close to solid walls is dominated by an ensemble of fluctuations of large and small spatial scales. Recent work by Mathis [J. Fluid Mech. 628, 311 (2009); Phys. Fluids 21, 111703 (20 ...

A new scaling for the streamwise turbulence intensity in wall-bounded turbulent flows and what it tells us about the “outer” peak

One recent focus of experimental studies of turbulence in high Reynolds number wall-bounded flows has been the scaling of the root mean square of the fluctuating streamwise velocity, but progress has largely been impaired by spatial resolution effects of hot-wire sensors. For the near-wall peak, recent results seem to have clarified the controversy; however, one of the remaining issues in this respect is the emergence of a second (so-called outer) peak at high Reynolds numbers. The present letter introduces a new scaling of the local turbulence intensity profile, based on the diagnostic plot by Alfredsson and Orlu [Eur. J. Mech. B/Fluids 42, 403 (2010)], which predicts the location and amplitude of the “outer” peak and suggests its presence as a question of sufficiently large scale separation.

On the fluctuating wall-shear stress in zero pressure-gradient turbulent boundary layer flows

Recent direct numerical simulation (DNS) results relating to the behavior of the fluctuating wall-shear stress τw,rms+ in turbulent boundary layer flows are discussed. This new compilation is motivated by a recent article [Wu and Moin, “Transitional and turbulent boundary layer with heat transfer,” Phys. Fluids 22, 085105 (2010)], which indicates a need for clarification of the value of τw,rms+. It is, however, shown here, based on other recent DNS data, that most results, both in boundary layer and channel geometry, yield τw,rms+≈0.4 plus a small increase with Reynolds number coming from the growing influence of the outer spectral peak. The observed discrepancy in experimental data is mainly attributed to spatial resolution effects, as originally described by Alfredsson et al. [“The fluctuating wall-shear stress and the velocity field in the viscous sublayer,” Phys. Fluids 31, 1026 (1988)].

The life of a vortex in an axisymmetric jet

Graphical Abstract

On determining characteristic length scales in pressure-gradient turbulent boundary layers

In the present work we analyze three commonly used methods to determine the edge of pressure gradient turbulent boundary layers: two based on composite profiles, the one by Chauhan et al. (Fluid Dy ...

Turbulent asymptotic suction boundary layers studied by simulation

The turbulent asymptotic suction boundary layer (ASBL) is studied using numerical simulations. Uniform suction is applied on the wall in order to compensate for the momentum loss inflicted by the wall friction. Four Reynolds numbers, defined as the ratio of free-stream velocity and suction rate, Re = 333, 400 and 500, are considered, whereas Re = 280 relaminarised. In agreement with previous studies, suction causes the fluctuation intensities to decrease, and the near-wall anisotropy to increase. The shape of the mean velocity profile is considerably changed yielding a decreased slope in the overlap region. It is shown that even for moderate suction rates large values for the friction Reynolds number Reτ = δ99+ are obtained; at Re = 333 a value of Reτ = 1900 is reached and Re = 400 yields Reτ = 5700. Artificially using smaller computational domains, limiting the size of the largest turbulent structures, gives unexpected results: The mean velocity profile starts to show a distinct wake region which only disappears for large enough domains. Moreover, the boundary layer thickness δ99 strongly depends on the chosen domain size. Spectral maps of the flow are analysed, showing an outer peak appearing at a spanwise size of about 0.6δ99, albeit with considerably lower amplitude compared to cases without suction. Visualisations of the flow are also discussed.

Dean vortices in turbulent flows: rocking or rolling?

Graphical Abstract

Time-resolved measurements with a vortex flowmeter in a pulsating turbulent flow using wavelet analysis

Vortex flowmeters are commonly employed in technical applications and are obtainable in a variety of commercially available types. However their robustness and accuracy can easily be impaired by environmental conditions, such as inflow disturbances and/or pulsating conditions. Various post-processing techniques of the vortex signal have been used, but all of these methods are so far targeted on obtaining an improved estimate of the time-averaged bulk velocity. Here, on the other hand, we propose, based on wavelet analysis, a straightforward way to utilize the signal from a vortex shedder to extract the time-resolved and thereby the phase-averaged velocity under pulsatile flow conditions. The method was verified with hot-wire and laser Doppler velocimetry measurements.