Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session EB: Turbulent Boundary Layers III |
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Chair: David R. Dowling, University of Michigan Room: Long Beach Convention Center 101B |
Sunday, November 21, 2010 4:10PM - 4:23PM |
EB.00001: Understanding and quantifying wall-turbulence: a new closure approach Zhen-Su She, Xi Chen, You Wu, Fazle Hussain A new closure approach - structure ensemble dynamics (SED) - is proposed for integrating the flow dynamics into a rigorous, quantitative description of the mean flow of wall-bounded turbulence. Starting with the ensemble-averaged Navier-Stokes (EANS) equations, it expresses the unknown effects of fluctuation structures in terms of a set of order functions (a concept in statistical physics describing transitions between different statistical states). A multi-layer picture of wall turbulence naturally arises as a formal, quantitative extension of traditional views in terms of sublayer, buffer layer, log layer and wake. The order functions capture transitions between the layers. SED theory, applied to turbulent channel flow, reveals a surprisingly simple structure of $1-z^4$ (where $z$ is the distance from the channel center) for a turbulent eddy length function $\ell_\nu$ in the bulk flow of the channel, where the traditional logarithmic layer near the wall is already included (without any matching): $\ell_\nu\propto y\equiv 1-z$ as $z\to 1$. A quantitative multi-layer model for relevant order functions is shown to give accurate description of the mean quantities like the profiles of mean velocity, Reynolds stress, kinetic energy, turbulence production and energy dissipation. Finally, a systematic procedure for evaluating numerical simulations using the SED theory is outlined. [Preview Abstract] |
Sunday, November 21, 2010 4:23PM - 4:36PM |
EB.00002: Scaling and interactions of inner and outer regions in wall-bounded turbulence Romain Mathis, Nicholas Hutchins, Ivan Marusic Recent investigations in wall-bounded turbulent flows have shown a nonlinear scale interaction, whereby the large-scale motion amplitude modulates the small-scale structures (Mathis~\emph{et al.}, \emph{J. Fluid Mech.} 628, 2009). Here, we present a comparison of the amplitude modulation effects between channels/pipes and boundary layers. It is found, that despite different large-scale structures in these internal and external wall-bounded flows, the amplitude modulation effects remain invariant in the inner region, whereas subtle differences appear in the outer region. Further details will be given on the close relationship between the wall-normal evolution of the amplitude modulation coefficient and the skewness as recently pointed out by Schlatter and Orlu (\emph{Phys. Fluids} 22, 2010). Scaling issues related to the inner and outer regions will also be considered by using a scale-decomposition approach. [Preview Abstract] |
Sunday, November 21, 2010 4:36PM - 4:49PM |
EB.00003: Time-resolved evolution of the wall-bounded vorticity cascade Adri\'an Lozano-Dur\'an, Javier Jim\'enez We study the temporal evolution of vortex clusters in turbulent channels with $Re_\tau=950$ and $1880$, using DNS sequences with temporal separations among fields short enough for individual structures to be tracked. From the geometric intersection of structures in consecutive fields, we build temporal connection graphs of all the cluster interactions, and, from their properties, distinguish the ``trunk'' of each evolution from less important ``branches.'' It is found that the lifetimes of the connected families of attached clusters are proportional to the cube roots of their maximum volumes, of the order of $Tu_\tau/h=0.25$ for the largest ones, and that they move approximately with the overall advection velocity. Especial attention is paid to the origin of the attached structures, and to their relation with an inverse cascade. They tend to be born below $y^+=100$, and to grow upward, although a similar study of the Reynolds stresses suggests interactions in both directions. Merging of comparable clusters is common, but splitting tends to involve smaller fragments. The creation and evolution of new clusters during the bursting events of the logarithmic layer are also studied. [Preview Abstract] |
Sunday, November 21, 2010 4:49PM - 5:02PM |
EB.00004: Direct simulation of the zero-pressure-gradient boundary layer up to $Re_\theta=6000$ Juan A. Sillero, Guillem Borrell, Ayse G. Gungor, Javier Jim\'enez, Robert D. Moser, Todd A. Oliver Preliminary results are presented from a direct simulation of the zero-pressure-gradient turbulent boundary layer in the range $Re_\theta=2500$--6000, approximately matching channels at $Re_\tau=2000$. Special emphasis is put on the effect of enforcing inflow conditions at a relatively-high Reynolds number, and on their influence on the streamwise development of the mean and fluctuating flow properties. [Preview Abstract] |
Sunday, November 21, 2010 5:02PM - 5:15PM |
EB.00005: Comparison of turbulence in a transitional boundary layer to turbulence in a developed boundary layer* G.I. Park, J. Wallace, X. Wu, P. Moin Using a recent DNS of a flat-plate boundary layer, statistics of turbulence in transition at $Re_\theta = 500$ where spots merge (distributions of the mean velocity, rms velocity and vorticity fluctuations, Reynolds shear stress, kinetic energy production and dissipation rates and enstrophy) have been compared to these statistics for the developed boundary layer turbulence at $Re_\theta = 1850$. When the distributions in the transitional region, determined in narrow planes $0.03 Re_\theta$ wide, exclude regions and times when the flow is not turbulent, they closely resemble those in the developed turbulent state at the higher Reynolds number, especially in the buffer and sublayers. The skin friction coefficient, determined in this conditional manner in the transitional flow is, of course, much larger than that obtained by including both turbulent and non-turbulent information there, and is consistent with a value obtained by extrapolating from the developed turbulent region. We are attempting to perform this data analysis even further upstream in the transitioning flow at $Re_\theta = 300$ where the turbulent spots are individuated. These results add further evidence to support the view that the structure of a developed turbulent boundary layer is little different from its structure in its embryonic form in turbulent spots. *CTR 2010 Summer Program research. [Preview Abstract] |
Sunday, November 21, 2010 5:15PM - 5:28PM |
EB.00006: Hairpin vortices in the transitional and developed turbulence of a flat-plate boundary layer* J. Wallace, X. Wu, I. Park, P. Moin The use of vortex lines to reveal vortical structures in turbulent shear flows has been in disfavor for some time, in spite of their successful use by Kim and Moin (1986, JFM 162) and Rogers and Moin (1987, JFM 176). This is because they are field lines that can be drawn wherever the flow is rotational, regardless of whether a true vortex exists in a part of the field or not. For this reason, it would be better to call them vorticity lines rather than vortex lines. A virtue that such lines have, however, is that the vortical structures they can reveal do not depend on setting a detection threshold, unlike all the vortex identifiers based on the velocity gradient tensor or based on a low pressure criterion. Furthermore, vorticity lines can be used to isolate a single vortical structure. We will show that individual hairpin vortices can be identified using vorticity lines in the transition region at $Re_\theta = 500$, where turbulent spots merge, and in the developed turbulence at $Re_\theta = 1850$ within a recent DNS of a flat-plate boundary layer, and that the vortices so identified have quite similar characteristics. These vortices emerge out of sheets of unorganized vorticity in the viscous sublayer. An attempt will be made to follow the temporal and spatial evolution of these vortical structures using simulation files closely separated in time. *CTR 2010 Summer Program research. [Preview Abstract] |
Sunday, November 21, 2010 5:28PM - 5:41PM |
EB.00007: Transitional and turbulent flat-plate boundary layers with heat transfer Xiaohua Wu, Parviz Moin We report on our direct numerical simulation of two incompressible, nominally zero-pressure-gradient flat-plate boundary layers from momentum thickness Reynolds number 80 to 1950. Heat transfer between the constant-temperature solid surface and the free-stream is also simulated with molecular Prandtl number$\Pr =1$. Throughout the entire flat-plate, the ratio of Stanton number and skin-friction $St/C_f $deviates from the exact Reynolds analogy value of 0.5 by less than 1.5{\%}. Turbulent Prandtl number $\Pr _t $ peaks at the wall. Preponderance of hairpin vortices is observed in both the transitional and turbulent regions of the boundary layers. In particular, the internal structure of merged turbulent spots is hairpin forest; the internal structure of infant turbulent spots is hairpin packet. Numerous hairpin vortices are readily detected in both the near-wall and outer regions of the boundary layers up to momentum thickness Reynolds number 1950. This suggests that the hairpin vortices in the turbulent region are not simply the aged hairpin forests convected from the upstream transitional region. Temperature iso-surfaces in the companion thermal boundary layers are found to be a useful tracer in identifying hairpin vortex structures. [Preview Abstract] |
Sunday, November 21, 2010 5:41PM - 5:54PM |
EB.00008: Very large-scale motions in a turbulent boundary layer Hyung Jin Sung, Jae Hwa Lee Direct numerical simulation of a turbulent boundary layer with \textit{Re}$_{\theta}$=2560 was performed to investigate the spatially coherent structures associated with very large-scale motions (VLSMs). Inspection of the three-dimensional instantaneous fields showed that groups of hairpin vortices are coherently arranged in the streamwise direction and that these groups create significantly elongated low- and high-momentum regions with large amounts of Reynolds shear stress. Adjacent packet-type structures combine to form the VLSMs; this formation process is attributed to continuous stretching of the hairpins, coupled with lifting-up and backward curling of the vortices. We employed the modified feature extraction algorithm to identify the properties of the VLSMs of hairpin vortices. Patches with lengths greater than 3$\sim $4\textit{$\delta $} account for more than 40{\%} of all the patches and these VLSMs contribute approximately 45{\%} of the total Reynolds shear stress. Finally, the application of linear stochastic estimation to the conditionally averaged flow field demonstrated the presence of packet organization in the form of a train of packets in the logarithmic layer. [Preview Abstract] |
Sunday, November 21, 2010 5:54PM - 6:07PM |
EB.00009: Multi-scale geometry of flow structures in a flat-plate turbulent boundary layer Ivan Bermejo-Moreno, Callum Atkinson, Sergei Chumakov, Julio Soria, Xiaohua Wu We study the geometry of structures educed from the enstrophy and dissipation fields obtained from a DNS of a flat-plate turbulent boundary layer (J. Fluid Mech. 630, 5-41, 2009) following the non-local multi-scale methodology introduced in J. Fluid Mech. 603, 101-135, 2008. We compare the results with those of homogeneous isotropic turbulence. In the present analysis, geometric parameters are combined with physical quantities associated with the flow structures. Their evolution in time is studied through a series of snapshots obtained from the simulation, following a moving subdomain. Individual structures are tracked in time, relating their physical and geometric properties at the local and structure levels. The validity of two local identification criteria for the eduction of vortex tubes and sheets in wall-bounded flows is also evaluated. [Preview Abstract] |
Sunday, November 21, 2010 6:07PM - 6:20PM |
EB.00010: Dynamic wall shear stress measurements in a turbulent channel flow Omid Amili, Julio Soria To quantify and understand the dynamics of near wall momentum transfer, high spatial resolution, time-resolved measurements of wall shear stress distribution are essential. In this study, a film-based shear stress sensor has been used to measure the time-resolved local wall shear stress distribution in a turbulent channel flow. Measurements have been undertaken in a turbulent channel flow at Reynolds numbers up to 130,000 based on the bulk velocity and channel height. The measured fluctuating wall shear stress distribution provides spatio-temporal information of the characteristics of near wall structures by detecting their footprints. The span-wise extent of the positive two-point correlation of the stream-wise shear stress fluctuations provides the average width in the order of 100 wall units for the near-wall coherent structures. An investigation of the topological features of the velocity gradient and rate of strain tensors enables us to show an intrinsic characteristic of the near wall flow, which follows a two-dimensional flow pattern. [Preview Abstract] |
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