Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session D20: Turbulent Boundary Layers II: Structures |
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Chair: Philipp Schlatter, KTH Room: 30A |
Sunday, November 18, 2012 2:15PM - 2:28PM |
D20.00001: Time-resolved evolution of coherent structures in turbulent channels Adrian Lozano-Duran, Javier Jimenez The temporal evolution of vortex clusters and of the structures responsible for the momentum transfer in turbulent channels at $Re_{\tau} = 950$, $2000$ and $4000$ are studied 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 objects and define main and secondary branches in a way that each branch represents the temporal evolution of one coherent structure. A family of evolutions is found with self-similar sizes and lifetimes that can be born at any height with respect to the wall, although the probability increases close to it. Especial attention is paid to the wall-normal displacement of the structures. Sweeps tend to go towards the wall whereas ejections move away from it. In all the cases, the vertical velocity is close to $u_{\tau}$ and the wall-normal displacement is proportional to the lifetime of the structures and to their sizes. Finally, direct and inverse physical cascades are defined, associated with the process of splitting and merging among structures. The direct cascade predominates, but both directions are roughly comparable. [Preview Abstract] |
Sunday, November 18, 2012 2:28PM - 2:41PM |
D20.00002: On the near-wall vortical structures at high Reynolds numbers Philipp Schlatter, Ramis Orlu, Qiang Li, Fazle Hussain, Dan Henningson A recent database from direct numerical simulation (DNS) of a turbulent boundary layer (TBL) up to $Re_\theta=4300$ has been analysed to educe the dominant flow structures in the near-wall region. In particular, the question of whether hairpin vortices are significant features of TBL is addressed. It is shown that during the initial phase of laminar-turbulent transition induced via tripping, hairpin vortices evolving from transitional vortices are numerous, and can certainly be considered as the dominant structure of the immediate post-transition stage of a boundary layer. This is in agreement with previous experiments and various low Reynolds-number simulations. At sufficient distances farther downstream from transition, the flow is dominated by a staggered array of quasi-streamwise vortices which are the same as observed in channel flow studies. It turns out that even quantitatively, no major differences between boundary layers and channels can be detected. The present results confirm that there is i) no reason why transitional hairpin vortices should persist in the fully developed turbulent region, ii) that the regeneration process does not involve hairpin vortices, and iii) that their dominant appearance as instantaneous outer structures is unlikely. [Preview Abstract] |
Sunday, November 18, 2012 2:41PM - 2:54PM |
D20.00003: Merging and auto-generation of vortices in wall bounded flow Gerrit Elsinga, Manu Goudar Vishwanathappa, Wim-Paul Breugem For channel flow, we explore how a hairpin eddy may reach a threshold strength required to produce additional hairpins by means of auto-generation. This is done by studying the evolution of two eddies with different initial strengths (but both below the threshold strength), initial sizes and initial stream-wise spacing between them. The numerical procedure followed is similar to Zhou et al [1]. The two eddies were found to merge into a single stronger eddy in case of a larger upstream and a smaller downstream eddy placed within a certain initial stream-wise separation distance. Subsequently, the resulting stronger eddy was observed to auto-generate new eddies. Merging of eddies thus is a viable explanation for the creation of the threshold strength eddies. \\[4pt] [1] J. Zhou, R.J. Adrian, S. Balachandar, and T.M. Kendall, Journal of Fluid Mechanics, 387:353-396, 1999. [Preview Abstract] |
Sunday, November 18, 2012 2:54PM - 3:07PM |
D20.00004: ABSTRACT WITHDRAWN |
Sunday, November 18, 2012 3:07PM - 3:20PM |
D20.00005: Application of vortex identification schemes to DNS data of transitional boundary layer Brian Pierce, Parviz Moin, Taraneh Sayadi We have demonstrated how various vortex identification and visualization criteria perform using DNS data from a transitional and turbulent boundary layer by Sayadi et al. (submitted to J. Fluid Mech.). The presence of well-known $\Lambda$ vortices in the transitional region provides a well defined and yet realistic benchmark for evaluation of various criteria. We investigate the impact of changing the threshold used for iso-surface plotting. [Preview Abstract] |
Sunday, November 18, 2012 3:20PM - 3:33PM |
D20.00006: Development and Interaction of Artificially Generated Hairpin Vortices Daniel Sabatino, Christopher McKenna The development and interaction of hairpin vortices are examined and categorized to better understand their role in fully turbulent boundary layers. Hairpin vortices are generated within an otherwise laminar boundary layer using a free surface water channel. Direct injection is the primary generation method and the behavior of the vortices is first examined using flow visualization. Hydrogen bubble wire is combined with dye injection to help clarify the role of the vorticity in the fluid immediately surrounding the hairpin vortex. PIV data is also used to classify the development and maturity of the vortices for a range of free stream and injection conditions. The interactions of two hairpin vortices of varying maturity are characterized to investigate the potential mechanisms for the formation of hairpin packets beyond autogeneration. Finally, the behavior of hairpin vortices generated with a new technique that uses a transient hemispherical protrusion is also examined. [Preview Abstract] |
Sunday, November 18, 2012 3:33PM - 3:46PM |
D20.00007: Two-point correlations for zero-pressure-gradient turbulent boundary layers and channels at $Re_\tau \approx 1000-2000$ Juan A. Sillero, Javier Jim\'enez, Robert D. Moser Two-point 5-dimensional correlations $C_{\xi\xi}(x; x'; y; y'; \Delta z)$ are investigated to educe the structure of the velocity and pressure fluctuations in zero-pressure-gradient turbulent boundary layers in the range $Re_\theta = 2780-6680$, and in matching channels at $Re_\tau \approx 1000-2000$. Eddies in channels are coherent over longer distances than in boundary layers, especially for $C_{uu}$ in the direction of the flow. At the 5\% level, the maximum streamwise length of $C_{uu}$ is $O(6\delta)$ for boundary layers and $O(15h)$ for channels. The corresponding lengths for the transverse velocities and for the pressure are shorter, $O(\delta\mbox{-}2\delta)$, and of the same order for both flows. Integral correlation lengths in the streamwise and spanwise directions grow away from the wall, except for $L_{uu,x}$, which peaks at $y\approx 0.6h$ in channels and at $y\approx 0.2 \delta$ in boundary layers, probably due to the outer intermittency in the latter. Above the buffer layer, $C_{uu}$ is inclined by $\approx 10-12^o$ from the wall, the wall-normal velocity and the pressure are roughly vertical, and $C_{ww}$ is inclined by $\approx 30^o$. Those features seem unaffected by the Reynolds number and by the type of flow. [Preview Abstract] |
Sunday, November 18, 2012 3:46PM - 3:59PM |
D20.00008: Near-wall turbulent fluctuation in the absence of the wide outer motions Yongyun Hwang Numerical experiments which filter out turbulent motions wider than $\lambda_z^+\simeq 100$ are carried out up to $Re_{\tau}=660$ in a turbulent channel. The mean-velocity profile shows very good agreement with that of full simulation below $y^+ \simeq 40$. The turbulent fluctuations mainly consist of two parts: the long streamwise velocity fluctuation confined to the near-wall region with $\lambda_x^+ \simeq 600 \sim 800$, and the short fluctuation of all the velocity components with $\lambda_x^+ \simeq 200 \sim 300$ inducing turbulence even above $y^+\simeq 100$. In the absence of the wide outer motions, the former remains almost unchanged with the Reynolds number, resulting in an almost constant value of the near-wall maximum streamwise velocity fluctuation at all the Reynolds numbers considered. On the other hand, the latter strongly interacts with the non-realistic mean shear in the outer region and induces non-negligible amounts of the fluctuation near the channel center even at $Re_{\tau}=660$. The removal of the wider structures also reveals significant amounts of drag reduction particularly at large Reynolds numbers, implying that the filtered wide outer structures are involved in generating turbulent skin friction. [Preview Abstract] |
Sunday, November 18, 2012 3:59PM - 4:12PM |
D20.00009: A comparison of spanwise vorticity fluctuations statistics over $\delta^+ = $ 350 - 8000 Caleb Morrill-Winter, Rachel Ebner, Rio Baidya, Petar Vukoslavcevic, Joseph Klewicki, James Wallace, Ivan Marusic The behaviors of the spanwise vorticity fluctuations ($\omega_z$) and their correlation with the wall normal and streamwise velocity fluctuations are central to describing the momentum and kinetic energy transport mechanisms in the turbulent boundary layer. To date, however, well-resolved laboratory measurements of $\omega_z$ have been confined to low Reynolds numbers. Compact four-element hot-wire probe measurements are used to explore the statistical structure of the spanwise vorticity fluctuations in zero pressure gradient turbulent boundary layers. Existing well-resolved laboratory data, $\delta^+ = $ 375, 970 \& 1500, along with recently acquired data from the University of New Hampshire's low-speed boundary layer wind tunnel and University of Melbourne's High Reynolds Number Boundary Layer Wind Tunnel are examined. In the present flows, the spatial resolution of the probe ranged from $l^+ = $ 4 - 12. The properties of individual velocity gradient contributions to $\omega_z$ are examined, along with the Reynolds number scaling behaviors of the first four statistical moments of $\omega_z$. The present results indicate that the motions bearing spanwise vorticity exhibit a significant Reynolds number dependence in the wake region of the boundary layer. [Preview Abstract] |
Sunday, November 18, 2012 4:12PM - 4:25PM |
D20.00010: Small-scale statistics in direct numerical simulation of turbulent channel flow up to $Re_\tau=5120$ Koji Morishita, Takashi Ishihara, Yukio Kaneda Small-scale statistics in high-Reynolds number turbulent channel flow are studied by direct numerical simulation (DNS) with $Re_\tau$ up to 5120. The DNS exhibits a range where the mean stream-wise velocity $U$ fits well to the log-law and the energy dissipation rate is nearly inversely proportional to the distance from the wall. The width of the range increases with the Reynolds number. In the range, the spectrum $E_{12}(k_1)$ of the cross correlation between the stream-wise and span-wise fluctuating velocity components fits well to the -7/3 power law in the inertial sub-range, where $k_1$ is the wave number in the stream wise direction. Its pre-factor is in good agreement with laboratory experiments of turbulent boundary layer by Saddoughi et al. (1994) and Tsuji (2003), and DNS of homogeneous turbulent shear flow by Ishihara et al.(2002). [Preview Abstract] |
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