Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session A20: Boundary Layers II: Structure |
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Chair: M. Malik, NASA Langley Room: 315 |
Sunday, November 24, 2013 8:00AM - 8:13AM |
A20.00001: Large- and very-large-scale motions in wall-bounded flows up to $\delta^+\approx2000$ Juan A. Sillero, Javier Jim\'enez Large- (LSM) and very-large-scale motions (VLSM) are investigated using DNSes of turbulent boundary layers and channels in very long computational domains, reaching $\delta^+\approx 2000$. Two-dimensional connected regions deviating above or below the local mean by more than a given threshold, are constructed in wall-parallel planes for the velocity components and for the pressure, and probability density functions are computed for their streamwise and spanwise lengths. Exponential tails are observed in the buffer, logarithmic and outer regions, that are characteristic of Poisson accretion processes, suggesting that the structures grow by merging smaller ones. In addition, analysis of the tails provides evidence for the presence of VLSMs in $u$, whereas only LSMs are present in the transversal velocities $(v,w)$ and in the pressure. Although the size of the structures depends on the thresholding value -- chosen here as the fraction of the standard deviation at each height that maximizes the number of regions -- it is found that boundary layers are shorter and narrower than channels, in agreement with two-point correlations measurements. [Preview Abstract] |
Sunday, November 24, 2013 8:13AM - 8:26AM |
A20.00002: An investigation of the very large scale motions in turbulent pipe flow Leo Hellstr\"om, Alexander Smits The very large sale motions (VLSM) in fully developed pipe flow were characterized using three component, time-resolved Stereoscopic Particle Image Velocimetry (SPIV). The work was conducted at Reynolds number of $50,000$ and $100,000$. In accordance with previous work, the VLSM have a characteristic $10-20R$ in pipe flow, meandering structures, contain $40-60\%$ of the TKE and $30-50\%$ of the Reynolds shear stress. Classical and snapshot POD were performed on the 3C fluctuating velocity field. It is shown that Proper Orthogonal Decomposition (POD) can be used as a low energy filter to extract the VLSM. The POD eigenfunctions and the reconstructed velocity field is then used to investigate the behavior and three-dimensional structure of the VLSM. [Preview Abstract] |
Sunday, November 24, 2013 8:26AM - 8:39AM |
A20.00003: Hairpin Vortices: Autogeneration and Interaction Daniel Sabatino, Rijan Maharjan, Andrew Sanders The regeneration of hairpin vortices is examined in a free-surface water channel where vortices are artificially generated by means of injection in a laminar boundary layer. The process is visualized with dye and hydrogen bubble-wire techniques. The strength of an isolated hairpin required to begin the autogeneration process is established by means of PIV measurements on the symmetry plane. Because hairpins are in close proximity in a fully-turbulent boundary layer, two hairpins are generated at different streamwise locations and allowed to interact at different stages of development. The relative position, strength and maturity of the interacting hairpins that generate secondary vortices are examined. The morphology of the generation process and of the resulting secondary hairpin for both the isolated and interacting cases are discussed and compared to previous work. [Preview Abstract] |
Sunday, November 24, 2013 8:39AM - 8:52AM |
A20.00004: A study of synthetic large scales in turbulent boundary layers Subrahmanyam Duvvuri, Mitul Luhar, Casey Barnard, Mark Sheplak, Beverley McKeon Synthetic spanwise-constant spatio-temporal disturbances are excited in a turbulent boundary layer through a spatially impulsive patch of dynamic wall-roughness. The downstream flow response is studied through hot wire anemometry, pressure measurements at the wall and direct measurements of wall-shear-stress made using a novel micro-machined capacitive floating element sensor. These measurements are phase-locked to the input perturbation to recover the synthetic large-scale motion and characterize its structure and wall signature. The phase relationship between the synthetic large scale and small scale activity provides further insights into the apparent amplitude modulation effect between them, and the dynamics of wall-bounded turbulent flows in general. Results from these experiments will be discussed in the context of the critical-layer behavior revealed by the resolvent analysis of McKeon \& Sharma (\emph{J Fluid Mech}, 2010), and compared with similar earlier work by Jacobi \& McKeon (\emph{J Fluid Mech}, 2011). Model predictions are shown to be in broad agreement with experiments. [Preview Abstract] |
Sunday, November 24, 2013 8:52AM - 9:05AM |
A20.00005: Spatial properties of large-scale structure in a turbulent boundary layer Jinyul Hwang, Jin Lee, Seo Yoon Jung, Tamer A. Zaki, Hyung Jin Sung Direct numerical simulation (DNS) database of a zero pressure-gradient turbulent boundary layer was scrutinized to investigate the spatial distribution of vortices around a streamwise-elongated low-speed structure. The turbulent flow field has been numerically produced by preceding the bypass transition simulation using the isotropic free-stream turbulence, in which the Reynolds number reaches up to $Re_{\theta} =3280.$ In the present study, the low-speed structures and vortices have been identified and tracked in order to obtain spatial properties of the large-scale structures. The information about the inclination angle, distance and population of individual vortices on the long streamwise structure has been investigated. Finally, the present study provides statistical evidence on the formation of large-scale packet-like structure and its variation along the downstream. [Preview Abstract] |
Sunday, November 24, 2013 9:05AM - 9:18AM |
A20.00006: ABSTRACT WITHDRAWN |
Sunday, November 24, 2013 9:18AM - 9:31AM |
A20.00007: Temporal evolution of Townsend's attached eddies Adrian Lozano-Duran, Javier Jimenez The temporal evolution of the eddies responsible for the momentum transfer in a turbulent channel are studied using time-resolved DNS data at $Re_\tau=4000$. The eddies are identified as connected regions of intense tangential Reynolds stress, and tracked in time. Once their evolutions are properly organized, they provide the necessary information to characterize eddies from birth to death. Eddies are born at all distances from the wall, although with higher probability near it, where the shear is strongest. Most of them stay small and do not last for long times. However, there is a family of eddies that become large enough to get attached to the wall while they reach into the logarithmic layer. They can be considered the best candidates for Townsend's attached eddies found until now. They are geometrically self-similar, with sizes and lifetimes proportional to their distance from the wall. Eddies associated with ejections move away from the wall with $dy/dt=u_\tau$, and their base attaches very fast at the beginning of their lives. Conversely, sweeps move towards the wall at $-u_\tau$, and attach later. In both cases, they remain attached for $2/3$ of their lives. In the streamwise direction, eddies are advected and sheared by the local mean velocity. [Preview Abstract] |
Sunday, November 24, 2013 9:31AM - 9:44AM |
A20.00008: Homage to Bob Brodkey at 85: ejections, sweeps and the genesis and extensions of quadrant analysis James Wallace Almost 50 years ago Bob Brodkey and his student, Corino, conceived and carried out a visualization experiment for the very near wall region of a turbulent pipe flow (JFM 37) that, together with the turbulent boundary layer visualization of Kline et al. (JFM 30), excited the turbulence community. Using a high speed movie camera mounted on a lathe bed that recorded magnified images in a moving frame of reference, they observed the motions of small particles in the sub- and buffer-layers. Surprisingly, these motion were not nearly so locally random as was the general view of turbulence at the time. Rather, connected regions of the near wall flow decelerated and then erupted away from the wall in what they called ``ejections.'' These decelerated motions were followed by larger scale connected motions toward the wall from above that they called ``sweeps.'' Brodkey and Corino estimated that ejections accounted for $70\%$ the Reynolds shear stress at $Re_d = 20,000$ while only occurring about $18\%$ of the time. Wallace et al. (JFM 54) attempted to quantify these visual observations by conceiving of and carrying out a quadrant analyisis in a turbulent oil channel flow. This paper will trace this history and describe the expanding use of these ideas in turbulence research today. [Preview Abstract] |
Sunday, November 24, 2013 9:44AM - 9:57AM |
A20.00009: Probability density function of pressure in turbulent boundary layers Yoshiyuki Tsuji, Yoshinobu Yamamoto We study the probability density function (PDF) inside turbulent boundary layer and discuss how their shapes vary depending on the Reynolds number and the distance from the wall. Pressure fluctuations are measured by small pressure probes developed by our research group so far. Experimental data are measured in high Reynolds number facilities. We measured both static pressure and wall pressure simultaneously in turbulent boundary layers up to Reynolds numbers based on the momentum thickness 44000. Direct numerical simulation of channel flow data is used for the present research. It is found that PDF shapes have slightly Re-number dependence but they are self-similar if normalized by their standard deviation. There is no great difference of PDF between channel and boundary layer flows expect near wall region. [Preview Abstract] |
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