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 A20: Turbulent Boundary Layers I: Large Structures |
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Chair: Nicholas Hutchins, University of Melbourne Room: 30A |
Sunday, November 18, 2012 8:00AM - 8:13AM |
A20.00001: The large-scale wall-to-wall interaction in fully developed turbulent channel flow Yong Seok Kwon, Kapil Chauhan, Charitha de Silva, Jason Monty, Nicholas Hutchins The geometry and boundary conditions in a turbulent channel flow ensure symmetry or anti-symmetry of the time-averaged turbulence statistics across the channel half-height ($y = h$). These statistics are conventionally studied over one half of the channel. However instantaneous fluctuating velocity fields often show coherent large-scale anti-symmetric features on either side of the centreline. This talk will present the interaction of such large-scale events with the turbulence originating on the opposite wall. Particle Image Velocimetry (PIV) is utilized to obtain two-component flow fields across the full channel height for $Re_\tau=hu_\tau/\nu\simeq1000-4000$. Ensemble averages of fluctuating velocity fields conditioned on clockwise and counter-clockwise swirl events at reference wall-normal positions ranging from $y=0.1h$ to $y=h$ reveal the presence of the wall-to-wall interaction of large-scale motions. Further results from conditional averaging such as fluctuating velocity contour plots and vector fields will be presented to show this wall-to-wall interaction in channel flow. A decomposition of flow fields into symmetric and anti-symmetric components shows that the anti-symmetry predominantly exists for very large-scale motions (VLSMs) of the order $10h$. [Preview Abstract] |
Sunday, November 18, 2012 8:13AM - 8:26AM |
A20.00002: Experimental investigation of coherent structures in turbulent pipe flow using a large-scale pipe flow facility David Dennis In recent years it has been shown by various researchers, using either experimental techniques or direct numerical simulations, that coherent structures (i.e. features of the flow that persist in space and time) such as hairpin vortices, vortex packets, and very large scale motions (or superstructures) play an important role in wall-bounded turbulent flows (boundary layers, pipes and channel flows). A large-scale recirculating pipe flow facility at the University of Liverpool has been developed to enable the investigation of large and very large scale coherent motions in turbulent pipe flow. The facility includes a 100mm-diameter working section, consisting of individual modules of precision-bore borosilicate glass tubes each 1.027m long, totalling 22 metres in length. Experimental measurements using high-speed stereoscopic particle image velocimetry at approximately 210 pipe diameters downstream of the inlet are made possible using a unique mechanical arrangement for performing the calibration. Reynolds numbers of up to $Re_D=10^5$ can be reached when the working fluid is water. [Preview Abstract] |
Sunday, November 18, 2012 8:26AM - 8:39AM |
A20.00003: Study of very-large-scale motions in turbulent pipe flow Jae Hwa Lee, Hyung Jin Sung Direct numerical simulation (DNS) of turbulent pipe flow was performed at \textit{Re}$_{\tau }$=544 to investigate the spatial organization of the very large-scale motions (VLSMs). The streamwise domain length employed here was 30$R$, where $R$ is the pipe radius. Inspection of the three-dimensional instantaneous fields showed that adjacent large-scale packet-like structures combine to form the VLSMs, and this formation process was attributed to continuous stretching of the hairpins coupled with lifting-up and backward curling of the vortices. To support our results found in the analysis of the instantaneous flow fields, we applied the spatial filter to decompose the signal into two length scales related to the VLSMs and smaller structures. The resulting streamwise length scale from the streamwise two-point correlations showed that the magnitude of the correlations for the VLSMs is larger than that from the large-scale motions (LSMs) through all directions. In addition, the mean inclination angle to the wall for the smaller scale structures was found to be larger than that of the VLSMs. These findings support the previous conjecture of Kim {\&} Adrian (1999) that the coherent alignment of LSMs creates the VLSMs. [Preview Abstract] |
Sunday, November 18, 2012 8:39AM - 8:52AM |
A20.00004: Structural organization of large and very-large scales in turbulent pipe flow simulation Jon Baltzer, Ronald Adrian, Xiaohua Wu The physical structures of velocity are examined in a recent DNS of fully developed incompressible turbulent pipe flow at $Re_{D}=24\ 580$ ($R^{+}=684.8$) with a periodic domain length of $30$ pipe radii $R$ (Wu, Baltzer, \& Adrian, J. Fluid Mech., 2012). In this simulation, the long motions of negative velocity fluctuation correspond to large fractions of energy present at very long streamwise wavelengths ($\geq 3R$). We study how long motions are composed of smaller motions. We characterize the spatial arrangements of very large scale motions (VLSMs) and find that they possess dominant helix angles (azimuthal inclinations relative to streamwise) that are revealed by 2D and 3D two-point spatial correlations of velocity. The correlations also reveal that the shorter, large scale motions (LSMs) that concatenate to comprise the VLSMs are themselves more streamwise aligned. We show that the largest VLSMs possess a form similar to roll cells and that they appear to play an important role in organizing the flow, while smaller scales of motion are necessary to create the strong streaks of velocity fluctuation that characterize the flow. [Preview Abstract] |
Sunday, November 18, 2012 8:52AM - 9:05AM |
A20.00005: Amplitude modulation by a synthetic very large-scale motion (VLSM) in the turbulent boundary layer I. Jacobi, B.J. McKeon A flat-plate boundary layer is dynamically forced with a spatially-impulsive, two-dimensional roughness patch, introducing a periodic velocity disturbance. This synthetic very large-scale motion (VLSM) is isolated by a phase-locked decomposition of the velocity field, and its relationship with other scales in the flow is studied in the context of the apparent amplitude modulation of small-scale motions by large scales. The modulation effect is described as a phase-relationship between the different scales, where the phase difference is identified by cross-correlation. The extent to which the synthetic VLSM can be treated as a linear superposition on the base flow is investigated, and it is shown that the phase measurements can be used to describe the dominant large-scale motions in the unperturbed flow as well as the non-equilibrium distortion of the boundary layer by the roughness. Cospectral techniques are employed to identify the particular large-scale motions dominant in the amplitude modulation in both perturbed and unperturbed flows, and it is shown that VLSMs are significant to the modulation process, even in the unperturbed boundary layer. This study is supported by the Air Force Office of Scientific Research under grant \#FA9550-09-1-0701(program manager John Schmisseur). [Preview Abstract] |
Sunday, November 18, 2012 9:05AM - 9:18AM |
A20.00006: Conditional analysis of the statistics near the turbulent/non-turbulent interface of turbulent boundary layers Hiroki Ogasawara, Takashi Ishihara Direct numerical simulations of zero-pressure-gradient turbulent boundary layer (TBL) along a flat plate are used to investigate the properties of turbulent/non-turbulent interface of the TBL. The Reynolds numbers based on the momentum thickness are from 800 to 2200. The interface is defined as the set of the outermost points of the region with the absolute values of vorticity greater than $\alpha U_{\infty} / \delta$, where $U_{\infty}$ is the free stream velocity, $\delta$ is the boundary layer thickness, and $\alpha = 0.3-0.7$ is used in our analysis. The analysis of conditional statistics of streamwise velocity $u$ and spanwise vorticity $\omega_{z}$ near the interface shows that there is a shear (a sharp change of $u$) due to a sharp change of the distribution of vorticity $\omega_{z}$ near the interface. These results are consistent with the experiments by Semin et al. (2011). DNS data also show that the shear rate near the interface is larger when the height of the interface is smaller. Visualization of the time series of the interfaces suggests that the interfaces are moving downstream without remarkable changes in their shape during a time period of the order of $\delta / U_{\infty}$. [Preview Abstract] |
Sunday, November 18, 2012 9:18AM - 9:31AM |
A20.00007: The Turbulent/Non-turbulent Interface and Entrainment in a Boundary Layer Kapil Chauhan, Jimmy Philip, Nicholas Hutchins, Charitha De Silva, Ivan Marusic The turbulent/non-turbulent (T/NT) interface in a zero pressure gradient turbulent boundary layer ($Re_\tau\approx8000$) is examined using particle image velocimetry. The interface is detected using local turbulent kinetic energy and proves to be an effective method for boundary layers. Statistically the interface exhibits a normal distribution characterizing the intermittency and has a fractal dimension of about 2.32. The presence of a T/NT superlayer is corroborated by the presence of a jump for the conditionally averaged streamwise velocity across the interface. The steep change in velocity is accompanied by a discontinuity in the vorticity and a jump in the Reynolds shear stress, in agreement with the governing equations within the superlayer. The analysis of the data indicates that the boundary layer entraiment is characterized by two distinctive length scales which appear to be associated with a two-stage entrainment process. [Preview Abstract] |
Sunday, November 18, 2012 9:31AM - 9:44AM |
A20.00008: Net Force Spectra of Wall Turbulence Ronald Adrian, Jon Baltzer In the mean momentum equation for turbulent flow the net force vector is the divergence of the turbulent Reynolds stress. In many ways it is a simpler and more fundamental quantity than the Reynolds stress. Wavenumber spectra of the streamwise net force in pipe and channel flows are shown to be similar. They define the layer structure more clearly than the mean velocity, and they show that long streamwise wavelength motions are predominantly accelerative near the wall, while short streamwise wavelengths are decelerative above the buffer layer. Spanwise wavenumber spectra provide a simpler map of the net force than the streamwise spectra, and in terms of them there is a simple boundary between accelerative and decelerative wavelengths as a function of distance above the wall. [Preview Abstract] |
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