Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session E19: Boundary Layers: Structure and Turbulence II |
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Chair: Beverley McKeon, California Institute of Technology Room: 207 |
Sunday, November 22, 2015 4:50PM - 5:03PM |
E19.00001: The relationship between amplitude modulation, coherent structure and critical layers in wall turbulence Beverley McKeon The importance of critical layers in determining aspects of the structure of wall turbulence is discussed. We have shown (Jacobi \& McKeon, 2013) that the amplitude modulation coefficient investigated most recently by Hutchins \& Marusic (2007) and co-authors, which describes the correlation between large scales above a (spatial) wavelength filter with the envelope of small scales below the filter, is dominated by very large scale motion (VLSM) at a single wavelength. The resolvent analysis of McKeon \& Sharma (2010) gives a suitable model for the three-dimensional, three-component form of the VLSM and energetic structure at other wavelengths. This model is used to identify the three-dimensional spatial variation of instantaneous critical layers in the presence of a mean velocity profile and to relate this to earlier observations of coherent structure in unperturbed flows (both experimental and via the resolvent model, Sharma \& McKeon, 2013); to the phase relationships between scales identified by Chung \& McKeon (2010, 2014); and to the structure of wall turbulence that has been modified by the addition of single synthetic scales, e.g. Jacobi \& McKeon (2011), Duvvuri \& McKeon (2015). [Preview Abstract] |
Sunday, November 22, 2015 5:03PM - 5:16PM |
E19.00002: Coinciding Features in a Turbulent Boundary Layer via Lagrangian Coherent Structures, Dynamic Mode Decomposition and Proper Orthogonal Decomposition Naseem Ali, Murat Tutkun, Rau'l Cal Low order decompositions and Lagrangian coherent structures are used to identify structures in a high-Reynolds-number turbulent boundary layer flow. Data are collected in Laboratoire de M\'{e}canique de Lille (LML) wind tunnel using time resolved stereo particle image velocimetry. Low-order descriptors are based on proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) frameworks to obtain energy content and frequency information of the flow, respectively. Repelling and attracting Lagrangian coherent structures (LCS)s reveal complex patterns within the flow field containing a hyperbolic behavior and the shapes of the attracting and repelling vary with advection time as result of the temporal coherence. The attracting and repelling LCSs are matched with POD and DMD modes to understand the relationship between the frameworks and respective representations. The POD is used as a low pass filtering of kinetic energy and then mode-dependent velocity reconstructions provide, firstly, the most coherent features of the flow and second are employed to generate new mode-based LCSs. This representations then provide clarity as to the organization of the LCS based on the energy contained in them and the dynamic coherence. [Preview Abstract] |
Sunday, November 22, 2015 5:16PM - 5:29PM |
E19.00003: An improved method to characterise the modulation of small-scale turbulent by large-scale structures Lionel Agostini, Michael Leschziner, Datta Gaitonde A key aspect of turbulent boundary layer dynamics is ``modulation,'' which refers to degree to which the intensity of coherent large-scale structures (LS) cause an amplification or attenuation of the intensity of the small-scale structures (SS) through large-scale-linkage. In order to identify the variation of the amplitude of the SS motion, the envelope of the fluctuations needs to be determined. Mathis et al(2009) proposed to define this latter by low-pass filtering the modulus of the analytic signal built from the Hilbert transform of SS. The validity of this definition, as a basis for quantifying the modulated SS signal, is re-examined on the basis of DNS data for a channel flow. The analysis shows that the modulus of the analytic signal is very sensitive to the skewness of its PDF, which is dependent, in turn, on the sign of the LS fluctuation and thus of whether these fluctuations are associated with sweeps or ejections. The conclusion is that generating an envelope by use of a low-pass filtering step leads to an important loss of information associated with the effects of the local skewness of the PDF of the SS on the modulation process. An improved Hilbert-transform-based method is proposed to characterize the modulation of SS turbulence by LS structures [Preview Abstract] |
Sunday, November 22, 2015 5:29PM - 5:42PM |
E19.00004: Long structures in a turbulent boundary layer with a rod-roughened wall Jung Hoon Kim, Jae Hwa Lee Direct numerical simulation (DNS) of a turbulent boundary layer (TBL) over a rod-roughened wall is performed with long streamwise domain to examine streamwise scale growth mechanism of streamwise velocity fluctuating structures. The surface roughness is the two-dimensional (2-D) rod arranged regularly in the streamwise direction. Inspection of the instantaneous flow fields and streamwise two-point spatial correlations of the streamwise velocity fluctuations shows that the streamwise and spanwise length scales of the structures over the rough wall are generally larger than those over the smooth wall, while the spanwise inclination angle to the streamwise direction (helix angle) is smaller over the rough wall. Inspection of time-evolving instantaneous fields clearly exhibits that adjacent long structures combine to form a longer structure by spanwise merging process over the rough wall, and the occurrence of the spanwise merging for the streamwise scale growth is expected to be more active over the rough wall due to turbulent scales generated by the 2-D roughness. [Preview Abstract] |
Sunday, November 22, 2015 5:42PM - 5:55PM |
E19.00005: Large-scale motions in a plane wall jet Ebenezer Gnanamanickam, Latim Jonathan, Bhatt Shibani The dynamic significance of large-scale motions in turbulent boundary layers have been the focus of several recent studies, primarily focussing on canonical flows - zero pressure gradient boundary layers, flows within pipes and channels. This work presents an investigation into the large-scale motions in a boundary layer that is used as the prototypical flow field for flows with large-scale mixing and reactions, the plane wall jet. An experimental investigation is carried out in a plane wall jet facility designed to operate at friction Reynolds numbers $Re_\tau > 1000$, which allows for the development of a significant logarithmic region. The streamwise turbulent intensity across the boundary layer is decomposed into small-scale (less than one integral length-scale $\delta$) and large-scale components. The small-scale energy has a peak in the near-wall region associated with the near-wall turbulent cycle as in canonical boundary layers. However, eddies of large-scales are the dominating eddies having significantly higher energy, than the small-scales across almost the entire boundary layer even at the low to moderate Reynolds numbers under consideration. The large-scales also appear to amplitude and frequency modulate the smaller scales across the entire boundary layer. [Preview Abstract] |
Sunday, November 22, 2015 5:55PM - 6:08PM |
E19.00006: Large scale structures in a turbulent boundary layer and their imprint on wall shear stress Rommel Pabon, Casey Barnard, Lawrence Ukeiley, Mark Sheplak Experiments were performed on a turbulent boundary layer developing on a flat plate model under zero pressure gradient flow. A MEMS differential capacitive shear stress sensor with a 1 mm $\times$ 1 mm floating element was used to capture the fluctuating wall shear stress simultaneously with streamwise velocity measurements from a hot-wire anemometer traversed in the wall normal direction. Near the wall, the peak in the cross correlation corresponds to an organized motion inclined 45$^\circ$ from the wall. In the outer region, the peak diminishes in value, but is still significant at a distance greater than half the boundary layer thickness, and corresponds to a structure inclined 14$^\circ$ from the wall. High coherence between the two signals was found for the low-frequency content, reinforcing the belief that large scale structures have a vital impact on wall shear stress. Thus, estimation of the wall shear stress from the low-frequency velocity signal will be performed, and is expected to be statistically significant in the outer boundary layer. Additionally, conditionally averaged mean velocity profiles will be presented to assess the effects of high and low shear stress. [Preview Abstract] |
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