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 D19: Boundary Layers: Structure and Turbulence I |
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Chair: Stephen Garrett, University of Leicester Room: 207 |
Sunday, November 22, 2015 2:10PM - 2:23PM |
D19.00001: Large scale structures in transitional pipe flow Leo Hellstr\"om, Bharathram Ganapathisubramani, Alexander Smits We present a dual-plane snapshot POD analysis of transitional pipe flow at a Reynolds number of 3440, based on the pipe diameter. The time-resolved high-speed PIV data were simultaneously acquired in two planes, a cross-stream plane (2D-3C) and a streamwise plane (2D-2C) on the pipe centerline. The two light sheets were orthogonally polarized, allowing particles situated in each plane to be viewed independently. In the snapshot POD analysis, the modal energy is based on the cross-stream plane, while the POD modes are calculated using the dual-plane data. We present results on the emergence and decay of the energetic large scale motions during transition to turbulence, and compare these motions to those observed in fully developed turbulent flow. [Preview Abstract] |
Sunday, November 22, 2015 2:23PM - 2:36PM |
D19.00002: Competing stability modes in vortex structure formation Stephen Garrett, J. Paul Gostelow, Aldo Rona, W. Andrew McMullan Nose cones and turbine blades have rotating components and represent very practical geometries for which the behavior of vortex structures is not completely understood. These two different physical cases demonstrate a common theme of competition between mode and vortex types. The literature concerning boundary-layer transition over rotating cones presents clear evidence of an alternative instability mode leading to counter-rotating vortex pairs, consistent with a centrifugal instability. This is in contrast to co-rotating vortices present over rotating disks that arise from crossflow effects. It is demonstrated analytically that this mode competes with the crossflow mode and is dominant only over slender cones. Predictions are aligned with experimental measurements over slender cones. Concurrent experimental work on the flow over swept cylinders shows that organized fine-scale streamwise vorticity occurs more frequently on convex surfaces than is appreciated. The conventional view of purely two-dimensional laminar boundary layers following blunt leading edges is not realistic and such boundary layers need to be treated three-dimensionally, particularly when sweep is present. The vortical structures are counter-rotating for normal cylinders and co-rotating under high sweep conditions. Crossflow instabilities may have a major role to play in the transition process but the streamline curvature mode is still present, and seemingly unchanged, when the boundary layer becomes turbulent. [Preview Abstract] |
Sunday, November 22, 2015 2:36PM - 2:49PM |
D19.00003: The relation between skin friction fluctuations and turbulent fluctuating velocities in turbulent boundary layers Carlos Diaz Daniel, Sylvain Laizet, John Christos Vassilicos The Townsend-Perry hypothesis of wall-attached eddies relates the friction velocity $u_{\tau}$ at the wall to velocity fluctuations at a position $y$ from the wall, resulting in a wavenumber range where the streamwise fluctuating velocity spectrum scales as $E(k) \sim k^{-1}$ and the corresponding structure function scales as $u_{\tau}^{2}$ in the corresponding length-scale range. However, this model does not take in account the fluctuations of the skin friction velocity, which are in fact strongly intermittent. A DNS of zero-pressure gradient turbulent boundary layer suggests a 10 to 15 degree angle from the lag of the peak in the cross-correlations between the fluctuations of the shear stress and streamwise fluctuating velocities at different heights in the boundary layer. Using this result, it is possible to refine the definition of the attached eddy range of scales, and our DNS suggests that, in this range, the second order structure function depends on filtered skin friction fluctuations in a way which is about the same at different distances from the wall and different local Reynolds numbers. [Preview Abstract] |
Sunday, November 22, 2015 2:49PM - 3:02PM |
D19.00004: Study of the near field wake of trips generating an artificially thick turbulent boundary layers Eduardo Rodriguez Lopez, Paul J.K. Bruce, Oliver R.H. Buxton The properties of an artificially thick turbulent boundary layer are influenced by its formation mechanism. Previous work has shown that wake or wall-driven mechanisms dominate boundary layer development depending on the trips’ aspect ratio. The current study characterizes these two formation mechanisms through the use of high-speed PIV in the near wake of obstacles arrays on a flat plate in a wind tunnel. The time resolved velocity field is studied using Optimal Mode Decomposition (OMD) generating a low order model which captures the representative motions. Results corroborate the original hypothesis and show that these mechanisms are divided in two families: (i) High aspect ratio trips (cylinders) generate vortices with a wall-normal axis which do not transfer information between the wall and the wake of the obstacle. In this case, the boundary layer growth is wall-driven entraining the low-momentum highly turbulent flow above it. (ii) Low aspect ratio trips generate spanwise vorticity increasing the influence of the obstacle’s wake in the wall region (wake-driven mechanism). A high level of correlation with the velocity fluctuations at the wall is maintained in case (ii) for the whole wake while in case (i) the correlation vanishes for heights smaller than half obstacle. [Preview Abstract] |
Sunday, November 22, 2015 3:02PM - 3:15PM |
D19.00005: Large scale motions of thermal transport in a turbulent channel Suranga Dharmarathne, Murat Tutkun, Guillermo Araya, Stefano Leonardi, Luciano Castillo The importance of large scale motions (LSMs) on thermal transport in a turbulent channel flow at friction number of 394 is investigated. Two-point correlation analysis reveals that LSM which significantly contribute to turbulence kinetic energy and scalar transport is a reminiscent of a hairpin packet. Low-order mode representation of the original fields using proper orthogonal decomposition (POD) unveils that the most dominant mode that transports $\left\langle u^{\prime 2}\right\rangle$ is 3-4 channel half-heights long and such structure which contribute to scalar transport is 2-4 channel half-heights long. Consequently, the study discloses that LSMs are effective in transporting both streamwise component of turbulence kinetic energy and scalar variances. [Preview Abstract] |
Sunday, November 22, 2015 3:15PM - 3:28PM |
D19.00006: Evolution of vortex-surface fields in the K-type temporal transition in channel flow Yaomin Zhao, Yue Yang, Shiyi Chen The vortex-surface field (VSF), a Lagrangian-based method (Yang and Pullin, \emph{J. Fluid Mech.}, 685, 2011), is used to study the evolution of vortical structures in the K-type temporal transition in channel flow. Iso-surfaces of an evolving VSF can represent vortex surfaces composed of vortex lines in evolution. Since the VSF was only used in simple flows with periodic boundary conditions, the validity of different wall boundary conditions for the VSF transport equation is first discussed, and then the Neumann boundary condition is applied in the implementation. The initial VSF is uniquely determined by proposed criteria, and its iso-surfaces are streamwise-spanwise planes. Compared with the evolution of material surfaces with the same initial scalar field, the VSF evolution can capture the topological changes of vortical structures that are induced by the interaction between different hairpin-like vortices. It is noted that the vortex reconnection is a critical mechanism for the breakdown of vortical structures in the late transitional stage, and is challenging to be characterized via Eulerian-based methods. [Preview Abstract] |
Sunday, November 22, 2015 3:28PM - 3:41PM |
D19.00007: Unified concepts in internal and external wall-turbulence Yong Seok Kwon, Cheng Chin, Nicholas Hutchins, Jason Monty Recently, Kwon, Hutchins and Monty (J. Fluid Mech., submitted) reported that the oscillation of turbulent/non-turbulent interface (TNTI) in a turbulent boundary layer can contaminate the fluctuating velocity component under the traditional Reynolds decomposition, which overestimates the extent of spatial coherence of velocity fluctuations. In order to overcome this issue, they proposed a new velocity decomposition method which removes the influence of the TNTI oscillation from the velocity fluctuations. Extension of their decomposition method to internal flows via the analogy between the free-stream and quiescent core (Kwon et al., J. Fluid Mech., vol. 751, 2014, pp. 228-254) reveals that both the scale and structure of turbulence are, in fact, similar in internal and external flows even up to the edge of the outer region. Based on this result, a new conceptual model for internal flows, that is similar to external flows, is proposed. [Preview Abstract] |
Sunday, November 22, 2015 3:41PM - 3:54PM |
D19.00008: Multi-scale geometric analysis of evolving Lagrangian structures in the compressible transitional boundary layer at $Ma=0.7$ Wenjie Zheng, Yue Yang, Shiyi Chen Evolutionary geometry of flow structures in a compressible transitional boundary layer at $Ma=0.7$ is investigated from a Lagrangian perspective. The Lagrangian structures in the transition are extracted from the Lagrangian scalar field by a moving window filter, and then their geometry is characterized by the multi-scale and multi-directional geometric analysis (Yang and Pullin, \emph{J. Fluid Mech.}, 674, 2011), including the averaged inclination and sweep angles at different scales ranging from one half of the boundary layer thickness to several viscous length scales $\delta_\nu$. The results show that averaged angles are almost unaltered for different scales before the transition. As the transition occurs, averaged inclination angles increase and sweep angles decrease rapidly with increasing reference time. Furthermore, the orientation changes more significantly for structures with small scales than large scales. In the late stage of transition, the averaged inclination angle of small-scale structures with the length scale $\sim O(10)\delta_\nu$ is $42\,^{\circ}$, and the averaged sweep angle in the logarithm law region is approximately $30\,^{\circ}$. [Preview Abstract] |
Sunday, November 22, 2015 3:54PM - 4:07PM |
D19.00009: Spatial organisation of large scale structures in turbulent boundary layers Felix Eich, Nico Reuther, Matthew Bross, Christian Kaehler The experimental investigation of the spatial organization of large-scale structures in a turbulent boundary layer at large Reynolds numbers is a difficult task due to the size of the turbulent structures and their mutual distance in streamwise and spanwise directions. However, by aligning various PIV systems side by side it was possible to resolve all relevant scales simultaneously in the Reynolds number range~${Re}_{\tau }=$4180-13355. This range of Reynolds numbers was selected to fully characterise the relevant scaling of these large structures. The measurements were performed in wall-parallel planes at several selected wall distances, to examine the variation of the average width and spanwise distance between the large scale flow structures. The acquired vector fields were analysed by means of two-point correlations. Form these correlations the average width and streamwise extent of large-scale structures was determined. Using conditional correlations, it was possible to separate and characterise the high and low speed structures. The statistical multipoint analysis shows that there are distinct variations between high and low speed structures as well as between the large-scale structures at different wall distances.~ [Preview Abstract] |
Sunday, November 22, 2015 4:07PM - 4:20PM |
D19.00010: Stress Boundary layer Development in Planar flow of Viscoelastic Fluids Nariman Ashrafi, Meysam Mohamadali Two-dimensional steady planar creeping flow of the nonlinear viscoelastic Upper Convected Maxwell (UCM) fluid along a flat plate is analyzed for high Weissenberg numbers, Wi. The viscoelastic boundary layer, formed in a thin region closer to the wall in which the relaxation terms are recovered. By means of similarity transformations the non-linear momentum and constitutive equations in each layer transform into a system of highly nonlinear coupled ordinary differential equations. The proper similarity variable is found that asymptotically matches each two adjacent layers. The numerical simulation shows that at the outer layer, the velocity profile changes linearly with the similarity variable meaning that no velocity boundary layer is developed. In general, the boundary layer is formed in all three stress components in different fashions. The stress boundary layer divides the flow into two separate regions of viscoelastic and elastic flows, in addition to the top outer flow. The viscoelastic region is completely bounded in two directions (x and y) for horizontal normal stress, T$_{\mathrm{xx}}$, and shear stress, T$_{\mathrm{xy}}$. Finally it is observed that the stress boundary layer for vertical stress, T$_{\mathrm{yy}}$, is formed only in x direction. [Preview Abstract] |
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