Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session D33: Turbulent Boundary Layers: Structures - I |
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Chair: Suranga Dharmarathne, Texas Tech University Room: Oregon Ballroom 202 |
Sunday, November 20, 2016 2:57PM - 3:10PM |
D33.00001: Effects of vortical motions on turbulence scalar transport in a turbulent channel flow Suranga Dharmarathne, Murat Tutkun, Ronald Adrian, Luciano Castillo Direct numerical simulations of a turbulent channel flow at Reynolds number, $Re_{\tau}=394$, (based on friction velocity and channel half height) were carried out to investigate the relationship between coherent vortices and turbulence scalar transport. Previous observations from three-dimensional two-point correlations of streamwise velocity fluctuations $(u)$ and scalar fluctuations $(\theta)$ suggest that three-dimensional iso-surfaces of correlations are distinctively different in all regions of the turbulent channel. Hence, it can be hypothesized that the scalars are more attracted to vortical structures of the flow and the velocity fluctuations reside in low-momentum regions induced by hairpin packets or vortex clusters. In order to test this, we first employ vortex identification methods ($\lambda_2$, $\lambda_{ci}$, and $Q-$ criterion) to capture the regions of high vortical activity. Then the three-dimensional correlations between scalar fluctuations and extracted vortices are computed to investigate the validity of the hypothesis. Behavior of the streamwise component of fluctuating flux term $(u\theta)$ and the wall-normal component of fluctuating flux term $(v\theta)$ are studied to quantify the relation between scalar fluxes and vortices. [Preview Abstract] |
Sunday, November 20, 2016 3:10PM - 3:23PM |
D33.00002: Hairpin vortices in the outer and near wall regions of the canonical turbulent boundary layer James wallace, Xiaohua Wu, Parviz Moin While the dominance of hairpin vortices and their significance for transport processes in the transitional and early turbulent regions of the canonical turbulent boundary layer has been widely accepted, opinion is divided about the developed flow downstream. Here we investigate the representative vortical structures in the outer and near wall regions for the momentum thickness Reynolds number, $Re_\theta$, of up to 3000 using the DNS database described in Phys. Fl. 26, 091104. This boundary layer grows spatially from a laminar state at $Re_\theta = 80$ beneath a freestream of continuous and nearly isotropic turbulence decaying from an intensity of 3 to 0.8$\%$. The vortical structures are visualized with the swirling strength, $\lambda_{ci}$. In the outer region hairpin vortices appear and are advected over distances corresponding to about 300 - 400 in $Re_\theta$ within the fully turbulent region, demonstrating that they are not remnants of transitional structures. The near wall vortical structures are more difficult to visualize and require careful tuning of the swirling strength and making invisible the flow above the near wall region of the flow. The hairpins in this region occur in intermittent clusters that have features remarkably similar to transitional turbulent spots. [Preview Abstract] |
Sunday, November 20, 2016 3:23PM - 3:36PM |
D33.00003: Spectral structure and linear mechanisms in a `rapidly' distorted boundary layer Sourabh Diwan, Jonathan Morrison A characteristic feature of a turbulent boundary layer (TBL) at high Reynolds numbers is the presence of coherent motions such as the `large scale motions' and `superstructures'. In this work we attempt to mimic such coherent motions and their spectral structure using a simplified experimental arrangement of a boundary layer flow over a flat plate subjected to grid-generated turbulence and/or localized patch of surface roughness. The velocity measurements done downstream of a grit roughness patch (in absence of grid turbulence) show that over a certain distance the energy spectrum of streamwise velocity fluctuations shows a bi-modal shape which resembles that found in a high-Re TBL. We also carry out experiments with both grid turbulence and grit roughness present and show that it is possible to `synthesize' the structure of a TBL in the wall-normal direction, in the limited context of streamwise coherent motions, using the present experimental design. These results indicate that the predictions of the Rapid Distortion Theory (RDT) can be applied to the present case in a region close to the plate leading edge, and we examine the linearized effects of `blocking' and `shear' on turbulent fluctuations near the edge of the boundary layer and close to the wall in the framework of the RDT. [Preview Abstract] |
Sunday, November 20, 2016 3:36PM - 3:49PM |
D33.00004: Relating instantaneous structures and mean flow characteristics of turbulent boundary layers Charitha de Silva, Jimmy Philip, Nicholas Hutchins, Ivan Marusic Recent works have highlighted the presence of thin interfacial layers of high shear that demarcate regions of relatively uniform streamwise momentum in turbulent boundary layers. Here, we aim to further our understanding of how such a zonal-like structural arrangement manifests in the averaged flow statistics. To this end, we start by identifying high shear interfaces in turbulent boundary layers employing particle image velocimetry databases that span more than an order of magnitude of friction Reynolds number ($Re_\tau = 10^3$--$10^4$). Inspection of these recurrent features reveal that their geometry is highly contorted and exhibits self-similarity across a wide range of scales. The Reynolds number dependence of these features is also investigated, together with their associated scaling. Based on these findings and the persistent presence of sharp changes in momentum in turbulent boundary layers, a simple model is presented towards reconstructing the mean velocity profile. [Preview Abstract] |
(Author Not Attending)
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D33.00005: Decomposition of multi-scale coherent structures in a turbulent boundary layer by variational mode decomposition Wenkang Wang, Chong Pan, Jinjun Wang Turbulent boundary layer (TBL) is believed to contain a wide spectrum of coherent structures, from near-wall low-speed streaks characterized by inner scale to log-layer large-scale coherent motions (LSM and VLSM) characterized by outer scale. Recent studies have evidenced the interaction between these multi-scale structures via either bottom-up or top-down mechanisms, which implies the possibility of identifying the coexistence of their footprints at medium flow layer. Here, we propose a Quasi-Bivariate Variational Mode Decomposition method (QB-VMD), which is an update of the traditional Empirical Mode Decomposition (EMD) with bandwidth limitation, for the decomposition of the PIV measured 2D flow fields with large ROI ($\Delta x\times \Delta z\sim 4\delta \times 1.5\delta $ ) at specified wall-normal heights ($y/\delta =0.05\sim 0.2$ ) of a turbulent boundary layer with $Re_{\tau } =3460$. The empirical modes identified by QB-VMD well capture the characteristics of log-layer LSMs as well as that of near-wall streak-like structures. The lateral scales of these structures are analyzed and their respective energy contribution are evaluated. [Preview Abstract] |
Sunday, November 20, 2016 4:02PM - 4:15PM |
D33.00006: Influence of large-eddy breakup device on near-wall turbulent structures in turbulent boundary layer Joon-Seok Kim, Jinyul Hwang, Min Yoon, Junsun Ahn, Hyung Jin Sung Direct numerical simulation of a large-eddy breakup (LEBU) device in a spatially developing turbulent boundary layer was performed to investigate the influence of outer structures on the near-wall turbulence. The thin and rectangular shaped LEBU device was placed on $y$/$\delta \quad =$ 0.8 and the device reduced the skin-friction coefficient ($C_{f})$ up to 17{\%}. Decomposition of $C_{f}$ showed that the contribution of the Reynolds shear stress decreased along the wall-normal direction. The reduction of the Reynolds shear stress was associated with the decrease of the ejection and sweep events, and in particular the latter was significantly reduced compared to the former in the near-wall region. The spanwise length scale of high-speed structures was more shortened than that of low-speed very near the wall ($y^{\mathrm{+}} \quad =$ 20). As a result, the dispersive motions induced by the outer sweeps were weakened leading to the reduction of $C_{f}$ even the LEBU device located far from the wall. [Preview Abstract] |
Sunday, November 20, 2016 4:15PM - 4:28PM |
D33.00007: Influence of large-scale low- and high-speed structures on the near-wall vortical motions in turbulent boundary layer Jinyul Hwang, Hyung Jin Sung Direct numerical simulation data of turbulent boundary layer ($Re_{\tau } =1000)$ are used to investigate the large-scale influences on the vortical structures in the near-wall region. The streamwise swirling strength ($\lambda_{x})$ depends on the strength of large-scale streamwise velocity fluctuations ($u_{l})$. The amplitude of $\lambda_{x}$ decreases under the negative $u_{l}$ rather than the positive $u_{l,}$ analogous to the amplitude modulation of the velocity fluctuations. The dependence of $\lambda_{x\thinspace }$on $u_{l}$ is due to the opposite spanwise motions in the footprints of low- and high-speed structures, which are congregative and dispersive, respectively. Conditionally averaged fields conditioned on $\lambda_{x}$ under the negative- and positive-$u_{l\thinspace }$events show that the swirling motions lie within the congregative and dispersive motions. The dispersive motion is more intense than the congregative motion because the sweep of high-momentum fluid under $u_{l} >0$ splats on the wall while the spanwise motions under $u_{l} <0$ decrease as the flows come close to each other. Due to the strong dispersive motion, the small-scale spanwise velocity fluctuations ($w_{s})$ are strengthened, whereas $w_{s}$ are relatively weak (attenuated) under $u_{l} <0$. As a result, the wall-normal components are also enhanced under $u_{l} >0$, which contributes to the amplification of $\lambda_{x}$. [Preview Abstract] |
Sunday, November 20, 2016 4:28PM - 4:41PM |
D33.00008: Spatio-temporal characteristics of large scale motions in a turbulent boundary layer from direct wall shear stress measurement Rommel Pabon, Casey Barnard, Lawrence Ukeiley, Mark Sheplak Particle image velocimetry (PIV) and fluctuating wall shear stress experiments were performed on a flat plate turbulent boundary layer (TBL) under zero pressure gradient conditions. The fluctuating wall shear stress was measured using a microelectromechanical 1mm $\times$ 1mm floating element capacitive shear stress sensor (CSSS) developed at the University of Florida. The experiments elucidated the imprint of the organized motions in a TBL on the wall shear stress through its direct measurement. Spatial autocorrelation of the streamwise velocity from the PIV snapshots revealed large scale motions that scale on the order of boundary layer thickness. However, the captured inclination angle was lower than that determined using the classic method by means of wall shear stress and hot-wire anemometry (HWA) temporal cross-correlations and a frozen field hypothesis using a convection velocity. The current study suggests the large size of these motions begins to degrade the applicability of the frozen field hypothesis for the time resolved HWA experiments. The simultaneous PIV and CSSS measurements are also used for spatial reconstruction of the velocity field during conditionally sampled intense wall shear stress events. [Preview Abstract] |
Sunday, November 20, 2016 4:41PM - 4:54PM |
D33.00009: Streak instability as an initiating mechanism of the large-scale motions in a turbulent channel flow. Matteo de Giovanetti, Hyung Jin Sung, Yongyun Hwang The large-scale motions (or bulges) have often been believed to be formed via merge and/or growth of the near-wall hairpin vortical structures. Here, we report our observation that they can be directly generated by an instability of the amplified streaky motions in the outer region (i.e. very-large-scale motions) through the self-sustaining process. We design a LES-based numerical experiment in turbulent channel flow for $Re_\tau=2000$ where a body forcing is implemented to artificially drive an infinitely long streaky motion in the outer layer. As the forcing amplitude is increased, it is found that a new energetic structure emerges at $\lambda_x\approx3\sim4h$ of the streamwise length ($h$ is the half height of channel) particularly in the wall-normal and spanwise velocities. A careful statistical examination reveals that this structure is likely to be linked with the sinuous-mode streak instability of the amplified streak, consistent with previous theoretical studies. Application of dynamic mode decomposition to this instability further shows that the phase speed of this structure scales with the outer velocity and it is initiated around the critical layer of the streaky flow. [Preview Abstract] |
Sunday, November 20, 2016 4:54PM - 5:07PM |
D33.00010: On validating Quasi-Steady Quasi-Homogeneous nature of the relationgship between large-scale and small-scale structures in a turbulent boundary layer Chi Zhang, Sergei Chernyshenko A formal definition to the two hypotheses of the quasi-steady and quasi-homogeneous (QSQH) theory was proposed. The theory is supposed to explain the phenomenon of the large-scale structures influencing the small-scale structures in a turbulent boundary layer. Multi-objective optimisations were performed to find the optimal cut-off parameters for the new large-scale filters. The new filters were proved to obtain much more clear large-scale structures than the filter suggested by the previous studies. Calculations and comparisons for a set of statistical flow properties extracted from the databases of the direct numerical simulations of a plane channel flow were conducted. The accuracy of the predictions based on the QSQH theory was observed improving when the Reynolds number increases. Extrapolations of $u_{rms}$ and two-points correlation from medium to high Reynolds number based on the QSQH approach were preformed and about $10\%$ accuracy was reported. The more interesting thing is that the QSQH theory implies a dependence of the mean profile log-law constants on the Reynolds number. The main overall result of the present work is the validations of the two hypotheses of the quasi-steady quasi-homogeneous theory in near-wall turbulent flows. [Preview Abstract] |
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