Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session G31: Boundary Layers: Structure and Turbulence |
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Chair: Ralph Volino, US Naval Academy Room: Georgia World Congress Center B403 |
Monday, November 19, 2018 10:35AM - 10:48AM |
G31.00001: Scale separation of velocity structures associated with large passive scalar gradients in a turbulent boundary layer Angeliki Laskari, Theresa A Saxton-Fox, Beverley J McKeon Planar particle image velocimetry measurements are performed in a thermal boundary layer with simultaneous measurements of the streamwise density gradient, averaged across the boundary layer height. Previous work on the topic (Saxton-Fox, 2018) revealed that large density gradients were linked to tall wall-normal velocity structures extending across the entire boundary layer height. It was postulated that they result from a wall-normal average of smaller scales, which reside at different locations from the wall, and contribute to the same density gradient change. Results from the present work support this model and show that, if a second condition on the vertical location of the identified features is imposed, one can extract wall-normal velocity structures that are localized in the vertical direction, their distance from the wall is linked to the phase of large-scale streamwise velocity fluctuations, and they convect with different velocities. Further, filtering of the density gradient signal reveals a strong scale separation in both wall-normal and streamwise velocities, allowing a multi-scale analysis of the velocity structures associated with large scalar gradients. |
Monday, November 19, 2018 10:48AM - 11:01AM |
G31.00002: Integral properties of turbulent-kinetic-energy production and dissipation in turbulent wall-bounded flows Tie Wei Integral properties of TKE production and dissipation are investigated in turbulent wall-bounded flows. The main findings include 1) the global integral identity for the TKE production is derived from the mean momentum balance equation. 2) At sufficiently high Reynolds number, the integrals of TKE production and dissipation are equally partitioned around the peak Reynolds shear stress location. 3) The integral of the TKE production and the integral of the TKE dissipation exhibit a logarithmic-like layer similar to that of the mean streamwise velocity. The logarithmic-like scaling of the global integral of TKE production and dissipation, the equal partition of the integrals of TKE production and dissipation around the peak Reynolds shear stress location, and the logarithmic-like layer in the integral of TKE production and dissipation are intimately related. The equal partition of the integral of the TKE production and dissipation around the peak Reynolds shear stress location underlines the important role of the meso-layer and meso-length scale in the dynamics of turbulent wall-bounded flows. |
Monday, November 19, 2018 11:01AM - 11:14AM |
G31.00003: Existence and properties of the logarithmic layer in oscillating flows Steven J Kaptein, Matias Duran-Matute, Vincenzo Armenio, Herman Clercx n this study, direct numerical simulations and wall-resolved large-eddy simulations are performed to test the logarithmic layer assumption in an oscillating boundary layer throughout the oscillation cycle. This is of interest for several industrial and environmental applications for which the wall-region is often bypassed in favour of a wall models to reduce computational time of numerical simulations of turbulent oscillating flows. Several configurations are tested by varying the Reynolds number, and the ratio between the water depth and the thickness of the Stokes boundary layer. In particular, we focus on how the existence of the logarithmic layer depends on the value of the Reynolds number, the depth to boundary layer thickness ratio, the phase, and in some cases, the oscillation cycle. Additionally, when the logarithmic layer exists, the universality of the von Karman constant and of the logarithmic layer intercept was thoroughly tested. |
Monday, November 19, 2018 11:14AM - 11:27AM |
G31.00004: Estimating the turbulent flow field of an atmospheric boundary layer from LIDAR data using LES-based 4D-Var Pieter Bauweraerts, Johan Meyers Recent advances in flow field measurement techniques, e.g. LIDAR, allow for a vast amount of information to be collected in the atmospheric boundary layer in the proximity (O(10 km)) of a sensor. This opens up possibilities for new short term turbulent flow field predictions techniques, with time scales up to an hour, which can for example be useful for wind farm power output estimates. The retrieved measurement data is typically spread highly irregular in space and time, due to the typical sweeping motion of the sensors. This makes the estimation of the current flow field state a challenging problem. To this end, we eploy 4D-Var data-assimilation, where we use a large-eddy simulation (LES) model, run on a coarse grid to limit computational complexity, as the sublying model. Virtual LIDAR measurements are taken from a high resolution LES simulation, which is an ideal setup for development and testing purposes. The 4D-Var optimization problem, is solved using L-BFGS, combined with an adjoint LES simulation for the gradient calculation, due to the shear amount of optimization variables. The methodology is illustrated on a pressure driven boundary layer for different case studies. |
Monday, November 19, 2018 11:27AM - 11:40AM |
G31.00005: Thermal and vortical layers in transcritical wall turbulence Jean-Pierre Hickey, Zeping Sun, Matthew Yao, Fan Duosi, Kukjin Kim, Carlo Scalo
Fully-developed, turbulent boundary layers are characterized by zones of uniform momentum which are delineated by internal interfacial layers (IIL). The uniform momentum zones (UMZ) have been shown to be related to the underlying coherent structures in wall-bounded turbulence. A novel IIL identification method, proposed by Duosi et al., allows the characterization of an analogous thermal layer in the transcritical turbulent channel flow. The thermal internal interfacial layer (TIIL) delineate regions of homogeneous thermodynamic properties. The thermal and vortical interfaces are correlated but not collocated in the turbulent boundary layer. The TIILs are statistically located halfway between the wall and the IIL. Their existence and location can be understood through the prism of the attached eddy model which is highlighted by a strong correlation between the heights of both layers. Conditional sampling at the TIIL shows the presence of a strong thermal gradient which is not observed in the mean flow. The understanding of the thermal layering in transcritical flow permits a simplification of the wall scaling under complex thermodynamic conditions which will be discussed in this presentation. |
Monday, November 19, 2018 11:40AM - 11:53AM |
G31.00006: Wall turbulence organization in the fully rough, very-high-Re logarithmic layer Michael Heisel, Jiarong Hong, Filippo Coletti, Michele Guala We present super-large-scale particle image velocimetry (SLPIV) measurements in the thermally-neutral atmospheric surface layer. The field site represents a canonical fully rough wall boundary layer with a Reynolds number of the order Re_{τ}∼10^{6}. The measurements capture the lowest 20 m of the surface layer and include the bottom of the logarithmic region. Structural features of the boundary layer observed in previous laboratory PIV studies are similarly present in our results at the atmospheric scale. We use the spatio-temporal SLPIV data to describe the spatial organization of wall turbulence in terms of vortex structures, zones of uniform momentum (UMZs), and internal shear layers. We compare the properties and wall-normal trends of these features to the characteristic scales of the logarithmic region, namely the friction velocity and the wall-normal distance. We discuss the findings in the context of existing theory such as the attached eddy hypothesis. |
Monday, November 19, 2018 11:53AM - 12:06PM |
G31.00007: Bursts in turbulent channel flows and the interaction between flow structures of multiple scales Yongseok Kwon, Javier Jimenez In the logarithmic region of turbulent channel flows, strong bursts of the wall-normal velocity, v, are known to play a crucial role in the production of turbulent kinetic energy (TKE). In turbulent channel flows, the only possible mechanism by which the wall-normal velocity can gain kinetic energy (v^{2}) is via the work of pressure (i.e. continuity) as represented by ⟨pdv/dy⟩ term in the TKE budget equations. Therefore, events of strong pdv/dy are used as a proxy for strong bursts of v. When the wall-normal locations of strong pressure source and burst are close, the underlying flow pattern is the backward leaning v structures as previously observed (Jimenez, Phys. Fluids 2013). However, when the wall-normal locations of strong pressure source and burst are far apart, bursts are associated with a certain alignment of flow structures of different sizes, revealing a mechanism in which the interaction between structures of multiple scales can lead to strong bursts in the log region of turbulent channel flows. |
Monday, November 19, 2018 12:06PM - 12:19PM |
G31.00008: Estimation of large scale motions in a turbulent boundary layer from direct wall shear stress measurements Rommel Jose Pabon, Lawrence Ukeiley, Mark Sheplak Direct measurements of the fluctuating wall shear stress are captured in a zero pressure gradient turbulent boundary layer at Re_{θ} ≈ 5000 with a 1 mm × 0.2 mm floating element capacitive sensing system. The velocity field is captured simultaneously in separate stereo particle image velocimetry (PIV) data planes, and the estimated velocity field is created based on correlations with targeted wall shear stress events. These events are characterized by instantaneous wall shear stress magnitude, local turbulence intensity via VITA-type approaches, and large scale patterns via low pass filtering. The estimated field is compared to the time-resolved hot-wire measurements of single point velocity to ascertain the effects of the frozen field hypothesis at this Reynolds number, using previous measurements of the convection velocity of the wall shear stress. In addition, the comparison between the spatial- and temporal- reconstruction is used to capture incipient attached eddy scaling at this Reynolds number. |
Monday, November 19, 2018 12:19PM - 12:32PM |
G31.00009: Effects of thermal winds on the structure and similarity of the atmospheric surface layer Khaled Ghannam, Mostafa Momen, Elie Bou-Zeid Atmospheric surface layer (ASL) flows are often modeled as an analogue to the inertial region of wall flows, where Townsend’s attached eddy model predicts that the distance from the wall and friction velocity are the proper length/velocity scales for the active motion. Monin-Obukhov Similarity Theory (MOST) accounts for the effects of stratification through stability correction functions. However, field experiments have shown contradicting results regarding the similarity and scaling laws predicted by these theories. The work here examines the effects of thermal winds (geostrophic shear/baroclinicity) on ASL flows as a possible cause for such departures. A suite of large eddy simulations spanning a range of stability conditions, and strength and rotation of the height-dependent geostrophic velocity vector was designed to explore how baroclinicity modifies the local shear production in the ASL, leading to increased importance of turbulent and pressure transport, and hence stronger coupling with the outer-layer eddies. The mechanisms behind such changes are explored through the budgets of velocity and temperature variances, covariances, and heat fluxes to understand how turbulence is modified and to explain closure problems in field experiments and climate models. |
Monday, November 19, 2018 12:32PM - 12:45PM |
G31.00010: Turbulence Structure in Non-zero Pressure Gradient Boundary Layers Ralph Volino Experimental measurements were made in non-equilibrium boundary layers on a smooth wall. In the test section, a zero pressure gradient (ZPG) development region was followed by a favorable pressure gradient (FPG) with constant acceleration parameter K, a ZPG recovery region, and an adverse pressure gradient region with constant K. The acceleration rates were as high as K=2×10^{-6} in the FPG and -1×10^{-6} in the APG. Velocity profiles were acquired at 12 streamwise stations using LDV. Velocity field measurements were acquired at the same locations using PIV in the streamwise-wall normal plane and two streamwise-spanwise planes at y/δ=0.15 and 0.4. Turbulence statistics, two point correlations of various turbulence quantities, and linear stochastic estimation were used to analyze the data. Strong FPG caused deviation from the law of the wall, suppression of the wake, significant lengthening of structures in the streamwise direction, and a reduction of inclination angles with respect to the wall. The ZPG recovery was rapid. APG behavior was opposite to the FPG with strong growth of the wake, but less pronounced dimensionless changes in the correlations. |
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