Bulletin of the American Physical Society
74th Annual Meeting of the APS Division of Fluid Dynamics
Volume 66, Number 17
Sunday–Tuesday, November 21–23, 2021; Phoenix Convention Center, Phoenix, Arizona
Session E19: Turbulence: Boundary layers |
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Chair: Ronald Adrian, Arizona State University Room: North 132 ABC |
Sunday, November 21, 2021 2:45PM - 2:58PM |
E19.00001: Energy transfer in the inner and outer layers of APG TBLs Taygun Recep R Gungor, Yvan Maciel, Ayse G Gungor The spectral distribution of energy, production and pressure-strain in channel flows, zero-pressure-gradient (ZPG) turbulent boundary layers (TBLs) and non-equilibrium adverse-pressure-gradient (APG) TBLs are investigated to understand the energy transfer in the inner and outer layer of APG TBLs with small and large velocity defect cases. In the inner layer, the inner-scaled spectra demonstrate that the relative size and wall-normal position of structures with respect to each other remain similar as the velocity defect increases from channel flow to APG TBL with a large velocity defect. Furthermore, the 2D spectral distributions show that the relative size of energy-carrying and energy-transferring structures does not profoundly change with velocity defect in the outer layer. These similarities, along with others that will be presented at the meeting, suggest that the energy transfer mechanisms in the inner and outer layers may not be significantly affected by the velocity defect and remain almost the same within one layer in channel flows, ZPG TBLs and APG TBLs. |
Sunday, November 21, 2021 2:58PM - 3:11PM |
E19.00002: Three unexpected properties of the turbulent boundary layer Paolo Luchini We shall discuss experimental and numerical evidence extracted from the literature of three unexpected features of the turbulent boundary layer: 1) The effect of pressure gradient upon a turbulent velocity profile is opposite in sign to its effect upon a laminar velocity profile, and as a consequence cannot be predicted by mixing-length or eddy-viscosity models. 2) The asymptotic decay of both Reynolds stresses and velocity defect at the outer edge of the turbulent boundary layer is faster than in a laminar boundary layer (and in a constant-eddy-viscosity model). 3) The verification of Clauser equilibrium (defect-layer self-similarity) through a careful interpolation highlights a different behavior of DNS and experimental results. |
Sunday, November 21, 2021 3:11PM - 3:24PM |
E19.00003: Probing the shape of inertial subrange eddies in boundary layer turbulence Michael Heisel, Charitha M De Silva, Gabriel G Katul, Marcelo Chamecki The inertial subrange of turbulent scales – whose signature is a power law in scale-dependent statistics – is widely studied in both theory and experiments. Yet the shape of these motions in physical space is often described in vague, ill-defined terms such as eddies or blobs. This presentation builds upon previous studies on the topic of fractal geometries to provide a more detailed picture of the instantaneous flow features that underpin the inertial subrange power law signature. Wind tunnel boundary layer measurements are used to assess the geometric properties of streamwise velocity isosurfaces. The size of "wrinkles" along each isosurface is statistically self-similar only within the inertial subrange. Simplified velocity signals are used to demonstrate that these wrinkles yield power laws in one-dimensional (point) statistics including the longitudinal structure function. The findings suggest that the presence of inertial subrange turbulent motions is reflected by the self-similar wrinkles of isosurfaces that are persistent throughout the flow field. |
Sunday, November 21, 2021 3:24PM - 3:37PM |
E19.00004: Scaling turbulence over rib-roughened permeable walls Okazaki Yuki, Yumeto Takase, Yusuke Kuwata, Kazuhiko Suga To clarify the combined effects of wall permeability and surface roughness on turbulence, PIV measurements of turbulent channel flows over porous media with structural surface roughness are conducted. Square porous ribs whose height k is 10% of the channel height are set at regular intervals at w/k=1, 3, 7, 9, 19. Three kinds of porous media applied have the same porosity, 0.9, but different mean pore diameters. The measured bulk Reynolds numbers are from 5000 to 20000. After fitting the mean velocity profiles to the logarithmic law, discussions of the log-law parameters are conducted. It is then confirmed that the zero-plane displacement has a linear relationship with the roughness scale in the case of w/k=1~9 and that the newly determined effective displacement well correlates to the mean pore diameter. Further discussions on the Kármán constant are required to find the universal law of the mean velocity although they generally correlate to the pore Reynolds number. |
Sunday, November 21, 2021 3:37PM - 3:50PM |
E19.00005: Wall-attached and detached structures in turbulent boundary layers Min Yoon, Hyung Jin Sung 3-D coherent u clusters are explored from the perspective of the attached-eddy model. We extract clusters of the streamwise velocity fluctuations (u) by using the connectivity of six-orthogonal neighbors in Cartesian coordinates without any assumptions. To this end, DNS datasets for adverse pressure gradient (APG) (β = 1.43) and zero pressure gradient (β = 0) turbulent boundary layers (TBLs) with Re_{τ} ≈ 800 are used. Here, β is the non-dimensional pressure gradient parameter. The identified structures can be decomposed into attached self-similar, attached non-self-similar, detached self-similar and detached non-self-similar structures with respect to y_{min} and l_{y} , which are the minimal distance from the wall and the height of clusters, respectively. The sizes of the attached self-similar structures (100 < l_{y}^{+} < 0.6δ^{+}) scale with l_{y}, and their population density is inversely proportional to l_{y} in the region l_{y}/δ = 0.4−0.58. Here, δ is the boundary layer thickness. They contribute to the logarithmic variation in the streamwise Reynolds stress and to the presence of the k_{z}^{−1} region in the pre-multiplied energy spectra of u, where k_{z} is the spanwise wavenumber. The attached self-similar structures are universal wall motions in the logarithmic region that are not dependent on the pressure gradient. The attached non-self-similar structures (l_{y}/δ > 0.6) are responsible for the enhanced large scales in the outer region in the presence of the APG. Their streamwise sizes are Ο(3−6δ), and some of them extend over 6δ in the streamwise direction and penetrate deeply into the near-wall region, reminiscent of very-large-scale motions. In addition, wall-detached structures are also decomposed into self-similar and non-self-similar ones. We establish new insights into coherent structures in TBLs and the development of the attached-eddy model. |
Sunday, November 21, 2021 3:50PM - 4:03PM |
E19.00006: Spectral coherence analysis of turbulent flows using stochastically forced linearized Navier-Stokes Armin Zare, Seyedalireza Abootorabi Recent data-driven efforts have utilized spectral decomposition techniques to uncover the geometrically self-similar features of dominant energy-containing motions in high-Reynolds number flows, thereby lending support to physics-based models that are derived from the attached-eddy hypothesis. In this study, we evaluate the predictive capability of the stochastically forced linearized Navier-Stokes equations in capturing such geometric signatures of turbulent flows. We overcome the shortcomings of linearized models by considering their statistical response to judiciously designed white- and colored-in-time stochastic forcing. This allows us to perfectly capture two-dimensional energy spectra at specific wall-normal locations and to provide good predictions of two-point correlations of the turbulent velocity field that comprise the cross-spectrum. We use the latter to construct spectral filters that can decontaminate the energy of logarithmic-layer turbulence from the contributions of wall-detached and very large-scale motions and study the geometric scaling of dominant flow structures that result from this spectral coherence analysis. |
Sunday, November 21, 2021 4:03PM - 4:16PM |
E19.00007: Reynolds stress budgets in orthogonal curvilinear coordinates for a turbulent flow over a curved ramp via DNS Abhiram B Aithal, Antonino Ferrante The physical mechanisms of turbulence in separated flows over curved walls are not yet well understood. In order to provide the necessary turbulence statistics to explain the physical mechanisms of such flows, we have derived analytically the equations for the mean flow, and for the budgets of turbulence kinetic energy (TKE) and Reynolds stress in orthogonal curvilinear coordinates. Also, we have recently developed an explicit, third-order Runge-Kutta based projection method, FastRK3, which requires solving the Poisson equation for pressure only once per time step to solve the incompressible Navier-Stokes equations in orthogonal curvilinear coordinates (Aithal & Ferrante, J. Comput. Phys, 2020). In the current work, using FastRK3, we have performed direct numerical simulations (DNS) of a spatially developing turbulent boundary layer over a curved ramp with adverse pressure-gradient and discuss the budgets of TKE and Reynolds stress in orthogonal curvilinear coordinates. |
Sunday, November 21, 2021 4:16PM - 4:29PM |
E19.00008: Reynolds-number independent modelling of low-order statistics in turbulent Ekman flow Cedrick Ansorge Modelling of low-order statistics, in particular of the mean profiles of wind speed and direction, is a crucial task for many applications in the boundary layer. We present here a formulation of the profiles of wind speed and direction in turbulent Ekman flow based on asymptotic theory and data from direct numerical simulation. The profile of the stream-wise component follows the classical viscous, logarithmic and wake scaling. In the outer layer, the velocity component proles can be described by an Ekman-spiral with adapted boundary conditions that result in a reduction of the spiral-like rotation. The span-wise component poses a conceptual challenge to the channel-ow analogy in the context of asymptotic matching; it exhibits a mixed scaling in the surface layer, but follows outer scaling for most of the outer layer. |
Sunday, November 21, 2021 4:29PM - 4:42PM |
E19.00009: Baroclinicity and directional shear explain departures from the logarithmic wind profile Elie R Bou-Zeid, Khaled Ghannam Similarity and scaling arguments underlying the existence of a logarithmic wind profile in the atmospheric surface layer (ASL) invoke two simplistic assumptions of negligible Coriolis effects (no wind turning) and vertically-uniform pressure gradients (barotropic PBL). In this talk, we relax these oversimplifications to provide a more realistic representation of the ASL where the common occurrence of baroclinicity (height-dependent pressure gradients) and wind turning, traditionally treated as outer-layer phenomena, are accounted for in the ASL. The constant-stress ASL assumption is first replaced by a model for the Reynolds stress derived from the mean momentum equations to incorporate the cross-isobaric angle (directional shear) and a dimensionless baroclinicity parameter (geostrophic shear). This results in a model for the wind profile, obtained from first-order closure principles, correcting the log-law with an additive term that is linear in height and accounts for the combined effects of wind turning and baroclinicity. Both the stress and wind models agree well with a suite of large-eddy simulations in the barotropic and baroclinic ABL. The talk concludes with the potential applications and further developments of the model. |
Sunday, November 21, 2021 4:42PM - 4:55PM |
E19.00010: Turbulence-Resolving Simulations of Atmosphere-Surface Coupling in the Marginal Ice Zone: The Interacting Effects of Temperature and Roughness Heterogeneity Joseph Fogarty, Elie R Bou-Zeid, Ming Pan, Andrey Grachev, Linette Boisvert, Mitchell Bushuk, Luc Deike, Jose D Fuentes The Arctic surface is undergoing rapid changes due to the warming of the Earth's atmosphere, including the reduction in perennial sea ice, creation of leads in early spring, and fractured sea ice in late spring and summer. The thermal contrast between warm sea water in leads and adjacent cold Arctic sea ice generates secondary circulations that drastically enhance the transport of heat, humidity, halogens, and aerosols. We propose a nondimensional framework to reduce the parameter space of this complicated turbulent atmospheric boundary layer flow over fractured sea ice. Large-eddy simulations are conducted to understand the non-linear interactions of surface temperature and roughness contrasts between ice and water surfaces. Idealized flow setups are considered where the surface wind is parallel, perpendicular, or oblique to the ice/water interface. The results elucidate the synergistic interactions of surface temperature and roughness in the context of heterogeneous Arctic sea ice, and the critical role of wind angle. The conclusions will inform the development of parametrizations for mesoscale and global models, underlining the importance of accounting for unresolved processes and feedbacks that give rise to the non-linear atmosphere-surface coupling. |
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