Bulletin of the American Physical Society
77th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 24–26, 2024; Salt Lake City, Utah
Session A39: Turbulent Shear Layers and Wakes |
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Chair: Giacomo Valerio Iungo, University of Texas at Dallas Room: 355 E |
Sunday, November 24, 2024 8:00AM - 8:13AM |
A39.00001: On the experimental study of 3D turbulent shear layers Dipendra Gupta, Vedant Kumar, Johan Larsson, Gregory P Bewley Flow skewing in turbulent boundary layers is reported to cause a reduction in maximum Reynolds stresses and drag. However, definitive conclusions regarding 3D turbulent shear layers, or skewed shear layers—where merging shear layers have not only different speeds but also different directions—remain elusive. We experimentally investigated the mean flow and turbulence statistics of 3D turbulent shear layers in a wind tunnel to find out if boundary layers and shear layers display similar effects. We generated 3D shear layers by skewing the mean flow with turning vanes at the trailing edge of a splitter plate, and used pitot-static tubes and x-wires to probe the flow at different cross-, span- and downstream distances. The findings indicate that 3D shear layers, like their 2D counterparts, have self-similar-like mean velocity profiles, and spread approximately linearly with downstream distance. However, the 3D shear layers have larger thickness and reduced maximum Reynolds stresses in near-downstream region, while opposite trend is observed far-downstream when compared to the 2D case, suggesting reduced mixing as in 3D boundary layers. |
Sunday, November 24, 2024 8:13AM - 8:26AM |
A39.00002: Three-dimensional effects in turbulent shear layers Vedant Kumar, Dipendra Gupta, Gregory P Bewley, Johan Larsson Canonical turbulent boundary layers are known to be affected by introducing a skew in the mean flow, with some observations including reduction in turbulent shear stress and drag in the initial transient. However, similar insights are lacking for turbulent shear layers. We are using direct numerical simulations (DNS) to study the qualitative and quantitative effects of three-dimensionality in shear layers. We consider an idealized version of the problem where two turbulent boundary layers with different freestream velocity vectors, say developing on either side of a splitter plate, come together to create a turbulent shear layer. In the long-time limit, the skewed shear layer behaves like its planar counterpart, albeit in a different frame of reference. This suggests that the three-dimensional effects, if any, are limited to the initial transient. Therefore, the current work is aimed at quantifying the extent and timescale of these transient three-dimensional effects. |
Sunday, November 24, 2024 8:26AM - 8:39AM |
A39.00003: Is the Taylor scale relevant to turbulent sublayer dynamics at the turbulent/non-turbulent interface? Zeeshan Saeed, Christopher M White, Tracy L Mandel The turbulent/non-turbulent interface (TNTI) is a thin layer separating rotational and irrotational flow that plays a critical role in the multi-scale process of entrainment. In this work, the thickness δT of the turbulent sublayer (TS) within the TNTI is assessed using a kinematic scaling analysis that matches the time scales between the interacting scales of motion from within the turbulent core and the TS. This yields a mixed-length scale parameter for scaling the TS thickness, which closely approximates the physical size of the TS, i.e δT/(η2/3λ1/3) ≈ 3 (where η and λ denote the Kolmogorov and the Taylor micro length scales, respectively). This highlights the practical utility of the scaling parameter from the perspective of resolving the TS in experiments and simulations. It is also shown that the new scaling parameter is quadratically related to the established scaling parameter η, through a weak Reλ dependence. The theory is then compared with the data available in the literature spanning from 60 < (Reλ=u'λ/ν) < 400 (where ν is the kinematic viscosity). The difference between the currently proposed scaling parameter (η2/3λ1/3), and the one previously proposed (η) is also discussed. This is done by comparing the Burger's vortex model (yielding δT ~ η) with the present time scale arguments which incorporate the coherent strains experienced by the operative scales of motion in the TS along their axial lengths. It is argued that the axial length scales of these coherent strains are on the same order of magnitude as the Taylor length scales. |
Sunday, November 24, 2024 8:39AM - 8:52AM |
A39.00004: Leveraging similarity in coherent structures to alleviate computational costs for high Reynolds number wake simulations Divyanshu Gola, Sutanu Sarkar Coherent structures in a turbulent wake can be obtained using Spectral Proper Orthogonal Decomposition (SPOD). Structural similarity in dominant coherent structures that are identified by SPOD analysis is demonstrated between Re = 5 × 103 and Re = 5 × 104 cases of a disk wake. The dominant SPOD modes in streamwise-constant planes are similar in shape between the two Re values although there is more high-mode energy at the higher Re. Cross-stream plane data for Re = 5 ×104 is reconstructed using the eigenmodes of Re = 5 × 103 and numerically modifying their energy. The reconstructed data is used to initialize body-exclusive spatial simulation at Re = 5 × 104 whose results are compared to those from body-inclusive simulations. This hybrid method of simulating a body-exclusive higher Reynolds number wake using body-inclusive simulation data from a lower Reynolds number leads to considerable savings in computational cost. |
Sunday, November 24, 2024 8:52AM - 9:05AM |
A39.00005: Yaw Influence on Unsteady Ship Airwakes and Turbulence Intensity Around the Landing Deck of SFS2 for Shipboard Helicopter Operations Sanjeev Kumar, Imran Afgan, Vladimir Parezanovic Unsteady airwake behind a maritime vessel superstructure is modelled using computational fluid dynamics (CFD), as an essential step in optimizing the ship/helipad design and helicopter operational envelopes. This study focuses on simulating the unsteady ship airwakes and turbulence intensity around the SFS2 model under varying wind yaw angles. The objective is to create a sensitivity map of major flow parameters above the flight deck which may be relevant to helicopter operations envelope. This database would subsequently be used to train Machine Learning (ML) algorithms which would predict the unsteady wake properties for different flow conditions. In the current study, wind yaw angles ranging from 0° (headwind) to 180° (tailwind), in 15° increments, are investigated to assess their impact on airwake patterns and turbulence intensity. The results reveal significant variations in airwake structure and turbulence intensity as the wind yaw angle changes. At 0° yaw, a symmetric wake pattern is observed as expected from a mean flow converged over long time-scales. The wake gradually transitions to more asymmetric patterns with increasing yaw angle, especially up to 45°. The major results will be presented in the form of sensitivity maps of velocity and turbulence magnitudes above the helipad, and compared with the existing ship-helo operational envelopes. |
Sunday, November 24, 2024 9:05AM - 9:18AM |
A39.00006:
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Sunday, November 24, 2024 9:18AM - 9:31AM |
A39.00007: Comparison of NATO-GD and SFS2 Ship Airwakes Using Low-Rank Models Guillermo Mazzilli, Zheng Zhang, Ebenezer P Gnanamanickam, John G Leishman Spectral analysis and reduced-order modeling were used to compare the airwake characteristics across multiple frequency bands of two generic Navy ship shapes, SFS2 and NATO-GD. Measurements were carried out in a low-speed wind tunnel at a Reynolds number of 3.2 million based on ship length. Two different incident wind conditions, namely a uniform headwind and a simulated atmospheric boundary layer (ABL) were considered. In the case of the uniform headwind, both airwakes exhibited similar turbulent kinetic energy (TKE) levels at low frequencies (St < 0.5 based on ship height) distributed among the first ten modes. When an ABL was introduced, the NATO-GD airwake distributed more TKE among higher frequencies but kept a uniform TKE distribution among the modes. The SFS2 airwake showed a significant TKE increase only in the first two modes at the lowest frequency band (St < 0.1), indicating a more substantial contribution from coherent structures. These opposite effects highlight significant differences in the sensitivity of each airwake to changes in upstream conditions. |
Sunday, November 24, 2024 9:31AM - 9:44AM |
A39.00008: Determination of the leading order eddy diffusivity in a turbulent wake using PIV data and the Macroscopic Forcing Method Hoyean Le, Cong Wang, Morteza Gharib, Ali Mani Dispersion of passive scalars, e.g., contaminants or heat, in turbulent wake flows is often mathematically modeled using a gradient diffusion model via the so-called eddy diffusivity. Such models assume locality and isotropy in dispersion closure operators and are often tuned to match reference data, but the validity of those assumptions is rarely assessed for realistic flow fields. In this study, we consider planar space-time resolved velocity fields measured via PIV in a turbulent wake flow behind a triangular wedge to quantitatively assess the eddy diffusivity closure for such flows. The measured velocity data are used to numerically solve the passive scalar transport equation using the Macroscopic Forcing Method (MFM). We will present the leading coefficients of the Kramers-Moyal expansion of the eddy diffusivity operator. The quantified leading order coefficients represent a local but anisotropic eddy diffusivity tensor field. Using a priori and a posteriori analyses of scalar transport against DNS solutions, we will present an assessment of the relative importance of nonlocality versus anisotropy in such flows. |
Sunday, November 24, 2024 9:44AM - 9:57AM |
A39.00009: Experimental study of the Reynolds number dependence of a sphere wake at fixed Froude number Madeline Samuell, Scott E Wunsch, Nerion Zekaj Wakes in density-stratified fluids radiate internal waves due to the random turbulent motions in the core of the wake (Meunier et al. JFM 2018; Rowe, Diamessis, and Zhou JFM 2020). To investigate the relationship between the core turbulent wake and the resulting internal waves, experiments with a dimpled sphere were conducted using particle image velocimetry. The Reynolds number was varied between 50,000 and 100,000 to investigate how wake characteristics would scale up and down and all experiments were conducted with a Froude number of about 80. The experiments extend prior unstratified wake results of Saunders et al. (PRF 2020; Exp. Fluids 2022) to the stratified regime. This presentation will focus on the core turbulent wake behind the sphere out to and the radiated internal waves extending to a dimensionless time of Nt ~ 50, where N is the buoyancy frequency. |
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