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 T37: Turbulence: General |
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Chair: Charitha De Silva, University of New South Wales Room: 355 C |
Monday, November 25, 2024 4:45PM - 4:58PM |
T37.00001: Probing the shape of turbulence through Lagrangian holographic vorticimetry Jiaqi Li, Xiang Yang, Jiarong Hong Turbulence is often perceived through a phenomenological lens, characterized by a chain of interacting vortices with energy cascading from large to small scales. However, traditional studies of turbulence rely on statistical averaging and stochastic dynamics due to the challenges in consistently tracking and quantifying the fast-evolving structures in turbulence. In this study, we present Lagrangian Holographic Vorticimetry (LHV) as an experimental approach to geometrically probe turbulence from the integral scale down to the Kolmogorov scale. Using LHV, we simultaneously track the translation and rotation of Kolmogorov-scale tracers, examining turbulence through changes in geometric properties such as deflection, rotation, and twist of the "ribbon" defined by the velocity and vorticity vectors. These properties are observed as tracers moving from one scale to another, characterized by surges in vorticity strength associated with encounters between tracers and high-vorticity filament regions in turbulence. Furthermore, by averaging these properties across different scales along the tracer trajectories, we provide a detailed geometric representation of scale similarities involved in the energy cascade within turbulence. |
Monday, November 25, 2024 4:58PM - 5:11PM |
T37.00002: An investigation of the distortion of turbulent structures of a turbulent inflow interacting with an airfoil’s leading edge. Charitha M De Silva, Ahmed Osama O Mahgoub, Chaoyang Jiang, Danielle Moreau, Con Doolan Turbulence interactions with solid structures play a critical role in an engineering system's aerodynamic and vibroacoustic performance. Therefore, understanding these interactions and the underlying physical mechanisms that govern them is essential. Here, we examine a NACA 0012 airfoil immersed in a turbulent inflow through experiments. The experimental framework measures the flow field using high-speed particle image velocimetry, and the unsteady surface pressure near the airfoil’s leading edge is measured using a remote microphone technique. Additionally, the experiments are complimented by a set of simulations at comparable flow conditions. Results characterising the turbulent inflow conditions will be presented. Furthermore, results on the distortion of the turbulent flow structure are investigated in the near vicinity of the leading edge of the airfoil. |
Monday, November 25, 2024 5:11PM - 5:24PM |
T37.00003: Manipulating the direction of spectral energy flux in a thin-layer flow Xinyu Si, Filippo De Lillo, Guido Boffetta, Lei Fang Manipulating turbulent energy flux among different scales can be a formidable task because the energy at any scale is not localized in physical space and vice versa. The ability to control energy flux and, thus, redistribute it among different length scales can be beneficial in many processes, such as chemical reactors. Here, we report a theoretical framework to manipulate the direction of energy flux in multi-scale flows. The theoretical framework's efficacy was tested experimentally with moving grids in an electromagnetically driven thin-layer shear flow and numerically with blinking force monopoles in a linear shear flow. Our results indicate that a physical perturbation with a designed geometric alignment with the background flow can generate desired directions of net energy flux, which can be applied for a wide range of natural or engineering applications. |
Monday, November 25, 2024 5:24PM - 5:37PM |
T37.00004: Study of fractal oscillating grid turbulence using Direct Numerical Simulation and Particle Image Velocimetry Valentin Musy, Hossameldin Abdelaziz, Anne-Lise Hantson, Diane Thomas, Jean-Christophe Baudez, Francesco Romano, Tom Lacassagne Oscillating grid flows have been classically studied by experimental methods [1] while fractal fixed grids turbulence has been analysed either by experimental [2] or numerical methods. Fractal oscillating grid flows were recently studied by an experimental PIV method and we propose here a complementary approach by direct numerical simulations of the turbulence generated by such a stirring system. A simple Cartesian and 3 fractal grids (fractal Cartesian, fractal square and fractal I-shaped) were considered experimentally and for the simulations. The DNS simulations were performed by using mesh morphing strategies to mimic the vertical sinusoidal movement of the grids. Both experimental data and data from numerical simulations in the statistically steady state were analysed thanks to the triple Reynolds decomposition [3]. |
Monday, November 25, 2024 5:37PM - 5:50PM |
T37.00005: Investigating intermittency in turbulent channel and wake flows using a novel time-frequency-based method Jibin Joy Kolliyil, Melissa Brindise Intermittent flow behaviors are commonly observed in turbulent flows. In general, this intermittency is associated with enhanced mixing and non-linear, chaotic behavior. Thus, investigating intermittency will reveal the non-uniform energy dissipation mechanism present in turbulent flows. In this work, we investigate the underlying energy dissipation mechanism associated with intermittency in both turbulent channel flow and wake flows using time-frequency analysis. A direct numerical simulation (DNS) dataset of channel flow over a rough wall surface and an experimental stereoscopic particle image velocimetry (PIV) dataset of wake flow over a cylindrical bluff body are considered. We extracted both instantaneous and spatially localized frequencies of the flows using our novel time-frequency method: the Fourier-decomposition wavelet transform. Further, we investigated the associated localized energy cascade and the corresponding scaling range to understand the scaling mechanism present in these turbulent flows. Finally, the extracted localized frequency structures are compared with traditional coherent structures to understand the energy dissipation pattern. |
Monday, November 25, 2024 5:50PM - 6:03PM |
T37.00006: Unsteady Periodic Forcing of Passive Grid-Generated Turbulence John A. Farnsworth, Martin Obligado Experimental wind tunnel measurements of grid-generated turbulence were carried out to better understand the dynamic response and to assess the scaling laws for non-stationary, non-equilibrium turbulence evolving within a flow composed of large, time-periodic fluctuations of the freestream velocity. The experiments were conducted within the unsteady low-speed wind tunnel at the University of Colorado Boulder using a conventional rectangular grid with a mesh size of M = 0.10 m and a blockage ratio of 44%. Synchronized measurements were collected from a single-component hotwire anemometer and a high-speed planar particle image velocimetry system at a streamwise position of x = 2.90 m downstream from the grid (x/M = 29). The freestream velocity within the wind tunnel test section was dynamically varied in a periodic fashion using a set of louvers situated far upstream of the test-section, producing a time-varying periodic velocity with a mean of U = 13.25 m/s and an amplitude of σu = 2.70 m/s at a frequency of f = 1 Hz. These conditions provided a mean turbulence Reynolds number based upon the Taylor microscale of Reλ ≈ 350. The hysteretic nature of grid-generated turbulence was analyzed and will be discussed, for example quantifying the velocity variances, turbulent kinetic energy, integral length scales, and Taylor microscale as a function of the unsteady periodic freestream velocity and the associated streamwise pressure gradient. In addition, the local equilibrium hypothesis (Gotto & Vassilicos, Fluid Dyn. Res., 2016) and unsteady dissipation scaling (Zheng et al., JFM, 2023) will also be applied and presented. |
Monday, November 25, 2024 6:03PM - 6:16PM |
T37.00007: Experimental Comparison of Turbulent Statistics in Superfluid 4He Coflow and Counterflow Tomoya Hirayama, Yoshiyuki Tsuji The flow in a quantum turbulent field is formed from normal and superfluid components. The two components are known to interact with each other, hence the flow is complex. In this presentation, we discuss the difference between 4He coflow and thermal counterflow flow with a focus on turbulent universality in small scales. We consider that while both components move in the same direction in the coflow, the thermal counter flow moves in the opposite direction, resulting in differences in turbulence statistic. Experimental images were taken of (i) thermal counterflow with a heat source, and (ii) coflow in a grid generated wind tunnel turbulence (Maximum grid Re = 6.46 ×104, RMS = 18.0 %). Small particles were added to visualize the flow. Lagrangian velocity was calculated for these data by applying the 2D PTV method. When analyzing these velocity data, we must keep in mind that the velocity can be affected by the particle size. Frequency spectra were calculated based on the time variation of the tracked particle velocities together with the wave number spectrum. We also calculated the longitudinal second-order structure function f (r) ≡ 〈δv‖2(r)〉 , where δv‖ is the radial component of the relative velocity in nearest particle pairs at the initial image. From this result, we attempt to discuss whether the Kolmogorov’s -5/3 law is observed or not in the spectrum. |
Monday, November 25, 2024 6:16PM - 6:29PM |
T37.00008: Coherent Structure Analysis in a Backward Facing Step Sriram P Kalathoor, Dogukan T Karahan, Cansu Uzay Karahan Lagrangian coherent structures are a useful tool for characterizing the dynamical behavior of a turbulent flow, in particular, to identify evolution of vortices, and assess turbulent mixing in practical flows. In this work, large-eddy simulation of incompressible flow in a backward facing step with an inlet Reτ of 395 is performed in order to identify and correlate formation and destruction of coherent structures with vortex shedding frequencies. The simulation is performed with OpenFOAM using second-order accurate spatial and temporal schemes. Forward and backward finite-time Lyapunov exponents are utilized for the identification of repelling and attractive structures, respectively. |
Monday, November 25, 2024 6:29PM - 6:42PM |
T37.00009: Enhanced dissipation in a vector convection diffusion equation Anuj Kumar An important characteristic of turbulent flow is that the rate of energy dissipation becomes independent of viscosity in the limit of vanishingly small viscosity, a phenomenon known as anomalous dissipation. Whether there exists a family of simple (steady or time-periodic) solutions to the Navier-Stokes equations that exhibit anomalous dissipation remains an important open problem in fluid mechanics. In this work, we address this problem by considering a passive vector equation, which is a more flexible variant of the Navier-Stokes equations. Designing solutions that exhibit anomalous dissipation is quite challenging even for this flexible equation. In this work, we construct a family of solutions to the passive vector equation where the rate of energy dissipation is proportional to the viscosity raised to the power of one-third. Notably, this rate of dissipation is significantly faster than what is observed in pure diffusion which implies enhanced dissipation. Our result is established through a design based on convection rolls and a variational principle for the rate of energy dissipation. |
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