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 J37: Turbulence: Experimental Methods |
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Chair: Michele Guala, University of Minnesota Room: 355 C |
Sunday, November 24, 2024 5:50PM - 6:03PM |
J37.00001: Comparison of the homogeneous turbulence created by a turbulence box to that of an active grid. Ankit Gautam, Tim Berk For the investigation of homogeneous turbulence, both turbulence boxes and active grids are widely used. In this study, we compare the design, operation, and performance of these two systems. Our turbulence box uses a unique actuation mechanism in which 64 actuators are individually controlled, enabling spatial variation in control parameters including power and frequency, similar to the capability of active grids. The synthetic jet turbulence box, featuring 64 speakers within a 24"×24"×24" cubical chamber, and the active grid, with a 12"×12" cross-sectional area and a 6×6 mesh operated by 12 stepper motors, were both controlled via Python code. The turbulence box operation involved adjusting power, frequency, and on/off timing, while the active grid's operation included modifications in shaft speed, acceleration/deceleration, angular displacement, flipping, and winglet position relative to the flow direction. Performance evaluation was based on turbulent energy, isotropy, integral length scale, integral time scale, Reynolds number based on Taylor microscale, and Kolmogorov scales. Additionally, the effect of mesh size on turbulence and shear turbulence was studied, comparing both generators. The research analyzed the variation of turbulence parameters under similar operational modes, including speed/power, winglet opening and closing frequency versus speaker frequency, periodic sinusoidal velocity variations, velocity steps and single gusts, spatial operation of the shaft versus the speakers' rows/columns operation, and shear generation. This study provides a comprehensive comparison and insight into the operational and performance characteristics of the synthetic jet turbulence box and the active grid with a wind tunnel. |
Sunday, November 24, 2024 6:03PM - 6:16PM |
J37.00002: Turbulent Coherent Structures and Circulation in Planar Contractions Abdullah A Alhareth, Vivek Mugundhan, Kenneth Langley, Sigurdur T Thoroddsen We measure time-resolved volumetric velocity fields in three 4:1 planar contractions with different contraction angles. We use four high-speed cameras and the Lagrangian particle tracking (LPT) technique for the measurements. Experiments are conducted in a gravity-driven water tunnel with interchangeable 2-D contraction sections. The planar contractions have different streamwise lengths which we call the short, intermediate, and long contractions. An active grid generates turbulence which is the advected through the contractions to study its evolution. The grids operated in the random mode at 210 rpm resulted in Taylor Reynolds numbers of 170-220 at the inlet to the contractions. Consistent with Prandtl’s theory for axisymmetric contractions, streamwise velocity fluctuations decay, while the transverse fluctuations are enhanced due to straining, for all three contractions. The strength of the preferential streamwise alignment of the coherent vortical structures is quantified using two measures, both based on the probability density function of cosine of angle between the structure and the mean flow, i.e. its cumulative probability and its moment of inertia. The intermediate contraction, which interestingly has a length almost equal to its inlet width, exhibits the strongest alignment. This has been reaffirmed by looking at the relative strengths of circulations Γx, Γy, and Γz computed in mutually perpendicular planes. |
Sunday, November 24, 2024 6:16PM - 6:29PM |
J37.00003: Fourier-based Proper Orthogonal Decomposition of Fully Turbulent Round Jet Vivek Mugundhan, Tiernan A Casey, Jun Sakakibara, Peter J Schmid, Sigurdur T Thoroddsen We use the Scanning-Tomographic Particle Image Velocimetry introduced by Casey et al. [1] to obtain time-resolved velocity field of a turbulent round jet with Re = 2640, 5280 and 10700, where the inlet turbulent jet is well-developed through a 100-diameter long pipe. A pulsed laser volume is scanned in 5-9 steps to obtain velocity vectors over a larger depth to span 100 mm-deep rectangular volume, centered in the self-similar region about 50 jet diameters downstream. This enables us to deduce polar velocity components in a cylindrical volume of radius 30-35 mm, and hence capture the large-scale azimuthal coherent structures. Casey et al. [1] observed azimuthal C-shaped coherent structures dominate at the lowest Re while observing smaller tubular structures at high Re using vorticity magnitude visualization criteria from the instantaneous reconstructions. By virtue of the jet’s symmetry, we decompose its velocity (ur, uθ, uz) or vorticity (ωr, ωθ, ωz) components into azimuthal Fourier modes. Then, we perform Proper Orthogonal Decomposition (POD) by taking the SVD of each Fourier mode to look at the evolution of coherent structures based on the dominant modes. The first azimuthal Fourier mode for all Re shows the appearance of azimuthal C-shaped structures from the inlet with different phases, which get broken down as they are advected by the mean flow. We characterize these structures by conditional averaging based on their phase, over many realizations. |
Sunday, November 24, 2024 6:29PM - 6:42PM |
J37.00004: Measurements of pressure-velocity correlations in variable density round jets John J Charonko, Tiffany R Desjardins Flows in which the density ratio and gradients are large create what is known as variable density turbulence. These physics are important to natural and engineered processes at many scales from the astrophysical (stellar evolution, supernovae) to the very small (inertial confinement fusion). We have been studying these problems at the Turbulent Mixing Tunnel at Los Alamos National Laboratory using simultaneous planar velocity and density measurements acquired via Particle Image Velocimetry and Laser Induced Fluorescence. However, an important part of the physics in these problems is the interaction of the velocity field with the instantaneous pressure, which has traditionally been very difficult to acquire with sufficient spatial resolution and accuracy for successful turbulence budget analysis. Building off measurements acquired with jets in air and SF6 at matching Reynolds numbers (At = 0.1 & 0.6, Re = 20,000), (Charonko and Prestridge, JFM 2016) we have applied recently developed GPU-accelerated omni-directional pressure integration schemes (Zigunov and Charonko, MST 2024) along with Taylor's frozen turbulence hypothesis to estimate the fluctuating pressure fields. Using this data, we have computed the resulting pressure-velocity terms in the turbulent kinetic energy budget. Differences caused by variable density conditions will be explored, and the experimentally measured values will be compared to models such as one proposed by Lumley for pressure-velocity correlations. |
Sunday, November 24, 2024 6:42PM - 6:55PM |
J37.00005: Scalar Interface in Turbulent Plane Wall Jets Pranav Sood, Shibani Bhatt, Harish Mangilal Choudhary, Abhishek Gupta, Neetesh S Raghuvanshi, Prajyot Sapkal, Thara Prabhakaran, Shivsai A Dixit A scalar interface (henceforth, interface) refers to a sharp and distinct boundary that separates regions marked by a passive scalar from the non-marked regions in a flow (Prasad and Sreenivasan 1989). Interfaces have been studied extensively in the literature and possess an important feature of fractal-like scaling behaviour over a certain range of scales. The present work studies interfaces in a gaseous plane wall jet experimental facility. A novel approach is implemented to identify the interface in gaseous flows. Low-speed (14 Hz) high-resolution (up to 0.6 η, where η is the mean Kolmogorov scale at the most probable location of the interface) image acquisition is performed to capture the interface. A robust method is devised that computes fractal dimensions from the box-counting algorithm with minimal subjectivity. Statistical and fractal properties of the detected interface are analysed. Results indicate that the interface in wall jets is more contorted (fractal dimension of 1.4 with standard error of 0.1%) compared to that in jets, wakes and boundary layers (fractal dimension of 1.36 with standard error of 0.2%). Possible reasons for this 'extra richness' are also discussed. |
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