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 Q30: Turbulent Jets and Shear Layers |
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Chair: Raul Cal, Portland State University Room: Georgia World Congress Center B402 |
Tuesday, November 20, 2018 12:50PM - 1:03PM |
Q30.00001: The small-scale kinematics of a variable-density turbulent jet Chris C.K. Lai, John J. Charonko, Kathy Prestridge Two recent experimental and theoretical studies have demonstrated that turbulent mixing in a variable-density jet is different from that found in a constant-density jet. The differences occurred at the jet centerline where high spatial-temporal density gradients were found. In particular, small turbulent eddies were deformed by the gradients of mean flow to become larger eddies that effectively constitutes an ``inverse’’ energy transfer such that the mean flow is strengthened at the expense of turbulent fluctuations. To better understand the relationship between these observations and the local flow topologies, analysis on volumetric data of both velocity and density fields are required. In this talk, we present our experimental setup and diagnostics used to obtain such data for a SF6 jet inside a coflowing wind tunnel. The technique is based on the application of Taylor’s frozen turbulence hypothesis to data obtained by planar stereoscopic particle image velocimetry (PIV) and laser-induced fluorescence (LIF) at a jet cross-section. We also show some preliminary results from a Q-R analysis of the velocity gradient tensor and momentum gradient tensor that can be used to explore the vortex-stretching and strain-amplification mechanisms in the variable-density jet. |
Tuesday, November 20, 2018 1:03PM - 1:16PM |
Q30.00002: Density Effects on Turbulent Kinetic Energy and Stress Budgets in Shear-Driven Mixing Layers Jon Baltzer, Daniel Livescu Direct Numerical Simulations of low-speed shear-driven mixing layers involving two streams of fluids with different densities reveal that the interface grows preferentially into the stream of lighter density. This effect strengthens as the density difference increases, with very little growth occurring into the heavy stream as the density difference (i.e., Atwood number) becomes large. Accompanying the interface growth, the turbulent kinetic energy and stresses are more intense in the light fluid. A suite of incompressible temporal simulations were performed involving two miscible fluids, with Atwood numbers of up to 0.87 in large domains of up to 6144 x 2048 x 1536 grid points. To explain the drift of intense turbulence to the light-fluid side, the effects of differing densities on the budgets of the turbulent kinetic energy and Reynolds stress components are analyzed. Dominant terms (production, transport, and dissipation) remain similar to those of the single-density case but become asymmetric, while additional variable-density terms appear and begin to make appreciable contributions at the highest Atwood numbers analyzed. |
Tuesday, November 20, 2018 1:16PM - 1:29PM |
Q30.00003: Scales in the near field of homogeneous turbulent jets Eric Ibarra, Franklin D. Shaffer, Ömer Savaş Turbulent jets are comprised of a cascade of scales, constantly evolving from the instance they are discharged. In previous work, images of the features visible at the turbulent/non-turbulent surface of the opaque jet were used to estimate an interfacial length scale for Re~4,500-50,000. The length scales show monotonic behavior soon after the discharge location. As an extension of that work, planar PIV measurements of the turbulent jets are used to calculate the velocity field and estimate the turbulence kinetic energy, energy dissipation rate, and Taylor microscale throughout the flow field of the homogeneous jets. A comparison of the previous interfacial length scale and the Taylor microscale (at the interface) with respect to their position along the axis of the jet are carried out. We expect to extend the study to non-homogeneous immiscible jets. |
Tuesday, November 20, 2018 1:29PM - 1:42PM |
Q30.00004: A Dynamical System Model for the Co-Flowing Buoyant Jet Daniel Israel The buoyant co-flowing jet breaks self-similarity in two ways: the transition from strong to weak jet, and the jet regime versus the plume regime. Previous work has shown that, for the self-similar jet, using an integral method the turbulence model equations can be reduced to a dynamical system with a single attracting fixed point that is the self-similar state. Here, the same technique is used to analyze the buoyant jet experiments of Charonko & Prestridge (JFM 2017). In this more complex case there appears to be a low-order attracting manifold which dominates the jet behavior, analogous to the fixed point in the self-similar case. Experimental data and results of RANS simulations are used to investigate this structure in both the model and the real world. |
Tuesday, November 20, 2018 1:42PM - 1:55PM |
Q30.00005: Multi-fractal properties in volcano-inspired variable density round jets Greg Sakradse, Sarah E Smith, Bianca Viggiano, Pradeep Ramasubramanian, Daniel Ringle, Naseem Ali, Stephen Solovitz, Raúl Bayoán Cal Geophysical flows appear over a large range of scales, with Reynolds and Richardson numbers occurring over several orders of magnitude. Jet flow of varying densities ejected vertically into a large ambient region are considered. Using particle image velocimetry, the velocity fields were measured for three different gases exhausting into air, specifically helium, air and argon. Experiments considered relatively low Reynolds numbers from approximately 1500 to 5500 with Richardson numbers near 0.001 in magnitude. These included a variety of flow responses, notably nearly laminar, turbulent and transitioning jet flows. The data are analyzed using multifractal framework. The multifractal spectrum showed the characteristics of the flow based on the evolution in the streamwise and wall-normal direction. The variation of the Holder exponent displays the asymmetry and intermittency of the flow. Entropy dimension is used to check the flow evolution with scales. Quadrant analysis based on the pointwise Holder exponent and streamwise velocity is achieved. Scale dependent behavior is considered by quantifying the influence of the scale interactions in terms of the pointwise Holder condition. |
Tuesday, November 20, 2018 1:55PM - 2:08PM |
Q30.00006: Proper orthogonal decomposition based feature identification on a round jet in turbulent cross-flow Grace Eliason, Graham Freedland, Stephen Solovitz, Raúl Bayoán Cal The complex interactions that occur within jets in cross-flow can be broken down into different regions of development. By identifying the most energetic regions, the global behavior of the system can be predicted and reconstructed. Experiments are performed in a closed-circuit wind tunnel during which a seeded round jet ejects particles orthogonally into the incoming cross-flow. Turbulence intensity is varied for these experiments using a passive and active grid system where winglets are rotated to excite the cross-flow to compare low and high turbulent inflow. Jet velocity is kept constant and several cross-flow velocities are explored. Mean flow statistics and Reynolds stresses are computed using snapshot particle image velocimetry (PIV) to create instantaneous flow fields and describe jet trajectory. The jet is traced to provide axes normal and tangential to the centerline and the Cartesian data is transformed. These relative instantaneous flow fields are put through proper orthogonal decomposition (POD) analysis to reconstruct Reynolds stresses using only the highest energy modes to highlight the most energetic regions of the jet. |
Tuesday, November 20, 2018 2:08PM - 2:21PM |
Q30.00007: Small scale characteristics of turbulent/non-turbulent interfaces of viscoelastic fluids Hugo Abreu, Fernando Pinho, Carlos Bettencourt da Silva New direct numerical simulations (DNS) of turbulent fronts bounded by irrotational regions in viscoelastic fluids are carried out in order to investigate the characteristics of the turbulent/non-turbulent interface (TNTI) layer. The viscoelastic fluid analysed here consists of a Newtonian solvent carrying a very small fraction of long chained polymer molecules, which is described using the finitely extensible nonlinear elastic constitutive equation closed with the Peterlin approximation (FENE-P), and the new simulations attain the highest Reynolds numbers yet observed for this fluid, in simulations or experiments. The work focusses on the small scale aspects associated with the entrainment mechanism that exists at the edges of wakes, jets, mixing layers and boundary layers. Specifically, we analyse the enstrophy and kinetic energy dynamics and the behaviour of the invariants of the velocity gradient tensor and their role in the turbulent entrainment mechanism in TNTI layers from viscoelastic fluids. |
(Author Not Attending)
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Q30.00008: Quantifying engulfment in Shear Free Turbulence, Jets and Wakes Ricardo Plath Xavier, Carlos Bettencourt Da Silva Turbulent entrainment (TE) is present in wakes, jets, mixing layers and boundary layers. It can be described as the process by which irrotational fluid is absorbed into the core turbulent region, by moving across the turbulent/non-turbulent interface (TNTI) layer. TE controls several important flow characteristics, such as the spreading rate of jets, wakes and mixing layers. This process is carried through by two types of phenomena: engulfing and nibbling. Nibbling is associated with small scale vorticity diffusion along the entire TNTI, whereas engulfment is associated with large scale swirling motions intermittently occurring at the TNTI. In the present work direct numerical simulations (DNS) of jets, wakes and shear free turbulence are used to quantify these two processes and their relative importance. |
Tuesday, November 20, 2018 2:34PM - 2:47PM |
Q30.00009: Turbulent entrainment in high Reynolds number planar wakes, jets and shear free turbulence. Marco Zecchetto, Carlos Bettencourt Da Silva The turbulent/non-turbulent interface (TNTI) is the sharp and highly contorted layer that exists at the edges of jets, wakes, and that separates the turbulent core from the irrotational flow region. The characteristics of the turbulent/non-turbulent interface (TNTI) are analysed through new high resolution direct numerical simulations (DNS) of shear free turbulence, planar turbulent jets and wakes. Using conditional statistic, performed in relation to the position of the interface, it's possible to highlight the flow features near the interface that otherwise would be masked. The differences and similarities between the TNTIs of this flows, as well how these affect the dynamics of the nearby quantities will be discussed. |
Tuesday, November 20, 2018 2:47PM - 3:00PM |
Q30.00010: Identification and modeling of acoustic bursts emitted from a supersonic turbulent jet Oliver T Schmidt, Peter J Schmid The acoustic radiation of turbulent jets occurs in form of intermittent bursts. This transient behavior complicates jet noise modeling endeavors, and explains the failure of linear jet noise models that do not take intermittency into account. In this study, we apply a conditional form of proper orthogonal decomposition (POD) to distill the waveform of the statistically most relevant, i.e. energetic, acoustic burst from a Large Eddy Simulation of a turbulent, hot, Mach 1.5 jet. The identified statistical burst event takes the form of spatially confined wavepacket that is emitted in the direction of the peak aft-angle noise. We identify the precursor of this loud event by defining a conditional space-time POD problem, which allows us to trace the time evolution of the statistical bust event—both forwards, and backwards in time. The precursor takes the form a compact wavepacket in the initial shear-layer of the jet. We demonstrate that this burst event and its temporal evolution can be modeled by means of mean-flow-based optimal linear growth theory. The identification and characterization of the precursor event is a first step towards model predictive control of loud events. |
Tuesday, November 20, 2018 3:00PM - 3:13PM |
Q30.00011: Modal Decomposition of Pulse Burst PIV Data Surabhi Singh, Lawrence Ukeiley Dynamic Mode Decomposition (DMD) has recently evolved as a useful modal decomposition technique for time-resolved data. In this work, DMD is applied to 2-D velocity datasets obtained from Pulse Burst PIV measurements of flow over an open cavity at free-stream Mach number, M∞ = 0.8*. Resolved modes are obtained by considering increasing number of datasets. Further, rank reduction is done to include higher energy modes. Spatio-temporal characteristics of these modes have been compared with modes obtained from application of Proper Orthogonal Decomposition (POD) to the same datasets. The modal shapes of wall-normal velocity fluctuations obtained by these methods closely relate to each other at specific Rossiter modes, which are the dominant tonal frequencies from surface pressure and velocity spectra. Reduced order representations of velocity fields are reconstructed at frequencies close to Rossiter modes revealing that flow dynamics can be captured by considering only a few modes at these frequencies. This comparative analysis helps in relating DMD and POD as modal reduction techniques for time-resolved velocity datasets. *Beresh et al., AIAA Paper 2016-1344 |
Tuesday, November 20, 2018 3:13PM - 3:26PM |
Q30.00012: Vorticity vs TNTI - Entrainment Fundamentals John F Foss, Kyle M Bade, Douglas Neal, Richard Prevost, Scott Morris A recent contribution by C. da Silva, et al. [1] presents results for the simulated flow: PJET, a temporally developing plane jet that could not be realized in a physical experiment. However, their results can be referenced to the PIV based vorticity observations of Foss, et al. [2]. Quantitatively similar results for the i) enstrophy distribution in [1] and ii) the mean square ωz distribution in [2] are found, relative to the border (IB) between the irrotational (entraining) and the vortical (interior) fluid domains. However, the physical features of interest: i) length of the IB with respect to its linear projection, ii) the intermittency distribution and iii) acceleration of the entraining fluid in its approach to the IB, are not available for the faux flow. The relatively high Reynolds number (Reλ) was 371 for [1], and higher: 590 for [2] using data from [3] for Reλ. [1] Silva, et al. “The scaling of the TNTI at high Reynolds numbers” JFM (2018) vol. 843 [2] Foss, et al. (2017) “Single Stream Shear Layer and the Viscous Superlayer”, 10 C 4, Tenth International Symposium on Turbulence and Shear Flow Phenomena [3] Morris, S.C. (2002) “The velocity and vorticity fields of a single stream shear layer” PhD Dissertation, Michigan State University |
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