Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session A22: Turbulence: Free Shear Layers and Jets |
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Chair: John Foss, Michigan State University Room: 210 |
Sunday, November 22, 2015 8:00AM - 8:13AM |
A22.00001: Vorticity Based Intermittency -- Low Speed Side of A Single Stream Shear Layer John Foss, Kyle Bade, Richard Prevost, Douglas Neal A single stream shear layer is one of the canonical free shear flows. It also appears as the near field of a planar jet well upstream of the merging shear layers. It has two entraining boundaries: i) high speed parallel entrainment and ii) low speed perpendicular entrainment. The latter region is given detailed study by the authors. The exceptionally careful design and fabrication (Morris and Foss (2003)) of the R(theta) $=$ 6.5x10**4 facility delivered irrotational entrainment fluid which allowed PIV images to identify the viscous super layer (VSL). The in-plane velocity components: u and v, as well as the transverse vorticity $\omega_{z}$ were obtained for all points in the image. The striking features of this region are manifest in the strong vortical motions with very weak streamwise motion: $\bar{u}$/$U_{0}\le $ 0.03. Measures of the vortical motions will be presented. S.C. Morris and J.F. Foss (November 2003) ``Turbulent boundary layer to single-stream shear layer: the transition region'', \textit{Jour. Fluid Mechanics}, \textbf{494}, pp. 187-221. [Preview Abstract] |
Sunday, November 22, 2015 8:13AM - 8:26AM |
A22.00002: On the role of the large-scale gradients in the scale interactions in a mixing layer Daniele Fiscaletti, Antonio Attili, Fabrizio Bisetti, Gerrit Elsinga The interaction between the small and large scales of turbulence is investigated in a mixing layer achieving a Reynolds number based on the Taylor microscale ($\rm{Re_{\lambda}}$) of 250. Positive fluctuations of the large-scale velocity correspond to large vorticity rms on the low-speed side of the mixing layer and to low vorticity rms on the high-speed side, respectively. The relationship between large and small scales thus depends on the position if the vorticity rms is correlated with the large-scale velocity fluctuations. However, when correlating the vorticity rms with the large-scale velocity gradients, the correlation coefficient is nearly constant throughout the mixing layer and close to unity. This observation reveals that large and small scales are characterized by a strong interaction independent of the flow position when the large-scale velocity gradients are considered instead of the large-scale velocity fluctuations usually employed in the existing literature on amplitude modulation. The vorticity from unfiltered (small scales) and from low-pass filtered velocity fields tend to be aligned when examined within vortical tubes, suggesting that part of the large-scale characteristics is not lost at the smallest scales. [Preview Abstract] |
Sunday, November 22, 2015 8:26AM - 8:39AM |
A22.00003: Surface roughness effects on vortex dynamics in turbulent impinging jet Wen Wu, Rayhaneh Banyassady, Ugo Piomelli Large-eddy simulations (LES) are used to study forced round jets impinging on rough surfaces at nozzle-to-plate distance $H/D=1$ ($D$ is the nozzle exit diameter) and Reynolds numbers $Re=U_oD/\nu= 6.6\times10^4$ ($U_o$ is the mean jet velocity). Our aim is to explore the roughness effects on the evolution of the vortices generated in the jet shear layer. Roughness is represented by uniformly distributed but randomly oriented ellipsoids which result in sand-grain-like surfaces with equivalent sand-grain heights $k_s/D = 0.02$ and $0.013$. An immersed boundary method is used. Results are compared to our previous LES simulations of jets impinging on a smooth surface. More rapid decay of the vortices is observed in the rough cases compared to the smooth one. The momentum deficit due to the roughness displacement effect plays a key role during this process. Secondary vorticity is amplified in magnitude and associated with larger dissipation. The turbulent statistics and structures near the wall are altered by the roughness. The extent of roughness effects is initially limited to the ``roughness sublayer". The modified near-wall flow, however, is advected outward by the secondary vorticity, and affects the evolution of the primary vortex in the outer shear layer of the wall jet. [Preview Abstract] |
Sunday, November 22, 2015 8:39AM - 8:52AM |
A22.00004: Turbulent Scalar Flux Modeling for Inclined Jets in Crossflow: an Optimization Approach Pedro Milani, Kevin Ryan, John Eaton Turbulent mixing for jets in crossflow is important in numerous applications. Reynolds-averaged models for turbulent scalar transport are usually based on the gradient diffusion hypothesis (GDH), with a scalar eddy diffusivity calculated from the model eddy viscosity. Such models are not accurate in the near jet region causing poor prediction of the scalar concentration distribution. We use 3D mean velocity and concentration data acquired using magnetic resonance imaging to infer improved diffusivity models. The transport equation is solved using the experimental velocity data and a prescribed functional form for the scalar diffusivity. An evolutionary algorithm then optimizes the model constants to minimize the difference between the calculated and measured scalar concentration fields. Tests of multiple model forms for seven different jet in crossflow configurations provide insight into the required characteristics of advanced models. The GDH with a weakly anisotropic diffusivity is very accurate beyond 4 hole diameters downstream of the injection point. However, standard turbulent diffusivity models overestimate turbulent mixing in the separation region; in most cases, the optimization procedure inferred counter-gradient diffusion in this region. New models that adjust automatically depending on the characteristics of the mean velocity and concentration fields are under development. [Preview Abstract] |
Sunday, November 22, 2015 8:52AM - 9:05AM |
A22.00005: Space-Time Correlations in a Turbulent Gas-Phase Jet Michael Papageorge, Jeffrey Sutton G.I. Taylor first hypothesized that the primary relationship between space and time was linear and arises from the convection of small eddies past a fixed point in space at velocity \textless U\textgreater . Recently it has been shown using DNS calculations in homogenous shear flows that a second-order Taylor's expansion of the space-time correlation function provides a better estimate of the relationship between space and time for velocity correlations [Zhao and He (Phys. Rev. E 2009)]. The second-order expansion leads to an elliptical relationship between space and time correlations. In this work, scalar field measurements, with both high resolution and dynamic range in space and time, from a round turbulent gas-phase jet were collected to examine if the proposed elliptical relationship holds for scalar fluctuations in a free-shear flow. Furthermore, the space-time correlations and the ``elliptical model'' of Zhao and He are used to understand the physical mechanisms by which scalar fluctuations decorrelate. This work is expected to lead to a better understanding of the relationship between space and time and the physical processes governing the decorrelation of scalar fluctuations in gas-phase turbulent jets. [Preview Abstract] |
Sunday, November 22, 2015 9:05AM - 9:18AM |
A22.00006: SPIV Measurements for Identifying Turbulence Structure in Swirling Jets Eric DeMillard, Pourya Nikoueeyan, Jonathan Naughton Swirling jets are of interest because they can enhance and control mixing and combustion. Several past studies have considered the turbulence statistics in swirling jets for a range of swirl number, and this behavior is now well characterized. However, there has been no attempt to date to link the statistical results to the turbulent structure in the jet. To address this, Stereoscopic Particle Image Velocimetry (SPIV) is being performed to capture instantaneous velocity fields. The resulting planes of three-component velocities are to be used in conjunction with Proper Orthogonal Decomposition (POD) to reconstruct turbulence structure. Using the POD results, comparisons can be made between the turbulence structure in the swirling jets to that of their non-swirling counterparts. Critical to this analysis are accurate two-point statistics that only result if proper care is taken during experiment setup. Errors can arise from misalignment of the laser sheet with the calibration plane, the selection of viewing angles and object distances for both cameras, and the improper selection of dual-image acquisition parameters. This work thus identifies the requirements for successful execution of SPIV measurements in swirling jets to be used for POD analysis. [Preview Abstract] |
Sunday, November 22, 2015 9:18AM - 9:31AM |
A22.00007: POD Mode Coupling from Two-Plane PIV Measurements of the Turbulent Round Jet Azur Hodzic, Clara Velte, William George Independent PIV measurements were performed in the streamwise- and crossplane of the turbulet axi-symmetric jet and the flow field was decomposed using the Lumley Projection approach in order to reveal the most dominant features of the flow [1,2]. However, in order to achieve a full spatial decomposition and to be able to couple the streamwise- and cross plane modes successfully, simultaneous PIV measurements in the streamwise- and crossplane of the jet are being acquired in the current work. These measurements provide opportunities to reveal dominant three-dimensional structures, their role in the energy production, their interactions to compose the flow field and reveal the energy transport phenomena characteristic of this particular flow. The experimental results are compared to DNS results of the turbulent jet performed with state of the art methods allowing high Reynolds-number simulations and providing the opportunity to verify the experimental data. Current status presentations of the experimental- and DNS results will be given.\\[4pt] [1] Hod\v zi\'c, PIV Measurements on Turbulent Jets, Master's thesis 2014.\\[0pt] [2] W\"anstr\"om, Spatial decompositions of a fully-developed turbulent round jet sampled with particle image velocimetry, Ph.D dissertation 2009 [Preview Abstract] |
Sunday, November 22, 2015 9:31AM - 9:44AM |
A22.00008: Analysis of the stability of jets in crossflow Marc Regan, Krishnan Mahesh Jets in crossflow (transverse jets) are a canonical fluid flow in which a jet of fluid is injected normal to a crossflow. A high-fidelity, unstructured, incompressible, DNS solver is shown to reproduce the complex shear layer instability seen in low-speed jets in crossflow experiments. A linear stability analysis extension to the unstructured grid DNS solver is developed. An iterative approach is used to solve for the dominant unstable eigenvalues and their associated eigenmodes in the linear regime. Details of the development along with validation studies for parallel flow, and 2-D and 3-D driven cavity flows. Results from ongoing work on applying the methodology to perform global stability analysis of transverse jets will be presented. [Preview Abstract] |
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