Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Fluid Dynamics
Volume 53, Number 15
Sunday–Tuesday, November 23–25, 2008; San Antonio, Texas
Session LC: Turbulent Shear Layers and Mixing |
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Chair: Laurent Mydlarski, McGill University Room: 002A |
Monday, November 24, 2008 3:35PM - 3:48PM |
LC.00001: Sensitivity Analysis of a Plane Mixing Layer using the Sensitivity Equation Method Mohsen Zayernouri, Meredith Metzger Sensitivity field evolution of the incompressible, two-dimensional mixing layer to perturbations in both Reynolds number, $Re_{\delta_0}$, and Prandtl number, $Pr$, has been examined using the sensitivity equation method. In this method, the sensitivity coefficients (i.e., the partial derivative of vorticity and temperature with respect to $Re_{\delta_0}$ and $Pr$) are obtained from direct numerical simulation of the sensitivity equations coupled with the governing equations of the fluid motion. This is achieved using an unsteady finite volume based fractional step algorithm. Coherent structures in the sensitivity field depict mechanisms responsible for enhanced vortex growth and scalar mixing with increasing $Re_{\delta_0}$ and $Pr$, respectively. Two distinct configurations were found in the sensitivity field of vorticity. The first configuration represents an increasing growth in the mixing layer as $Re_{\delta_0}$ increases, while the second configuration depicts the saturation state for the vorticity field. The sensitivity of the temperature field to changes in $Pr$ exhibits a third configuration describing enhanced scalar with increasing $Pr$. These interpretations are confirmed with calculations of integral quantities, namely the rate of growth of the mixing layer with $Re_{\delta_0}$ and the evolution of the probability density function of the scalar field with $Pr$. [Preview Abstract] |
Monday, November 24, 2008 3:48PM - 4:01PM |
LC.00002: Reduced Order Modeling for Beam Propagation through a Shear Layer Jurgen Seidel, Stefan Siegel, Casey Fagley, Thomas McLaughlin The performance of airborne platforms emitting or receiving light beams is severely hampered by the flow field around the turret mounted on the air vehicle. From a fluid dynamics point of view, the flow separating from the turret develops large, coherent structures. These structures are associated with density variations, which, from an optical point of view, result in large optical distortions. The goal of this research is to improve system performance by mitigating these structures using feedback flow control. While developing a feedback flow control system is a multi-step process, one important step is the design of a Reduced Order Model of the flow field under consideration. For the canonical flow field of a shear layer behind a backward facing step, both simulations results and experiments data are compared and used to provide input training data for the development of a neural network model based on Proper Orthogonal Decomposition of the flow field. The design of the neural network, based on the time coefficients obtained from Proper Orthogonal Decomposition, is shown and analyzed. The model of the flow field is then utilized to develop feedback control strategies to mitigate the optically detrimental coherent structures. [Preview Abstract] |
Monday, November 24, 2008 4:01PM - 4:14PM |
LC.00003: Experimental Investigation of Optical Beam Propagation through a free Shear Layer Stefan Siegel, Juergen Seidel, Casey Fagley, Thomas McLaughlin The performance of airborne platforms emitting or receiving light beams is severely hampered by the flow field around the turret mounted on the air vehicle. From a fluid dynamics point of view, the flow separating from the turret develops large, coherent structures. From an optical point of view, these structures due to their associated density variations, cause large optical distortions since the index of refraction is a function of density. The goal of this research is to reduce optical distortions by mitigating these structures using feedback flow control. For the canonical flow of a shear layer behind a backward facing step, both experimental measurements using hot film and Malley Probes and high resolution simulations are used to provide input data for model development. Based on the time coefficients obtained from Proper Orthogonal Decomposition of multiple open loop forced reference cases, the design of a global neural network based model of the flow field will be presented. A comparison between experiments, simulations and reduced order model will be presented. [Preview Abstract] |
Monday, November 24, 2008 4:14PM - 4:27PM |
LC.00004: Fine-scale features in the far-field of a turbulent jet Oliver Buxton, Bharathram Ganapathisubramani The structure of a fully turbulent axisymmetric jet, at Reynolds number based on jet exit conditions of 5000, is investigated with cinematographic (1 kHz) stereoscopic PIV in a plane normal to the jet axis. Taylor's hypothesis is employed to calculate all three velocity gradients in the axial direction. The technique's resolution allows all terms of the velocity gradient tensor, hence strain rate tensor and kinetic energy dissipation, to be computed at each point within the plane. The data reveals that the vorticity field is dominated by high enstrophy tube-like structures. Conversely, the dissipation field appears to consist of sheet-like structures. Several criteria for isolating these strongly swirling vortical structures from the background turbulence were employed. One such technique involves isolating points in which the velocity gradient tensor has a real and a pair of complex conjugate eigenvectors. Once identified, the alignment of the various structures with relation to the vorticity vector and the real velocity gradient tensor eigenvector is investigated. The effect of the strain field on the geometry of the structures is also examined. [Preview Abstract] |
Monday, November 24, 2008 4:27PM - 4:40PM |
LC.00005: Measurements in a High Reynolds Number Wake Marcus Hultmark, Juan Jimenez, Sean Bailey, Alexander Smits Experiments were conducted in the Princeton/ONR HRTF windtunnel with highly pressurized air. The wake of a DARPA SUBOFF submarine model was measured over a large range of Reynolds numbers at 5 different downstream locations. The model is an axisymmetric body without appendages (fins) supported by a streamlined support, mimicking a semi-infinite sail. For all Reynolds numbers studied, the mean velocity distribution becomes self-similar between 3 and 6 diameters, $D$, downstream for the side where the support is not located. In contrast, self-similarity in the Reynolds stresses is not reached at the furthest downstream location ($x/D=15$). The spectra reveal two peaks in the near-wake. The lower wavenumber peak corresponds to a Strouhal number based on diameter and freestream velocity of about 0.22, suggesting that it is associated with an azimuthal or helical shedding mode in the wake. This mode is evident at all Reynolds numbers, at all cross-stream positions, indicating that it is unlikely to be due to the interference of the support wake with the model wake. The mode is seen only for $x/D<15$, suggesting that it plays a partial role in the approach to self-similarity of the turbulent stresses. [Preview Abstract] |
Monday, November 24, 2008 4:40PM - 4:53PM |
LC.00006: Turbulent entrainment in jets: the role of the large and small scales of motion Carlos B. da Silva, Jose Fernandes Pereira This work analysis several large and small scale aspects of the turbulent entrainment mechanism that exists in mixing layers, wakes, and jets. The turbulent entrainment mechanism takes place across the turbulent/nonturbulent (T/NT) interface dividing the irrotational (nonturbulent - NT) from the turbulent (T) region in these flows. Recently da Silva and Pereira (Phys. Fluids, 20, 055101, 2008) using DNS of a turbulent plane jet analyzed the invariants of the velocity-gradient, rate-of-strain, and rate-of-rotation tensors across the T/NT interface in order to characterize the small scale dynamics near the T/NT interface. In the present work we focus on the intense vorticity structures (IVS) from the flow in order to analyze the interplay between the large and small scales of the flow during the turbulent entrainment. An interesting result is the existence of non negligible viscous dissipation rate outside the turbulent region. It turns out that this interesting phenomena is caused the the presence of IVS near the T/NT interface. The presentation will focus on how the presence of these IVS commands the evolution of many small scale quantities and ultimately imposes the entrainment rate. [Preview Abstract] |
Monday, November 24, 2008 4:53PM - 5:06PM |
LC.00007: Three-Component Turbulence Measurement in Three Dimensional Wall jet Lhendup Namgyal, Joseph Hall The lateral width of the turbulent 3-D wall jet is 5 to 8 times larger than the vertical height and is due to strong secondary flows. This makes the wall jet very difficult to model. The source of this streamwise vorticity is as yet unclear, although it has been linked to the anisotropy in the Reynolds stresses. The goal of this investigation is to simultaneously measure all three turbulent velocity components on and off the jet centerlines, so the sources of the streamwise vorticity can be determined. Here, the flow issues from a contoured nozzle with a diameter of 1.5 inches and a jet exit Reynolds number of around 2.5 X105. The three velocity components are measured with a four-wire-hot-wire probe in the far-field region. [Preview Abstract] |
Monday, November 24, 2008 5:06PM - 5:19PM |
LC.00008: Experimental Model of Contaminant Transport by a Moving Wake Inside an Aircraft Cabin Stephane Poussou, Paul Sojka, Michael Plesniak The air cabin environment in jetliners is designed to provide comfortable and healthy conditions for passengers. The air ventilation system produces a recirculating pattern designed to minimize secondary flow between seat rows. However, disturbances are frequently introduced by individuals walking along the aisle and may significantly modify air distribution and quality. Spreading of infectious aerosols or biochemical agents presents potential health hazards. A fundamental study has been undertaken to understand the unsteady transport phenomena, to validate numerical simulations and to improve air monitoring systems. A finite moving body is modeled experimentally in a 10:1 scale simplified aircraft cabin equipped with ventilation, at a Reynolds number (based on body height) of the order of 10,000. Measurements of the ventilation and wake velocity fields are obtained using PIV and PLIF. Results indicate that the evolution of the typical downwash behind the body is profoundly perturbed by the ventilation flow. Furthermore, the interaction between wake and ventilation flow significantly alters scalar contaminant migration. [Preview Abstract] |
Monday, November 24, 2008 5:19PM - 5:32PM |
LC.00009: Dependence of higher-order passive-scalar structure functions on the scalar-field initial conditions Jason Lepore, Laurent Mydlarski To investigate the dependence of structure function scaling exponents of a turbulent passive scalar on the (scalar-field) initial conditions, higher-order structure functions of temperature are measured in the turbulent wake of a circular cylinder ($R_\lambda = 388$). The turbulent scalar field is generated by two different means. It is first created by heating the cylinder. It is then produced using a mandoline.\footnote{Warhaft and Lumley, 1978, {\it J. Fluid Mech.}, {\bf 88}, 659-684.} Though the second-order statistics ({\it e.g.}, power spectra, second-order structure functions) of the scalar field are experimentally indistinguishable (in the inertial and dissipative ranges) for the two cases, we observe notable differences in the inertial-range scaling exponents ($\zeta_n$) of the n$^{th}$-order passive-scalar structure functions at higher orders ($n \geq 4$). The implication is that the variations in previous estimates of $\zeta_n$ observed in different experiments may be i) attributable to differences in the scalar field initial conditions, and therefore ii) inconsistent with a universal nature of the small-scale statistics of a passive-scalar field. [Preview Abstract] |
Monday, November 24, 2008 5:32PM - 5:45PM |
LC.00010: Mixing Characteristics of Strongly-Forced Jet Flames in Crossflow Kevin Marr, Noel Clemens, Ofodike Ezekoye The effects of high frequency, large-amplitude forcing on the characteristics of a non-premixed jet flame in crossflow (JFICF) at mean Reynolds numbers of 3,200 and 4,850 are studied experimentally. Harmonic forcing of the jet fuel results in a drastic decrease in flame length and complete suppression of soot luminosity. Visualization by planar laser Mie scattering shows that forced JFICF, similar to forced free or coflow jet flames, are characterized by ejection of high-momentum, deeply penetrating vortical structures. These structures rapidly breakdown and promote intense turbulent mixing in the near region of the jet. The rapid mixing resembles a ``one-step'' process going from a fuel rich state far in the nozzle to a well-mixed, but significantly diluted, state just a few diameters from the jet exit plane. Exhaust gas emissions measurements indicate a decrease in NOx, but increases in CO and unburned hydrocarbons with increasing forcing amplitude. Acetone PLIF measurements are used to investigate the effect of partial-premixing on these emissions findings. [Preview Abstract] |
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