ALBERT

Notes: Abstract Noncircular jets have been the topic of extensive research in the last fifteen years. These jets were identified as an efficient technique of passive flow control that allows significant improvements of performance in various practical systems at a relatively low cost because noncircular jets rely solely on changes in the geometry of the nozzle. The applications of noncircular jets discussed in this review include improved large- and small-scale mixing in low- and high-speed flows, and enhanced combustor performance, by improving combustion efficiency, reducing combustion instabilities and undesired emissions. Additional applications include noise suppression, heat transfer, and thrust vector control (TVC). The flow patterns associated with noncircular jets involve mechanisms of vortex evolution and interaction, flow instabilities, and fine-scale turbulence augmentation. Stability theory identified the effects of initial momentum thickness distribution, aspect ratio, and radius of curvature on the initial flow evolution. Experiments revealed complex vortex evolution and interaction related to self-induction and interaction between azimuthal and axial vortices, which lead to axis switching in the mean flow field. Numerical simulations described the details and clarified mechanisms of vorticity dynamics and effects of heat release and reaction on noncircular jet behavior. The research on noncircular jets has also led to technology transfer. A topic that started as an academic curiosity-an interesting flow phenomenon-subsequently has had various industrial applications. The investigations reviewed include experimental, theoretical, numerical, and technological aspects of the subject.

Notes: The near-field of an azimuthally excited round jet was investigated in a combined computational/experimental study. The reaction zones in the jet were visualized using OH Planar-Laser- Induced-Fluorescence (PLIF) diagnostics. Both axisymmetric and azimuthal modes of the jet were excited to stabilize its spatial structure. Three-dimensional flame visualization of the laboratory jet reconstructed from multiple two-dimensional images acquired at constant phase angle, reveal a complex structure of the reaction zone. Time-dependent numerical simulations provided insight into the underlying fluid-dynamical processes leading to this flame structure. Simulations of reactive and non-reactive free jets used a Monotonically Integrated Large-Eddy-Simulation (MILES) approach, multi-species diffusive transport, global finite-rate chemistry and appropriate inflow/outflow boundary conditions. The flow visualizations of the experimental and computational jets strongly resemble each other, revealing tight coupling between axisymmetric vortex rings and braid (rib) vortices. The jet vorticity evolution is dominated by the dynamics of vortex-ring self-deformation induced by the azimuthal excitation imposed at the jet exit, the dynamics of rib vortices forming in the braid regions between undulating vortex rings, and strong interactions between rings and ribs. The observed topological features of the flow are directly related to the nearly-inviscid jet vorticity dynamics. These processes affect the mixing pattern of the jet, resulting in localized regions of high fuel concentration leading to combustion inactive regions in the flame, and other regions with enhanced mixing and a proper air-to-fuel ratio in the flame where the combustion process is intense. The vorticity dynamics and ensuing mixing processes determine the regions of combustion within the flame and thus the overall heat release pattern. © 1996 American Institute of Physics.

Notes: We report results of time-dependent numerical simulation of spatially developing free square jets initialized with a thin square vortex-sheet with slightly rounded corner-regions. The studies focus on the near field of jets with Mach number 0.3–0.6 and moderately high Reynolds numbers. A monotonically-integrated large-eddy-simulation approach is used, based on the solution of the unfiltered inviscid equations and appropriate inflow/outflow open boundary conditions. The simulations show that the initial development of the square jet is characterized by the dynamics of vortex rings and braid vortices. Farther downstream, strong vortex interactions lead to the breakdown of the vortices, and to a more disorganized flow regime characterized by smaller scale elongated vortices and spectral content consistent with that of the Kolmogorov (K41) inertial subrange. Entrainment rates significantly larger than those for round jets are directly related to the enhanced fluid and momentum transport between jet and surroundings determined by the vortex dynamics underlying the axis-rotation of the jet cross-section. The first axis-rotation of the jet cross-section can be directly correlated with self-induced vortex-ring deformation. However, subsequent jet axis-rotations are the result of strong interactions between ring and braid vortices, rather than being correlated with successive self-induced vortex-ring deformations, as previously conjectured based on laboratory observations. The interaction between braid and ring vortices has the effect of inhibiting the periodic self-induced axis-rotations observed in the case of isolated square vortex rings. © 1996 American Institute of Physics.

Notes: Results of a combined numerical and experimental investigation of the near field of low-subsonic air square jets are presented. The study focuses on examining the role of initial conditions and other features of the jet dynamics in determining the nature and frequency of occurrence of axis switching and the related mechanisms which enhance entrainment, mixing, and turbulence production. Three different experimental square jet facilities were utilized, including orifice jets with low and high initial turbulence level, and pipe jets. Unsteady, spatially developing jets were investigated computationally using direct and monotonically-integrated large-eddy simulation approaches, and appropriate inflow/outflow boundary conditions. Insight on the axis-switching process was obtained using the detailed database from the simulations to investigate how the unsteady vorticity dynamics reflects on the time-averaged properties of the jet cross sections. The different experimental jets were chosen such that important parameters affecting the initial conditions could be tested. Depending on the particular initial conditions of the subsonic jets studied, several or no axis switchings were observed in the first few diameters of jet development. Observed trends reported, include the effects of initial conditions such as, ratio of equivalent diameter to characteristic-momentum-thickness, turbulence level, nonuniform azimuthal momentum-thickness distributions, and Reynolds number. © 1995 American Institute of Physics.

Notes: Two-dimensional numerical simulations of compressible, subsonic, planar shear flows, are used to investigate the role of feedback in the reinitiation of vortex roll-up. The study deals with unforced, spatially evolving mixing layers for which the propagation of acoustic disturbances can be resolved and boundary effects are ensured to be negligible. The calculated pattern of coherent structures shows global self-sustaining instabilities in which new vortex roll-ups are triggered in the initial shear layer by pressure disturbances originating in the fluid accelerations downstream. This reinitiation mechanism, absent in the linear treatments of stability, is demonstrated conclusively here and examined as a function of Mach number and free-stream velocity ratio. The global instability becomes less efficient in reinitiating vortex roll-up in the initial shear layer when the acoustic propagation velocities on the sides of the mixing layer approach each other, i.e., as the incompressible regime is approached, and as the free-stream velocity ratios approach unity.

Notes: Self-excited instabilities in supersonic, countercurrent (CC) jets are demonstrated based on numerical simulations, and it is argued that they can be used to explain unresolved discrepancies between laboratory and theoretical analysis. Self-excitation is based on upstream feedback mechanisms acting on the subsonic outer jet regions, and associated with underexpanded initial conditions or partial confinement effects introduced by a collar surrounding the CC jet shear layer. Recognition of these instabilities provides new insights on the role played by the collar in the laboratory CC jet systems, suggesting that practical approaches to the active control of CC jets might be based on suitable direct excitation of the shear layer within the region of influence of the collar. © 2002 American Institute of Physics.

Notes: Results are reported from numerical studies of a compressible, subsonic reactive mixing layer, on the the effects of chemical-reaction exothermicity on the shear-layer development, and the dependence of these effects on initial conditions. The model solves the unsteady, conservation equations for mass, momentum, energy, and species concentrations. The convective transport equations are solved using the flux-corrected transport (FCT) algorithm and appropriate inflow and outflow boundary conditions. A one-step, irreversible, Arrhenius chemical reaction rate, and realistic (species- and temperature-dependent) modeling of diffusive transport are coupled with the convective transport using time-step splitting. The system studied consists of nonpremixed coflowing streams, where both the fuel (faster stream, hydrogen) and the oxidizer (slower stream, oxygen) are diluted in nitrogen. To facilitate the analysis of the results the flow is organized by low-level, single-frequency velocity perturbation at the inflow. The simulations show that energy release has the effect of reducing the shear layer growth and the amount of chemical product formed−relative to the corresponding cases for which exothermicity is not accounted for, in qualitative agreement with results from previous investigations. The relative mixing-layer growth reduction becomes more pronounced for larger energy release and lower Re, and is significant in terms of both, Reynolds stress, ρu'v', and the velocity-fluctuation correlation u'v'.In spite of the relatively fast flows studied, for the regimes considered, the results on the initial mixing-layer growth are significantly sensitive to diffusive transport effects−more so in terms of Reynolds stress, than in terms of product formation. With larger energy release here associated to larger free-stream reactant molar fractions c0, the amount of chemical product in absolute terms is found to increase with energy release−but slower than c0, so that the product formation becomes effectively less efficient. The results of the present work highlight the difficulties involved in making general statements about the effects of exothermicity on the mixing-layer growth, indicating that a careful conceptualization of these properties in terms of initial conditions and other characteristic parameters of the reactive systems under study is required.

Notes: Abstract Rational approximants are introduced for expansions in orthogonal polynomials. They are proposed as a method to obtain fast convergent approximations. We investigate this possibility, by summing the partial wave expansion of the scattering amplitude in two cases, for which the exact phase shifts may be determined. In the first case, we consider the repulsive inverse square potential, as an example of the long range interactions usual in atomic collision processes. The results show, that when using this approximants, the knowledge of a few phase shifts is enough to obtain a degree of accuracy that requires several hundreds of them, when partial sums are used for the calculations. In the second case, we have considered the delta shell potential. Here the two approaches give similar results.

Notes: Abstract The regularization factor technique is considered as a summation method for divergent, oscillating and slowly convergent partial wave expansions of the non-forward scattering amplitude for long range central potentials. Its convergence properties are studied and its efficiency is compared with that of the Punctual Padé approximant method in the cases of the Kratzer and Lennard-Jones potentials.

Notes: Abstract Coherent structures (CS) are educed using a conditional sampling technique involving alignment of vorticity patches of largest size and strength; hence we educe dominant CS. A numerically simulated spatially evolving wake of a thick flat plate is used as the database, and the inflow condition for the simulated wake includes random velocity perturbations which emulate turbulent conditions at a plate exit in the laboratory. In addition to previously educed properties such as coherent vorticity and production, and incoherent Reynolds stress and turbulence intensity, other measures such as coherent pressure and passive scalar distributions are also studied. In spite of the geometry difference, the near-wake dynamics of the plate seem quite similar to that of a cylinder. For example, turbulence is mostly produced by vortex stretching of the ribs at the saddle and then advected to the structure center, where it accumulates, and is balanced by incoherent dissipation. The distribution of coherent passive scalar indicates that mixing occurs in the saddle regions and that the mixed fluid is advected into the structure center.