Fernando F. Grinstein

New insights into large eddy simulation

Fluid dynamic turbulence is one of the most challenging computational physics problems because of the extremely wide range of time and space scales involved, the strong nonlinearity of the governing equations, and the many practical and important applications. While most linear fluid instabilities are well understood, the nonlinear interactions among them makes even the relatively simple limit of homogeneous isotropic turbulence difficult to treat physically, mathematically, and computationally. Turbulence is modeled computationally by a two-stage bootstrap process. The first stage, direct numerical simulation, attempts to resolve the relevant physical time and space scales but its application is limited to diffusive flows with a relatively small Reynolds number (Re). Using direct numerical simulation to provide a database, in turn, allows calibration of phenomenological turbulence models for engineering applications. Large eddy simulation incorporates a form of turbulence modeling applicable when the large-scale flows of interest are intrinsically time dependent, thus throwing common statistical models into question. A promising approach to large eddy simulation involves the use of high-resolution monotone computational fluid dynamics algorithms such as flux-corrected transport or the piecewise parabolic method which have intrinsic subgrid turbulence models coupled naturally to the resolved scales in the computed flow. The physical considerations underlying and evidence supporting this monotone integrated large eddy simulation approach are discussed.

Dynamics of coherent structures and transition to turbulence in free square jets

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.

Vortex dynamics and entrainment in rectangular free jets

Simulations of low-aspect-ratio, rectangular free jets are presented. The investigations focus on the entrainment and transitional vortex dynamics in compressible (subsonic) jets initialized with laminar conditions, a thin vortex sheet with slightly rounded-o corner regions, and uniform initial momentum thickness. A monotonically integrated large-eddy simulation approach based on the solution of the unsteady flow equations with high-resolution monotone algorithms is used. Inherent uncertainties in the jet entrainment measurement process are addressed using the database from laboratory experiments and simulations. Vorticity geometries characterizing the near flow eld of low aspect-ratio (A) rectangular jets are demonstrated, involving: (i) self-deforming and (ii) splitting vortex rings; interacting ring and braid (rib) vortices including (iii) single ribs aligned with corner regions (A> 2) and (iv) rib pairs aligned with the corners (A = 1); (v) a more disorganized flow regime in the far jet downstream, where the rotational-fluid volume is occupied by a relatively weak vorticity background with strong, slender tube-like lament vortices lling a small fraction of the domain { as observed in fully developed turbulent flows. The near eld entrainment properties of low-A rectangular jets are shown to be largely determined by the characteristic A-dependent coupling geometry of interacting rib and ring vortices and by vortex-ring axis-switching times.

Near field dynamics of subsonic free square jets. A computational and experimental study

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 p...

Simulation of transition and turbulence decay in the Taylor–Green vortex

Conventional large-eddy simulation (LES) and monotone integrated LES (MILES) are tested in emulating the dynamics of transition to turbulence in the Taylor–Green vortex (TGV). A variety of subgrid scale (SGS) models and high-resolution numerical methods are implemented in the framework of both incompressible and compressible fluid flow equations. Comparisons of the evolution of characteristic TGV integral measures are made with previously reported and new direct numerical simulation (DNS) data. The computations demonstrate that the convective numerical diffusion effects in the MILES methods can consistently capture the physics of flow transition and turbulence decay without resorting to an explicit SGS model, while providing accurate prediction of established theoretical findings for the kinetic energy dissipation, energy spectra, enstrophy and kinetic energy decay. All approaches tested provided fairly robust computational frameworks.

Self‐induced vortex ring dynamics in subsonic rectangular jets

The development in space and time of vortex rings in low aspect‐ratio (AR) rectangular jets is investigated. By design, the present studies isolate the self‐induced ring dynamics from effects of unsteady events otherwise present upstream and downstream of the rings in developed jets. The simulations show that the vortex rings undergo quite regular self‐induced nonplanar deformations, approximately recovering their shape and flatness with axis rotated with respect to their initial configuration. The axis‐rotation periods are in good agreement with previously reported data for pseudoelliptic rings, and exhibit nearly linear growth rate as a function of AR. For the larger aspect‐ratio case studied (AR=4), bifurcation of the ring due to vortex reconnection into roughly round rings is observed, followed by collision of the split rings and a new reconnection process, suggesting pathways for transition to turbulence based on self‐induced vortex deformations and reconnections.

Numerical simulations of asymmetric mixing in planar shear flows

Numerical simulations were performed of the evolution of the Kelvin-Helmholtz instability in planar, free shear layers, resulting from coflow past a splitter plate. The calculations solved the time-dependent inviscid compressible conservation equations. New algorithms were developed and tested for inflow and outflow boundary conditions. Since no turbulence subgrid modelling was included, only the large-scale features of the flow are described. The transition from laminar flow was triggered by transverse pressure gradients and subsequent vorticity fluctuations at the shear layer, near the tip of the splitter plate. The calculations were performed for a range of free-stream velocity ratios and sizes of the chamber enclosing the system. The simulations showed that the resulting mixing layers have more of the faster fluid than the slower fluid entrained in the roll-ups. This effect is in general agreement with the results of recent splitter-plate experiments of Koochesfahani, Dimotakis & Broadwell (1983). The calculated mixing asymmetry is more apparent when the velocity ratio of the two streams is larger, and does not significantly on the separation between the walls of the chamber.

Streamwise and spanwise vortex interaction in an axisymmetric jet. A computational and experimental study

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 ...

From canonical to complex flows: Recent progress on monotonically integrated LES

Large-eddy simulation (LES) based on subgrid-scale modeling implicitly provided by the discretization algorithm has been the subject of considerable recent interest. In the monotonically integrated LES approach, flux-limiting schemes emulate the flow features in the high-wavenumber end of the inertial range region of turbulence.

Flow dynamics in a swirl combustor

A hybrid simulation approach is used to investigate the flow patterns in an axisymmetric swirl combustor configuration. Effective inlet boundary conditions are based on velocity data from Reynolds-averaged Navier-Stokes or actual laboratory measurements at the outlet of a fuel-injector nozzle, and large eddy simulations are used to study the unsteady non-reactive swirl flow dynamics downstream. Case studies ranging from single-swirler to more complex triple-swirler nozzles are presented to emphasize the importance of initial inlet conditions on the behaviour of the swirling flow entering a sudden expansion area, including swirl and radial numbers, inlet length and characteristic velocity profiles. Swirl of sufficient strength produces an adverse pressure gradient which can promote flow reversal or vortex breakdown, and the coupling between swirl and sudden expansion instabilities depends on the relative length of the inlet. The flow is found to be very sensitive to the detailed nature of the velocity radi...