George Constantinescu

Numerical investigations of flow over a sphere in the subcritical and supercritical regimes

The flow field around a sphere in an uniform flow has been analyzed numerically for conditions corresponding to the subcritical (laminar separation) and supercritical (turbulent separation) regimes spanning a wide range of Reynolds numbers (104–106). Particular attention has been devoted to assessing predictions of the pressure distribution, skin friction, and drag as well as to understanding the changes in the wake organization and vortex dynamics with the Reynolds number. The unsteady turbulent flow is computed using detached-eddy simulation, a hybrid approach that has Reynolds-averaged Navier–Stokes behavior near the wall and becomes a large eddy simulation in the regions away from solid surfaces. For both the subcritical and supercritical solutions, the agreement with experimental measurements for the mean drag and pressure distribution over the sphere is adequate; differences in skin friction exist due to the simplistic treatment of the attached boundary layers in the computations. Improved agreement in the skin-friction distribution is obtained for the supercritical flows in which boundary layer transition is fixed at the position observed in experiments conducted at the same Reynolds numbers. For the subcritical flows the Strouhal number, St, associated with the large-scale shedding is predicted at St∼0.195 along with a higher frequency component associated with the development of the Kelvin–Helmholtz instabilities in the detached shear layers. If in the subcritical regime the wake assumes a helical-like form due to the shedding of hairpin-like vortices at different azimuthal angles, in the supercritical regime the wake structure is characterized by “regular” shedding of hairpin-like vortices at approximately the same azimuthal angle and at a much higher frequency (St∼1.3) that is practically independent of the Reynolds number and not sensitive to the position of laminar-to-turbulent transition.

Large-Eddy Simulation of Reacting Turbulent Flows in Complex Geometries

Large-eddy simulation (LES) has traditionally been restricted to fairly simple geometries. This paper discusses LES of reacting flows in geometries as complex as commercial gas turbine engine combustors. The incompressible algorithm developed by Mahesh et al. (J. Comput. Phys., 2004, 197, 215-240) is extended to the zero Mach number equations with heat release. Chemical reactions are modeled using the flamelet/progress variable approach of Pierce and Moin (J. Fluid Mech., 2004, 504, 73-97). The simulations are validated against experiment for methane-air combustion in a coaxial geometry, and jet-A surrogate/air combustion in a gas-turbine combustor geometry.

Analysis of the flow and mass transfer processes for the incompressible flow past an open cavity with a laminar and a fully turbulent incoming boundary layer

The three-dimensional incompressible flow past a rectangular two-dimensional shallow cavity in a channel is investigated using large-eddy simulation (LES). The aspect ratio (length/depth) of the cavity is

LES and DES Investigations of Turbulent Flow over a Sphere at Re = 10,000

L/D\,{=}\,2

Turbulence Modeling Applied to Flow over a Sphere

and the Reynolds number defined with the cavity depth and the mean velocity in the upstream channel is 3360. The sensitivity of the flow around the cavity to the characteristics of the upstream flow is studied by considering two extreme cases: a developing laminar boundary layer upstream of the cavity and when the upstream flow is fully turbulent. The two simulations are compared in terms of the mean statistics and temporal physics of the flow, including the dynamics of the coherent structures in the region surrounding the cavity. For the laminar inflow case it is found that the flow becomes unstable but remains laminar as it is convected over the cavity. Due to the three-dimensional flow instabilities and the interaction of the jet-like flow inside the recirculation region with the separated shear layer, the spanwise vortices that are shed regularly from the leading cavity edge are disturbed in the spanwise direction and, as they approach the trailing-edge corner, break into an array of hairpin-like vortices that is convected downstream the cavity close to the channel bottom. In the fully turbulent inflow case in which the momentum thickness of the incoming boundary layer is much larger compared to the laminar inflow case, the jittering of the shear layer on top of the cavity by the incoming near-wall coherent structures strongly influences the formation and convection of the eddies inside the separated shear layer. The mass exchange between the cavity and the main channel is investigated by considering the ejection of a passive scalar that is introduced instantaneously inside the cavity. As expected, it is found that the ejection is faster when the incoming flow is turbulent due to the interaction between the turbulent eddies convected from upstream of the cavity with the separated shear layer and also to the increased diffusion induced by the broader range of scales that populate the cavity. In the turbulent case it is shown that the eddies convected from upstream of the cavity can play an important role in accelerating the extraction of high-concentration fluid from inside the cavity. For both laminar and turbulent inflow cases it is shown that the scalar ejection can be described using simple dead-zone theory models in which a single-valued global mass exchange coefficient can be used to describe the scalar mass decay inside cavity over the whole ejection process.

LES and DES investigations of turbulent flow over a sphere

Large Eddy Simulation (LES) using a dynamic Smagorinsky type subgridstress (SGS) model and Detached Eddy Simulation (DES) are applied toprediction and investigation of the flow around a sphere at a Reynoldsnumber of 104 in the subcritical regime. In this regime the boundarylayers at separation are laminar, and transition to turbulence occursfarther downstream in the separated shear layers via Kelvin–Helmholtz(K–H) instabilities. The dynamic eddy viscosity model of Germano et al.(Physics of Fluids3 (1991) 1760–1765) is used in the LES, while the current implementation of the DESemploys a formulation based on the Spalart–Allmaras (S–A) model. DES isa hybrid approach in which the closure is a modification to theproduction/destruction term of the original Reynolds-AveragedNavier–Stokes (RANS) model, reducing to RANS in the attached regions,and to LES away from the wall. In the present work where we simulate theflow over a sphere in the subcritical regime in which the boundarylayers at separation are laminar, DES can be viewed as LES with adifferent SGS model. Effects of the discretization scheme used toapproximate the convective terms are considered, along with sensitivityof predictions to changes in the additional model coefficient, CDES, in the DES formulation. DES and LES yield similar predictions of the wakestructure, large-scale vortex shedding and the Strouhal numberassociated with the low frequency mode in the wake. Predictions ofquantities such as the drag coefficient, wake frequencies, position oflaminar separation on the sphere, and the mean pressure andskin-friction distributions along the sphere are in good agreement withthe measurements of Achenbach (Journal of Fluid Mechanics54 (1972) 565–575). Predictions of the primaryReynolds shear stress, turbulent kinetic energy, eddy viscosity, andturbulent dissipation for the two models are also similar. In addition,both models successfully resolve the formation of the vortex tubes inthe detached shear layers along with the value of the Strouhal numberassociated with the high frequency instability mode, provided that thelevel of numerical dissipation introduced by the discretization schemeremains sufficiently low. Flow physics investigations are focused onunderstanding the wake structure in the subcritical regime.

Flow and bathymetry in sharp open-channel bends: Experiments and predictions

Numerical simulations of the subcritical flow over a sphere are presented. The primary aim is to compare prediction of some of the main physics and flow parameters from solutions of the unsteady Reynolds-averaged Navier-Stokes (URANS) equations, large-eddy simulation (LES), and detached-eddy simulation (DES). URANS predictions are obtained using two-layer κ-e, κ-ω, ν 2 -f, and the Spalart-Allmaras model. The dynamic eddy viscosity model is used in the LES. DES is a hybrid technique in which the closure is a modification to the Spalart-Allmaras model, reducing to RANS near solid boundaries and LES in the wake. The techniques are assessed by evaluating simulation results against experimental measurements, as well as through their ability to resolve time-dependent features of the flow related to vortex shedding. Simulation are performed at a Reynolds number of 10 4 , where laminar boundary-layer separation occurs at approximately 83 deg

Numerical simulations of lock-exchange compositional gravity current

Large Eddy Simulation (LES) and Detached Eddy Simulation (DES) are applied to prediction and investigation of the ow around a sphere. DES is a hybrid approach in which the closure is a simple modi cation to the Spalart-Allmaras model, reducing to Reynolds-Averaged Navier-Stokes (RANS) in attached regions, and LES away from the wall . Calculations are performed at a Reynolds number, Re = 10, in the sub-critical regime where there is a laminar boundary layer separation from the sphere. A fth-order upwind-biased treatment of convection is su cient for capturing the development of the separated shear layers and for the velocity spectra to display an inertial range without excessive decay at the smallest resolved scales. The closest agreement with LES predictions is obtained in DES with the model constant CDES = 0:65. Both techniques yield predictions of the drag, position of laminar separation, and the mean pressure and skin-friction distributions along the sphere are in good agreement with measurements .

An investigation of the dynamics of coherent structures in a turbulent channel flow with a vertical sidewall obstruction

This paper focuses on experiments and simulations conducted in very sharp open-channel bends with flat and equilibrium bathymetry, corresponding to the initial and final phases of the erosion and deposition processes, respectively. The study of flow in curved open bends is relevant for flow in natural river configurations, as most river reaches are not straight. The configuration considered in the present work was designed as a test case in which the role of the cross-sectional flow is more severe than in meandering natural river reaches (radius of curvature of the channel is close to the channel width) and, thus, can serve for validation of numerical models used to predict flow and sediment transport in river engineering applications. This paper presents detailed new experimental data on the equilibrium bathymetry as well as depth-averaged distributions, vertical profiles, and cross-sectional patterns of the streamwise velocity, the cross-stream circulation, streamwise vorticity, and the turbulent kinetic energy at the initial and final stages of the erosion and deposition processes. The numerical simulations are performed using a three-dimensional nonhydrostatic RANS model for flow, sediment transport, and bathymetry, which employs fine meshes, accounts for the effect of small bed forms, and avoids the use of the law of the wall. The model predicts, rather accurately, the distribution of the streamwise velocity, the cross-stream circulation, and the turbulent kinetic energy in the simulations conducted with a fixed (flat and deformed bed corresponding to equilibrium conditions) prescribed bathymetry. In the case of a simulation conducted with loose bed, the model predicts satisfactorily the main features of the bathymetry at equilibrium conditions, despite the fact that including the interaction between the flow and the bathymetry increases the overall uncertainty in the model predictions. Results indicate that both improvements in the level of turbulence modeling and in the modeling of the sediment transport would allow further improvement in the predictive capabilities of morphodynamic models.

Detached Eddy Simulation Investigation of Turbulence at a Circular Pier with Scour Hole

Compositional gravity current flows produced by the instantaneous release of a finite-volume, heavier lock fluid in a rectangular horizontal plane channel are investigated using large eddy simulation. The first part of the paper focuses on the evolution of Boussinesq lock-exchange gravity currents with a large initial volume of the release during the slumping phase in which the front of the gravity current propagates with constant speed. High-resolution simulations are conducted for Grashof numbers = 3150 (LGR simulation) and = 126000 (HGR simulation). The Grashof number is defined with the channel depth h and the buoyancy velocity u b = ( g ′ is the reduced gravity). In the HGR simulation the flow is turbulent in the regions behind the two fronts. Compared to the LGR simulation, the interfacial billows lose their coherence much more rapidly (over less than 2.5 h behind the front), which results in a much faster decay of the large-scale content and turbulence intensity in the trailing regions of the flow. A slightly tilted, stably stratified interface layer develops away from the two fronts. The concentration profiles across this layer can be approximated by a hyperbolic tangent function. In the HGR simulation the energy budget shows that for t > 18 h / u b the flow reaches a regime in which the total dissipation rate and the rates of change of the total potential and kinetic energies are constant in time. The second part of the paper focuses on the study of the transition of Boussinesq gravity currents with a small initial volume of the release to the buoyancy–inertia self-similar phase. When the existence of the back wall is communicated to the front, the front speed starts to decrease, and the current transitions to the buoyancy–inertia phase. Three high-resolution simulations are performed at Grashof numbers between = 3 × 10 4 and = 9 × 10 4 . Additionally, a calculation at a much higher Grashof number ( = 10 6 ) is performed to understand the behaviour of a bottom-propagating current closer to the inviscid limit. The three-dimensional simulations correctly predict a front speed decrease proportional to t −α (the time t is measured from the release time) over the buoyancy–inertia phase, with the constant α approaching the theoretical value of 1/3 as the current approaches the inviscid limit. At Grashof numbers for which > 3 × 10 4 , the intensity of the turbulence in the near-wall region behind the front is large enough to induce the formation of a region containing streaks of low and high streamwise velocities. The streaks are present well into the buoyancy–inertia phase before the speed of the front decays below values at which the streaks can be sustained. The formation of the velocity streaks induces a streaky distribution of the bed friction velocity in the region immediately behind the front. This distribution becomes finer as the Grashof number increases. For simulations in which the only difference was the value of the Grashof number ( = 4.7 × 10 4 versus = 10 6 ), analysis of the non-dimensional bed friction velocity distributions shows that the capacity of the gravity current to entrain sediment from the bed increases with the Grashof number. Past the later stages of the transition to the buoyancy–inertia phase, the temporal variations of the potential energy, the kinetic energy and the integral of the total dissipation rate are logarithmic.