Ananias G. Tomboulides

Three-dimensional simulations of premixed hydrogen/air flames in microtubes

The dynamics of fuel-lean (equivalence ratio φ=0.5) premixed hydrogen/air atmospheric pressure flames are investigated in open cylindrical tubes with diameters of d=1.0 and 1.5 mm using three-dimensional numerical simulations with detailed chemistry and transport. In both cases, the inflow velocity is varied over the range where the flames can be stabilized inside the computational domain. Three axisymmetric combustion modes are observed in the narrow tube: steady mild combustion, oscillatory ignition/extinction and steady flames as the inflow velocity is varied in the range 0.5≤ U IN ≤ 500 cm s -1 . In the wider tube, richer flame dynamics are observed in the form of steady mild combustion, oscillatory ignition/extinction, steady closed and open axisymmetric flames, steady non-axisymmetric flames and azimuthally spinning flames (0.5 ≤ U IN ≤ 600 cm s -1 ). Coexistence of the spinning and the axisymmetric modes is obtained over relatively wide ranges of U IN . Axisymmetric simulations are also performed in order to better understand the nature of the observed transitions in the wider tube. Fourier analysis during the transitions from the steady axisymmetric to the three-dimensional spinning mode and to the steady non-axisymmetric modes reveals that the m = 1 azimuthal mode plays a dominant role in the transitions.

Lattice Boltzmann method for direct numerical simulation of turbulent flows

We present three-dimensional direct numerical simulations (DNS) of the Kida vortex flow, a prototypical turbulent flow, using a novel high-order lattice Boltzmann (LB) model. Extensive comparisons of various global and local statistical quantities obtained with an incompressible-flow spectral element solver are reported. It is demonstrated that the LB method is a promising alternative for DNS as it quantitatively captures all the computed statistics of fluid turbulence.

Direct numerical simulation of multiple cycles in a valve/piston assembly

The dynamics and multiple-cycle evolution of the incompressible flow induced by a moving piston through the open valve of a motored piston-cylinder assembly was investigated using direct numerical simulation. A spectral element solver, adapted for moving geometries using an Arbitrary Lagrange/Eulerian formulation, was employed. Eight cycles were simulated and the ensemble- and azimuthally-averaged data were found to be in good agreement with experimentally determined means and fluctuations at all measured points and times. During the first half of the intake stroke the flow field is dominated by the dynamics of the incoming jet and the vortex rings it creates. With decreasing piston speed a large central ring becomes the dominant flow feature until the top dead center. The flow field at the end of the previous cycle is found to have a dominant effect on the jet breakup and the vortex ring dynamics below the valve and on the observed significant cyclic variations. Based on statistical averaging, the evolut...

Investigation of wall heat transfer and thermal stratification under engine-relevant conditions using DNS

Unsteady wall heat transfer and thermal stratification during the compression stroke under engine relevant conditions are investigated using direct numerical simulations (DNS). In order to avoid artificial initial and boundary conditions the initial conditions are obtained from a separate DNS of the intake stroke involving thermal and composition mixing. The dynamically changing thermodynamic properties were found to strongly affect turbulence and wall heat transfer during the compression stroke. The increasing pressure results in a strongly reduced kinematic viscosity, and thus in significantly reduced length scales in the flow and temperature fields towards the top dead center (TDC). This has a direct impact on wall heat transfer, since reduced length scales lead to increased temperature gradients at the walls. Hence the heat transfer coefficient, which expresses the hydrodynamic influence on the heat transfer, increases by a factor of approximately five during compression. For the simulated conditions, the heat transfer coefficient extracted from the DNS data is found to agree reasonably well with the global correlation by Hohenberg but deviates strongly from the Woschni correlation. The influence of the boundary layers is not limited to the region close to walls, since close to TDC it affects the temperature distribution in the cylinder core. Vortical structures are identified, which transport cold gases from the boundary layer into the inner cylinder indicating that the assumption of an isentropic core temperature in the inner cylinder is not valid.

Investigation of cycle-to-cycle variations in an engine-like geometry

The multiple-cycle direct numerical simulation data of the flow in the valve/piston assembly investigated in Schmitt et al. [“Direct numerical simulation of multiple cycles in a valve/piston assembly,” Phys. Fluids 26, 035105 (2014)] is revisited to identify the relevant flow features leading to the observed cyclic variations. These are found to be the radial velocity at top dead center (TDC) remaining from the previous cycle, the location of the center of the hollow jet during intake and the strength and orientation of the vortex ring at bottom dead center. Comparisons between these features showed strong correlations in the flow field within a cycle and between two consecutive cycles. The trajectory of the hollow jet during intake is strongly influenced by the remaining radial velocity from the previous cycle. Subsequently, the hollow jet forms a vortex ring whose orientation and strength influences the radial velocity at TDC of the next cycle. This has in turn an effect on the jet trajectory of the following cycle. The results in this simplified geometry are a first attempt to understand the origin of cause-and-effect relationships of cycle-to-cycle variations (CCVs) in the flow field and can serve as a base for investigating CCVs in more realistic engine geometries. Moreover, the reported correlations are a useful validation platform for large Eddy simulation models.

Recent developments in spectral element simulations of moving-domain problems

Presented here are recent developments in spectral element methods for simulations of incompressible and low-Mach-number flows in domains with moving boundaries. Features include PDE-based mesh motion, implicit treatment of fluid–structure interaction based on a Green’s function decomposition, and an arbitrary Lagrangian-Eulerian formulation for low-Mach-number flows that includes an evolution equation for the background thermodynamic pressure. Several examples illustrate the basic principles introduced in the text.

The Turbulent Flame Speed of Premixed Spherically Expanding Flames

Premixed syngas/air flames expanding in turbulent flow fields are investigated using large scale direct numerical simulations. A parametric analysis is performed in circular and spherical geometries for the detailed investigation of the interaction between the flame front and the turbulent flow field. A stoichiometric syngas-air mixture with molar ratio \({ CO}/H_2=3\) is considered at conditions relevant to internal combustion engines. The dependence of the integral heat release rate on the characteristics of the flow field (integral length scale and turbulent intensity) is discussed. The long-term evolution of important global flame quantities is analyzed, and the mechanisms that dominate the growth of the flame kernel are identified. An expression for the speed of turbulent premixed spherical flames is formulated, based on the rate of change of the surface area of the flame.

An Operator-Integration-Factor Splitting (OIFS) method for Incompressible Flows in Moving Domains

In this paper, we present a characteristic-based numerical procedure for simulating incompressible flows in domains with moving boundaries. Our approach utilizes an operator-integration-factor splitting technique to help produce an efficient and stable numerical scheme. Using the spectral element method and an arbitrary Lagrangian-Eulerian formulation, we investigate flows where the convective acceleration effects are non-negligible. Several examples, ranging from laminar to turbulent flows, are considered. Comparisons with a standard, semi-implicit time-stepping procedure illustrate the improved performance of the scheme.

A Novel Variant of the K-ω URANS Model for Spectral Element Methods: Implementation, Verification, and Validation in Nek5000

Unsteady Reynolds-averaged Navier-Stokes (uRANS) models can provide good engineering estimates of wall shear and heat flux at a significantly lower computational cost compared with LES simulations. In this paper, we discuss the implementation of two novel variants of the k-ω turbulence model, the regularized k-ω standard and the regularized k-ω SST model, in a spectral element code, Nek5000. We present formulation for the specific dissipation rate (ω) in the standard k-ω model, which would obviate the need for ad hoc boundary conditions of ω on the wall. The regularized approach is designed to lead to grid-independent solutions as resolution is increased. We present a detailed comparison of these novel methods for various standard problems including the T-junction benchmark problem. The two approaches presented in this work compare very well with the standard k-ω model and experimental data for all the cases studied.Copyright