Abstract

This paper presents the Particle Finite Element Method (PFEM) and its application to multi-fluid flows. Key features of the method are the use of a Lagrangian description to model the motion of the fluid particles (nodes) and that all the information is associated to the particles. A mesh connects the nodes defining the discretized domain where the governing equations, expressed in an integral form, are solved as in the standard FEM.We have extended the method to problems involving several different fluids with the aim of exploiting the fact that Lagrangian methods are specially well suited for tracking any kind of interfaces.

Abstract

A numerical approximation for the one‐dimensional Burgers equation is proposed by means of the orthogonal subgrid scales–variational multiscale (OSGS‐VMS) method. We evaluate the role of the variational subscales in describing the Burgers “turbulence” phenomena. Particularly, we seek to clarify the interaction between the subscales and the resolved scales when the former are defined to be orthogonal to the finite‐dimensional space. Direct numerical simulation is used to evaluate the resulting OSGS‐VMS energy spectra. The comparison against a large eddy simulation model is presented for numerical discretizations in which the grid is not capable of solving the small scales. An accurate approximation to the phenomena of turbulence is obtained with the addition of the purely dissipative numerical terms given by the OSGS‐VMS method without any modification of the continuous problem.

Abstract

Abstract

This work presents a numerical study on the effect of flame-wall interaction (FWI) from the viewpoint of flame dynamics. For that purpose, direct numerical simulations (DNS) employing detailed calculations of reaction rates and transport coefficients have been applied to a 2D premixed methane/air flame under atmospheric condition. Free flame (FF) and side-wall quenching (SWQ) configurations are realized by defining one lateral boundary as either a symmetry plane for the FF or a cold wall with fixed temperature at 20 oC for the SWQ case. Different components of flame stretch and Markstein number regarding tangential, normal (due to curvature) and total stretch, Kas , Kac and Katot = Kas + Kac, as well as their correlations with respect to the local flame consumption speed SL have been evaluated. It has been shown that the FWI zone is dominated by negative flame stretch. In addition, SL decreases with decreasing normal stretch due to curvature Kac while approaching the cold wall. However, SL increases with decreasing Kac while approaching the symmetry boundary for the free flame case, leading to an inversion of the Markstein number Matot based on Katot from positive in the free flame case to negative in the SWQ case. The quenching distance evaluated based on wall-normal profiles of SL has been found to be approximately equal to the unstretched laminar flame thickness, which compares quantitatively well with measured data from literature. The flame speed has been confirmed to scale quasi-linearly with the stretch in the FWI zone. The results reveal a distinct correlation during transition between FWI and FF regarding flame dynamics, which brings a new perspective for modeling FWI phenomena by means of flame stretch and Markstein number. To do this, the quenching effect of the wall may be reproduced by a reversed sign of the Markstein number from positive to negative in the FWI zone and by applying the general linear Markstein correlation (SL/SL,0 = 1− Ma ·Ka), leading to a decrease of the flame speed or the reaction rate in the near-wall region.

Abstract

Laminar flow separation has detrimental effects on the aerodynamics and performance of low pressure turbines (LPT). Flow separation is caused by the presence of adverse pressure gradient condition on the upper side of the blade past the suction peak, and is followed by laminar-to-turbulent transition and the subsequent turbulent mean reattachment due to the enhanced mixing. These phenomena characterise the size and dynamics of the separated flow, which are primarily dominated by the laminar-turbulent process. This study examines the influence of periodically-varying inflow conditions on the separated flow over a bump geometry at low Reynolds numbers. The geometry and flow conditions represent the upper surface of small LPT during high-altitude of flight. Direct numerical simulations are performed, in which a harmonic variation of the inlet total pressure is imposed, as a rough approximation of the passage of the upstream blade's wake. Three different frequencies with identical amplitude of the total pressure are simulated. The dynamics of the separated shear layer and the transition process are studied by separating the flow components correlated and un-correlated to the inflow frequency. Even moderate frequencies are found to have a strong effect in reducing the averaged size of the separated flow region, thus reducing the losses.