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We propose a predictive model of the turbulent burning velocity S T in homogeneous isotropic turbulence (HIT) based on Lagrangian statistics of propagating surfaces. The propagating surfaces with a constant displacement speed are initially arranged on a plane, and they evolve in non-reacting HIT, behaving like the propagation of a planar premixed flame front. The universal constants in the model of S T characterize the enhancement of area growth of premixed flames by turbulence, and they are determined by Lagrangian statistics of propagating surfaces. The flame area is then modelled by the area of propagating surfaces at a truncation time. This truncation time signals the statistical stationary state of the evolutionary geometry of propagating surfaces, and it is modelled by an explicit expression using limiting conditions of very weak and strong turbulence. Another parameter in the model of S T characterizes the effect of fuel chemistry on S T , and it is pre-determined by very few available data points of S T from experiments or direct numerical simulation (DNS) in weak turbulence. The proposed model is validated using three DNS series of turbulent premixed flames with various fuels. The model prediction of S T generally agrees well with DNS in a wide range of premixed combustion regimes, and it captures the basic trends of S T in terms of the turbulence intensity, including the linear growth in weak turbulence and the 'bending effect' in strong turbulence.

We extend the vortex-surface field (VSF), a Lagrangian-based structure identification method, to investigate the vortex reconnection in temporally evolving transitional pipe flows. In the direct numerical simulation (DNS) of round pipe flows, a radial wave-like velocity disturbance is imposed on the inlet region to trigger the transition. The VSF isosurfaces are vortex surfaces composed of vortex lines, and they are concentric tubes with different wall distances at the initial time. The VSF evolution is calculated by the two-time method based on the DNS velocity field, and it is effective to identify the vortex reconnection. In the early stage of transition, the vortex surfaces are first corrugated with streamwise elongated bulges. The escalation and descent of vortex surfaces characterize the generation of high- and low-speed streaks and streamwise vortex pairs, along with the surge of the wall-friction coefficient. The resultant highly coiled and stretched vortex loops then reconnect with each other under the viscous cancelation mechanism. Subsequently, successive vortex reconnections occur via a “greedy snake” mechanism. The streamwise vortex loops consecutively capture the secondary vortex rings pinched off with self-reconnection, forming long helical vortex loops spanning over ten pipe radii in the streamwise direction. Finally, the Kelvin–Helmholtz instability of the shear layer at the trailing edge breaks down the streamwise helical vortex loops into turbulent spots.

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We report large-eddy simulation (LES)/probability density function (PDF) modeling of piloted turbulent dimethyl ether (DME)/air jet flames with a skeletal chemical mechanism. The modeled PDF transport equation with three different implementations of molecular transport in the interaction-byexchange-with-the-mean (IEM) mixing model is solved using the NGA/HPDF code in order to assess the a posteriori performance of these LES/PDF methodologies applied to the DME flames D and F proposed in the TNF Workshop. The three implementations considered are the classical randomwalk model (IEM-RW), the mean-drift model with a single molecular diffusivity (IEM-MD), and the mean-drift model with differential diffusion (IEM