Sensitivity analysis of LES-CMC predictions of piloted jet flames

The sensitivity of Large Eddy Simulation with Conditional Moment Closure (LES-CMC) simulations of the Sandia piloted jet Flames D and F to various parameters have been investigated. It was found that while an LES grid may sufficiently resolve velocity fields, the conditional scalar dissipation rate obtained may still be affected by grid size due to the calculation of sub-grid scalar dissipation rate, and this can affect the degree of localised extinction predicted. A study of the relative size of the terms in the CMC equation during an extinction/reignition event showed that transport, including in the cross stream direction, plays a key role. The results are sensitive to the choice of inlet boundary conditions as extinction is only observed when the inert-mixing distributions in mixture fraction space are used as inlet conditions for the CMC equation in the primary jet and air jets

Aerosol nucleation and growth in a turbulent jet using the Stochastic Fields method

The Stochastic Fields transported PDF method for turbulent reacting flows has been used to model the nucleation and growth of dibutyl phthalate particles in a hot, turbulent jet in a colder background for which experimental data is available. The aerosol population is modelled using an assumed log-normal size distribution. It has been found that neglecting the effect of turbulent fluctuations leads to the peak particle concentration being predicted too close to the jet and the concentration downstream underpredicted. However, this effect was small compared to that of adjusting modelled surface tension. Only by adjusting this was it possible to reproduce correctly the downstream evolution of particle number found in experiment. Particle mass mean diameter was significantly underpredicted at the centre of the jet, which may be due to the inability of log-normal size distribution to capture the distribution in detail. Taking account of turbulent fluctuations leads to increased mean particle size at the edge of the plume. The extent of this increase is strongly dependent on the choice of micromixing timescale

Numerical simulation of oxy-fuel jet flames using unstructured LES-CMC

A finite volume Conditional Moment Closure (CMC) formulation has been developed as an LES sub-grid combustion model. This allows unstructured meshes to be used for both LES and CMC grids making the method more applicable to complex geometry. The method has been applied to an oxy-fuel jet flame. This flame offers new challenges to combustion modelling due to a high CO2 content in the oxidiser stream and significant H2 content in the fuel stream. The density ratio of the two streams is of the order 5 and the viscosity of the two streams will also differ. All the flames simulated showed localised extinction in the region around 3-5 jet diameters downstream of the nozzle, which is in very good agreement with the experiment. Trends for conditional and unconditional statistics with changing levels of H2 in the fuel are correctly captured by the LES-CMC method, although different levels of agreement are observed for different species and temperature and possible reasons for this are discussed. The degree of extinction is also correctly predicted to increase as the H2 content of the jet is reduced, showing the ability of the CMC method to predict complex turbulence-chemistry interaction phenomenon in the presence of changing fuel composition

Capturing localised extinction in Sandia Flame F with LES-CMC

A Large Eddy Simulation (LES) using the Conditional Moment Closure (CMC) as a sub-grid turbulence-chemistry model has been applied to piloted jet diffusion flames (Sandia D&F). A 3D CMC grid was used which allowed different CMC boundary conditions to be applied in the jet and pilot streams. The code was found to give very good agreement with experiment in the low extinction case of Flame D. For Flame F transient extinction and re-ignition events were observed with LES–CMC which lead to reductions in averaged unconditional and conditional temperature consistent with experimental observations. Further analysis revealed that the CMC extinction/ignition events were the result of a combination of high conditional scalar dissipation rate and transport in the CMC grid

A coupled level set and volume of fluid method for automotive exterior water management applications

Motivated by the need for practical, high fidelity, simulation of water over surface features of road vehicles a Coupled Level Set Volume of Fluid (CLSVOF) method has been implemented into a general purpose CFD code. It has been implemented such that it can be used with unstructured and non-orthogonal meshes. The interface reconstruction step needed for CLSVOF has been implemented using an iterative ‘clipping and capping’ algorithm for arbitrary cell shapes and a reinitialisation algorithm suitable for unstructured meshes is also presented. Successful verification tests of interface capturing on orthogonal and tetrahedral meshes are presented. Two macroscopic contact angle models have been implemented and the method is seen to give very good agreement with experimental data for a droplet impinging on a flat plate for both orthogonal and non-orthogonal meshes. Finally the flow of a droplet over a round edged channel is simulated in order to demonstrate the ability of the method developed to simulate surface flows over the sort of curved geometry that makes the use of a non-orthogonal grid desirable

A robust two-node, 13 moment quadrature method of moments for dilute particle flows including wall bouncing

For flows where the particle number density is low and the Stokes number is relatively high, as found when sand or ice is ingested into aircraft gas turbine engines, streams of particles can cross each other’s path or bounce from a solid surface without being influenced by inter-particle collisions. The aim of this work is to develop an Eulerian method to simulate these types of flow. To this end, a two-node quadrature-based moment method using 13 moments is proposed. In the proposed algorithm thirteen moments of particle velocity, including cross-moments of second order, are used to determine the weights and abscissas of the two nodes and to set up the association between the velocity components in each node. Previous Quadrature Method of Moments (QMOM) algorithms either use more than two nodes, leading to increased computational expense, or are shown here to give incorrect results under some circumstances. This method gives the computational efficiency advantages of only needing two particle phase velocity fields whilst ensuring that a correct combination of weights and abscissas are returned for any arbitrary combination of particle trajectories without the need for any further assumptions. Particle crossing and wall bouncing with arbitrary combinations of angles are demonstrated using the method in a two-dimensional scheme. The ability of the scheme to include the presence of drag from a carrier phase is also demonstrated, as is bouncing off surfaces with inelastic collisions. The method is also applied to the Taylor-Green vortex flow test case and is found to give results superior to the existing two-node QMOM method and in good agreement with results from Lagrangian modelling of this case

An iterative interface reconstruction method for PLIC in general convex grids as part of a Coupled Level Set Volume of Fluid solver

Reconstructing the interface within a cell, based on volume fraction and normal direction, is a key part of multiphase flow solvers which make use of piecewise linear interface calculation (PLIC) such as the Coupled Level Set Volume of Fluid (CLSVOF) method. In this paper, we present an iterative method for interface reconstruction (IR) in general convex cells based on tetrahedral decomposition. By splitting the cell into tetrahedra prior to IR the volume of the truncated polyhedron can be calculated much more rapidly than using existing clipping and capping methods. In addition the root finding algorithm is designed to take advantage of the nature of the relationship between volume fraction and interface position by using a combination of Newton's and Muller's methods. In stand-alone tests of the IR algorithm on single cells with up to 20 vertices the proposed method was found to be 2 times faster than an implementation of an existing analytical method, while being easy to implement. It was also found to be 3.4–11.8 times faster than existing iterative methods using clipping and capping and combined with Brent's root finding method. Tests were then carried out of the IR method as part of a CLSVOF solver. For a sphere deformed by a prescribed velocity field the proposed method was found to be up to 33% faster than existing iterative methods. For simulations including the solution of the velocity field the maximum speed up was found to be approximately 52% for a case where 12% of cells lie on the interface. Analysis of the full simulation CPU time budget also indicates that while the proposed method has produced a considerable speed-up, further gains due to increasing the efficiency of the IR method are likely to be small as the IR step now represents only a small proportion of the run time

A computational and experimental investigation into the effects of debris on an inverted double wing in ground effect

Cars in several motor sports series, such as Formula 1, make use of multi-element front wings to provide downforce. These wings also provide onset flows to other surfaces that generate downforce. These elements are highly loaded to maximise their performance and are generally operating close to stall. Rubber debris, often known as marbles, created from the high slip experienced by the soft compound tyres can become lodged in the multiple elements of a front wing. This will lead to a reduction in the effectiveness of the wing over the course of a race. This work will study the effect of such debris, both experimentally and numerically, on an inverted double element wing in ground effect at representative Reynolds numbers. The wing was mounted at two different ride heights above a fixed false-floor in the Loughborough University wind tunnel and the effect of debris blockage modelled by closing sections of the gap between elements with tape. The reduction in downforce compared to the clean wing was measured and the sensitivity to the size and position of the blockage studied. It was found that debris near the centre of the element has a greater impact. CFD simulations were also carried out that were able to correctly predict the trend of downforce with blockage position. The CFD was also used to computationally remove the effects of the tunnel. This confirmed the result seen in experiment that the blockage has more effect on a more highly loaded wing

On the acoustic response of a generic gas turbine fuel injector passage

A current trend in the design of modern aero engines is the transition towards leaner combustion as a solution to satisfy increasingly stringent emission regulations. Lean combustion systems are often more susceptible to thermoacoustic instability and the fuel injector can play a critical role. This paper presents an analytical study on the unsteady air flow through a generic injector passage in response to incident acoustic waves. The injector passage is represented by a simplified geometry which comprises the main geometrical passage features. The unsteady flow through the passage is obtained by combining the elemental solutions for different parts of the passage. This enables the transfer impedance of the injector passage to be determined and the effects of different design parameters on the sensitivity of the air flow to acoustic perturbations to be examined. The convective wave associated with the unsteady swirl vane wakes is also visited and compared with the results from the numerical simulations obtained in previous works. In addition to helping derive design practices for injector passages from the perspective of thermoacoustic instability, the current analysis can also be applied as a preliminary design tool to assess the acoustic characteristics for an injector passage of the axial swirler type

Simulation of the evolution of aircraft exhaust plumes including detailed chemistry and segregation

The Field Monte Carlo or Stochastic Fields (SF) method for turbulent reacting flows has been applied to the chemical evolution of the early part of a hot jet with bypass flow producing 7kN of thrust, using a 23 species chemical mechanism. This is done to broadly approximate a turbofan engine at idle thrust setting. Much of the chemistry was found to take place inside the core of the jet before mixing occurs, as there is no reactant gradient there, considering segregation makes little difference. Radical concentrations, however, were found to be changed. The reaction between NO and ambient O 3, which is slow compared to the fast mixing timescale of the turbulent jet, is unaffected by segregation. The local Damköhler number was calculated based on an estimate of the chemical timescale and the local large-eddy timescale. It was found that only those species which had local Da greater than five were affected by segregation. In this work we have applied the SF method the early part of the plume, however the method developed here could equally be employed to study the plume over a longer distance