Andrew Garmory

Experimental and Computational Study of Vehicle Surface Contamination on a Generic Bluff Body

This paper focuses on methods used to model vehicle surface contamination arising as a result of rear wake aerodynamics. Besides being unsightly, contamination, such as self-soiling from rear tyre spray, can degrade the performance of lighting, rear view cameras and obstruct visibility through windows. In order to accurately predict likely contamination patterns, it is necessary to consider the aerodynamics and multiphase spray processes together. This paper presents an experimental and numerical (CFD) investigation of the phenomenon. The experimental study investigates contamination with controlled conditions in a wind tunnel using a generic bluff body (the Windsor model.) Contamination is represented by a water spray located beneath the rear of the vehicle. The aim is to investigate the fundamentals of contamination in a case where both flow field and contamination patterns can be measured, and also to provide validation of modelling techniques in a case where flow and spray conditions are known. CFD results were obtained using both steady RANS and unsteady URANS solvers, combined with particle tracking methods. Steady RANS does not capture the wake structures accurately and this affects the contamination prediction. URANS is able to recover the large-scale wake unsteadiness seen in the experimental data, but the difference between the experimental and computational contamination distributions is still notable. The CFD is also able to provide further insight by showing the behaviour of particles of different sizes. Large particles are found to take on a ballistic trajectory and penetrate the wake. In contrast, small particles are shown to be less likely to become entrained into the wake.

Conditional Moment Closure LES Modelling of an Aero-Engine Combustor at Relight Conditions

A Conditional Moment Closure (CMC) approach embedded in an LES CFD framework is presented for simulation of the reactive flow field of an aero-engine combustor operating at altitude relight conditions. Before application to the combustor geometry, the CMC model was validated on the standard lab-scale Sandia flame D. For the combustor simulation, a global mechanism for n-heptane was used along with a Lagrangian approach for the spray, to which a secondary break-up model was applied. The simulation modelled a multi-sector sub-atmospheric rig that was used to verify the altitude relight capability of the combustor. A comprehensive suite of diagnostics was applied to the rig test, including high-speed OH and kerosene PLIF as well as high speed OH* chemiluminescence. The CMC-based CFD simulation was able to predict well the position of the flame front and fuel distribution at the low pressure, low temperature conditions typical of altitude relight. Furthermore, the simulation of the ignition showed strong similarities with OH* chemiluminescence measurements of the event. An EBU-based LES was run too and showed to be unable to capture the flame front as well as the CMC model could. This work demonstrates that CMC LES can be an effective tool to support assessment of the relight capability of aero-engine combustors.Copyright

Coupled Level-Set Volume of Fluid Simulations of Water Flowing Over a Simplified Drainage Channel With and Without Air Coflow

The motivation for this paper is to predict the flow of water over exterior surfaces of road vehicles. We present simulations of liquid flows on solid surfaces under the influence of gravity with and without the addition of aerodynamic forces on the liquid. This is done using an implementation of a Coupled Level Set Volume of Fluid method (CLSVOF) multiphase approach implemented in the open source OpenFOAM CFD code. This is a high fidelity interface-resolving method that solves for the velocity field in both phases without restrictions on the flow regime. In the current paper the suitability of the approach to Exterior Water Management (EWM) is demonstrated using the representative test cases of a continuous liquid rivulet flowing along an inclined surface with a channel located downstream perpendicular to the oncoming flow. Experimental work has been carried out to record the motion of the rivulet in this case and also to measure the contact angle of the liquid with the solid surface. The measurements of the liquid/solid characteristics such as equilibrium and dynamic contact angles are described along with the analytical expression for contact angle vs. capillary number used in the CFD code. The results from the simulations are compared to experimental measurements. The simulations are carried out with air co-flows of 0, 0.5 and 10 m/s. The simulations are seen to reproduce physical phenomena such as the liquid pinning at sharp corners and the longitudinal stretching of the rivulet with higher air velocity.

A Parametric Study of Automotive Rear End Geometries on Rear Soiling

The motivation for this paper is to consider the effect of rear end geometry on rear soiling using a representative generic SUV body. In particular the effect of varying the top slant angle is considered using both experiment and Computational Fluid Dynamics (CFD). Previous work has shown that slant angle has a significant effect on wake shape and drag and the work here extends this to investigate the effect on rear soiling. It is hoped that this work can provide an insight into the likely effect of such geometry changes on the soiling of similarly shaped road vehicles. To increase the generality of results, and to allow comparison with previously obtained aerodynamic data, a 25% scale generic SUV model is used in the Loughborough University Large Wind Tunnel. UV doped water is sprayed from a position located at the bottom of the left rear tyre to simulate the creation of spray from this tyre. Having a single source of contamination simplifies the configuration of both experimental tests and simulations. It also improves analysis by allowing the soiling pattern from only one wheel to be seen in isolation. In order to provide further insight into the flowfield and its interaction with the spray CFD simulations are also performed at the same scale. A Detached Eddy Simulation approach is used, specifically the Spalart Allmaras formulation of the IDDES CFD model. Lagrangian particle tracking is used to model the dispersed phase. This CFD methodology has been found to give good agreement for soiling pattern with experiment for baseline cases.

Simulation of Particle Flow in Inertial Particle Separators with Eulerian VR-QMOM Method

This paper presents research into practical simulations of particle flow in inertial particle separators typically used in helicopter and tilt-rotor aircraft propulsion systems. The flowfield of the carrier gas is predicted by a Reynolds-averaged Navier–Stokes computational-fluid-dynamics method with the Reynolds-Averaged turbulence model. An Eulerian methodology is used to trace the trajectories of foreign particles such as droplets, ice, and sand. To predict the characteristics of particle wall bouncing in dilute particle flow, the velocity-reassociated two-node quadrature-based method of moments is used. The particle distributions in the inertial particle separator are predicted for various particle sizes, and these are compared with results from a Lagrangian particle-tracking method. The particle–wall interactions and the separation efficiencies are studied for solid particles bouncing off perfectly elastic walls and an inertial particle separator shell coated with the M246 alloy, which changes the co...

Micromixing effects in air pollution modelling

Predicting the dispersion of reacting pollutants close to their source is a topic of importance in Air Quality Modelling. The conventional method of neglecting species concentration fluctuations is not valid for such small-scale problems. Various methods that incorporate segregation are reviewed here and their use for typical atmospheric dispersion problems is illustrated through numerical simulations of a simplified problem. By comparison with experimental data, it is found that micromixing can affect the evolution of the mean reaction rate and that the models presented here are more accurate than if segregation were not included. Further work should focus on the interfacing of these models with practical Air Quality calculations.

An Objective Measure for Automotive Surface Contamination

Surface contamination, or soiling, of the exterior of road vehicles can be unsightly, reduce visibility and customer satisfaction and, with the increasing application of surface mounted sensors, can degrade the performance of advanced driver assistance systems. Experimental methods of evaluating surface contamination are increasingly used in the product development process, but the results are generally subjective. The use of computational methods for predicting contamination make objective measures possible, but comparable data from experiment is an important validation requirement. This paper describes the development of an objective measure of surface contamination arising during experiments. A series of controlled experiments using Ultra Violet (UV) dye doped water are conducted to develop a robust methodology. This process is then applied to a simplified contamination test. An image of a surface, illuminated by an UV lamp, is captured after every test along with a calibration vessel with known fluid depth. The image is processed to remove the influence of variation in incident illumination. The total mass of contamination deposited is then calculated using the calibration vessel to provide the required local fluid depths. The paper includes validation of the technique.

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.

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

Abstract 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 Newtons and Mullers 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 Brents 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.

The Identification and Prediction of Helical Modes Induced by a Multi-Passage Swirl Stabilised Lean Burn Aero-Engine Fuel Injector Under Steady State and Acoustically Forced Conditions

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