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Faculty of Engineering and Biult Enviroment School of Mechanical,Industrial And Aeronautical Engineering 9807537d efunemarc@hotmail.com / Drag over a wide range of shapes is well established for steady flow conditions. Drag in unsteady flow, however, is for the most part not well understood. The research presented herein examines the drag over cones in unsteady compressible flow. This was achieved by constraining cones, with half-vertex angles ranging from 15° to 30°, in a shock tube and passing shock waves over them. The resulting drag was measured directly using a stress wave drag balance (SWDB). Tests were run at shock Mach numbers between 1.12 and 1.31 with corresponding post-shock Reynolds numbers between 2 × 105 and 6 × 105. The drag on the four cone geometries as well as one sphere geometry was modelled numerically. Density contours of the flow fields, obtained from the numerical simulations were used to visualise the shock/model interactions and deduce the causes of any variations in drag. It was thus proved that post-shock fluctuations are due to shock wave reflections off the shock tube walls and the model support. The maximum unsteady drag values measured experimentally ranged from 53.5 N for the 15° cone at a Mach number of 1.14 to 148.6 N for the 30° cone at a Mach number of 1.29. The drag obtained numerically agreed well with experimental results, showing a maximum deviation in peak drag of 9.6%. The drag forces on the conical models peaked as the shock wave reached the base of the cone whereas the drag on the sphere peaked just before the shock reached the equator of the sphere. The negative drag and large post-shock drag fluctuations on a sphere measured by Bredin (2002) were present in the numerical results and thus confirm that these features were not due to balance error. The large post-shock drag fluctuations were also present on the cones. The unsteady drag was shown to increase as both the shock wave Mach number and the cone angle were increased. The ratio of the maximum unsteady drag to the compressible steady state drag varied from v 4.4:1 to 9.8:1, while the ratio of the maximum unsteady drag to the incompressible steady state drag varied from 8.3:1 to 22.2:1. The steady state drag values were shown to be of the same order of magnitude as the post shock unsteady drag. Further numerical work is recommended to confirm that drag fluctuations are in fact due to shock reflections and to better establish the relationship between the unsteady drag and the cone angle.

The purpose of this study is to experimentally and numerically investigate flame acceleration in an obstructed channel. The motivation for this research is for the development of Pulse Detonation Engines (PDEs), which are unsteady propulsion devices that utilize the detonative mode of combustion. A literature survey on flame acceleration in the context of PDEs is presented, which covers a wide range of combustion regimes including laminar combustion, turbulent combustion, and finally detonation. An overview of current numerical modeling strategies is also presented along with a selection of recent numerical studies focused on flame acceleration in obstructed channels. Experimentally, the effect of obstacle blockage ratio on flame acceleration was investigated in a modular channel. The channel had a square cross-section and obstacles were mounted onto the top and bottom surfaces. Schlieren images were used to study the flame shape and the centerline flame velocity. A novel visualization technique has been developed to study the unburned gas flow ahead of the flame front. Flame propagation at speeds above the speed of sound in the reactants was also studied as compression waves formed in the unburned gas. It was found that shock reflection from obstacle surfaces and subsequent flame interaction dominates flame acceleration at these higher flame speeds. The unburned gas flow field ahead of the flame front was simulated using Large Eddy Simulation (LES) and was compared to the visualization technique developed experimentally. The detailed unsteady calculation was used to further study the development of recirculation zones behind the obstacle surfaces and the generation of turbulence in the shear layers. The unburned gas flow field was investigated to give insight into the speed and shape of the flame as it propagates into these regions. Flame propagation was modeled using a flame surface density combustion model and simulations showed flame interactions with the turbulent flow field and how three-dimensional vortical structures augmented the flame shape and increase total area. / Thesis (Ph.D, Mechanical and Materials Engineering) -- Queen's University, 2009-09-21 10:10:36.38

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A novel approach to modelling highly compressible and reactive flows is formulated to provide high resolution closure of turbulent-scale reaction rates in the presence of very rapid transients in pressure and energy. For such flows, treatment of turbulent-micro scales are generally unattainable through traditional modelling techniques. To address this, the modelling strategy developed here is based on the Linear Eddy Model for Large Eddy Simulation (LEM-LES); a technique which has only previously been applied to weakly compressible flows. In the current formulation of the Compressible LEM-LES (CLEM-LES), special treatment of the energy balance on the model subgrid is accounted for in order for the model reaction rates to respond accordingly to strong shocks and rapid expansions, both of which may be present in reactive and supersonic flow fields. In the current study, the model implemented is verified and validated for various 1D and 2D flow configurations in a compressible Adaptive Mesh Refinement (AMR) framework. In 1D test cases, laminar and turbulent flame speeds and structure have been reproduced. Also, detonation speeds and initiation events are also captured with the model. For 2D model validation, unsteady and turbulent detonation propagation and initiation events, in a narrow channel, are simulated. Both test cases involve premixed methane-oxygen mixture at low pressures. The model is found to capture well the two-dimensional detonation cellular structure, behaviour, and initiation events that are observed in corresponding shock tube experiments. Furthermore, the effect of turbulent mixing rates is investigated though a single tuning constant. It was found that by increasing the intensity of turbulent fluctuations present, detonations exhibit larger and more irregular cell structures. Furthermore, the intensity of turbulent fluctuations is found to also have an effect on initiation events.

abstract: The process of designing any real world blunt leading-edge wing is tedious andinvolves hundreds, if not thousands, of design iterations to narrow down a single design. Add in the complexities of supersonic flow and the challenge increases exponentially. One possible, and often common, pathway for this design is to jump straight into detailed volume grid computational fluid dynamics (CFD), in which the physics of supersonic flow are modeled directly but at a high computational cost and thus an incredibly long design process. Classical aerodynamics experts have published work describing a process which can be followed which might bypass the need for detailed CFD altogether. This work outlines how successfully a simple vortex lattice panel method CFDcode can be used in the design process for a Mach 1.3 cruise speed airline wing concept. Specifically, the success of the wing design is measured in its ability to operate subcritically (i.e. free of shock waves) even in a free stream flow which is faster than the speed of sound. By using a modified version of Simple Sweep Theory, design goals are described almost entirely based on defined critical pressure coefficients and critical Mach numbers. The marks of a well-designed wing are discussed in depth and how these traits will naturally lend themselves to a well-suited supersonic wing. Unfortunately, inconsistencies with the published work are revealed by detailedCFD validation runs to be extensive and large in magnitude. These inconsistencies likely have roots in several concepts related to supersonic compressible flow which are explored in detail. The conclusion is made that the theory referenced in this work by the classical aerodynamicists is incorrect and/or incomplete. The true explanation for the perplexing shock wave phenomenon observed certainly lies in some convolution of the factors discussed in this thesis. Much work can still be performed in the way of creating an empirical model for shock wave formation across a highly swept wing with blunt leading-edge airfoils. / Dissertation/Thesis / Masters Thesis Engineering 2020

Core engine icing has been recognized to affect a wide variety of engines since the 1990's. This previously unrecognized form of icing occurs in flights through high altitude convective regions and vicinity of thunderstorms. Engine icing events involve power loss or damage associated to the engine core, namely instabilities such as compressor surge, stall, engine rollback and even combustor flameout events. The effects on compressor performance are significant in understanding the response of the engine to atmospheric ice ingestion. A one-dimensional axisymmetric flow model is used to simulate the continuous phase through the compressor. The steady state operation of dry air is validated with an industrial database. By changing an exit throttle, the point where the dry compressor mass flow rate slowly starts to drop, is predicted. The stage that is the first to locally collapse, causing the remaining stages and eventually the complete compressor failure, is determined. The continuous flow model is then coupled with a Lagrangian model for the discrete phase in a framework that conserves mass, momentum and energy. From numerical simulations of the coupled, continuous-discrete phase flow model, it is observed that a rematching of the stages across the compressor occurs with increasing ice flow rates to accommodate loss of energy to the ice flow. The migration of the operating point towards the stall point at the rear stage eventually causes the compressor to stall. The onset of stall is characterized by initial oscillations followed by a rapid decay of pressures of the last stage with the instability traveling quickly towards the front of the compressor. Effectively, a reduction in the compressor stall margin is observed as the ice flow rate increases. Further, the relevance of factors such as blockage due to discrete particles and break/splash semi-empirical models in the icing physics, are analyzed through parametric studies. Conclusions are drawn that underscore the influence of the assumptions and models in prediction of the flow behavior in the presence of ice ingestion. Smaller ice crystal diameters have a greater influence on the gas flow dynamics in terms of a higher reduction in surge margin. The break empirical model for ice crystals and splash model for the droplets that are used to calculate the secondary particle size upon impact with rotor blades have a significant influence on the gas flow predictions.

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Large-eddy simulation is rapidly becoming the preferred method for calculations involving turbulent phenomena. However, filtering equations as performed in traditional LES procedures leads to significant problems. In this work we present some key components in the construction of a novel LES solver for compressible turbulent flow, designed to overcome most of the problems faced by traditional LES procedures. We describe the construction of the large-scale algorithm, which employs fairly standard numerical techniques to solve the Navier{Stokes equations. We validate the algorithm for both transonic and supersonic ow scenarios. We further explicitly show that the solver is capable of capturing boundary layer effects. We present a detailed derivation of the chaotic map termed the \compressible poor man's Navier{Stokes (CPMNS) equation" starting from the Navier{Stokes equations themselves via a Galerkin procedure, which we propose to use as the fluctuating component in the SGS model. We provide computational results to show that the chaotic map can produce a wide range of temporal behaviors when the bifurcation parameters are varied over their ranges of stable behaviors. Investigations of the overall dynamics of the CPMNS equation demonstrates that its use increases the potential realism of the corresponding SGS model.