Analysis and modelling of a rotary forming process for cast aluminum alloy A356

Spinning of a common aluminum automotive casting alloy A356 (Al-7Si-0.3 Mg) at elevated temperatures has been investigated experimentally with a novel industrial-scale apparatus. This has permitted the implementation of a fully coupled thermomechanical finite element model aimed at quantifying the processing history (stress, strain, strain-rate and temperature) and predicting the final geometry. The geometric predictions of this model have been compared directly to the geometry of the workpieces obtained experimentally. This study is novel in regards to both the size and shape of the component as well as the constitutive material representation employed. The model predictions are in reasonable agreement with experimental results for small deformations, but errors increase for large deformation conditions. The model has also enabled the characterization of the mechanical state which leads to a common spinning defect. Suggestions for improving the accuracy and robustness of the model to provide a predictive tool for industry are discussed

Global instability of low-density jets

The global stability of laminar axisymmetric low-density jets is investigated in the low Mach number approximation. The linear modal dynamics is found to be characterised by two features: a stable arc branch of eigenmodes and an isolated eigenmode. Both features are studied in detail, revealing that, whereas the former is highly sensitive to numerical domain size and its existence can be linked to spurious feedback from the outflow boundary, the latter is the physical eigenmode that is responsible for the appearance of self-sustained oscillations in low-density jets observed in experiments at low Mach numbers. In contrast to previous local spatio-temporal stability analyses, the present global analysis permits, for the first time, the determination of the critical conditions for the onset of global instability, as well the frequency of the associated oscillations, without additional hypotheses, yielding predictions in fair agreement with previous experimental observations. It is shown that under the conditions of those experiments, viscosity variation with composition, as well as buoyancy, only have a small effect on the onset of instability

Thermo-Mechanical Characterization of Glass and its effect on Predictions of Stress State, Birefringence and Fracture in Precision Glass Molded Lenses

The Precision Glass Molding (PGM) process was established as an economical and sustainable option for the production of aspherical glass optics to satisfy the increased industrial demand. Applications of precision molded aspherical lenses range from consumer electronics products such as cell phone cameras to defense and medical systems. An aspherical lens can eliminate the spherical and optical aberrations as compared to a spherical lens thus making the lens system more compact and lighter. In spite of being a clean and environmentally friendly process, the lens molding operation suffers from a few drawbacks such as lens profile deviation, stress birefringence/refractive index drop and lens cracking. Prior research has identified a lack in accurate and reliable thermo-mechanical characterization of optical glasses as an obstacle to the application of computational mechanics to resolve these issues. The work presented in this dissertation addresses the importance of a precise determination of the thermo-mechanical material property inputs of optical glass for an accurate prediction of the state of stress during the complex thermo-mechanical loading of a glass preform during the Precision Glass Molding (PGM) process. In addition to an accurate prediction of the residual stress state in a lens, birefringence and fracture were also considered as these are direct consequences of stress. Due to the complexity of glass behavior in the relatively large temperature range where the material behavior transitions from that of an elastic solid to a viscous fluid, it is essential to characterize accurately the time and temperature dependence of the stress relaxation behavior. After understanding the weaknesses in existing stress relaxation characterizations, a set of careful experiments was designed that utilized Parallel Plate Viscometer (PPV) to perform the cylinder compression test on a glass sample. It was determined that the uniaxial compression of a cylindrical sample at an uniform temperature, with a known friction condition at the interface, yields a high quality creep data that was used to determine accurate viscosity and viscoelastic constants of two moldable glasses - L-BAL35 and NBK-7 glass at the given temperature. Comparison of the computational solutions with closed form approximations used in an ASTM standard, revealed deficiencies at viscosity near and above 108 Pa*s due to specimen bulging and interface slip, and led to the development of an approximate expression for a reasonable estimate of viscosity above 108 Pa*s for the full range of interface friction behavior. This study highlighted the importance of an accurate characterization of the stress relaxation function of a moldable glass which enabled the numerical examination of the effect of different levels of modeling detail of the relaxation function on the lens molding simulations. The choice of the material model and the level of detail required in performing the creep and relaxation experiments, is dependent on the problem being solved. The use of simplest viscoelastic stress relaxation function with a single exponential relaxation time that lacks much of the transient effects present in a full viscoelastic relaxation, showed minimal effect on the profile deviation of a lens but leads to an over-estimate of residual stress for the two lens shapes studied. A similar effect was observed on the stress birefringence of a lens after molding. Using numerical experiments, residual stresses were shown to be sensitive to the lower temperature limit of the viscoelastic assumption (TL). The fracture assessment inside a molded lens was made for both radial and circumferential crack configurations. The stress state for the two configurations revealed that the radial crack orientation was more prone to failure among the two. The full viscoelastic relaxation assumption also led to higher crack tip opening displacement (CTOD) values than the simplified relaxation assumptio

Experimental and Computational Study of Underexpanded Jet Impingement Heat Transfer

An experiment was performed to assess CFD modeling of a hypersonic-vehicle breach, boundary-layer flow ingestion and internal surface impingement. Tests were conducted in the NASA Langley Research Center 31-Inch Mach 10 Tunnel. Four simulated breaches were tested and impingement heat flux data was obtained for each case using both phosphor thermography and thin film gages on targets placed inside the model. A separate target was used to measure the surface pressure distribution. The measured jet impingement width and peak location are in good agreement with CFD analysis

Thin film dynamics on a vertically rotating disk partially immersed in a liquid bath

The axisymmetric flow of a thin liquid film is considered for the problem of a vertically rotating disk that is partially immersed in a liquid bath. A model for the fully three-dimensional free-boundary problem of the rotating disk, that drags a thin film out of the bath is set up. From this, a dimension-reduced extended lubrication approximation that includes the meniscus region is derived. This problem constitutes a generalization of the classic drag-out and drag-in problem to the case of axisymmetric flow. The resulting nonlinear fourth-order partial differential equation for the film profile is solved numerically using a finite element scheme. For a range of parameters steady states are found and compared to asymptotic solutions. Patterns of the film profile, as a function of immersion depth and angular velocity are discussed.Comment: 31 pages, 19 figures accepted: Applied Mathematical Modellin

Simulation of the fountain flow effect by means of the radial functions method (RFM)

1 CD-ROM : il.The purpose of this work is to simulate the fountain flow effect using a meshless technique (RFM) and, therefore, to explore the possibilities that the method offers for free surface problems. To the knowledge of the author, the fountain ow effect has not been simulated in the past by means of meshless techniques. In this thesis, a steady state was assumed for all simulations. This assumption is common in the literature and can be found for instance in [28, 27]. It originates from the consideration of a reference system that moves with the average velocity of the flow. For the flow in a slit, a power law model model with n varying between 0:6 and 1:1 was used to describe the variation of the viscosity with the shear rate. Another case considered in this thesis is the fountain flow of a Newtonian uid in an axisymmetric tube including the force of gravity. The organization of the thesis is as follows: Chapter 2 presents a brief overview of the literature corresponding to the simulation of the fountain flow effect. In Chapter 3, the Radial Functions Method is presented using the solution of the Poisson equation as an example. Chapter 4 deals with the implementation of the method to simulate the fountain flow effect; this entails the representation of the motion and continuity equations, and the appropriate boundary conditions (including the free boundary) in terms of Radial Basis Functions. The thesis ends with chapters corresponding to the conclusions derived from the simulations and a presentation of possible lines of research for future work.1 Introduction -- 2 Overview of the Literature -- 3 The Radial Functions Method (RFM)-- 4 Implementation and Results -- 4.1 Newtonian Case in a Slit -- 4.1.1 Modeling -- 4.1.2 Numerical Implementation -- 4.1.3 Results -- 4.2 Non-Newtonian Case in a Slit -- 4.2.1 Modeling and Numerical Implementation -- 4.3 Newtonian-Axisymmetric Case with Body Forces -- 4.3.1 Modeling and Numerical Implementation -- 4.3.2 Results --4.4 Fountain Flow E ect on Fiber Matrix Separation during Manufacturing of Short Fiber Filled Injection Parts -- 5 Conclusions -- 6 Future Work -- List of Figures -- List of Table

The onset of purely elastic and thermo-elastic instabilities in the Taylor-Couette flow: Influence of gap ratio and fluid thermal sensitivity

Linear stability analysis of Taylor-Couette flow of dilute polymeric solutions has been performed by using two prototypical constitutive equations for polymeric solutions namely, the Oldroyd-B and the FENE-P models. The hydrodynamic stability characteristics of the flow in presence and absence of thermal effects and in the limit of vanishing fluid inertia have been determined using an eigenvalue analysis. Particular attention has been paid to accurate determination of the instability onset conditions as a function of fluid thermal sensitivity and gap ratio. We observe a reduction in the critical Deborah, Dec for the instability onset as the gap ratio and fluid thermal sensitivity is enhanced. In particular, under non-isothermal conditions, Dec is reduced by almost an order of magnitude for all gap ratios. Our results suggest that recent experiments leading to observations of “purely elastic turbulence” in the Taylor-Couette flow at order (1) De by Steinberg and Groisman (reference 17) were not performed under isothermal conditions. Hence, this new flow state should be labeled as “thermo-elastic turbulence”

Review of fluid flow and convective heat transfer within rotating disk cavities with impinging jet

International audienceFluid flow and convective heat transfer in rotor-stator configurations, which are of great importance in different engineering applications, are treated in details in this review. The review focuses on convective heat transfer in predominantly outward air flow in the rotor-stator geometries with and without impinging jets and incorporates two main parts, namely, experimental / theoretical methodologies and geometries/results. Experimental methodologies include naphthalene sublimation techniques, steadystate (thin layer) and transient (thermochromic liquid crystals) thermal measurements, thermocouples and infra-red cameras, hot-wire anemometry, laser Doppler and particle image velocimetry, laser plane and smoke generator. Theoretical approaches incorporate modern CFD computational tools (DNS, LES, RANS etc). Geometries and results part being mentioned starting from simple to complex elucidates cases of a free rotating disk, a single disk in the crossflow, single jets impinging onto stationary and rotating disk, rotor-stator systems without and with impinging single jets, as well as multiple jets. Conclusions to the review outline perspectives of the further extension of the investigations of different kinds of the rotor-stator systems and their applications in engineering practice