Pore-scale modeling of fluid-particles interaction and emerging poromechanical effects

A micro-hydromechanical model for granular materials is presented. It combines the discrete element method (DEM) for the modeling of the solid phase and a pore-scale finite volume (PFV) formulation for the flow of an incompressible pore fluid. The coupling equations are derived and contrasted against the equations of conventional poroelasticity. An analogy is found between the DEM-PFV coupling and Biot's theory in the limit case of incompressible phases. The simulation of an oedometer test validates the coupling scheme and demonstrates the ability of the model to capture strong poromechanical effects. A detailed analysis of microscale strain and stress confirms the analogy with poroelasticity. An immersed deposition problem is finally simulated and shows the potential of the method to handle phase transitions.Comment: accepted in Int. Journal for Numerical and Analytical Methods in Geomechanic

Using a one-dimensional spray model to improve liquid length and ignition delay estimations for diesel flames

[EN] In the current paper, a methodology based on the combination of a one-dimensional spray model and experimental correlations has been proposed to predict the physical time associated with ignition delay in diesel diffusion flames. This physical time depends significantly on the nozzle geometry, and its influence is not captured in traditional Arrhenius-like correlation. To assess this influence, three multi-hole nozzles with different degrees of conicity (expressed in terms of k-factor) have been tested onan optically accessible 2-stroke single-cylinder engine. First, the hydraulic behavior of the nozzles is assessed from the point of view of injection rate and spray momentum. Later, the effect of the geometry on vapor spray angle has been analyzed through a Schlieren visualization technique. Mie-scattering has allowed to determine the stabilized liquid length. Then, chemiluminescence imaging was used to obtain the temporal and spatial appearance of OH radicals, which are used as indicators to the ignition delay. Finally, all the results are combined with a one-dimensional spray model to determine the physical induction time and include it into a new ignition delay correlation, which shows up to 4% accuracy improvement compared to a traditional Arrhenius equation. (C) 2017 Elsevier Ltd. All rights reserved.This work was partly sponsored by "Ministerio de Economia y Competitividad", of the Spanish Government, in the frame of the Project "Estudio de la interaccion chorro-pared en condiciones realistas de motor", Reference TRA2015-67679-c2-1-R.Payri, R.; Salvador, F.; De La Morena, J.; Pagano, V. (2017). Using a one-dimensional spray model to improve liquid length and ignition delay estimations for diesel flames. Applied Thermal Engineering. 124:1090-1102. https://doi.org/10.1016/j.applthermaleng.2017.06.1021090110212

Local sensitivity analysis for compositional data with application to soil texture in hydrologic modelling

Compositional data, such as soil texture, are hard to deal with in the geosciences as standard statistical methods are often inappropriate to analyse this type of data. Especially in sensitivity analysis, the closed character of the data is often ignored. To that end, we developed a method to assess the local sensitivity of a model output with resect to a compositional model input. We adapted the finite difference technique such that the different parts of the input are perturbed simultaneously while the closed character of the data is preserved. This method was applied to a hydrologic model and the sensitivity of the simulated soil moisture content to local changes in soil texture was assessed. Based on a high number of model runs, in which the soil texture was varied across the entire texture triangle, we identified zones of high sensitivity in the texture triangle. In such zones, the model output uncertainty induced by the discrepancy between the scale of measurement and the scale of model application, is advised to be reduced through additional data collection. Furthermore, the sensitivity analysis provided more insight into the hydrologic model behaviour as it revealed how the model sensitivity is related to the shape of the soil moisture retention curve

Numerical Simulation (CFD) to Explore Optimal Vortex Generator Array Configurations in Air Cooled Condensers

Heat exchangers such as evaporators and condensers are a major component in applications of air conditioning, refrigeration and power conversion systems. Air-cooled condensers (ACC) used in power stations reject heat to the environment very similar to air conditioning systems. Rejection of heat to the ambient air is not very efficient and a more effectives design is sought in the fin component of the condenser. The current project proposes the use of vortex generator arrays in the plain fin of a flat tube heat exchanger to enhance heat transfer performance and increase system efficiency by achieving a breakthrough design. In the field of heat exchanger design, prior research has shown that vortex generators can be used for improving air side heat transfer. Elsherbini and Jacobi (2002) obtained 31% heat transfer enhancement with a pressure drop penalty of 10% for leading edge delta-wing VGs on a plain-fin-and-tube heat exchanger. Joarder and Jacobi (2005) assessed leading edge delta wings on flat-tube, louvered-fin compact heat exchanger and obtained an average heat transfer augmentation of 20% and a pressure drop of less than 7%. These results were obtained from full scale testing of VGs in prototype heat exchangers. It is anticipated that creating arrays of delta-wings will generate further enhancement of heat transfer while not increasing the pressure drop. In order to explore this further, a numerical simulation of such vortex generators deployed in different array configurations is proposed. Two challenges arise in designing such systems. The wings must be spaced far apart to avoid destructive interference but close enough to enhance as much surface area as possible. Another challenge is to test for various Reynolds numbers based on wing size to generate a vortex that can flow cleanly through the passage. The proposed work is focused on the deployment of such vortex generators in different array configurations in such a way that constructive interference of the vortices occurs. First, flow visualization experiments were performed in a water tunnel to guide the design and placement of delta winglets. Based on the conclusions from these experiments, different array deployments are being simulated numerically on commercial finite volume based CFD software (ANSYS Fluent). Different parameters such as angle of attack, number of delta winglets, their placement relative to each other and to the walls of the fin, etc. will be varied to determine the greatest enhancement effect and the results of heat transfer enhancement versus the pressure drop penalty will be reported in the final manuscript. Recommendations will be made on an optimal vortex generator configuration in order to maximize the heat transfer while not increasing the pressure drop significantly. REFERENCES [1] Elsherbini, A. and A.M. Jacobi, 2002, J. HVAC&R Res., 8:357-370. [2] Joarder, A., and A.M. Jacobi, 2005, Int. J. Heat Mass Trans., 48:1480

Analisis Gerakan Osilasi Pancaran Jet Nosel Kovergen-divergen Akibat Screech Noise

Nosel merupakan suatu alat yang berfungsi alat bantu untuk menyalurkan atau mengalirkan fluida cair maupun fluida gas. Karakteristik aliran fluida yang keluar melalui nosel (pancaran jet) mempunyai bentuk yang berbeda untuk setiap perubahan tekanan dan kecepatan aliran. Mekanisme screech noise merupakan suatu pusaran (loop) gelombang akustik yang menjalar ke hilir diantara lapisan batas (shear layer) pancaran jet pada kecepatan suara lokal dan gelombang menjalar ke hulu pada kecepatan akustik di bagian luar atau sekitar pancaran nosel (jet). Fenomena loop yang muncul pada meja analogi merupakan gelombang yang tidak stabil (instability waves). Teknik visualisasi dengan metode bayangan, digunakan untuk melihat dan mengamati gerakan osilasi pancaran jet nosel. Analisis gambar diidentifikasi berdasarkan kontur distribusi intensitas yang diperoleh pada visualisasi gambar. Gerakan pancaran jet yang tidak stabil (berosilasi kekanan dan kekiri) pada rasio head Rh = 1,85 atau ratio tekanan  Rp = 3,4. Pancaran jet menunjukkan gerakan yang relatif stabil pada kondisi rasio head Rh < 1,85 dan gerakan osilasi pancaran jet terjadi ketika kapasitas aliran meningkat dengan rasio head antara Rh = 1,85 sampai Rh = 2,1. Kapasitas aliran dinaikkan hingga rasio head mencapai Rh > 2,1, maka gerakan pancaran jet mulai stabil. Pengoperasian jet nosel dimana screech noise pada kondisi dominan, maka secara visual aliran jet nosel menunjukkan fenomena gerakan osilasi secara lateral

Modeling of Induced Hydraulically Fractured Wells in Shale Reservoirs Using Branched Fractals

Imperial Users onl

Stability and Vortex Shedding of Bluff Body Arrays

The primary purpose of this study was to develop an understanding of the stability of laminar flow through bluff body arrays, and investigate the nature of the unsteady vortex shedding regime that follows. The flow was numerically investigated using a specially developed multi-domain spectral element solver. Important criteria in the solver development were flexibility, efficiency, and accuracy. Flexibility was critical to the functionality of the code, as arrays of varying geometry were investigated. Efficiency with a high degree of accuracy was also of primary importance, with the code implemented to run efficiently on today's massively parallel architectures. Numerical two-dimensional stability analysis of the flow in several configurations of inline and staggered array geometries was performed. The growth rate, eigenfunction, and frequency of the disturbances were determined. The critical Reynolds number for flow transition in each case was identified and compared to that of flow over a single body. Based on the solutions of the laminar flow, a one-dimensional analytical analysis was performed on selected velocity profiles in the wake region. The results of this analysis were used to guide the interpretation of the two dimensional results and formulate a general theory of stability of inline and staggered bluff body arrays. The nature of the flow in the unsteady regime following the onset of instability was examined for an inline and a staggered arrangement. Particular attention was focused on the vortex shedding which was visualized and quantified through computation of the flow swirl, a quantity which identifies regions of rotary motion. The conditions required for the generation of leading edge vortex shedding were identified and discussed. Finally, a third geometry related to the inline and staggered arrays was considered. Flow solution data for this geometry is presented and its suitability as a model for louvered arrays was discussed.Air Conditioning and Refrigeration Project 11

Determination of strength and debonding energy of a glass-concrete interface for encapsulation-based self-healing concrete

This paper presents a combined experimental-numerical analysis to assess the strength and fracture toughness of a glass-concrete interface. This interface is present in encapsulation-based self-healing concrete. There is absence of published results of these two properties, despite their important role in the correct working of this self-healing strategy. Two setups are used: uniaxial tensile tests to assess the bonding strength and four point bending tests to get the interfacial energy. The complementary numerical models for each setup are conducted using the finite element method. Two approaches are used: cohesive zone model to study the interface strength and the virtual crack closure technique to analyze the interfacial toughness. The models are validated and used to verify the experimental interpretations. It is found that a glass-concrete interface can develop a maximum strength of approximately 1 N/mm^2 with fracture energy of 0.011 J/m^2

Aeronautical Engineering: A special bibliography with indexes, supplement 67, February 1976

This bibliography lists 341 reports, articles, and other documents introduced into the NASA scientific and technical information system in January 1976

Use of Machine Learning for Automated Convergence of Numerical Iterative Schemes

Convergence of a numerical solution scheme occurs when a sequence of increasingly refined iterative solutions approaches a value consistent with the modeled phenomenon. Approximations using iterative schemes need to satisfy convergence criteria, such as reaching a specific error tolerance or number of iterations. The schemes often bypass the criteria or prematurely converge because of oscillations that may be inherent to the solution. Using a Support Vector Machines (SVM) machine learning approach, an algorithm is designed to use the source data to train a model to predict convergence in the solution process and stop unnecessary iterations. The discretization of the Navier Stokes (NS) equations for a transient local hemodynamics case requires determining a pressure correction term from a Poisson-like equation at every time-step. The pressure correction solution must fully converge to avoid introducing a mass imbalance. Considering time, frequency, and time-frequency domain features of its residual’s behavior, the algorithm trains an SVM model to predict the convergence of the Poisson equation iterative solver so that the time-marching process can move forward efficiently and effectively. The fluid flow model integrates peripheral circulation using a lumped-parameter model (LPM) to capture the field pressures and flows across various circulatory compartments. Machine learning opens the doors to an intelligent approach for iterative solutions by replacing prescribed criteria with an algorithm that uses the data set itself to predict convergence