Mechanical Engineering
Permanent URI for this communityhttp://hdl.handle.net/1903/2263
Browse
11 results
Search Results
Item Impact of Polymeric Drops on Drops and Films of a Different but Miscible Polymer(2024) Bera, Arka; Das, Siddhartha; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The fluid mechanics of a liquid drop impacting on another stationery (or spreading) liquid drop or on a liquid film (of thickness comparable, or smaller, or larger than the impacting drop) has attracted significant attention over the past several years. Such problems represent interesting deviations from the more widely studied problems of liquid drops impacting on solid surfaces having different wettabilities with respect to the impacting drops. These deviations stem from the fact that the resting liquid (in the form of the drop or the film) itself undergoes deformation on account of the drop impact and can significantly affect the overall combined drop-drop or drop-film dynamics. The problem becomes even more intriguing depending on the rheology of the drop(s) and the film as well as the (im)miscibility of the impacting drop with the underlying drop or the film. Interestingly, the majority of such drop-impact-on-drop or drop-impact-on-film studies have considered Newtonian drop(s) and films, with little attention to polymeric drop(s) and films. This thesis aims to bridge that void by studying, using Direct Numerical Simulation (DNS) based computational methods, the impact-driven dynamics of one polymeric drop on another (different but miscible) polymeric drop or film. As specific examples, we consider two separate problems. In the first problem, we consider the impact of a PMMA (poly-methyl methacrylate) drop on a resting PVAc (polyvinyl acetate) drop as well as the impact of a PVAc drop on a resting PMMA drop. In the second problem, we consider the impact of a PMMA drop on a PVAc film as well as the impact of a PVAc drop on a PMMA film. For the first problem, the wettability of the resting drop (on the resting surface), the Weber number of the impacting drop, the relative surface tension values of the two polymeric liquids (PVAc and PMMA), and the miscibility (or how fast the two liquids mix) dictate the overall dynamics. PVAc has a large wettability on silicon (considered as the underlying solid substrate); as a result, during the problem of the PMMA drop impacting on the PVAc drop, the PVAc drop spreads significantly and the slow mixing of the two liquids ensures that the PMMA drop spreads as a thin film on top of the PVAc film (formed as the PVAc drop spreads quickly on silicon). Depending on the Weber number, such a scenario leads to the formation of transient liquid films (of multitudes of shapes) with stratified layers of PMMA (on top) and PVAc (on bottom) liquids. On the other hand, for the case of the PVAc drop impacting on the PMMA drop, a combination of the weaker spreading of the PMMA drop on silicon and the “engulfing” of the PMMA drop by the PVAc drop (stemming from the PVAc having a smaller surface tension than PMMA) ensures that the impacting PVAc drop covers the entire PMMA drop and itself interacts with the substrate giving rise to highly intriguing transient and stratified multi-polymeric liquid-liquid structures (such as core-shell structure with PMMA core and PVAc shell). For both these cases, we thoroughly discuss the dynamics by studying the velocity field, the concentration profiles (characterizing the mixing), the progression of the mixing front, and the capillary waves (resulting from the impact-driven imposition of the disturbance). In the second problem, we consider a drop of the PMMA (PVAc) impacting on a film of the PVAc (PMMA). In addition to the factors dictating the previous problem, the film thickness (considered to be either identical or smaller than the drop diameter) also governs the overall droplet-impact-driven dynamics. Here, the impact, being on the film, the dynamics is governed by the formation of crown (signifying the pre-splashing stage) and a deep cavity (the depth of which is dictated by the film thickness) on the resting film. In addition to quantifying these facets, we further quantify the problem by studying the velocity and the concentration fields, the capillary waves, and the progression of the mixing front. For the PMMA drop impacting on the thin film, a noticeable effect is the quick thinning of the PMMA drop on the PVAc film (or the impact-driven cavity formed on the PVAc film), which gives rise to a situation similar to the previous study (development of transient multi-polymeric-liquid structures with stratified polymeric liquid layers). For the case of the PVAc drop impacting on the PMMA film, the PVAc liquid “engulfs” the deforming PMMA film, and this in turn, reduces the depth of the cavity formed, the extent of thinning, and the amplitude of the generated capillary waves. All these fascinating phenomena get captured through the detailed DNS results that are provided. The specific problems considered in this thesis have been motivated by the situations often experienced during the droplet-based 3D printing processes (e.g., Aerosol jet printing or inkjet printing). In such printing applications, it is commonplace to find one polymeric drop interacting with an already deposited polymeric drop or a polymeric film (e.g., through the co-deposition of multiple materials during multi-material printing). The scientific background for explaining these specific scenarios routinely encountered in 3D printing problems, unfortunately, has been very limited. Our study aims to fill this gap. Also, the prospect of rapidly solidifying these polymeric systems (via methods such as in-situ curing) can enable us to visualize the formation of solidified multi-polymeric structures of different shapes (by rapidly solidifying the different transient multi-polymeric-liquid structures described above). Specifically, both PMMA and PVAc are polymers well-known to be curable using in-situ ultraviolet curing, thereby establishing the case where the present thesis also raises the potential of developing PMMA-PVAc multi-polymeric solid structures of various shapes and morphologies.Item MULTI-FIDELITY PARAMETRIC SENSITIVITY FOR LARGE EDDY SIMULATION(2023) Oberoi, Nikhil; Larsson, Johan Prof.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Designing engineering systems involving fluid flow under uncertainty or for optimality often requires performing many computational fluid dynamics (CFD) calculations. For low-fidelity turbulence modeling simulations such as Reynolds-averaged Navier-Stokes (RANS), such a framework has been established and is in use. However, for high-fidelity turbulence-resolving simulations such as large eddy simulations (LES), the relatively high computational cost of even a single calculation hinders the development of such a framework. The overarching goal of this work is to aid LES in becoming a usable engineering design tool. In this thesis, a computationally affordable approach to estimate parametric sensitivities of engineering relevant quantities of interest in an LES is explored. The method is based on defining a RANS problem that is constrained to reproduce the LES mean flow field. The proposed method is described and assessed for a shock/boundary layer interaction problem, where the shock angle and wall temperature are considered variable or uncertain. In the current work, a proof-of-concept of the proposed method is demonstrated. The method offers qualitative improvements to the sensitivity prediction of certain flow features as compared to standalone RANS simulations, while using a fraction of the LES cost. Different cost functions to infer auxiliary RANS variables are also examined and their influence on the sensitivity estimation is assessed. Overall, the results serve as an important proof-of-concept of the method and suggests the most promising path for future developments.Item Development and Application of Solid-Liquid Lattice Boltzmann Model for Phase Change Material in Heat Exchanger(2022) Chen, Dongyu; Radermacher, Reinhard; Riaz, Amir; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Phase change materials (PCMs) are widely used in thermal energy storage systems, as they can absorb and release a large amount of heat during the phase change process. Numerical simulations can be used for parametric studies and analysis of the thermal performance of the PCM heat exchanger (HX) to produce an optimal design. Among various numerical methods, the lattice Boltzmann method (LBM), a mesoscopic approach that considers the molecular interactions at relatively low computation costs, offers certain key advantages in simulating the phase change process compared with the conventional Navier-Stokes-based (NS-based) methods. Moreover, LBM is ideal for parallel computing, by which numerical analysis can be efficiently performed. Therefore, a comprehensive solid-liquid phase change model is developed based on LBM which is capable of accurately and efficiently simulating the process of convective PCM phase change with and without porous media in both Cartesian and axisymmetric domains. Double distribution functions (DDF) coupled with a multi-relaxation-time (MRT) scheme are utilized in the LBM formulation for the simulation of the fluid flow and the temperature field. A differential scanning calorimetry (DSC) correlated equation is applied in LBM to model enthalpy, by which the solid-liquid interface can be automatically tracked. The source term in the MRT scheme is modified to eliminate numerical errors at high Rayleigh numbers. Moreover, the conjugate thermal model is adopted for the consideration of heat transfer fluid (HTF) flow and conducting fins. The new model is verified and validated by various case studies. The results indicate that the new model can successfully predict the process of PCM phase change with errors confined to less than 10\%. Parametric studies are then performed using the validated model to quantitatively evaluate the effect of convection on PCM melting, from which the acceleration rates (\(a_c\)) of PCM melting and the threshold Rayleigh numbers (\(Ra_{dc}\)) at various aspect ratios are defined and quantified. Furthermore, PCM melting in porous cylindrical HX is also investigated. The results indicate that the acceleration of melting could reach 95\% compared to that in pure PCM at 60\% energy storage. Moreover, the negative effect of uneven temperature distributions on thermal performance of the HX caused by convection is quantified and analyzed. A modified cylindrical HX that offsets this negative effect by varying the geometry is also evaluated. The results indicate that the modified geometry can successfully enhance heat transfer and balance the uneven temperature distributions.Item A MULTI-FIDELITY APPROACH TO SENSITIVITY ESTIMATION IN LARGE EDDY SIMULATIONS(2022) Arias Ramírez, Walter; Larsson, Johan; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)An approach to compute approximate sensitivities in a large eddy simulation (LES) is proposed and assessed.The multi-fidelity sensitivity analysis (MFSA) solves a linearized mean equation, where the mean equation is based on the LES solution. This requires closure modeling which makes the computed sensitivities approximate. The closure modeling is based on inferring the eddy viscosity from the LES data and in predicting the change in turbulence (or the perturbed eddy viscosity) using a simple algebraic model. The method is assessed for the flow over a NACA0012 airfoil at a fixed angle of attack, with the Reynolds number as the varying parameter and the lift, drag, skin friction, and pressure coefficients as the quantities-of-interest. The results show the importance of accurate closure modeling, specifically that treating the eddy viscosity as "frozen" is insufficiently accurate. Also, predictions obtained using the algebraic model for closing the perturbed eddy viscosity are closer to the true sensitivity than results obtained using the fully RANS-based method which is the state-of-the-art and most common method used in industry. The proposed method aims to complement, rather than replace, the current state-of-the-art method in situations in which sensitivities with higher fidelity are required.Item ATOMISTIC EXPLORATION OF DENSELY-GRAFTED POLYELECTROLYTE BRUSHES: EFFECT OF APPLIED ELECTRIC FIELD AND MULTIVALENT SCREENING COUNTERIONS(2022) Pial, Md Turash Haque; Das, Siddhartha; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Polyelectrolyte (PE) or charged polymers are ubiquitous under biological and synthetic conditions, ranging from DNA to advanced technologies. PE chains can be grafted on a surface and they extend into solution to form a "brush"-like configuration if the grafting density is high. PE brushes respond to external stimuli by changing their conformation and chemical details, which make them very attractive for numerous applications. Multivalent counterions (neutralizing PE charges) and external electric fields are known to significantly affect the brush behavior. Obtaining fundamental insights into PE brush’s response to ions and electric filed is of utmost importance for both industrial and academic research. In this dissertation, we use atomistic tools to improve our understanding of the PE brushes grafted on a single surface and two inner walls of a nanochannel under these two stimuli.We start by developing an all-atom molecular dynamics simulation framework to test the behavior of the PE brushes (grafted on a single surface) in the presence of externally applied electric fields. It is discovered that the charge density of PE monomers can have significant influence on their response; a smaller monomer charge density helps the brush to tilts along the electric field, while the PE brush with higher monomer charge density bends and shrinks. We found that counterion condensation to PE chains has a substantial impact in controlling these responses. In the subsequent study we discuss the effect of counterion size and valence in dictating counterion mediated bridging interaction of two or more negative monomers. By examining the solvation behavior, we identify that bridging interactions are not a sole function of the counterion valence. Rather, it depends on the counterion condensation on the PE chain, as well as the size of the counterion solvation shell. We also test the dynamic properties of the counterions and associated bridges. Later, we proceeded to simulate PE brush-grafted nanochannels to explore equilibrium and flow behavior in presence of nanoconfinement. We identify the onset of overscreening: there are a greater number of coions than counterions in the bulk liquid outside the brush layer. This specific ion distribution ensures that the overall electroosmotic flow is along the direction of the coions. Furthermore, for a large electric field, some of the counterions leave the PE brush layer into the bulk, resulting in disappearance of overscreening. If the number of counterions is greater than coions, electroosmotic flow reverses its direction and follows the motion of counterions. Finally, we discover that counterion-monomer interactions control the ion distribution. As a result, a diverse range of electroosmotic flow is found for counterions with different valence and size.Item A FLAMELET APPROACH FOR LARGE EDDY SIMULATIONS OF COUPLED COMBUSTION AND RADIATION IN TURBULENT BUOYANT DIFFUSION FLAMES(2021) Xu, Rui; Trouvé, Arnaud; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In traditional computational fluid dynamics (CFD) descriptions of fires, the combustion and radiation models generally rely on a global combustion equation and the assumption of a linear relationship between radiative power and heat release rate. These models may lead to a crude treatment of important phenomena such as flame extinction, formation of soot and toxic species, and the change of radiant emissions in response to evolving fire conditions. The general objective of this Ph.D. study is to evaluate the potential of advanced combustion and radiation models for large eddy simulations (LES) of fires. A flamelet-based modeling framework is proposed that considers established or modified steady and unsteady flamelet formulations. This study is part of an international collaborative project between the University of Maryland and the University of Poitiers (France) aimed at providing a fundamental understanding of coupled combustion-radiation phenomena in fires. It consists of two parts. The objective of the first part is to bring fundamental information on the coupling between combustion and thermal radiation occurring in laminar flames. The study considers a simplified configuration corresponding to one-dimensional counterflow planar laminar diffusion flames subjected to time-evolving moderate-to-slow mixing conditions that are representative of fires. The analysis demonstrates that for conditions far from the extinction limits, the flame belongs to the semi-unsteady regime in which mixing processes occurring in the outer diffusive layers of the flame are unsteady whereas heat release processes occurring in the inner reactive layer remain quasi-steady. The objective of the second part is to develop and validate a fully coupled flow-flame-radiation fire modeling framework. A novel unsteady flamelet model is developed that includes: detailed information on combustion chemistry through a tabulated chemistry approach; a careful description of the combustion-radiation coupling; a description of subgrid-scale turbulence-radiation interactions; and a description of non-grey radiation effects through a Weighted-Sum-of-Grey-Gases (WSGG) model. This new combustion/radiation model is then incorporated into the LES solver FireFOAM (developed by FM Global) and is evaluated by comparisons with experimental data obtained in a turbulent line burner experiment previously studied at the University of Maryland. Results on the global radiant fraction (GRF) obtained in cases with nitrogen dilution suggest that provided that the WSGG radiation model is used, the new modeling framework is capable of simulating changes in the flame radiative emissions with the predicted GRF within 20% of the measured values. Comparisons between the grey and WSGG options in the flamelet model show that, with the WSGG model, the simulated flame is no longer optically thin (the ratio of global absorption divided by global emission is close to 40%). Note that while the flamelet combustion model presented in this study has provided unique insights into the micro physics of fires, it is not a modeling approach that is recommended for engineering-level simulations of fires. First, the flamelet combustion modeling approach assumes the availability of a detailed chemical kinetic mechanism to describe fuel oxidation and this type of mechanism is typically not available for practical fuels in fire problems. Second, the flamelet combustion modeling approach treats the heat release rate implicitly and numerical tests show that the implicit heat release rate is described with limited accuracy (the error in the simulated global heat release rate ranges takes values between a few percent up and 20% in the present work). This limited accuracy on the description of the fire power is viewed as a strong limitation of current tabulated chemistry approaches for engineering-level simulations of fires.Item Numerical and Experimental Studies on Dynamic Interactions of Robot Appendages with Granular Media(2021) Ravula, Preethi; Balachandran, Balakumar; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Terramechanics plays an important role in the design and control of robots moving on granular surfaces. Traction capabilities, slippage, and sinkage of a robot are governed by the interaction of a robot's appendage (such as wheel, track or leg) with the operating terrain and how the terrain motion happens with respect to the appendage during such an interaction. In this dissertation work, dynamics of robot appendages interaction with granular media is explored through numerical and experimental studies. A two dimensional (2D) numerical model, constructed using the Discrete Element Method (DEM), is adapted to simulate lugged wheel interaction with granular media. Parametric studies on wheel performance are conducted for two different control schemes, namely, a slip-based control scheme and an angular velocity-based wheel control scheme. Furthermore, the soil flow pattern under the wheel is studied by examining the force distribution and evolution of force networks during the course of wheel travel.An experimental setup is designed to study the particle motion and force networks inside the media during dynamic forcing. Two different designs of robot appendages, a lugged and a single actuator pendulum are investigated. High speed imaging of photo-elastic particles under polarized light is used to visualize the force distributions inside the media. Qualitative behavior of force chains/networks evolution during interaction with the lugged wheel and pendulum is presented. In addition, quantitative measures of the interaction between appendage and granular media, such as, the driving torque values, appendage velocity, and particle motion are inferred from the experimental findings. Based on this work, insights can be gained into the design influences of robot appendages on performance and further understanding can be obtained on the behavior of granular media across different length scales. Furthermore, the numerical and experimental techniques developed and outcomes of this dissertation can serve as an important foundation for optimal design and control of different robot appendages interacting with deformable surfaces.Item PHYSICS-BASED AND DATA-DRIVEN MODELING OF HYBRID ROBOT MOVEMENT ON SOFT TERRAIN(2020) Wang, Guanjin; Balachandran, Balakumar; Riaz, Amir; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Navigating an unmapped environment is one of the ten biggest challenges facing the robotics community. Wheeled robots can move fast on flat surfaces but suffer from loss of traction and immobility on soft ground. However, legged machines have superior mobility over wheeled locomotion when they are in motion over flowable ground or a terrain with obstacles but can only move at relatively low speeds on flat surfaces. A question to answer is as follows: If legged and wheeled locomotion are combined, can the resulting hybrid leg-wheel locomotion enable fast movement in any terrain condition? To investigate the rich physics during dynamic interactions between a robot and a granular terrain, a physics-based computational framework based on the smoothed particle hydrodynamics (SPH) method has been developed and validated by using experimental results for single robot appendage interaction with the granular system. This framework has been extended and coupled with a multi-body simulator to model different robot configurations. Encouraging agreement is found amongst the numerical, theoretical, and experimental results, for a wide range of robot leg configurations, such as curvature and shape. Real-time navigation in a challenging terrain requires fast prediction of the dynamic response of the robot, which is useful for terrain identification and robot gait adaption. Therefore, a data-driven modeling framework has also been developed for the fast estimation of the slippage and sinkage of robots. The data-driven model leverages the high-quality data generated from the offline physics-based simulation for the training of a deep neural network constructed from long short-term memory (LSTM) cells. The results are expected to form a good basis for online robot navigation and exploration in unknown and complex terrains.Item PHYSICS OF LAMINAR PREMIXED CH4 − O2 FLAMES AT CRYOGENIC CONDITIONS - A COMPUTATIONAL STUDY(2019) Gopal, Abishek; Larsson, Johan; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)With increased commercial spaceflight activity, methane has found adoption in the next generation of liquid rocket engines (LREs). In a liquid rocket engine with cryogenic propellants, such as methane and oxygen, the propellants are stored in their tanks at low temperatures. As they are injected into the combustion chamber at high pressures, the fluid is close to its thermodynamic critical point where there are drastic changes in fluid properties like density, heat capacity, surface tension, and solubility. The ideal gas law is inapplicable at such extreme conditions, and real gas thermodynamic and transport properties are required to accurately model the combustion physics at supercritical conditions. Much of the previous work applying real gas models in computational simulations of reacting flows have focused on non-premixed flames or cold-flow mixing configurations. In this study, we investigate the effects of real gas property estimation on planar, unstretched, laminar premixed methane-oxygen flames at transcritical conditions. The computational framework used in this study integrates real gas property estimation into the steady-state, freely-propagating flame solver available in the Cantera combustion suite. The Peng-Robinson equation of state provides thermodynamic property closure. High-pressure transport properties are modeled by the Chung and Takahashi correlations, respectively. The effects on laminar flame structure are presented. We find that enhanced real gas reactant densities have a significant impact on flame propagation, lowering flame speeds by a factor of ∼ 5 near the critical region. Real gas caloric properties lower mass burning rates by 10%. The consequence of using low-pressure transport properties with the Peng-Robinson EOS at variable Lewis numbers is discussed.Item ERROR ESTIMATION, GRID SELECTION AND CONVERGENCE VERIFICATION IN LARGE EDDY SIMULATION(2019) Toosi, Siavash; Larsson, Johan; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Large eddy simulation (LES) is a modeling approach to simulation of turbulence, in which the large and energy containing eddies are directly resolved, while the smaller scales are modeled. The ``coarse-graining'' length scale (the length scale below which the turbulent eddies are modeled) is an important modeling parameter that is directly tied to the computational grid. As a result, the LES grid controls both the numerical and modeling errors and in most cases (given that the LES model is consistent) becomes the most important factor in determining the accuracy of the solution. The main goal of this dissertation is to enable a systematic approach to grid selection and convergence-verification in LES. Systematic grid selection consists of five essential ingredients: (i) an ``error-indicator'' that identifies the regions of error generation, (ii) some knowledge of the directional structure of error generation (i.e., an anisotropic measure of error generation at each location), (iii) a model that describes the connection between the error generation and the filter/grid resolution (i.e., how it changes with a change in the resolution), (iv) criteria that describe the most ``optimal'' distribution of the error-indicator in space and in direction, and (v) a robust method for convergence-verification. Items (i), (ii), (iv) and (v) are all addressed in this dissertation, while item (iii) has not been a subject of extensive research here (because of its somewhat lower importance compared to the other four). Three error-indicators are introduced that are different in terms of their underlying assumptions, complexity, potential accuracy, and extensibility to more complex flows and more sophisticated formulations of the problem of ``optimal'' grid selection. Two of these error-indicators are inherently anisotropic, while the third one is only a scalar but can be combined with either of the other two to enable anisotropic error-estimation. The ``optimal'' distributions of these error-indicators are discussed in detail, that, combined with a model to connect the error-indicator and the grid/filter resolution, describe our ``optimal'' grid selection criteria. Additionally, a more robust approach for convergence-verification in LES is proposed, and is combined with error-estimation and ``optimal'' grid selection/adaptation to form a systematic algorithm for large eddy simulation. The proposed error-estimation, grid selection, and convergence-verification methods are tested on the turbulent channel flow and the flow over a backward-facing step, with good results in all cases, and grids that are quite close to what is know as ``best practice'' for LES of these flows.