Browsing by Author "Ahmadi, Mojtaba"

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    Developing a New Powered Seed Delivery System with Constant Seed Release Speed Using Two Confronting Belts
    (North Dakota State University, 2018) Ahmadi, Mojtaba
    Seed delivery devices are aimed to carry seeds down to open furrow that might affect uniformity of plant stand and spacing. The objective of this study was to develop and evaluate a new seed delivery system utilizing a two-belt mechanism configuration for sunflower (Helianthus annuus). A prototype were fabricated and tested with MeterMax® Ultra Test Stand in order to evaluate the new seed delivery system. The outcomes show that dependent variables like seed population, singulation, skips, and multiples rates were not affected by planting speed levels (Pvalues > 0.05), while planting speed had a negative effect on seed spacing consistency (P-values < 0.05). In addition, due to facing broken seeds during test process, multiples rate were between 8.0 to 9.5% and consequently affected other variables as well. To improve this mechanism and avoid the systematic error that caused by broken seeds, this system should be redesigned in a single-belt form.
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    Mechanics of Surface Instabilities of Soft Nanofibers and Nonlinear Contacts of Hydrogels
    (North Dakota State University, 2020) Ahmadi, Mojtaba
    The research of this dissertation is formulated in two fields, i.e., the theoretical and computational studies of circumferential wrinkling on soft nanofibers and the swelling mechanics study of a bi-layered spherical hydrogel containing a hard core. Continuous polymer nanofibers have been massively produced by means of the low-cost, top-down electrospinning technique. As a unique surface instability phenomenon, surface wrinkling in circumferential direction is commonly observed on soft nanofibers in electrospinning. In this study, a theoretical continuum mechanics model is developed to explore the mechanisms of circumferential wrinkling on soft nanofibers under uniaxial stretching. The model is able to examine the effects of elastic properties, surface energy, and fiber radius on the critical axial stretch to trigger circumferential wrinkling and to discover the threshold fiber radius to initiate spontaneous wrinkling. In addition, nonlinear finite element method (FEM) is further adopted to predict the critical mismatch strain to evoke circumferential wrinkling in core-shell polymer nanofibers containing a hard core, as a powerful computational tool to simulate controllable wrinkling on soft nanofibers via co-electrospinning polymer nanofibers incorporated with nanoparticles as the core. The studies provide rational understanding of surface wrinkling in polymer nanofibers and technical approaches to actively tune surface morphologies of polymer nanofibers for particular applications, e.g. high-grade filtration, oil-water separation, polymer nanocomposites, wound dressing, tissue scaffolding, drug delivery, and renewable energy harvesting, conversion, and storage, etc. Furthermore, hydrogels are made of cross-linked polymer chains that can swell significantly when imbibing water and exhibit inhomogeneous deformation, stress, and, water concentration fields when the swelling is constrained. In this study, a continuum mechanics field theory is adopted to study the swelling behavior of a bi-layered spherical hydrogel containing a hard core. The problem is reduced into a two-point boundary value problem of a 2nd-order nonlinear ordinary differential equation (ODE) and solved numerically. Effects of material properties on the deformation, stress, and water concentration fields of the hydrogel are examined. The study offers a rational route to design and regulate hydrogels with tailorable swelling behavior for practical applications in drug delivery, leakage blocking, etc.