A. James Clark School of Engineering
Permanent URI for this communityhttp://hdl.handle.net/1903/1654
The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item EFFECTS OF 3D PRINTED VASCULAR NETWORKS ON HUMAN MESENCHYMAL STEM CELL VIABILITY IN LARGE BONE TISSUE CONSTRUCTS(2015) Ball, Owen Matthew; Fisher, John P; Bioengineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)There is a significant clinical need for engineered bone graft substitutes that can quickly, effectively, and safely repair segmental bone defects. One emerging field of interest involves the growth of engineered bone tissue in vitro within bioreactors, the most promising of which, are perfusion bioreactors. Utilizing a tubular perfusion system bioreactor, which allows media to perfuse freely around alginate scaffolds laden with human mesenchymal stem cells, large-scale bone constructs can be created by simply aggregating these beads together in the desired shape. However, these engineered constructs lack inherent vasculature and quickly develop a necrotic core, where no nutrient exchange occurs. Through the use of 3D printed vascular structures, used in conjunction with a TPS bioreactor, cell viability after just one day of aggregation was found to increase by as much as 50 percent in the core of these constructs, with in silico modeling predicting construct viability at steady state.Item DEVELOPMENT OF A LAGRANGIAN-LAGRANGIAN METHODOLOGY TO PREDICT BROWNOUT DUST CLOUDS(2012) Syal, Monica; Leishman, J. Gordon; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A Lagrangian-Lagrangian dust cloud simulation methodology has been developed to help better understand the complicated two-phase nature of the rotorcraft brownout problem. Brownout conditions occur when rotorcraft land or take off from ground surfaces covered with loose sediment such as sand and dust, which decreases the pilot's visibility of the ground and poses a serious safety of flight risk. The present work involved the development of a comprehensive, computationally efficient three-dimensional sediment tracking method for dilute, low Reynolds number Stokes-type flows. The flow field generated by a helicopter rotor in ground effect operations over a mobile sediment bed was modeled by using an inviscid, incompressible, Lagrangian free-vortex method, coupled to a viscous semi-empirical approximation for the boundary layer flow near the ground. A new threshold model for the onset of sediment mobility was developed by including the effects of unsteady pressure forces that are induced in vortically dominated rotor flows, which can significantly alter the threshold conditions for particle motion. Other important aspects of particle mobility and uplift in such vortically driven dust flows were also modeled, including bombardment effects when previously suspended particles impact the bed and eject new particles. Bombardment effects were shown to be a particularly significant contributor to the mobilization and eventual suspension of large quantities of smaller-sized dust particles, which tend to remain suspended. A numerically efficient Lagrangian particle tracking methodology was developed where individual particle or clusters of particles were tracked in the flow. To this end, a multi-step, second-order accurate time-marching scheme was developed to solve the numerically stiff equations that govern the dynamics of particle motion. The stability and accuracy of this scheme was examined and matched to the characteristics of free-vortex method. One-way coupling of the flow and the particle motion was assumed. Particle collisions were not considered. To help reduce numerical costs, the methodology was implemented on graphic processing units, which gave over an order of magnitude reduction in simulation time without any loss in accuracy. Validation of the methodology was performed against available measurements, including flow field measurements that have been made with laboratory-scale and full-scale rotors in ground effect operations. The predicted dust clouds were also compared against measurements of developing dust clouds produced by a helicopter during taxi-pass and approach-to-touchdown flight maneuvers. The results showed that the problem of brownout is mostly driven by the local action of the rotor wake vortices and the grouping or bundling of vortex filaments near the sediment bed. The possibilities of mitigating the intensity of brownout conditions by diffusing the blade tip vortices was also explored. While other means of brownout mitigation may be possible, enhancing the diffusion of the tip vortices was shown to drastically reduce the quantity of mobilized particles and the overall severity of the brownout dust cloud.