Mesh Based Physical Simulation Methods And Their Applications In Digital Fabrication

Physically based simulation is a technique to virtually reproduce natural phenomena of the real world in computers based on physical principles.This technique is widely used in many fields,including aerospace/mechanical engineering,and medical science,due to its high precision as well as low cost.Together with physically based rendering,physically based simulation can be used for making special effects in movies,such as flood,hurricane and fire.With the development of 3D printing in recent years,the research topic of digital fabrication has attracted much attention.Physically based simulation can improve the quality of 3D printed products.By performing stress analysis before printing,we can make the structure of the printed model more stable.By mechanical analysis,we can even print models whose physical property will satisfy certain constraints.Surface mesh plays an important role in simulation.First,thin shell objects(i.e.paper,cloth,shell,bubbles)are usually represented by surface mesh directly.Second,the shape of most objects are represented by surface meshes,which are widely used for collision detection and real-time rendering.Third,dynamic mesh can be used to track the surface of fluid,which has no static shape.Although all objects in real world possess finite volumes,the topology and computation of volumetric mesh are usually very complex.If the computational model can be approximated with surface mesh,the computational cost can be greatly reduced,thus improving the performance of simulation.In this thesis,we modeled three kinds of materials:water droplet,viscous sheet and plastic sheet,using surface mesh.The shape of water droplet is mainly influenced by surface tension,so we can simulate the motion of water droplet using a deformable surface model while ignoring the water flow inside.Merging and splitting of water droplets can be simulated through Boolean operations and mesh optimization.Surface mesh is very suitable for the simulation of viscous sheet and plastic sheet,as they are both thin shells.We apply these simulation methods in the virtualization of two traditional fabrication techniques:hydrographics and thermoforming,proposing "Computational Hydrographic Printing" and "Computational Thermoforming",which can be used for 3D surface coloring.We built prototype systems of these new techniques,and validated their usability and robustness through physical experiments.The contributions of this thesis are as follows.·We proposed a mesh based real-time water droplet simulation method.Using a deformable surface model,this method is able to simulate droplet motion,the hydrophilic effect,and droplet flowing on solid surfaces.This method reduced water droplet simulation from 3D volume to surface meshes,thus greatly reduced computational cost,making the whole system real-time and allows user interaction.·We model the viscous sheet floating on water using surface mesh,and simulate the phenomenon of viscous sheet stretches and wraps around the object as it is immersed into the water.We apply this simulation in the virtualization of hydrographics,which is a traditional surface coloring technique.We proposed "Computational Hydrographic Printing" method,which solved the problem of precise alignment between color pattern and 3D model.We built a prototype system using off-the-shelf hardware,which combined virtual simulation,system calibration and controlled immersion.We further extended this technique to multiple immersions to solve the problem of color deviation due to large stretch.In each immersion,we only color a certain part of the model,and finally get a seamless texture after immersing in all directions.·We model plastic sheet using surface mesh,and simulate its stretching under air pressure,which finally adhere to the mold surface.We apply this simulation in the virtualization of thermoforming,which is widely used in industry to make thin shell plastic products.We proposed "Computational Thermoforming" method,which is able to make digital 3D models into physical objects,with the help of 3D printing.First,we simulate the thermo-forming process,taking the digital 3D model as mold.We calculate a pre-distorted pattern according to the stretch of the simulated plastic sheet,and print it on the transparent plastic sheet physically.By simulating the venting process,we detect potential air pockets between the plastic sheet and mold,setting vacuum holes accordingly,and print vented mold using 3D printing.We improved a small-scale vacuum forming machine,making it possible to align pattern on plastic sheet with mold.After vacuum forming,the printed pattern will adhere to the mold surface,forming a physical reproduction of the digital model.The plastic sheet will give the pattern a protective layer.