Immersogeometric analysis of moving objects in incompressible flows

We deploy the immersogeometric approach for tracking moving objects. The method immerses objects into non-boundary-fitted meshes and weakly enforces Dirichlet boundary conditions on the object boundaries. The object motion is driven by the integrated surface force and external body forces. A residual-based variational multiscale method is employed to stabilize the finite element formulation for incompressible flows. Adaptively refined quadrature rules are used to better capture the geometry of the immersed boundaries by accurately integrating the intersected background elements. Treatment for the freshly-cleared nodes (i.e. background mesh nodes that are inside the object at one time step, but are in the fluid domain at the next time step) is considered. We assess the accuracy of the method by analyzing object motion in different flow structures including objects freely dropping in viscous fluids and particle focusing in unobstructed and obstructed micro-channels. We show that key quantities of interest are in very good agreements with analytical, numerical and experimental solutions. We also show a much better computational efficiency of this framework than current commercial codes using adaptive boundary-fitted approaches. We anticipate deploying this framework for applications of particle inertial migration in microfluidic channels

An immersogeometric formulation for free-surface flows with application to marine engineering problems

An immersogeometric formulation is proposed to simulate free-surface flows around structures with complex geometry. The fluid–fluid interface (air–water interface) is handled by the level set method, while the fluid–structure interface is handled through an immersogeometric approach by immersing structures into non-boundary-fitted meshes and enforcing Dirichlet boundary conditions weakly. Residual-based variational multiscale method (RBVMS) is employed to stabilize the coupled Navier–Stokes equations of incompressible flows and level set convection equation. Other level set techniques, including re-distancing and mass balancing, are also incorporated into the immersed formulation. Adaptive quadrature rule is used to better capture the geometry of the immersed structure boundary by accurately integrating the intersected background elements. Generalized-α role= presentation style= box-sizing: border-box; margin: 0px; padding: 0px; display: inline-block; line-height: normal; font-size: 16.2px; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative; \u3eα method is adopted for time integration, which results in a two-stage predictor multi-corrector algorithm. GMRES solver preconditioned with block Jacobian matrices of individual fluid and level set subproblems is used for solving the coupled linear systems arising from the multi-corrector stage. The capability and accuracy of the proposed method are assessed by simulating three challenging marine engineering problems, which are a solitary wave impacting a stationary platform, dam break with an obstacle, and planing of a DTMB 5415 ship model. A refinement study is performed. The predictions of key quantities of interest by the proposed formulation are in good agreement with experimental results and boundary-fitted simulation results from others. The proposed formulation has great potential for wide applications in marine engineering problems

Buoyancy-driven flow and fluid-structure interaction with moving boundaries

We deploy the residual-based variational multi-scale (VMS) method in the sense of large-eddy simulation (LES) in finite element method to buoyancy-driven flow in enclosures and consider an extensive range of Rayleigh number from laminar ($10^3$) to turbulent ($10^{10}$) in a 2D benchmark Rayleigh--B\'enard problem. 3D simulations for a laminar and a turbulent case are performed and comparisons including mean profiles as well as fluctuation profiles with other numerical and experimental results are successfully carried out. A weakly imposed boundary conditions method is employed for both velocity and temperature, and it produces reasonable results with a much coarser mesh compared with the traditional imposition of boundary conditions. This suggests that the VMS framework with the weak imposition of boundary conditions is a computationally efficient approach to model buoyancy-driven flows in complex indoor environments. In addition to the flow fields, we deploy the immersogeometric analysis (IMGA) method in the sense of the immersed boundary method (IBM) for objects moving in fluids onto an unstructured framework. The finite element formulation is stabilized by the VMS method in an unstructured background mesh. Weak imposition of boundary conditions is used to impose no-slip boundary condition on the immersed boundary. Adaptively refined quadrature rules are used to better capture the geometry of the immersed boundary and accurately integrate the background elements that intersect the immersed boundary. Treatment for the freshly-cleared nodes is considered. We assess the accuracy of the moving IMGA framework by analyzing object motion in a variety of flow structures, including freely dropping cylinder/sphere in viscous fluids and particle focusing in (un)obstructed channels. We show the quantities of interests are in good agreements with other analytical, numerical and experimental solutions. Advantages of this moving IMGA framework in computational cost and efficiency are indicated by the comparison with the body-fitted method using a commercial computational fluid dynamic (CFD) software. The framework of moving IMGA is capable to be deployed in applications of particle control and manipulation in microfluidic channels. The moving IMGA on the unstructured framework is further deployed to a scalable, adaptively refined, octree-based finite element approach for a better computational performance to track object motion. This enables using a parallel, hierarchically refined octree mesh as the background mesh, with a variationally consistent IMGA formulation on this background mesh. We integrate the unstructured framework of moving IMGA to the octree-based framework. We show good scaling results of the coupled framework on Stampede2, TACC. This illustrates the potential of the moving IMGA on the coupled framework to efficiently track complex particles in flows.</p

Immersogeometric analysis of moving objects in incompressible flows

We deploy the immersogeometric approach for tracking moving objects. The method immerses objects into non-boundary-fitted meshes and weakly enforces Dirichlet boundary conditions on the object boundaries. The object motion is driven by the integrated surface force and external body forces. A residual-based variational multiscale method is employed to stabilize the finite element formulation for incompressible flows. Adaptively refined quadrature rules are used to better capture the geometry of the immersed boundaries by accurately integrating the intersected background elements. Treatment for the freshly-cleared nodes (i.e. background mesh nodes that are inside the object at one time step, but are in the fluid domain at the next time step) is considered. We assess the accuracy of the method by analyzing object motion in different flow structures including objects freely dropping in viscous fluids and particle focusing in unobstructed and obstructed micro-channels. We show that key quantities of interest are in very good agreements with analytical, numerical and experimental solutions. We also show a much better computational efficiency of this framework than current commercial codes using adaptive boundary-fitted approaches. We anticipate deploying this framework for applications of particle inertial migration in microfluidic channels.This is a manuscript of an article published as Xu, Songzhe, Fei Xu, Aditya Kommajosula, Ming-Chen Hsu, and Baskar Ganapathysubramanian. "Immersogeometric analysis of moving objects in incompressible flows." Computers & Fluids 189 (2019): 24-33. DOI: 10.1016/j.compfluid.2019.05.018. Posted with permission.</p

An immersogeometric formulation for free-surface flows with application to marine engineering problems

An immersogeometric formulation is proposed to simulate free-surface flows around structures with complex geometry. The fluid–fluid interface (air–water interface) is handled by the level set method, while the fluid–structure interface is handled through an immersogeometric approach by immersing structures into non-boundary-fitted meshes and enforcing Dirichlet boundary conditions weakly. Residual-based variational multiscale method (RBVMS) is employed to stabilize the coupled Navier–Stokes equations of incompressible flows and level set convection equation. Other level set techniques, including re-distancing and mass balancing, are also incorporated into the immersed formulation. Adaptive quadrature rule is used to better capture the geometry of the immersed structure boundary by accurately integrating the intersected background elements. Generalized-α" role="presentation" style="box-sizing: border-box; margin: 0px; padding: 0px; display: inline-block; line-height: normal; font-size: 16.2px; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;">α method is adopted for time integration, which results in a two-stage predictor multi-corrector algorithm. GMRES solver preconditioned with block Jacobian matrices of individual fluid and level set subproblems is used for solving the coupled linear systems arising from the multi-corrector stage. The capability and accuracy of the proposed method are assessed by simulating three challenging marine engineering problems, which are a solitary wave impacting a stationary platform, dam break with an obstacle, and planing of a DTMB 5415 ship model. A refinement study is performed. The predictions of key quantities of interest by the proposed formulation are in good agreement with experimental results and boundary-fitted simulation results from others. The proposed formulation has great potential for wide applications in marine engineering problems.This is a manuscript of an article published as Zhu, Qiming, Fei Xu, Songzhe Xu, Ming-Chen Hsu, and Jinhui Yan. "An immersogeometric formulation for free-surface flows with application to marine engineering problems." Computer Methods in Applied Mechanics and Engineering (2019): 112748. DOI: 10.1016/j.cma.2019.112748. Posted with permission.</p

An Integrated Experimental-Computational Investigation of Connected Spaces as Natural Ventilation Typologies

This paper investigates the impact of spatial composition on the effectiveness of passive cooling by natural ventilation in a comparative study of the conical roofed Harran houses in Turkey and a passive solar home in the Midwest of the United States. While the projects are distinct and are situated in two extreme climate zones (hot and arid and continental humid) both projects have in common open variable configurations of multiple interconnected spaces. Computational fluid dynamics (CFD) simulations using OpenFoam were used to investigate the fundamental airflow characteristics and the resulting interior temperature and velocity profiles. The simulations were initialized as well as validated with measured field data. Subsequently, we tested the impact of the interconnected spatial composition of the buildings on their cooling potentials. This was accomplished by simulating variations of the spatial connections with reduced flow path connectivity compared to the original validated cases. Preliminary results regarding changes in temperature and air velocity show higher temperatures and lower velocities in the less connected cell-like spaces and indicate the importance of spatial connectivity for effective cooling by natural ventilation based on variable interaction of vents and flow path.This conference proceeding is published as Ulrike Passe, Mirka Deza, Baskar Ganapathysubramanian, Shan He, Kyle Vansice, Songzhe Xu2, An Integrated Experimental-Computational Investigation of Connected Spaces as Natural Ventilation Typologies. at the 2016 Proceedings of the Symposium on Simulation for Architecture and Urban Design. London United Kingdom, May 16-18, 2016; Session 1; 59-66. Posted with permission.</p