I am a researcher in the Applied Numerical Algorithms Group in CRD. I focus on scalable numerical and mathematical algorithms applied to problems where the accuracy of dynamics and multi-scale interactions are important.
I have also spent 12 years in IT consulting and architecture in startups and the financial services industry, which included roles in strategic planning, vendor management, software project leadership, business process and data analytics. I have a Ph.D. in Mechanical Engineering from the University of California, Berkeley.
See my LinkedIn profile for more information.
Current research projects and collaborations:
- Global atmospheric dynamics simulations. Developing a Chombo-based time-accurate, adaptive mesh refinement (AMR) dynamical core on the cubed sphere, with appropriate treatment of acoustic waves. See our recent conference poster.
- Multi-scale modeling combined with optimization of urban energy usage. Developing a proof of concept for combined weather and energy simulation and optimization across different scales.
- Higher-order Finite Volume methods. Under the DoE ASCR Applied Math program, developing scalable higher-order methods for complex PDE's. Examples include 4th-order adaptive methods for advection-diffusion, higher-order Embedded Boundary methods, and adaptive mapped multi-block discretizations.
- Embedded Boundary methods. Ongoing research into "cut-cell" approaches to PDE's using EB Chombo, and the mathematical theory avoiding issues with "small cells," with applications to complex geometries and multi-material problems.
Ongoing research interests:
- Scientific software and evolving HPC architectures. The future of HPC platforms may be GPU/manycore systems, globally distributed cloud-type computing, or both. Coming up with approaches, algorithms and libraries that can solve bigger problems faster is an ongoing challenge. Work supported by DoE ASCR and an NSF SI2 project.
- Software Architecture in Scientific Computing. My background as a systems and software architect in commercial settings contrasts dramatically with how things are done in most scientific computing teams, so I'm developing approaches to bridge those two worlds.
Nishant Nangia, Hans Johansen, Neelesh A. Patankar, Amneet Pal Singh Bhalla, "A moving control volume approach to computing hydrodynamic forces and torques on immersed bodies", Journal of Computational Physics, June 29, 2017, doi: 10.1016/j.jcp.2017.06.047
Dharshi Devendran, Daniel T. Graves, Hans Johansen,Terry Ligocki, "A Fourth Order Cartesian Grid Embedded Boundary Method for Poisson's Equation", Communications in Applied Mathematics and Computational Science, edited by Silvio Levy, May 12, 2017, 12:51-79, doi: DOI 10.2140/camcos.2017.12.51
- Download File: poisson-eb-4th-order.pdf (pdf: 1.1 MB)
Jared O. Ferguson, Christiane Jablonowski, Hans Johansen, Peter McCorquodale, Phillip Colella, Paul A. Ullrich, "Analyzing the adaptive mesh refinement (AMR) characteristics of a high-order 2D cubed-sphere shallow-water model", Mon. Wea. Rev., November 9, 2016, 144:4641–4666, doi: 10.1175/MWR-D-16-0197.1
P. McCorquodale, P.A. Ullrich, H. Johansen, P. Colella, "An adaptive multiblock high-order finite-volume method for solving the shallow-water equations on the sphere", Comm. App. Math. and Comp. Sci., 2015, 10:121-162, doi: 10.2140/camcos.2015.10.121
D. Devendran, D. T. Graves, H. Johansen, "A higher-order finite-volume discretization method for Poisson's equation in cut cell geometries", submitted to SIAM Journal on Scientific Computing (preprint on arxiv), 2015,
Peter Schwartz, Julie Percelay, Terry J. Ligocki, Hans Johansen, Daniel T. Graves, Dharshi Devendran, Phillip Colella, Eli Ateljevich, "High-accuracy embedded boundary grid generation using the divergence theorem", Communications in Applied Mathematics and Computational Science 10-1 (2015), 83--96. DOI 10.2140/camcos.2015.10.83, March 31, 2015,
Q. Zhang, H. Johansen and P. Colella, "A Fourth-Order Accurate Finite-Volume Method with Structured Adaptive Mesh Refinement for Solving the Advection-Diffusion Equation", SIAM Journal on Scientific Computing, Vol. 34, No. 2. (2012), B179, doi:10.1137/110820105, 2010,
- Download File: O4AdvDiff.pdf (pdf: 599 KB)
McCorquodale, P., Colella, P., Johansen, H., "A Cartesian Grid Embedded Boundary Method for the Heat Equation on Irregular Domains", J. Comput. Phys. Vol.173 (2001), pp. 620-635, 2001,
- Download File: A148.pdf (pdf: 652 KB)
Johansen, H., Colella, P., "A Cartesian Grid Embedded Boundary Method for Poisson's Equation on Irregular Domains", J. Comput. Physics, Vol.147, No.1, pp. 60-85, November 1998,
- Download File: A135.pdf (pdf: 390 KB)
Tuowen Zhao, Mary Hall, Hans Johansen, Samuel Williams, "Improving Communication by Optimizing On-Node Data Movement with Data Layout", PPoPP, February 2021,
- Download File: PPoPP-Bricks-MPI-final.pdf (pdf: 864 KB)
Tuowen Zhao, Mary Hall, Samuel Williams, Hans Johansen, "Exploiting Reuse and Vectorization in Blocked Stencil Computations on CPUs and GPUs", Supercomputing (SC), November 2019,
- Download File: SC19-VectorScatter-final.pdf (pdf: 1019 KB)
Tuowen Zhao, Samuel Williams, Mary Hall, Hans Johansen, "Delivering Performance Portable Stencil Computations on CPUs and GPUs Using Bricks", International Workshop on Performance, Portability and Productivity in HPC (P3HPC), November 2018,
- Download File: p3hpc-bricks-final.pdf (pdf: 1.3 MB)
Tuowen Zhao, Mary Hall, Protonu Basu, Samuel Williams, Hans Johansen, "SIMD code generation for stencils on brick decompositions", Proceedings of the 23rd ACM SIGPLAN Symposium on Principles and Practice of Parallel Programming (PPoPP), February 2018,
Bryce Adelstein Lelbach, Hans Johansen, Samuel Williams, "Simultaneously Solving Swarms of Small Sparse Systems on SIMD Silicon", Parallel and Distributed Scientific and Engineering Computing (PDSEC), June 2017,
Dharshi Devendran, Suren Byna, Bin Dong, Brian van Straalen, Hans Johansen, Noel Keen, and Nagiza Samatova,, "Collective I/O Optimizations for Adaptive Mesh Refinement Data Writes on Lustre File System", Cray User Group (CUG) 2016, May 10, 2016,
Xiaocheng Zou, David A Boyuka II, Dhara Desai, Daniel F Martin, Suren Byna, Kesheng Wu, "AMR-aware in situ indexing and scalable querying", Proceedings of the 24th High Performance Computing Symposium, January 1, 2016, 26,
Bin Dong, Suren Byna, Kesheng Wu, Hans Johansen, Jeffrey N Johnson, Noel Keen, others, "Data elevator: Low-contention data movement in hierarchical storage system", 2016 IEEE 23rd international conference on high performance computing (HiPC), January 1, 2016, 152--161,
- Download File: 201612-DataElevator-HiPC2016-Bin-Byna.pdf (pdf: 765 KB)
Xiaocheng Zou, Kesheng Wu, David A. Boyuka, Daniel F. Martin, Suren Byna, Houjun, Kushal Bansal, Terry J. Ligocki, Hans Johansen, and Nagiza F. Samatova, "Parallel In Situ Detection of Connected Components Adaptive Mesh Refinement Data", Proceedings of the Cluster, Cloud and Grid Computing (CCGrid) 2015, 2015,
J. Ferguson, C. Jablonowski, H. Johansen, R. English, P. McCorquodale, P. Colella, J. Benedict, W. Collins, J. Johnson, P. Ullrich, "Assessing Grid Refinement Strategies in the Chombo Adaptive Mesh Refinement Model", AGU Fall Meeting, San Francisco, CA, December 15, 2014,
Gunther H. Weber, Hans Johansen, Daniel T. Graves, Terry J. Ligocki, "Simulating Urban Environments for Energy Analysis", Proceedings Visualization in Environmental Sciences (EnvirVis), 2014, LBNL 6652E,
Dharshi Devendran, Daniel T. Graves, Hans Johansen, "A Hybrid Multigrid Algorithm for Poisson's equation using an Adaptive, Fourth Order Treatment of Cut Cells", LBNL Report Number: LBNL-1004329, November 11, 2014,
- Download File: multigrid.pdf (pdf: 221 KB)
Cartesian Grid Embedded Boundary Finite Difference Methods for Elliptic and Parabolic Partial Differential Equations on Irregular Domains, Johansen, H., 1997,
- Download File: HansJohansenThesis1997.pdf (pdf: 8.8 MB)
Hans Johansen, Daniel Martin, Esmond Ng, "High-resolution Treatment of Topography and Grounding Line Dynamics in BISICLES", AGU 2019 Fall Meeting, December 13, 2019,
D.F. Martin, H.S. Johansen, P.O. Schwartz, E.G. Ng, "Improved Discretization of Grounding Lines and Calving Fronts using an Embedded-Boundary Approach in BISICLES", European Geosciences Union General Assembly, April 10, 2019,
- Download File: Martin-EGU2019-final.pdf (pdf: 1.2 MB)