About

Paul Fischer is a professor at the Siebel School of Computing and Data Science at the University of Illinois Urbana-Champaign. His research areas include Scientific Computing, with recent courses taught in Numerical Analysis, Numerical Methods for PDEs, Iterative & Multigrid Methods, and Computational Fluid Mechanics. Fischer has contributed to high-performance computing research, including work on the nation's best supercomputers, and has been involved in projects utilizing large-scale simulations such as flow inside internal combustion engines and models for nuclear reactors. His work has earned recognition through awards like the R&D 100 Award for simulation software. Fischer's expertise encompasses numerical analysis, fluid mechanics, and computational methods, and he collaborates with institutions like Argonne National Laboratory to advance computational science.

Research topics

• Computer Science
• Computational science
• Parallel computing
• Physics
• Distributed computing
• Mechanics
• Aerospace engineering
• Operating system
• Mathematical optimization
• Nuclear physics
• Programming language
• Theoretical computer science
• Mathematics

Selected publications

• Nek5000/RS performance on advanced GPU architectures

  Frontiers in High Performance Computing · 2025-02-21 · 1 citations

  articleOpen access

  The authors explore performance scalability of the open-source thermal-fluids code, NekRS, on the U.S. Department of Energy's leadership computers, Crusher, Frontier, Summit, Perlmutter, and Polaris. Particular attention is given to analyzing performance and time-to-solution at the strong-scale limit for a target efficiency of 80%, which is typical for production runs on the DOE's high-performance computing systems. Several examples of anomalous behavior are also discussed and analyzed.

  PublisherDOI

• Deciphering boundary layer dynamics in high-Rayleigh-number convection using 3360 GPUs and a high-scaling in-situ workflow

  ArXiv.org · 2025-01-22

  preprintOpen access

  Turbulent heat and momentum transfer processes due to thermal convection cover many scales and are of great importance for several natural and technical flows. One consequence is that a fully resolved three-dimensional analysis of these turbulent transfers at high Rayleigh numbers, which includes the boundary layers, is possible only using supercomputers. The visualization of these dynamics poses an additional hurdle since the thermal and viscous boundary layers in thermal convection fluctuate strongly. In order to track these fluctuations continuously, data must be tapped at high frequency for visualization, which is difficult to achieve using conventional methods. This paper makes two main contributions in this context. First, it discusses the simulations of turbulent Rayleigh-Bénard convection up to Rayleigh numbers of $Ra=10^{12}$ computed with NekRS on GPUs. The largest simulation was run on 840 nodes with 3360 GPU on the JUWELS Booster supercomputer. Secondly, an in-situ workflow using ASCENT is presented, which was successfully used to visualize the high-frequency turbulent fluctuations.

  PublisherOA PDFDOI

• A Robust Spectral Element Implementation of the K – Τ Rans Model in Nek5000/Nekrs

  SSRN Electronic Journal · 2024-01-01 · 2 citations

  preprintOpen accessSenior author
  PublisherDOI

• Video: On the Transition to Turbulence in a Human Thoracic Aorta : Direct Numerical Simulation

  2024-11-21

  articleOpen access
  PublisherDOI

• PULPo: Probabilistic Unsupervised Laplacian Pyramid Registration

  Lecture notes in computer science · 2024-01-01 · 3 citations

  book-chapter
  PublisherDOI

• A robust spectral element implementation of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si238.svg" display="inline" id="d1e1988"><mml:mrow><mml:mi>k</mml:mi><mml:mo linebreak="goodbreak" linebreakstyle="after">−</mml:mo><mml:mi>τ</mml:mi></mml:mrow></mml:math> RANS model in Nek5000/NekRS

  International Journal of Heat and Fluid Flow · 2024-12-12 · 3 citations

  articleSenior author
  PublisherDOI

• Exascale Simulations of Fusion and Fission Systems

  arXiv (Cornell University) · 2024-09-27 · 2 citations

  preprintOpen access

  We discuss pioneering heat and fluid flow simulations of fusion and fission energy systems with NekRS on exascale computing facilities, including Frontier and Aurora. The Argonne-based code, NekRS, is a highly-performant open-source code for the simulation of incompressible and low-Mach fluid flow, heat transfer, and combustion with a particular focus on turbulent flows in complex domains. It is based on rapidly convergent high-order spectral element discretizations that feature minimal numerical dissipation and dispersion. State-of-the-art multilevel preconditioners, efficient high-order time-splitting methods, and runtime-adaptive communication strategies are built on a fast OCCA-based kernel library, libParanumal, to provide scalability and portability across the spectrum of current and future high-performance computing platforms. On Frontier, Nek5000/RS has achieved an unprecedented milestone in breaching over 1 trillion degrees of freedom with the spectral element methods for the simulation of the CHIMERA fusion technology testing platform. We also demonstrate for the first time the use of high-order overset grids at scale.

  PublisherOA PDFDOI

• SC29.02 UNCERTAINTY ESTIMATION FOR AI-BASED DOSE MODELLING IN MR-GUIDED RADIOTHERAPY USING ENSEMBLE LEARNING AND MONTE CARLO DROPOUT

  Physica Medica · 2024-09-01

  article
  PublisherDOI

• Energy Exascale Computational Fluid Dynamics Simulations With the Spectral Element Method

  Journal of Fluids Engineering · 2024-02-06 · 7 citations

  articleOpen access

  Abstract Development and application of the open-source GPU-based fluid-thermal simulation code, NekRS, are described. Time advancement is based on an efficient kth-order accurate timesplit formulation coupled with scalable iterative solvers. Spatial discretization is based on the high-order spectral element method (SEM), which affords the use of fast, low-memory, matrix-free operator evaluation. Recent developments include support for nonconforming meshes using overset grids and for GPU-based Lagrangian particle tracking. Results of large-eddy simulations of atmospheric boundary layers for wind-energy applications as well as extensive nuclear energy applications are presented.

  PublisherOA PDFDOI

• Accelerating transients with NekRS: GPU overlapping domain implementation and multi-rate timestepping

  2024-09-30

  reportOpen accessSenior author

  The simulation of nuclear transients using Computational Fluid Dynamics (CFD) presents significant computational challenges due to the inherent complexity and the wide separation in temporal scales between various flow physical phenomena. These disparities lead to high computational costs, often making the simulation of transients impractical without advanced techniques. Consequently, multiple research initiatives are being pursued by the NEAMS thermal-hydraulic area, some driven by academic institutions and some by national laboratories. Overall, they are exploring novel methods to make transient simulations more feasible and efficient. This report delves into recent advancements within the CFD code NekRS, specifically those achieved in Fiscal Year 2024 under the CONNECT effort, aimed at improving the performance and feasibility of transient simulations. The first major advancement involves the porting of NekRS to Aurora, one of the Department of Energy’s (DOE) most powerful supercomputers. Additionally, the report discusses the implementation of an overlapping domain capability within NekRS. This novel GPU-accelerated capability allows different spatial regions of the domain to be solved independently, enhancing the code’s efficiency, particularly when running large-scale simulations in complex domains. The scalability of this approach is demonstrated, highlighting its potential to transform how transients are approached in CFD simulations. Lastly, the report focuses on how this overlapping domain capability specifically accelerates transient simulations through multi-rate timestepping. By decoupling different regions and facilitating faster computations, this method offers a promising pathway to making nuclear transient simulations more computationally feasible, addressing one of the critical bottlenecks in the field. Together, these advancements represent a significant leap forward in transient simulation technology, bringing closer the possibility of handling highly complex nuclear scenarios with greater efficiency and accuracy.

  PublisherOA PDFDOI

Recent grants

• CMG Collaborative Research: A New Modeling Framework for Nonhydrostatic Simulations of Small-Scale Oceanic Processes

  NSF · $78k · 2006–2009

• Collaborative Research: Three-Dimensional Numerical Investigation of Density Currents

  NSF · $100k · 2002–2006

• CMG Collaborative Research: Ocean Modeling by Bridging Primitive and Boussinesq Equations

  NSF · $191k · 2010–2014

Frequent coauthors

• Elia Merzari

  Argonne National Laboratory

  86 shared

• Misun Min

  54 shared

• Ananias Tomboulides

  47 shared

• Yu-Hsiang Lan

  27 shared

• Aleksandr Obabko

  26 shared

• Stefan Kerkemeier

  24 shared

• Tim Warburton

  22 shared

• Thilina Rathnayake

  20 shared

Labs

• Siebel School of Computing and Data SciencePI

Awards & honors

• R&D 100 Award for simulation software