Scientific Computing Department Seminars
06 Jun 2019
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Ysfsff ghou are invited to an SCD Seminar by –

 

Lei Wu, University of Strathclyde

Host: Xiaojun Gu, Engineering and Environment, SCD

 

Friday 11 October 2019 at 11:00-12:00

CR3 DL and via VC to CR02- R89 (S44) in RAL

Please join the seminar via Zoom, see instructions below

 

Can we find steady-state solutions to multiscale rarefied gas flows within dozens of iterations?

 

One of the central problems in the study of rarefied gas dynamics is to find the steady-state solution of the Boltzmann equation quickly. When the Knudsen number is large, the conventional iterative scheme can lead to convergence within a few iterations. However, when the Knudsen number is small, hundreds of thousands iterations are needed, and yet the ``converged'' solutions are prone to be contaminated by accumulated error and large numerical dissipation. Recently, we put forward a general synthetic iterative scheme (GSIS) to find the steady-state solutions of general rarefied gas flows within dozens of iterations at any Knudsen number. The key ingredient of our scheme is that the macroscopic equations, which are solved together with the Boltzmann equation and help to adjust the velocity distribution function, not only asymptotically preserve the Navier-Stokes limit in the framework of Chapman-Enskog expansion, but also contain Newton's law for stress and Fourier's law for heat conduction explicitly. The GSIS does not rely on the specific kinetic model/collision operator, it can be naturally extended to quickly find converged solutions for mixture flows and even flows involving chemical reactions. These two superior advantages are expected to accelerate the slow convergence in simulation of near-continuum flows via the direct simulation Monte Carlo method and its low-variance version.

 

../Documents/leiwu.jpgLei Wu is a Senior Lecturer at the University of Strathclyde. His research interest is in Rarefied (non-equilibrium) Gas Dynamics, in particular to construct efficient and accurate numerical schemes to solve Boltzmann equation for dilute gases and Enskog equation for dense gas/liquid-vapour multiphase flows, as well as kinetic modelling for polyatomic gas flows, with applications in high-altitude aerothermodynamics of space vehicles, MEMS, shale gas transportation, granular flows, multi-scale heat transfer in crystals, and thermal motion of quantum gases.

 

 

 

 

 

 

 

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You are invited to an SCD Seminar by –

 

Lei Wu, University of Strathclyde

Host: Xiaojun Gu, Engineering and Environment, SCD

 

Friday 11 October 2019 at 11:00-12:00

CR3 DL and via VC to CR02- R89 (S44) in RAL

Please join the seminar via Zoom, see instructions below

 

Can we find steady-state solutions to multiscale rarefied gas flows within dozens of iterations?

 

One of the central problems in the study of rarefied gas dynamics is to find the steady-state solution of the Boltzmann equation quickly. When the Knudsen number is large, the conventional iterative scheme can lead to convergence within a few iterations. However, when the Knudsen number is small, hundreds of thousands iterations are needed, and yet the ``converged'' solutions are prone to be contaminated by accumulated error and large numerical dissipation. Recently, we put forward a general synthetic iterative scheme (GSIS) to find the steady-state solutions of general rarefied gas flows within dozens of iterations at any Knudsen number. The key ingredient of our scheme is that the macroscopic equations, which are solved together with the Boltzmann equation and help to adjust the velocity distribution function, not only asymptotically preserve the Navier-Stokes limit in the framework of Chapman-Enskog expansion, but also contain Newton's law for stress and Fourier's law for heat conduction explicitly. The GSIS does not rely on the specific kinetic model/collision operator, it can be naturally extended to quickly find converged solutions for mixture flows and even flows involving chemical reactions. These two superior advantages are expected to accelerate the slow convergence in simulation of near-continuum flows via the direct simulation Monte Carlo method and its low-variance version

Computational Mathematics and Applications seminar

Date and Time: Thursday 20th February 2020 at 2pm

Location: CR03 - RL, R61 @ RAL and CR5 @ DL

Speaker: Francesco Tudisco, GSSI Gran Sasso Science Institute

Title: Learning with nonlinear Perron eigenvectors

Abstract:

In this talk I will present a Perron-Frobenius type result for nonlinear eigenvector problems which allows us to compute the global maximum of a class of constrained nonconvex optimization problems involving multihomogeneous functions.

I will structure the talk into three main parts:

First, I will motivate the optimization of homogeneous functions from a graph partitioning point of view, showing an intriguing generalization of the famous Cheeger inequality.

Second, I will define the concept of multihomogeneous function and I will state our main Perron-Frobenious theorem. This theorem exploits the connection between optimization of multihomogeneous functions and nonlinear eigenvectors to provide an optimization scheme that has global convergence guarantees.

Third, I will discuss a few example applications in network science and machine learning that require the optimization of multihomogeneous functions and that can be solved using nonlinear Perron eigenvectors.


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All welcome. You are also invited to come and join us for lunch in the Rutherford restaurant beforehand.

Please let me know if you plan to attend so that the relevant security arrangements can be made. On arrival, visitors should report to the Main Rutherford Reception.

For information about travelling to the Rutherford Appleton Laboratory, please see
http://www.stfc.ac.uk/about-us/where-we-work/rutherford-appleton-laboratory/how-to-get-to-rutherford-appleton-laboratory/
or
http://harwellcampus.com/about/contact
Parking is available if needed.

If travelling from Oxford
The Science Transit Shuttle runs every 30 minutes from various pickup points in Oxford (including the Science area) directly to RAL. Details are here: http://sciencetransitshuttle.co.uk/
The Thames Travel X32 bus travels between Oxford and the Harwell Campus (~ 60 minute journey, with a bus every hour).

If travelling by Train
The Thames travel X32/32A/X34 buses travel between Didcot Parkway Station and the Harwell Campus (~ 15 minute journey, with a bus every 15 minutes). Taxis are also available at Didcot Parkway Station.

For further information, contact Michael Wathen (michael.wathen@stfc.ac.uk).

 

Dr Michael Wathen
Scientific Computing Department
STFC Rutherford Appleton Laboratory
R71, Harwell Oxford, Didcot, OX11 0QX England

email: michael.wathen@stfc.ac.uk

phone: 01235 394217

 

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Contact: Lomas, Georgia (STFC,DL,SC)