Monday, November 20, 2017

Machine Learning for CFD Turbulence Closures

I wrote a couple previous posts on some interesting work using deep learning to accelerate topology optimization, and a couple neural network methods for accelerating computational fluid dynamics (with source). This post is about a use of machine learning in computational fluid dynamics (CFD) with a slightly different goal: to improve the quality of solutions. Rather than a focus on getting to solutions more quickly, this post covers work focused on getting better solutions. A better solution is one that has more predictive capability. There is usually a trade-off between predictive capability, and how long it takes to get a solution. The most well-known area for improvement in predictive capability of state-of-the-practice, industrial CFD is in our turbulence and transition modeling. There are a proliferation of approaches to tackling that problem, but the overall strategy that seems to be paying off is for CFD'ers to follow the enormous investment being made by the large tech companies in techniques, open source libraries, and services for machine learning. How can those free / low-cost tools and techniques be applied to our problems?

The authors of Machine Learning Models of Errors in Large Eddy Simulation Predictions of Surface Pressure Fluctuations used machine learning techniques to model the error in their LES solutions. See an illustration of the instantaneous density gradient magnitude of the developing boundary layer from that paper shown to the right. Here's the abstract,
We investigate a novel application of deep neural networks to modeling of errors in prediction of surface pressure fluctuations beneath a compressible, turbulent flow. In this context, the truth solution is given by Direct Numerical Simulation (DNS) data, while the predictive model is a wall-modeled Large Eddy Simulation (LES
). The neural network provides a means to map relevant statistical flow-features within the LES solution to errors in prediction of wall pressure spectra. We simulate a number of flat plate turbulent boundary layers using both DNS and wall-modeled LES to build up a database with which to train the neural network. We then apply machine learning techniques to develop an optimized neural network model for the error in terms of relevant flow features

Monday, November 13, 2017

Deep Learning to Accelerate Computational Fluid Dynamics

Lat-Net: Compressing Lattice Boltzmann Flow Simulations using Deep Neural Networks
I posted about a surprising application of deep learning to accelerate topology optimization. The thing I like about that approach is it's a strategy that could be applied to accelerate many different solvers that we use to simulate all sorts of continuum mechanics based on partial differential equations (i.e. computational fluid dynamics, structural mechanics, electrodynamics, etc.). With a bit of help from Google I found a neat paper and project on github doing exactly that for a Lattice-Boltzmann fluid solver.

Friday, November 10, 2017

Deep Learning to Accelerate Topology Optimization

Topology Optimization Data Set for CNN Training
Neural networks for topology optimization is an interesting paper I read on arXiv that illustrates how to speed up the topology optimization calculations by using a deep learning convolution neural network. The data sets for training the network are generate in ToPy, which is an Open Source topology optimization tool.

Saturday, March 25, 2017

Innovation, Entropy and Exoplanets

I enjoy Shipulski on Design for the short articles on innovation. They are generally not technical at all. I like to think of most of the posts as innovation poetry to put your thoughts along the right lines of effort. This recent post has a huge, interesting technical iceberg riding under the surface though.
If you run an experiment where you are 100% sure of the outcome, your learning is zero. You already knew how it would go, so there was no need to run the experiment. The least costly experiment is the one you didn’t have to run, so don’t run experiments when you know how they’ll turn out. If you run an experiment where you are 0% sure of the outcome, your learning is zero. These experiments are like buying a lottery ticket – you learn the number you chose didn’t win, but you learned nothing about how to choose next week’s number. You’re down a dollar, but no smarter.

The learning ratio is maximized when energy is minimized (the simplest experiment is run) and probability the experimental results match your hypothesis (expectation) is 50%. In that way, half of the experiments confirm your hypothesis and the other half tell you why your hypothesis was off track.
Maximize The Learning Ratio

Tuesday, March 7, 2017

NASA Open Source Software 2017 Catalog

NASA has released its 2017-2018 Software Catalog under their Technology Transfer Program. A pdf version of the catalog is available, or you can browse by category. The NASA open code repository is already on my list of Open Source Aeronautical Engineering tools. Of course many of the codes included in that list from PDAS are legacy NASA codes that were distributed on various media in the days before the internet.

Saturday, December 17, 2016

Hybrid Parallelism Approaches for CFD

This previous post, Plenty of Room at Exascale, focuses on one specific commercial approach to scaling CFD to large problems on heterogeneous hardware (CPU & GPU) clusters. Here's some more references I found interesting reading on this sort of approach.


Recent progress and challenges in exploiting graphics processors in computational fluid dynamics provides some general strategies for using multiple levels of parallelism accross GPUs, CPU cores and cluster nodes based on that review of the literature:
  • Global memory should be arranged to coalesce read/write requests, which can improve performance by an order of magnitude (theoretically, up to 32 times: the number of threads in a warp)
  • Shared memory should be used for global reduction operations (e.g., summing up residual values, finding maximum values) such that only one value per block needs to be returned
  • Use asynchronous memory transfer, as shown by Phillips et al. and DeLeon et al. when parallelizing solvers across multiple GPUs, to limit the idle time of either the CPU or GPU.
  • Minimize slow CPU-GPU communication during a simulation by performing all possible calculations on the GPU.

Wednesday, July 6, 2016

Exciting 3D Print Service Developments

It has never been easier to go from a design in your head to parts in your hand. The barriers to entry are low on the front end. There are all sorts of free and open source drawing, mesh editing, modeling or CAD applications. On the fabrication end of things, services like shapeways, and imaterialise continue to improve their delivery times, material options and prices.

One of the recent developments that deserves some attention is in metal 3D printing. imaterialise has offered parts in DMLS titanium for a while, but they've been pretty pricey. They have now significantly reduced the prices on Ti parts, and are now offering a trial with aluminum. Not to be left out, Shapeways has graduated SLM aluminum from its pilot program.

It's great to see such thriving competition in this space. I'm working on some models specifically for this metal powder-bed fusion technology. What will you print?