# Computational Science Stack Exchange Community Digest

## Top new questions this week:

### What can a computational scientist do in the fourth industrial revolution?

This question is neither scientific nor technical but more career related. I am at a junction in my professional life where I need to make a decision with regard to the future of my career. At the ...

career-development industry4.0 computational-science-careers asked by Dude 9 votes answered by Biswajit Banerjee 15 votes

### FEM : energy minimization VS PDE solving

Engineering FEM When I studied engineering, I learned the traditional approach for finite elements for elasticity. The point was to solve the PDE $-div(\sigma)=f$ as: Multiply your PDE with a test ...

finite-element optimization pde simulation computer-vision asked by Txnda 6 votes

### Roundoff errors in FEM computations - generalized eigenvalues

This is a continuation of my previous question. I am trying to effectively compute a bound for the roundoff errors in some FEM computation (2d polygons, triangular meshes). Below I will write some of ...

finite-element eigenvalues error-estimation asked by Beni Bogosel 4 votes answered by user7440 1 vote

### Is there any reliable free/open source tool for structured mesh smoothing?

I have been using Pointwise for grid generation and found the quality of smoothed grids to be stunning. I am not aware of any free/open source alteranative that offers the same capabilities for ...

mesh-generation grid smoothing asked by Pet but Ape 3 votes

### 3D Cooley-Tukey FFT

To compute the $N$-point DFT $$X[k] = \sum_{n=0}^{N-1} x[n] W_N^{kn}$$ where $N = N_1 N_2$, we can write the indices as $n = N_2 n_1 + n_2$ and $k = k_1 + N_1 k_2$, (effectively packing the data ...

fourier-transform asked by Brian 3 votes

### When do not use preconditioners for sparse linear system of equations?

I'm implementing a solver of Finite Element Method, and to solve the linear system of equations I'm using gmres from MKL of Intel. Exists the option with and without a preconditioning. In what case it ...

iterative-method preconditioning linear-system gmres asked by yemino 3 votes answered by Anton Menshov 11 votes

### What determines the order of a finite volume scheme?

I often hear that cell centred finite volume is second order accurate but at the same time I come across notions of high order FVM flux schemes. Is there a distinction between the two? If I were to ...

finite-volume asked by CuteCompute 3 votes

## Greatest hits from previous weeks:

### What kinds of problems lend themselves well to GPU computing?

So I've got a decent head for what problems I work with are best one in serial, and which can be managed in parallel. But right now, I don't have much of an idea of what's best handled by CPU-based ...

gpu asked by Fomite 89 votes answered by Max Hutchinson 66 votes

### Which algorithm is more accurate for computing the sum of a sorted array of numbers?

Given is an increasing finite sequence of positive numbers $z_{1} ,z_{2},.....z_{n}$. Which of the following two algorithms is better for computing the sum of the numbers? ...

algorithms floating-point asked by CryptoBeginner 24 votes

### How much better are Fortran compilers really?

This question is an extension of two discussions that came up recently in the replies to "C++ vs Fortran for HPC". And it is a bit more of a challenge than a question... One of the most often-heard ...

fortran c blas benchmarking asked by Pedro 78 votes answered by Jack Poulson 37 votes

### How to discretize the advection equation using the Crank-Nicolson method?

The advection equation needs to be discretized in order to be used for the Crank-Nicolson method. Can someone show me how to do that? asked by pandoragami 8 votes answered by boyfarrell 20 votes

### Periodic boundary condition for the heat equation in ]0,1[

Let us consider a smooth initial condition and the heat equation in one dimension : $$\partial_t u = \partial_{xx} u$$ in the open interval $]0,1[$, and let us assume that we want to solve it ...

pde boundary-conditions parabolic-pde asked by bela83 14 votes answered by Doug Lipinski 10 votes

### How mature is the "Julia" scientific computing language project?

I'm considering learning a new language to use for numerical/simulation modelling projects, as a (partial) replacement for the C++ and Python that I currently use. I came across Julia, which sounds ...

parallel-computing languages julia asked by Nathaniel 59 votes answered by Chris Rackauckas 56 votes

### Why is my iterative linear solver not converging?

What can go wrong when using preconditoned Krylov methods from KSP (PETSc's linear solver package) to solve a sparse linear system such as those obtained by discretizing and linearizing partial ...

linear-algebra petsc preconditioning krylov-method iterative-method asked by Jed Brown 28 votes

## Can you answer these questions?

### How do you correctly implement Scipy's FFT procedures to produce a low-pass filter - image processing

I'm following this low-pass filter example in the text "Image Operators: Image Processing in Python 1st Edition" by Jason M. Kinser, but can't seem to duplicate their results. The text's ...

python scipy fourier-transform image-processing asked by Lagreeni 1 vote

### 2D DFT for lower frequencies only; is there something significantly faster than numpy.fft.fft2 (throwing away high frequencies)?

I do a lot of 2D discrete FFT in python using np.fft.fftshift(np.fft.fft2(y)), then throw away 90% or more of the array, keeping only the central low-frequency area....

python fourier-transform asked by uhoh 1 vote

### Is it possible to use a fixed point iteration for solving this nonlinear system?

Consider the following differential equation \begin{align} \frac{\partial f(u)}{\partial x} &= g(x), \ \ x\in [x_{L},x_{R}] \label{Eq2.2} \\ u(x_{L}) &= g_{1} \end{align} where $f(u)$ is a ...

nonlinear-equations discontinuous-galerkin fixed-point asked by TheComander 1 vote
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