Boring stuff

Posted on 29 October, 2018 by ddcampayo

The previous equation is still too general, and a connection between stress and strain is still needed. Here we consider the case in which there is a linear relationship between both, which involves the coefficient of viscosity.

To begin with, let us consider a simple case in which a fluid is confined between two planes. One of them moves sideways with a certain speed $u_0$ , while the other is kept fixed. After a certain transient, some force is needed in order to keep this shearing. The simplest expression is

$F= \mu A \frac{u_0}{L}.$

The force is proportional to the area and to the velocity difference between the planes. It is also inversely proportional to their separation, L (this fact being the least obvious). Finally, a constant of proportionality is given by $\mu$ , the viscosity coefficient, or

simply “the viscosity”. This constant may vary with temperature, density, pressure, but the point with Newtonian fluids is that it does not vary with the velocity field (or its derivatives).

Later, in section …, this flow will be solved as a solution of the Navier-Stokes equations, the Couette flow. There, it will be shown that the velocity is everywhere in the direction of the force exerted on the upper plane, let us call it $x$, and varies linearly between the planes, in the y direction. Therefore, the only components of the strain rate tensor are $\epsilon_{xy} = \epsilon_{yx} = u_0 / ( 2 L )$ . We therefore have

$\tau_{xy} = \mu \epsilon_{xy}.$

With this in mind, let us look for a general relationship between $\tau$ and $\epsilon$ . This is much easier if we go to the principal strain axes. These are the coordinates on which the strain rate is diagonal. Such coordinate system always exist, since the strain rate tensor is symmetric. Notice that in these system strains are not due to shear, only to dilations.

Better with stock pictures

Posted on 29 October, 2018 by ddcampayo

“A connection is needed”. Photo by Pixabay on Pexels.com

Holy cow, is honey viscous or what. Photo by Pixabay on Pexels.com

$F= \mu A \frac{u_0}{L}.$

action balls black and white illustration

Newton’s craddle. Another of this guy’s creations. Photo by Pixabay on Pexels.com

simply “the viscosity”. This constant may vary with temperature, density, pressure, but the point with Newtonian fluids is that it does not vary with the velocity field (or its derivatives).

What a wave. Its strain tensors must be on fire. Photo by Emiliano Arano on Pexels.com

$\tau_{xy} = \mu \epsilon_{xy}.$

With this in mind, let us look for a general relationship between $\tau$ and $latex

What a bunch of math. This is so hard. Photo by Lum3n.com on Pexels.com

\epsilon$. This is much easier if we go to the principal strain axes. These are the coordinates on which the strain rate is diagonal. Such coordinate system always exist, since the strain rate tensor is symmetric. Notice that in these system strains are not due to shear, only to dilations.

Tired of that “TeX” look?

Posted on 13 March, 2018 by ddcampayo

TeX has been using computer modern (CM) font since its inception. But that “TeX” look may become a bit tiring. Of course, TeX is a typesetting engine, it is not limited to CM fonts. On the other hand, there aren’t so many fonts around for both the text and the math. (If you have no math, xeTex makes it easy to use most fonts you can imagine, including the Microsoft and google families).

I found a very clear review of existing alternatives at the Font usage post, by Ryosuke Iritani (入谷亮介). I have taken his suggestions and created a gallery, with a simple sample of text and equations.

More elegant Palatino

\usepackage[sc]{mathpazo}
\linespread{1.05} % Palladio needs more leading (space between lines)
\usepackage[T1]{fontenc}

Kpfonts (Palatino-like)

\usepackage{kpfonts}

mathpazo

Libertine

Used e.g. in Wikipedia on each sectioning

\usepackage{libertine}
\usepackage{libertinust1math}
\usepackage[T1]{fontenc}

STIX

Scientific and Technical Information Exchange; Times-based but much more elegant than txfonts package.

\usepackage[T1]{fontenc}
\usepackage{stix}

styx

Garamond

It’s a bit thin and less friendly

\usepackage[urw-garamond]{mathdesign}
\usepackage[T1]{fontenc}

Utopia (Adobe)

\usepackage[adobe-utopia]{mathdesign}
\usepackage[T1]{fontenc}

Charter

\usepackage[charter]{mathdesign}

Crimson (with math support)

\usepackage[T1]{fontenc}
\usepackage{cochineal}
\usepackage[cochineal,varg]{newtxmath}

Baskervald

Baskerville-based, thicker font

\usepackage[lf]{Baskervaldx} % lining figures
\usepackage[bigdelims,vvarbb]{newtxmath} % math italic letters from Nimbus Roman
\usepackage[cal=boondoxo]{mathalfa} % mathcal from STIX, unslanted a bit
\renewcommand*\oldstylenums[1]{\textosf{#1}}

Helvetica

So far, the only font not included the Iritani’s Font usage post!

\usepackage{helvet}
\usepackage{sansmath}

\usepackage{titlesec}  % this enforces helvetica in section and chapter titles
\titleformat{\chapter}[display]
  {\normalfont\sffamily\huge\bfseries}
  {\chaptertitlename\ \thechapter}{20pt}{\Huge}
\titleformat{\section}
  {\normalfont\sffamily\Large\bfseries}
  {\thesection}{1em}{}

% In main text, at the beginning:
\fontfamily{phv}\selectfont

% before the first equation:
\sansmath

helvetica

The code

All the above was produced with variations of this file. I just run latex on it, then dvips to get a ps file, which I then crop and export as PNG using the GIMP. Of course, depending on the system, some LaTeX packages may be needed, as well as fonts (I had to install urw-garamond on my arch linux system, for example.)

\documentclass{article}

\newcommand{\bfr}{\mathbf{r}}
\newcommand{\bfu}{\mathbf{u}}
\newcommand{\bfq}{\mathbf{q}}

\usepackage{amsmath}

\usepackage{libertine}
\usepackage{libertinust1math}
\usepackage[T1]{fontenc}

\usepackage{lipsum}% for filler text

\begin{document}

\section{A section}

\lipsum[10]

Equations:

\begin{equation}
\frac{d \mathbf{u}}{d t} = - \nabla p + \nu \nabla^2 \mathbf{u},
\end{equation}

\begin{equation}
\begin{split}
E &= m c^{2},\\
T &= 2\pi \sqrt{\frac{m}{k}}
\end{split}
\end{equation}

\begin{equation}
\iint \phi = - \oint p
\end{equation}
\end{document}

A markdown test

Posted on 26 February, 2018 by ddcampayo

This is a test of markdown blog writing. The writing comes straight from my website on CFD methods. These were written in markdown under reveal.js, for quick and nice lecture slides. Some changes had to be made:

LaTeX must start as “dollar sign latex” … “dollar”
Links to local files (such as pictures) don’t work
Lists (such as this one) do not seem to work well

Muy a menudo, se parte de las EDPs, conocidas, por ejempo:

$\frac{\partial u}{\partial t} + c \frac{\partial u}{\partial x} = 0$

Estas se discretizan: sustituyendo las derivadas por diferencias.

Sin embargo, este es un proceso de ida y vuelta, porque
las EDPs se deducen a nivel discreto.

Deducción

Se suponen cambios de un campo $u$ sólo
en la dirección $x$

Esquema de convección en 1D

conveccion_1d

parts_bw_values_250

El cambio en la cantidad total $A \Delta x \, u_i$ será:

$\frac{d }{d t} (A \Delta x \, u_i ) = \Phi_{i-1/2} - \Phi_{i+1/2}$

Flujos, convección

Antes los flujos por las caras venían dados por:

$\Phi_{i-1/2} = A c \, u_{i-1/2}$

$\Phi_{i+1/2} = A c \, u_{i+1/2}$

$\frac{d }{d t} (A \Delta x \, u_i ) = A c \, u_{i-1/2} - A c \, u_{i+1/2}$

Sound waves with attenuation

Posted on 20 November, 2015 by ddcampayo

Just a simple derivation of the role of attenuation in the standard sound wave equation. Original work: Stokes, 1845.

Starting with the Navier-Stokes momentum equation

$\frac{\partial }{\partial t} \mathbf{u} + \mathbf{u} \nabla \mathbf{u} = - \frac{1}{\rho} \nabla p + \frac{\mu}{\rho} \nabla^2 \mathbf{u} + \left(\frac{\lambda+\mu}{\rho}\right)\nabla (\nabla\cdot\mathbf{u}) ,$

where $\lambda$ is a Lamé viscosity coefficient. The bulk viscosity coeficient is defined as $\zeta = \lambda + (2/3) \mu$ . The last term is often neglected, even in compressible flow, but sound attenuation is one of the few cases where it may have some influence. All viscosities are assumed to be constant, but in this case this is a safe assumption, since we are going to assume small departures about equilibrium values.

Continue reading →

Moving to python for science

Posted on 28 November, 2013 by ddcampayo

OK, it’s final.

After some thinking, reading here and there, I have convinced myself to move to python as far as science is concerned.

One of the mean reasons is that there is this environment (“ecosystem” they call it), SciPy, in which my main concerns are answered.

How many times have I looked for information on a language only to find about something I don’t care about. Like address books, what’s up with those? SciPy goes straight to the point: linear algebra, plotting and analysis, symbolic analysis…
Already in the first lines, they are already discussing software I use: matlab, octave, emacs, xmgrace (one of the reasons of moving is the lack of progress in this fine 2D program)

Not long ago I wrote a list of scientific software that was interesting to have in linux. Now, if things go as planed I will only need:

GRAPHS

SciPy (matplotlib)
Keep, just in case:
- gnuplot (it’s a standard, after all)
Not replaceable:
- ParaView
- geogebra

MATH, NUMERICS

SciPy (numpy)
Keep, just in case:
- octave

MATH,SYMBOLIC

SciPy (SymPy)

CFD

Not replaceable:
ParaView
openfoam

LIBRARIES

SciPy and numpy
Not replaceable:
- CGAL, Computational Geometry Algorithms Library

PROGRAMMING

SciPy
Not replaceable:
- Good ole C and C++

PUBLISHING

LaTeX
emacs (don’t forget the AucTeX extension for LaTeX)

STATISTICS

SciPy

Easy LaTeX tables

Posted on 6 June, 2012 by ddcampayo

Who wouldn’t want to type tables in LaTeX without worrying about how text is treated in each column. Something as simple as:

\begin{a_table} {|20%|60%|20%|}\hline

bla blaaaa & bleee bleeeeh & babble babble \\

etc…

Well, it can be almost as simple!

Continue reading →

Annotum, the future of scholarly publising

Posted on 16 March, 2012 by ddcampayo

Annotum is “an open-source, open-process, open-access scholarly authoring and publishing platform based on WordPress.” This sound very interesting if you are into scholarly publishing, like the open-* ideas, and work with collaborators – or just work in different places.

It supports LaTeX, figure upload with captions, bibliography management (quite easy through DOIs), and I guess some other features that I have not explored. I am particularly interested in linking all sorts of files (e.g. data), and export capabilities (specially, into straight LaTeX).

This is the reference to a trial article I have written: Daniel Duque. A trial: Annotum trial [Internet]. Version 3. Annotum trial. 2012 Mar 16.

Lost pdf author and title with latex

Posted on 3 November, 2011 by ddcampayo

¿Lost pdf author and pdf title? Just go ahead, edit the pdf file, and perhaps (YMMV) you’ll find a readable section with the relevant info. Look for “\Title”. This happened to me while running latex + dvips -Ppdf + dvips… the author and title info from the hyperref section was lost in the process (pdflatex seems work ok.)

The relevant section of my pdf:

<</Producer(dvips + GPL Ghostscript 9.0)
/CreationDate(Tue May 31 19:00:02 2011)
/ModDate(D:20111103111644+01’00’)
/Creator(gnuplot 4.4 patchlevel 0)
/Title(Proyecto Investigador)
/Subject(Proyecto)
/Author(Daniel Duque)
/Keywords(proyecto)>>endobj

vi, emacs quick macros

Posted on 25 October, 2011 by ddcampayo

vi

Some may be used to the magical “.” (period) command in vi, which magically repeats “the last thing you did”. Since this last thing can be quite complicated, this is some kind of easy macro in practice. Imagine you want to add a period at the end of every sentence of a list. Simple: type “A-<esc>” (i.e. A-hyphen-escape key) on the first sentence. Then go j.j.j.j.j.j.j. like mad. You can also type “4.” to do the thing 4 times (not very useful in this case).

emacs

But you know all this if you use vi anyway (why do you use it otherwise?). What I guess many people don’t know is that emacs also has quick macros. Type “C-x (” (i.e. control-x open parenthesis) and read the warning “defining keyboard macro”. “Keyboard” means this is not defined in some other way (e.g. in a config file). Now, type your macro. In the previous case you could type now C-a, “-“, down-arrow. Them “C-x )” to end your macro. Notice this includes the “down-arrow”, which moves to the next line (C-n is a more traditional way to do this). Type now C-e to execute the macro once. Type C-n C-x to do it several times (n is a number).

Daniel Duque Campayo

web-ish blog

Category Archives: computer publishing