Re: [Boost-users] [Units] derived units of SI system (newbie)

Subject: Re: [Boost-users] [Units] derived units of SI system (newbie)
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2010-12-10 10:52:52

AMDG

On 12/10/2010 4:52 AM, André Berthe wrote:
> I would like to calculate a shear stress based on a shear rate and a 
> viscosity using Boost.Units. Here I have problems using derived 
> dimensions and conversion factors based on a existing system. The 
> error messages of my compiler are in German, so I tried to document my 
> problems with a test code:
>
> <main.cpp>
> #include <boost/units/systems/si/dynamic_viscosity.hpp>
> #include "stress.h"
> #include "shearrate.h"
>
> namespace bu = boost::units;
> namespace SI = bu::si;
>
> main(){
>   //Define a quantity of unit stress. Unfortunately there is no
>   //realization of the defined physical dimension stress in si.
>   //So a user defined type from stress.h (see below) is used:
>   bu::quantity<SI::stress> WSS_x;
>
>   //Define a shear rate. I did not understand how to use a   //inverse 
> time as dimension. Therefore I defined the derived
>   //dimension per_time in shearrate.h. Is it possible to add it to
>   //the si system  (as I tried) or do I have to define
>   //my own system and a conversion factor?
>   bu::quantity<SI::per_time> WSR_x(1*per_second);

The inverse of time is frequency.

>   //There is a existing si type for the dynamic viscosity.
>   //However, there is no defined constant. How is the
>   //correct way to assign a value? Should I define this
>   //constant?
>   bu::quantity<SI::dynamic_viscosity> eta;

You can define the constant or you can combine other
units: si::kilograms/(si::meter*si::second).

>   //Calculation of the shear stress. It seems that this calculation
>   //tries to use a »sizeof« on eta, that seems to have a
>   // incomplete type
>   //(»boost::STATIC_ASSERTION_FAILURE<false>«). And there
>   //seems to be something wrong with the conversion.   //Do I have to 
> define a conversion factor? Or is this error
>   //caused by my previous mistakes?
>   WSS_x=WSR_x/eta;

The dimensions of stress are L^-1 M T^-2.
Dynamic viscosity is M L^-1 T^-1.  Thus, the
ratio WSR_x/eta should be (1/T)/(M L^-1 T^-1) = M^-1 L.
You get an error because the dimensions don't
match.

In Christ,
Steven Watanabe