flightgear/src/WeatherCM/FGWeatherUtils.h

/*****************************************************************************

 Header:       FGWeatherUtils.h	
 Author:       Christian Mayer
 Date started: 28.05.99

 -------- Copyright (C) 1999 Christian Mayer (fgfs@christianmayer.de) --------

 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
 Foundation; either version 2 of the License, or (at your option) any later
 version.

 This program is distributed in the hope that it will be useful, but WITHOUT
 ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
 FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more
 details.

 You should have received a copy of the GNU General Public License along with
 this program; if not, write to the Free Software Foundation, Inc., 59 Temple
 Place - Suite 330, Boston, MA  02111-1307, USA.

 Further information about the GNU General Public License can also be found on
 the world wide web at http://www.gnu.org.

FUNCTIONAL DESCRIPTION
------------------------------------------------------------------------------
Utilities for the weather calculation like converting formulas

HISTORY
------------------------------------------------------------------------------
02.06.1999 Christian Mayer      Created
08.06.1999 Christian Mayer      Changed sat_vp
16.06.1999 Durk Talsma		Portability for Linux
20.06.1999 Christian Mayer	added lots of consts
11.10.1999 Christian Mayer	changed set<> to map<> on Bernie Bright's 
				suggestion
19.10.1999 Christian Mayer	change to use PLIB's sg instead of Point[2/3]D
				and lots of wee code cleaning
17.01.2000 Christian Mayer	Added conversion routines make it easier for
				JSBsim to use the weather database.
18.05.2000 Christian Mayer      Added function for geting the density when
                                temperature and pressure are given
*****************************************************************************/

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/* SENTRY                                                                   */
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#ifndef FGWeatherUtils_H
#define FGWeatherUtils_H

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/* INCLUDES								    */
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#include <math.h>
		
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/* DEFINES								    */
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/*assuming as given:							    */
/*									    */
/* t:	    temperature in <20>C						    */
/* p:	    preasure in mbar						    */
/* //abs_hum: absoloute humidity in g/m^3,				    */
/* act_vp:  actual vapor pressure pascal				    */
/*									    */
/* Calculated vaues:							    */
/* //max_hum: maximum of humidity in g/m^3,				    */
/* sat_vp:  saturated vapor pressure in pascal				    */
/* rel_hum: relative humidity in %					    */
/* dp:	    dew point in <20>C						    */
/* wb:	    approximate wetbulp in <20>C					    */
/*									    */
/* NOTE: Pascal is the SI unit for pressure and is defined as Pa = N/m^2    */
/*       1 mbar = 1 hPa = 100 Pa					    */
/* NOTE: <20>C isn't a SI unit, so I should use <20>K instead. But as all	    */
/*       formulas are given in <20>C and the weather database only uses	    */
/*       'normal' temperatures, I've kept it in <20>C			    */
/****************************************************************************/

#define SAT_VP_CONST1 610.483125
#define SAT_VP_CONST2 7.444072452
#define SAT_VP_CONST3 235.3120919

inline WeatherPrecision sat_vp(const WeatherPrecision temp)
{
    //old:
    //return 6.112 * pow( 10, (7.5*dp)/(237.7+dp) );    //in mbar

    //new:
    //advantages: return the result as SI unit pascal and the constants
    //are choosen that the correct results are returned for 0<>C, 20<32>C and
    //100<30>C. By 100<30>C I'm now returning a preasure of 1013.25 hPa
    return SAT_VP_CONST1 * pow( 10, (SAT_VP_CONST2*temp)/(SAT_VP_CONST3+temp) );    //in pascal
}

inline WeatherPrecision rel_hum(const WeatherPrecision act_vp, const WeatherPrecision sat_vp)
{
    return (act_vp / sat_vp) * 100;	//in %
}

inline WeatherPrecision dp(const WeatherPrecision sat_vp)
{
    return (SAT_VP_CONST3*log10(sat_vp/SAT_VP_CONST1))/(SAT_VP_CONST2-log10(sat_vp/SAT_VP_CONST1)); //in <20>C 
}

inline WeatherPrecision wb(const WeatherPrecision t, const WeatherPrecision p, const WeatherPrecision dp)
{
    WeatherPrecision e = sat_vp(dp); 
    WeatherPrecision tcur, tcvp, peq, diff;
    WeatherPrecision tmin, tmax;

    if (t > dp)
    {
	tmax = t;
	tmin = dp;
    }
    else
    {
	tmax = dp;
	tmin = t;
    }

    while (true) 
    { 
	tcur=(tmax+tmin)/2;
	tcvp=sat_vp(tcur);
	
	peq = 0.000660*(1+0.00155*tcur)*p*(t-tcur);
	
	diff = peq-tcvp+e;
	
	if (fabs(diff) < 0.01) 
	    return tcur;	//in <20>C
	
	if (diff < 0) 
	    tmax=tcur; 
	else 
	    tmin=tcur; 
    }; 

}

// Assume that we've got an ideal gas in normal altitudes
inline WeatherPrecision Density(const WeatherPrecision AirPressure, const WeatherPrecision Temperature )
{
    const float rho0 = 1.293; /*density for air in normal altitudes at 0<>C and 1013 mbar*/

    return rho0 * 273.15 * AirPressure / (101300.0 * Temperature);
}

inline WeatherPrecision Celsius		    (const WeatherPrecision celsius)
{
    return celsius + 273.15;				//Kelvin
}

inline WeatherPrecision Fahrenheit	    (const WeatherPrecision fahrenheit)
{
    return (fahrenheit * 9.0 / 5.0) + 32.0 + 273.15;	//Kelvin
}

inline WeatherPrecision Kelvin2Celsius	    (const WeatherPrecision kelvin)
{
    return kelvin - 273.15;				//Celsius
}

inline WeatherPrecision Kelvin2Fahrenheit   (const WeatherPrecision kelvin)
{
    return ((kelvin - 273.15) * 9.0 / 5.0) + 32.0;	//Fahrenheit
}

inline WeatherPrecision Celsius2Fahrenheit  (const WeatherPrecision celsius)
{
    return (celsius * 9.0 / 5.0) + 32.0;		//Fahrenheit
}

inline WeatherPrecision Fahrenheit2Celsius  (const WeatherPrecision fahrenheit)
{
    return (fahrenheit - 32.0) * 5.0 / 9.0;		//Celsius
}

inline WeatherPrecision Torr2Pascal	    (const WeatherPrecision torr)
{
    return (101325.0/760.0)*torr;			//Pascal
}

inline WeatherPrecision Rankine2Kelvin	    (const WeatherPrecision Rankine)
{
    return (5.0 / 9.0) * Rankine;			//Kelvin
}

inline WeatherPrecision JSBsim2SIdensity    (const WeatherPrecision JSBsim)
{
    return JSBsim / 0.0019403203;			//kg / cubic metres
}

inline WeatherPrecision psf2Pascal	    (const WeatherPrecision psf)
{
    return psf / 0.020885434;				//lbs / square foot (used in JSBsim)
}

inline WeatherPrecision Kelvin2Rankine	    (const WeatherPrecision kelvin)
{
    return (9.0 / 5.0) * kelvin;			//Rankine (used in JSBsim)
}

inline WeatherPrecision SIdensity2JSBsim    (const WeatherPrecision SIdensity)
{
    return 0.0019403203 * SIdensity;			//slug / cubic feet (used in JSBsim)
}

inline WeatherPrecision Pascal2psf	    (const WeatherPrecision Pascal)
{
    return 0.020885434 * Pascal;			//lbs / square feet (used in JSBsim)
}

/****************************************************************************/
/* CLASS DECLARATION							    */
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#endif /*FGWeatherUtils_H*/