src/FDM/SP/BalloonSim.cpp

   1 /*****************************************************************************
   2
   3  Module:       BalloonSim.cpp
   4  Author:       Christian Mayer
   5  Date started: 01.09.99
   6  Called by:
   7
   8  -------- Copyright (C) 1999 Christian Mayer (fgfs@christianmayer.de) --------
   9
  10  This program is free software; you can redistribute it and/or modify it under
  11  the terms of the GNU General Public License as published by the Free Software
  12  Foundation; either version 2 of the License, or (at your option) any later
  13  version.
  14
  15  This program is distributed in the hope that it will be useful, but WITHOUT
  16  ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
  17  FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more
  18  details.
  19
  20  You should have received a copy of the GNU General Public License
  21  along with this program; if not, write to the Free Software
  22  Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
  23
  24  Further information about the GNU General Public License can also be found on
  25  the world wide web at http://www.gnu.org.
  26
  27 FUNCTIONAL DESCRIPTION
  28 ------------------------------------------------------------------------------
  29 A hot air balloon simulator
  30
  31 HISTORY
  32 ------------------------------------------------------------------------------
  33 01.09.1999 Christian Mayer      Created
  34 03.10.1999 Christian Mayer      cleaned  the code  by moveing  WeatherDatabase
  35                                 calls inside the update()
  36 *****************************************************************************/
  37
  38 /****************************************************************************/
  39 /* INCLUDES                                                                 */
  40 /****************************************************************************/
  41
  42 #ifdef HAVE_CONFIG_H
  43 #  include "config.h"
  44 #endif
  45
  46 #include <stdio.h>
  47 #include <math.h>
  48
  49 #include <simgear/constants.h>
  50
  51 #include "BalloonSim.h"
  52
  53 /****************************************************************************/
  54 /********************************** CODE ************************************/
  55 /****************************************************************************/
  56
  57 /****************************************************************************/
  58 /*                                                                          */
  59 /*  Constructor:                                                            */
  60 /*  Set the balloon model to some values that seem reasonable as I haven't  */
  61 /*  got much original values                                                */
  62 /*                                                                          */
  63 /****************************************************************************/
  64 balloon::balloon()
  65 {
  66     dt = 0.1;
  67     ground_level = 3400.0;
  68
  69     gravity_vector = SGVec3f(0.0, 0.0, -9.81);
  70     velocity = SGVec3f(0.0, 0.0, 0.0);
  71     position = SGVec3f(0.0, 0.0, 0.0);
  72     hpr = SGVec3f(0.0, 0.0, 0.0);
  73
  74     /************************************************************************/
  75     /* My balloon  has a  radius of  8.8 metres  as that  gives a  envelope */
  76     /* volume of about 2854 m^3  which is  about 77000 ft^3,  a very common */
  77     /* size for hot air balloons                                            */
  78     /************************************************************************/
  79
  80     balloon_envelope_area = 4.0 * (8.8 * 8.8) * SGD_PI;
  81     balloon_envelope_volume = (4.0/3.0) * (8.8 * 8.8 * 8.8) * SGD_PI;
  82
  83     wind_facing_area_of_balloon = SGD_PI * (8.8 * 8.8);
  84     wind_facing_area_of_basket = 2.0;   //guessed: 2 m^2
  85
  86     cw_envelope=0.45;                   //a sphere in this case
  87     cw_basket=0.8;
  88
  89     weight_of_total_fuel = 40.0;        //big guess
  90     weight_of_envelope = 200.0;         //big guess
  91     weight_of_basket = 40.0;            //big guess
  92     weight_of_cargo = 750.0;            //big guess
  93
  94     fuel_left=1.0;
  95     max_flow_of_fuel_per_second=10.0*1.0/3600.0; //assuming 10% of one hour of total burn time
  96     current_burner_strength = 0.0;      //the throttle
  97
  98     lambda = 0.15;                      //for plasic
  99     l_of_the_envelope = 1.0/1000.0;     //the thickness of the envelope (in m): 1mm
 100
 101     T = 273.16 + 130.6;                 //Temperature in the envelope => still at ground level
 102 }
 103
 104 void balloon::update()
 105 {
 106     /************************************************************************/
 107     /* I'm  simplifying  the  balloon by  reducing the  simulation  to  two */
 108     /* points:                                                              */
 109     /* the center of the basket (CB) and the center of the envelope (CE)    */
 110     /*                                                                      */
 111     /*                                 ce                                   */
 112     /*                                 I                                    */
 113     /*                                 I                                    */
 114     /*                                 cg (=center of gravity)              */
 115     /*                                 I                                    */
 116     /*                                 cb                                   */
 117     /*                                                                      */
 118     /* On each center  are forces acting:  gravity and  wind resitance.  CE */
 119     /* additionally got the lift  (=> I need to calculate the weight of the */
 120     /* air inside, too)                                                     */
 121     /*                                                                      */
 122     /* The weight of the air  in the envelope is  dependant of the tempera- */
 123     /* ture. This temperature is decreasing over the time that is dependant */
 124     /* of the insulation  of the envelope  material (lambda),  the gas used */
 125     /* (air) and the wind speed. For a plane surface it's for air:          */
 126     /*                                                                      */
 127     /*    alpha = 4.8 + 3.4*v   with v < 5.0 m/s                            */
 128     /*                                                                      */
 129     /* The value k that takes all of that into account is defined as:       */
 130     /*                                                                      */
 131     /*    1 / k = 1 / alpha1 + 1 / alpha2 + l / lambda                      */
 132     /*                                                                      */
 133     /* with 'l' beeing the 'length' i.e. thickness of the insulator, alpha1 */
 134     /* the air inside and alpha2 the air outside of the envelope.  So our k */
 135     /* is:                                                                  */
 136     /*                                                                      */
 137     /*    k = 1 / [1/4.8 + 1/(4.8+3.4v) + l/lambda]                         */
 138     /*                                                                      */
 139     /* The energy lost by this process is defined as:                       */
 140     /*                                                                      */
 141     /*    dQ = k * A * t * dT                                               */
 142     /*                                                                      */
 143     /* with Q being  the energy,  k that value  defined above,  A the total */
 144     /* area of the envelope,  t the time (in hours)  and dT the temperature */
 145     /* difference between the inside and the outside.                       */
 146     /* To get the temperature of the air in the inside I need the formula:  */
 147     /*                                                                      */
 148     /*    dQ = cAir * m * dT                                                */
 149     /*                                                                      */
 150     /* with cAir being the specific heat capacity(?)  of air (= 1.00 kcal / */
 151     /* kg * degree),  m the mass of the air and  dT the temperature change. */
 152     /* As the envelope is open I'm  assuming that the  same air pressure is */
 153     /* inside and  outside of  it  (practical there  should  be  a slightly */
 154     /* higher air pressure  in the inside or the envelope  would collapse). */
 155     /* So it's easy to calculate the density of the air inside:             */
 156     /*                                                                      */
 157     /*    rho = p / R * T                                                   */
 158     /*                                                                      */
 159     /* with p being  the pressure,  R the gas constant(?)  which is for air */
 160     /* 287.14 N * m / kg * K and T the absolute temperature.                */
 161     /*                                                                      */
 162     /* The value returned by this function is the displacement of the CB    */
 163     /************************************************************************/
 164
 165     /************************************************************************/
 166     /* NOTE:  This is the simplified version:  I'm assuming that  the whole */
 167     /* balloon consists only of the envelope.I will improove the simulation */
 168     /* later, but currently was my main concern to get it going...          */
 169     /************************************************************************/
 170
 171    // I realy don't think there is a solution for this without WeatherCM
 172    // but this is a hack, and it's working -- EMH
 173    double mAir = 1;
 174    float Q = 0;
 175
 176     // gain of energy by heating:
 177     if (fuel_left > 0.0)        //but only with some fuel left ;-)
 178     {
 179         float fuel_burning = current_burner_strength * max_flow_of_fuel_per_second * dt * weight_of_total_fuel; //in kg
 180
 181         //convert to cubemetres (I'm wrongly assuming 'normal' conditions; but that's correct for my special case)
 182         float cube_metres_burned = fuel_burning / 2.2;  //2.2 is the density for propane
 183
 184         fuel_left -= fuel_burning / weight_of_total_fuel;
 185
 186         // get energy through burning.
 187         Q += 22250.0 * cube_metres_burned;  //22250 for propan, 29500 would be butane and if you dare: 2580 would be hydrogen...
 188     }
 189
 190     // calculate the new temperature in the inside:
 191     T += Q / (1.00 * mAir);
 192
 193     //calculate the masses of the envelope and the basket
 194     float mEnvelope = mAir + weight_of_envelope;
 195     float mBasket   = weight_of_total_fuel*fuel_left + weight_of_basket + weight_of_cargo;
 196
 197     float mTotal = mEnvelope + mBasket;
 198
 199     //calulate the forces
 200     SGVec3f fTotal, fFriction, fLift;
 201
 202     fTotal = mTotal*gravity_vector;
 203
 204     // fTotal += fLift;     //FIXME: uninitialized fLift
 205     // fTotal += fFriction; //FIXME: uninitialized fFriction
 206
 207     //claculate acceleration: a = F / m
 208     SGVec3f aTotal, vTotal, dTotal;
 209
 210     aTotal = (1.0 / mTotal)*fTotal;
 211
 212     //integrate the displacement: d = 0.5 * a * dt**2 + v * dt + d
 213     vTotal = dt*velocity;
 214     dTotal = (0.5*dt*dt)*aTotal; dTotal += vTotal;
 215
 216     //integrate the velocity to 'velocity': v = a * dt + v
 217     vTotal = dt*aTotal; velocity += vTotal;
 218
 219     /************************************************************************/
 220     /* VERY WRONG STUFF: it's just here to get some results to start with   */
 221     /************************************************************************/
 222
 223     // care for the ground
 224     if (position[2] < (ground_level+0.001) )
 225         position[2] = ground_level;
 226
 227     //return results
 228     position += dTotal;
 229
 230     //cout << "BallonSim: T: " << (T-273.16) << " alt: " << position[2] << " ground: " << ground_level << " throttle: " << current_burner_strength << "\n";
 231 }
 232
 233 void balloon::set_burner_strength(const float bs)
 234 {
 235     if ((bs>=0.0) && (bs<=1.0))
 236     current_burner_strength = bs;
 237 }
 238
 239 void balloon::getVelocity(SGVec3f& v) const
 240 {
 241     v = velocity;
 242 }
 243
 244 void balloon::setVelocity(const SGVec3f& v)
 245 {
 246     velocity = v;
 247 }
 248
 249 void balloon::getPosition(SGVec3f& v) const
 250 {
 251     v = position;
 252 }
 253
 254 void balloon::setPosition(const SGVec3f& v)
 255 {
 256     position = v;
 257 }
 258
 259 void balloon::getHPR(SGVec3f& angles) const //the balloon isn't allways exactly vertical
 260 {
 261     angles = hpr;
 262 }
 263
 264 void balloon::setHPR(const SGVec3f& angles)  //the balloon isn't allways exactly vertical
 265 {
 266     hpr = angles;
 267 }
 268
 269 void balloon::setGroundLevel(const float altitude)
 270 {
 271     ground_level = altitude;
 272 }
 273
 274 float balloon::getTemperature(void) const
 275 {
 276     return T;
 277 }
 278
 279 float balloon::getFuelLeft(void) const
 280 {
 281     return fuel_left;
 282 }
 283
 284 /*
 285 void main(void)
 286 {
 287     bool burner = true;
 288     balloon bal;
 289     bool exit = false;
 290     sgVec3 pos={0.0, 0.0, 0.0};
 291     char c;
 292     float acc_dt = 0.0;
 293     float alt;
 294
 295 bool hysteresis = false; // moving up
 296     for (;!exit;)
 297     {
 298         for (int i=0; i<100; i++)
 299         {
 300             bal.update(0.1); acc_dt += 0.1;
 301             bal.getPosition(pos);
 302             alt = pos[2];
 303
 304             if (alt > 3010)
 305             {
 306                 hysteresis = true;
 307                 burner = false;
 308             }
 309             if ((alt < 2990) && (hysteresis == true))
 310             {
 311                 hysteresis = false;
 312                 burner = true;
 313             }
 314             if ((bal.getTemperature()-273.16)>250.0)
 315                 burner = false; //emergency
 316         }
 317
 318         // toogle burner
 319         c = getch();
 320         if (c==' ')
 321             burner=!burner;
 322         //if (c=='s')
 323         //    show=!show;
 324
 325         //printf("Position: (%f/%f/%f), dP: (%f/%f/%f), burner: ", pos[0], pos[1], pos[2], dp[0], dp[1], dp[2]);
 326         printf("%f         \t%f         \t%f         \t%f\n", acc_dt/60.0, bal.getTemperature()-273.16, pos[2], bal.getFuelLeft());
 327         if (burner==true)
 328         {
 329             //printf("on\n");
 330             bal.set_burner_strength(1.0);
 331         }
 332         else
 333         {
 334             //printf("off\n");
 335             bal.set_burner_strength(0.0);
 336         }
 337
 338     }
 339 }
 340 */