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CHAPTER IV EARTHS ATMOSPHERE EARTHS ATMOSPHERE

VIEWS: 8 PAGES: 4

									                                                          INTRODUCTION
                                              • Aircraft and launch vehicles fly mainly in
                                                the earth’s atmosphere.
        CHAPTER IV
    EARTH’S ATMOSPHERE                        • A knowledge of the pressure, temperature
                                                and density of the aircraft’s immediate
                                                surrounding is very essential for proper
                                                flight.

                                              • This chapter deals with a model of the
                                                earth’s atmosphere.




    EARTH’S ATMOSPHERE                              EARTH’S ATMOSPHERE
• The earth’s atmosphere consists of the
  following layers,

     • Troposphere
     • Stratosphere
     • Mesosphere
     • Ionosphere
     • Exosphere




            TROPOSPHERE                                  STRATOSPHERE
• This is the region closest to the earth’s   • Extends from tropopause up to about
  surface, characterized by turbulent air.
                                                50km.

• Height varies from 9km at poles to 16km
  at equator.                                 • It has high velocity winds ( but no gusts)

• Temperature drops at a nearly constant      • Temperature remains constant up to 25km
  rate of 6.5 degrees Celsius per km.           and then increases (simplified model).

• The highest point is the Tropopause.        • The highest point is the stratopause.




                                                                                             1
             MESOSPHERE                                      IONOSPHERE
                                                • It extends from the mesopause to about
• This extends from the stratopause upto
                                                  500km.
  80km.

• Temperature decreases. Pressure and           • It consists mainly of ions and free
  density are very low, but air retains the       electrons.
  composition at sea level.
                                                • Temperature increases in this region.
• The region near the stratopause contains
  the ozone layer.
                                                • Phenomena like aurora borealis occur
                                                  here.




                                                  INTERNATIONAL STANDARD
              EXOSPHERE
                                                      ATMOSPHERE (ISA)
• This is the outermost region of the earth’s   • The ISA is a simplified model of the earth’s
  atmosphere.                                     atmosphere using which preliminary
                                                  prediction of the earth’s atmosphere can
• It contains very few air molecules and          be made.
  merges into interplanetary space.
                                                • This does not represent the earth’s
• The mean free path of the molecules is of       atmosphere exactly but only an averaged
  the order of a few meters.                      earth’s atmosphere.




 ISA - STANDARD CONDITIONS                       ISA - STANDARD CONDITIONS
• The following are defined as the standard     • Air is treated as a perfect dry gas.
  reference conditions at sea level
• Standard Temperature = 288.15 K               • The composition of air is for practical
• Standard Pressure = 101325 Pa                   purposes 78% Nitrogen, 22% Oxygen and
• Temperature lapse rate = 6.5K/km upto           traces of other gases like Hydrogen,
  11km, 0 from 11km to 25km (simple)              Carbon dioxide etc.
• Note : from these, the standard density is
  got as 1.2256 kg/m3.




                                                                                                 2
            BASIC EQUATIONS                                                       BASIC EQUATIONS
• The two main equations using which all
  the variations can be derived are                                 • Using these two equations along with the
                                                                      characteristics of each region (like
     • The equation of state, p = ρRT, where
                                                                      constant temperature in the lower
       p: pressure, ρ: density,
                                                                      stratosphere), the variations of pressure,
       T:Temperature, R: gas constant.
                                                                      temperature and density in each region
     • Variation of pressure, dp/dh=-ρg,                              can be derived.
       where g is the acceleration due to
       gravity. (is g constant?)




  BASIC EQUATIONS (Linear T)                                         BASIC EQUATIONS (constant T)

                         T = T0 − Lh                                                              T = T0
                                       g
                                                                                                          g
                          p ⎛T ⎞      LR                                                      p     −   ( h − h0 )
                            =⎜ ⎟
                          p0 ⎜ T0 ⎟
                                                                                                 = e RT
                             ⎝ ⎠                                                              p0
                                      g
                                           −1                                                    ρ    p
                         ρ ⎛ T ⎞ LR                                                                 =
                            =⎜ ⎟                                                                 ρ 0 p0
                         ρ 0 ⎜ T0 ⎟
                             ⎝ ⎠

   T0, p0, ρ0 at bottom of layer, h measured from bottom of layer        T0, p0, ρ0 at bottom of layer, h measured from bottom of layer




     Standard (1976 US) model                                                     SIMPLIFIED MODEL
• http://www.pdas.com/coesa.htm
  (geo-potential altitude)                                          • This is the final set of equations modeling
  h1 (km)                h2 (km)                  L (K/km)            the earth’s atmosphere,
  0                      11                       -6.5              • For h<11km, T=288.15-0.0065h,
                                                                      p=101.325(T/288.15)^5.256
  11                     20                       0
                                                                    • For 11km<h<25km,T=216.65,
  20                     32                       +1.0
                                                                      p=22.63exp(1.735-0.0001577h)
  32                     47                       +2.8
                                                                    In the above, T:Kelvin, p:kPa, h:meters.
  47                     51                       0
                                                                    (see http://www.aeromech.usyd.edu.au/aero/atmos/atmtab.html
  51                     71                       -2.8
                                                                      http://www.desktopaero.com/appliedaero/appendices/stdatm.html )
  71                     84.85                    -2.0




                                                                                                                                          3
SIMPLIFIED MODEL




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