Frequently Asked Questions
What is the International Standard Atmosphere (ISA)?
It is a standard model of altitude, temperature, pressure, and density defined by ICAO (International Civil Aviation Organization). Reference values at sea level: pressure 101325 Pa, temperature 288.15 K (15°C), density 1.225 kg/m³. It is defined in multiple layers: the troposphere (0–11 km) where temperature decreases at 6.5 K/km, the lower stratosphere (11–20 km) is isothermal at 216.65 K, and the upper stratosphere (20–32 km) where temperature increases at 1 K/km, etc.
What is the atmospheric pressure at 8848 m (Mount Everest)?
According to the ISA model, it is about 314 hPa (about 31% of sea level pressure), and the temperature is about −42°C. The partial pressure of oxygen is also about 31% of sea level, making it extremely difficult to summit without supplemental oxygen. Hillary and Tenzing used oxygen cylinders when they first climbed in 1953. ISA is an average for a standard year, so actual values vary with season and latitude.
How does the speed of sound change with altitude?
The speed of sound is given by $a = \sqrt{\gamma RT}$ and depends only on temperature (γ=1.4, R=287 J/kgK), not directly on pressure or density. In the troposphere, as altitude increases, temperature decreases and the speed of sound also decreases. Sea level: 340 m/s → lower stratosphere (T=216.65 K): about 295 m/s. The Mach number of a high-speed aircraft is the ratio of its speed to this local speed of sound, so at the same flight speed, the Mach number is higher at higher altitudes.
What are the characteristics of the mesosphere and thermosphere?
The mesosphere (50–80 km) sees temperature drop again, reaching the lowest atmospheric temperature (about −90°C) near 80 km. It is also the layer where meteors burn up. The thermosphere (above 80 km) absorbs extreme ultraviolet (EUV) from the sun and can reach over 1000°C, but the air is so thin that the heat content is low. The ISS flies in the thermosphere (around 400 km altitude).
How to use the ISA model for aircraft CFD analysis?
In aircraft CFD, ISA values at the flight altitude (pressure p, temperature T, density ρ, dynamic viscosity μ) are set as far-field boundary conditions. Dynamic viscosity is calculated using Sutherland's formula: $\mu = \mu_0(T/T_0)^{3/2}(T_0+S)/(T+S)$. The boundary conditions for compressible flow, determined by Mach number M = V/a, also change with altitude. In OpenFOAM, this can be set with the `freestream` boundary condition; in Ansys Fluent, pressure correction for altitude can be done under 'Operating Conditions'.