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A convergent/divergent duct in the exhaust system of a gas turbine engine, where the remaining gas energy is converted into velocity to produce thrust. In a gas turbine engine, the exit velocity of gases is subsonic at low-thrust conditions only. During most operating conditions, in relation to the exhaust gases temperature, the exit velocity reaches the speed of sound, resulting in the choking of the propelling nozzle (i.e., there is no increasing velocity unless the temperature increases). As the upstream total pressure increases above the choking pressure, the static pressure of gases at the exit increases above atmospheric pressure. This is referred to as pressure thrust and is effective over the nozzle exit area. This is in addition to the thrust provided by the momentum change of the gas stream. To avoid a surge, variable nozzles are used to ensure the correct balance of pressure, temperature, and thrust. See exhaust nozzle.