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Approximations
Sound production by flows occurs at relatively high Reynolds numbers. When considering wave propagation in air at audio frequencies, we can neglect friction and heat transfer over distances of the order of the wave length. Neglecting friction, heat transfer and heat production, the energy equation (11) becomes:
(28)
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The momentum equation (7) reduces to the Euler equation:
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In this isentropic flow approximation dQ = 0, so that it follows from the first law of thermodynamics:
