Linear acoustic calculations in gas are very similar to heat transfer calculations. Indeed, whereas the governing equation for heat transfer reads
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(14) |
(
contains the conduction coefficients,
is the
density,
the heat generation per unit of mass and
is the specific
heat), the pressure variation in a space with uniform basis pressure
and
density
(and consequently uniform temperature
due to the gas law) satisfies
where
is the second order unit tensor (or, for simplicity,
unit matrix) and
is the speed of sound satisfying:
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(16) |
is the ratio of the heat capacity at constant pressure devided by the
heat capacity at constant volume (
for normal air),
is the
specific gas constant (
for normal air) and
is the
absolute basis temperature (in K). Furthermore, the balance of momentum reduces to:
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(17) |
For details, the reader is referred to [11] and [2]. Equation (15) is the well-known wave equation. By comparison with the heat equation, the correspondence in Table (1) arises.
heat quantity | gas quantity |
T | p |
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Notice, however, that the time derivative in the heat equation is first order, in the gas momentum equation it is second order. This means that the transient heat transfer capability in CalculiX can NOT be used for the gas equation. However, the frequency option can be used and the resulting eigenmodes can be taken for a subsequent modal dynamic analysis. Recall that the governing equation for solids also has a second order time derivative ([10]).
For the driving terms one obtains:
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(18) |
which means that the equivalent of the normal heat flux at the boundary is the basis density multiplied with the accelaration. Consequently, at the boundary either the pressure must be known or the acceleration.