### 14.2.3. Quadrupole Sources

Quadrupole sources arise in the interior of the flow and result from turbulence fluctuations in wakes, shear layers, or interaction of flow features. Because many of these features are three-dimensional in nature, quadrupole sources are very expensive to evaluate and store. This is mainly because transient data for a symmetric tensor is needed over some three dimensional region of the CFD calculation.

Usually the density fluctuation term in the Lighthill tensor Equation 14–4 is neglected because for turbulent flows, it is much smaller than the combined instantaneous Reynolds and compressive stresses.

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