The onset of flow instability and the fluctuating periodicity are studied for a polymeric liquid flowing through an abrupt contraction.
The shear rate at the onset of flow instability decreases as the contraction ratio of a channel increases. And the fluctuating periodicity decreases as the shear rate or the contraction ratio increases. The frequency of fluctuation depends on the geometric mean of mean shear rates above and below the contraction. In addition, the frequency of fluctuation increases as the angle of contraction decreases.
In an asymmetric contraction channel as well as in a symmetric one, the flow instability occurs. And the shear rate at the onset of instability is larger in the asymmetric channel than in the symmetric one.

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