Spin-down in rotating Hagen–Poiseuille flow: a simple criterion to detect the onset of absolute instabilities

What is it about?

We conduct experiments in a pipe flow with constant flow rate is subjected to a constant rotation. In this setup, we detect experimentally instabilities in a rotating pipe flow that were predicted theoretically decreasing in a certain time, the rotation speed. In particular, these instabilities travel upstream though the flow rate is positive. This kind of rotating flows could appear in applications such us (static) pipe flows with sudden contractions (e.g. nuclear plants) or photobioreactors for production of algae.

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Why is it important?

Our findings show that the values given by flowmeters could oscillate. For this reason, the presence of these instabilities could be negative in some industrial applications. Conversely, the mixing can be quite efficient in devices in which the ratio between flow rate and angular speed is well controlled.

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