Irreversibility in an ideal fluid

TL;DR

This paper demonstrates that irreversibility in fluid flow can arise from limited control over initial conditions, even in ideal, time-reversible flows, challenging the notion that dissipation is necessary for irreversibility.

Contribution

It shows that asymmetry between inflow and outflow in ideal fluids results from experimental limitations, not viscous dissipation, highlighting a new perspective on irreversibility.

Findings

Outflow and inflow exhibit asymmetric flow patterns despite ideal, reversible equations.

Irreversibility can emerge from control limitations rather than dissipation.

The experiment illustrates how microscopic reversibility does not imply macroscopic reversibility.

Abstract

When a real fluid is expelled quickly from a tube, it forms a jet separated from the surrounding fluid by a thin, turbulent layer. On the other hand, when the same fluid is sucked into the tube, it comes in from all directions, forming a sink-like flow. We show that, even for the ideal flow described by the time-reversible Euler equation, an experimenter who only controls the pressure in a pump attached to the tube would see jets form in one direction exclusively. The asymmetry between outflow and inflow therefore does not depend on viscous dissipation, but rather on the experimenter's limited control of initial and boundary conditions. This illustrates, in a rather different context from the usual one of thermal physics, how irreversibility may arise in systems whose microscopic dynamics are fully reversible.