We investigate the hydrodynamic flow of strongly interacting Dirac electrons in a nozzle geometry, which can for instance be realized with graphene. We show that a nozzle can induce a transition from subsonic to supersonic flow. This transition causes a shock wave of the electrons downstream of the throat of the nozzle, which is a distinct signature of hydrodynamic transport. We demonstrate that this effect is visible in the voltage profile along the nozzle when applying a bias and thus represents a suitable experimental probe of the hydrodynamic regime. In particular, there is a section of the nozzle with pronounced negative local resistance and a discontinuity of the local voltage induced by the shock wave.
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