Instabilities on a turbulent background

We present a review of several experimental results that concern the problem of hydrodynamic instabilities occurring in turbulent flows. The first experiment is related to the generation of a magnetic field by a turbulent flow of liquid sodium, i.e. the dynamo effect. We show how the bifurcation to the dynamo regime is affected by turbulent fluctuations. Above the dynamo threshold, we study the reversal dynamics of the magnetic field and present a model showing the respective contributions of deterministic and stochastic aspects. The second experiment is a nearly two-dimensional turbulent flow forced in a confined domain. We study a sequence of bifurcations that involve a large scale flow that develops over a turbulent background and displays random reversals. We emphasize both the similarities and differences with the magnetic reversal problem.