**3. Analysis techniques**

The above experimental or modelling investigations have to be analysed with suited tools in order to get information on flow instabilities. These consist mainly of signal processing techniques, which are applied to raw data, such as LDA recordings of the instantaneous velocity, in order to gain information on the characteristics of flow instabilities.

Two kinds of information have been extracted so far:


Among the techniques which have been employed in literature for the characterization of flow instabilities in stirred vessels, there are (see Fig. 1b):


Whereas the first two techniques have been largely used for the determination of the flow instability frequency, the latter has been used to evaluate the impact of flow instabilities on the motion through the analysis of the most energetic modes of the flow.
