Saturday, 26 June 2010

Campbell Diagram

Campbell diagram is widelly used in rotordynamics to plot eigenfrequencies vs rotating speed (RPM) but you can find it in other applications such as vibroacoustics too. In my proffessional career I've seen several types of Campbell diagrams, mainly used to plot interaction between eigenfrequencies and excitation force.

Let's look at Campbell plot of turbine blade.

On the picture are 3 eigen modes and each mode has also Low and High frequency. This is because of variability in production processes. Any two components you manufacture have some differencies because of tollearances and material properties. In this case Low and High frequencies show frequency spread of turbine blades. If you measure turbine with 11 blades, you'll get 11 blade frequencies. If you measure 10 turbines, each turbine 11 blades, you'll get 110 frequencies.

Mode 1 and Mode 2 is in interaction with all three orders, it means that our blades will be excited 3 times per revolution. Mode 3 is in interaction only with 3rd order, so Mode 3 will be excited only one time per revolution. In addition to that only Mode 3 lower frequencies are in interaction with 3rd order, but blades with frequencies higher then 30 kHz are not in interaction, which means that they won't be excited by first 3 orders. In that case we can say that Modes higher then 30 kHz are out of the running range.

See the more detailed article about Campbell Diagram here.

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