You said what I was thinking! it reflects what I’m seeing when the pump is in operation. At lower frequency (for example as low as 1 Hz), the fluid moves much more per stroke than at high frequencies.
But of course there is a balance between how many strokes per second and amount of fluid displaced per stroke to get the highest flow rate (which is what the 80 Hz seems to be). As the frequency goes much higher than 100 Hz, the water doesn’t seem to have enough time to react just like you said, before the next cycle begins.
I hadn’t thought of the fact that the resonant frequency of the disk changes when clamped, very good point. I was getting the resonance frequency of 3.6 kHz from the disc data sheet, but this is most likely when the disc is not fixed to anything, and in contact with air only…
Final note, that other papers that deal with piezo-pumps also mention optimum driving frequencies of around 100 Hz.
Thank you again you’re a great help!