The idea:
You do not need the second resistor, there is enough direct coupling between the piezos.
For 1 - 100 km/h wind speed to be in the range -90 degrees to +90 degrees phase shift, the transmitter and receiver must be really close together. few cm actually,. The only way to get the wind to have an unobstructed effect is use reflections.
Test setup (sort of reproduces from a desk full of stuff):
I do measure changes, but PC fan is not really that fast, and the blades cause reflections by themselves.
Then as justification not to use it (grapes are too high sort of thing) well pinging, or 40 kHz carrier all the time, is not stealth. LOL
Still waiting for the BMP180 modules... (for differential pressure method).
Using acoustic reflections is not new, others do it too:
but look at the price...