Although I haven’t posted anything in the past month, I have been working away at the AUV here and there between work and some much-needed vacation. I’m making some decent progress in design and setting up for manufacture. A couple of the big things I’m working on are some detailed design work on an acoustic modem, and putting some finishing touches on my CNC machines to start building some of the mechanical components for this project. I’ll post an update on those topics when I have some more substantial progress.
In the meantime, I was digging for some files and came across some earlier CFD results, specifically pertaining to the Doppler Velocity Log (DVL) and the flow of water around it. A DVL works by sending sonar pulses out at multiple off-nadir angles and measuring the doppler shift in the return signal. If the AUV is stationary, there is no Doppler shift, but if there is motion a Doppler shift is induced which can be measured to determine the motion of the AUV underwater in lieu of not being able to get a GPS lock.
Due to the relatively small diameter of the AUV, and the size of the DVL transducers, they needed to be pocketed in the hull. I ran some simulations to determine the way water would flow, and as expected there is some recirculation induced by the pockets. This not only adds drag but potentially increases flow noise on the transducer itself.
As such, I’ve modified the design slightly and will attempt to put a LDPE window to hopefully improve the flow. LDPE is a type of plastic which has an acoustic impedance fairly close to water, so should be mostly transparent to the sonar wave. Modeling the shape proved an interesting challenge, but haveing done so will enable to me to fairly easily cut out the shape from flat sheet using the CNC.
Hopefully, some more updates will be forthcoming in the next couple of weeks!