What are some scientific fields that require math/technical expertise but aren't as brutal as theoretical fluid dynamics.

So I finished a Master's degree in Engineering. I was supposed to do a thesis with this prof who is well known for his work in theoretical fluid dynamics.
I ended up not doing the thesis. His theoretical fluid classes were brutal and not very intuitive and I struggled a lot.
Right now I'm thinking of shifting to experimental fluids work. I do like math and science and I want to stay in the field, but i don't think I'm cut out for the more theoretical work. What do you guys think? What are some other fields I can consider?