So, I suggest you think about which field of Fluid Dynamic application you are most interested in?
Do you like biology? If so I recommend you take interest in the application of Fluid Dynamics to the circulation system of humans/animals. A lot of fluid dynamic research is aimed at the medical industry so we can develop stents to be put in people's arteries to keep them open. These stents need to be designed with fluid dynamics in mind as to maximise the efficiency of the blood flow through the stent. Why do arteries collapse, from a fluid dynamic point of view? Etc
Are you interested in aviation? Then perhaps you'd rather look at the Fluid Dynamics as it occurs over the surface of a wing. How does the fluid dynamics over a wing produce a lift force which keeps the aircraft in the sky?
Are you interested in nature and animals? How do natural fliers (birds, insects, etc) use make use of fluid properties to propel themselves through the air? How do fish and sea-dwellers make use of fluid dynamics to propel themselves through the water?
Interested in civil engineering? What fluid dynamic issues must be taken into account when designing and implementing a bridge over open water in a particularly windy area? Not only is there the fluid dynamics of the air on the bridge, but also the fluid dynamics of the water on the supports. Or how about tall skyscrapers, and their design with regards to fluid dynamics?
What about automobiles? The hydraulic systems involved in the braking systems of cars, etc. What are is the involvement of fluid dynamics in the design of such systems? How do we change the design to deal with the specifications of the vehicle? (is it a pick up truck, or an F1 car?), how do these specifications affect the fluid dynamics concerns?
Or perhaps you're more interested in specific topics of fluid dynamics in general without specific application?
What about the Boundary Layer? The boundary layer is the result of the no-slip condition, which states that if a viscous fluid flows over a surface, the velocity at the surface is zero, and the velocity increases towards the free stream velocity the further you get from the surface? This layer of fluid between 0 velocity and free stream velocity is the boundary layer and it has many profound consequences in ALL of the topics I mentioned above ^^^.
What turbulent/laminar flows? What is the difference between a laminar flow and a turbulent flow? How do we tell when a flow will be turbulent and when it will be laminar? What affects this? (Hint: Look up the Reynolds Number!). How does a fluid change from laminar to turbulent and back again? How does the laminar regime and the turbulent regime affect the boundary layer? Again, this has profound consequences for all of the above applications ^^^.
How about... the speed of the flow? Supersonic flows (greater than the speed of sound), Subsonic flows (less than the speed of sound), Transonic flows (the transition between the two? What features do each of these flows have and how do they form? What affect does it have on a surface in the flow? How do we design things like aeroplane wings and space shuttle bodies to deal with the fact that they will be travelling supersonically, and will have to deal with the affects and features of supersonic flow? How do we accelerate a flow from subsonic to supersonic speeds? (Hint: Look up convergent-divergent nozzles!). What is the Mach number and why is it significant? What are the effects of low speed flows? What is separation of flow and what has it got to do with turbulence, the boundary layer, the mach number and the Reynolds number?
Feel free to add me to msn and we'll chat further. Mushet@hotmail.co.uk