Recap of last week
-Frame/Forks/Wheels: Sanding and priming the battery box and oil tank.
-Electrical: Figuring out the circuit of the bike. Model circuit. Need to start laying out the wires and start making connectors.
-Clutch: Taking exploded pictures of parts to get ready to set up the bike. Cleaning up parts.
-Bottom End: Finished inventory. Laying everything out and finding missing parts. Glen replaced one of the gears.
-Top End: Exploded view of parts. Missing two small springs.
Prof. Martinelli
Current Work: Interest in aerodynamics and fluid flows. Did a lot of sailing. Specializes in external aerodynamics. Deciding shapes that minimize drag. Optimal aerodynamic performance: Wings. Key is to achieve performance goals, not to delve into the specific physics and theory. Went from unsophisticated pen and paper models and formulas to much more powerful computer models that can perform complex calculations of a series of non-linear partial differential equations.
Initial assumption was that velocity flow was a potential field. Aerodynamics and Electromagnetism very similar in some regards.
Involved in the development of numerical methods and their application to computational science with regard to aero and fluid dynamics. Different bottlenecks: e.g. not enough knowledge on the physics, or not enough knowledge on the numerical methods, inability to put a system together.
Worked with blended wing body, large twin engine. E.g. Beechcraft, Embraer-180. Also for boats. America’s Cup.
Questions:
Downforces on a car: Front Wing, Rear Wing, Diffuser on the bottom of the car. Diffuser uses the bottom of the car to form a Venturi with the ground. The Diffuser is mainly the design of the curve of the exit at the back of the car bottom.
Venturi: Pipe with area changes. Area of pipe decreases and then expands. Area of intake different from area at throat.
Mach number: Relation between the velocity and the speed of sound.
When Mach number < 0.28 , flow is separated from the thermodynamics.
When Mach number 0.3< M < 0.5, flow is affected by thermodynamics.
When Mach number 0.5< M < 0.9, flow is transonic
When M > 1, supersonic.
Mass flow rate has to remain constant (when density is constant). When the velocity of the fluid is faster, the pressure is lower beneath the car and therefore the car is pulled down.
Same applies for airplane wings. Airflow faster at top of wing (depending on angle of attack) and therefore suction on the top of the wing generates lift.
Carburetor works the same way. Venturi system creates area of low pressure through which fuel is injected.
Drafting: When a fluid flows over a body, it is most desirable for the streamlines to be completely reflected and for the pressure to return to the same after the object passes through, and there is minimum drag. However, streamlines are disturbed at the back end of the flow and there is significant drag. Dimples in golf balls allow streamlines to flow better and decrease form drag, but increase viscosity drag. When drafting, the velocity of the air after the first car is very low, so the second car enjoys less resistance.
For a truck, the drag region is about the same as one truck length.
For bikes, the resistance is decreased for those in the pack compared to the leading riders.
Tips of airplanes (or birds’ wings) have a strong tip vortex on the tips of the wings. These create a drag on the airplane. When birds or planes are in formation, the tip vortices cancel each other out as they are spinning the opposite ways. Winglets also reduce vortex tip drag.
The way that motorcycle riders position themselves is also to reduce form drag.
Choked flow: In the intake flow the air is colder, and the speed of sound is less. The intake pressure differential is also much less than the exhaust pressure differential. Therefore the intake valve tends to be much larger.