round vs pointed.

[y2kbug]

explain to me: why a the front of a rounded object is more aerodynamic than the front of a pointed object?

[aerohead]

Originally Posted by y2kbug

explain to me: why a the front of a rounded object is more aerodynamic than the front of a pointed object?

Only in 'super-critical Reynolds number, 'low-speed' , 'incompressible', subsonic flow, does it work.
The air is accelerating from high-static pressure 'dead' air, towards a flow constriction where the flow is 'faster' and at 'lower' pressure ( high-to-low ) an ideal setup as far as flow goes.
Any other shape would have too little, or too much surface area to get the job done, with either non-optimum pressure, or non-optimum friction characteristics.
If you put a cardboard refrigerator carton, longwise, in a wind tunnel, and observe smoke flow approaching from ahead, you'll notice that the flow will automatically 'create' a 'phantom' bulbous nose of dead-air for the flow to follow.
Once you get above 250-mph, you enter 'transonic' flow, where the air begins to 'compress', and shockwave drag can initiate. Now your 'pointy' nose becomes more 'favorable.'
It's a Reynolds number phenomena.

[freebeard]

[aerohead beat me to it but here's mine:]

I'm not going to have the whole story, but it has to do with Reynolds number.

While bluff bodies are preferred at road speeds, as you get into a transonic regime, the pointed nose becomes important.

The air has to move sideways (or up/down). The angle of incidence seems to scale with the speed.

[Vman455]

There's some confusion in the replies above as to the definition of "transonic" flow etc.

"Transonic" refers to the speed at which the local flow velocity on some part of the vehicle reaches Mach 1 and shock waves start to develop (usually the upper surface of the wing on a plane with low sweep). In aerospace, we refer to this as the "critical Mach number" and for most aircraft it's around 0.78-0.9 (higher for low aspect ratio, high sweep aircraft; lower for high aspect ratio, low sweep). Airliners typically travel just higher than this speed but below what we call "drag divergence Mach number" where drag coefficient starts to rise as a result of these transonic effects.

Below ~Mach 0.3 we normally approximate flow as incompressible (note: this does not mean it is incompressible, only that the approximation is good enough to solve numerical problems with that assumption). At sea level and standard temperature, Mach 0.3 is just over 228 mph and goes down with altitude e.g. at 5,000 ft it's around 224 mph. Above ~Mach 0.3, numerical simulations must include density as a function of other flow parameters (and typically do even in basic CFD such as inviscid Euler flow models--with some notable exceptions such as the Athena Vortex Lattice solver. I'm actually coding a compressible Euler solver for one of my final projects right now). This is well below the speed at which transonic effects come into play.

Pointed noses are used to manage oblique shocks around the vehicle, as the shock angle is influenced by the angle of the disturbance i.e. the nose of the plane. We use nose shaping to try and put as much of the wing as possible in subsonic flow where it can make lift more efficiently. This only matters if the freestream flow is supersonic, as can be seen again on airliners and subsonic military transport aircraft which typically have rounded noses. At subsonic speeds, the rounded nose can support negative static pressures outside of the stagnation area which contribute a small thrust force, reducing drag--which applies to cars as well as aircraft.

[skyking]

The other factor is relative velocities. A simplification:
We live in a moving sea of air. When the object's velocity is so great that any cross wind is a small fraction, then that pointy nose will work as intended, typically in that transonic flow regime as outlined above.
However at road speeds, the air is rarely either still or not a crosswind. In a crosswind, the pointy thing sucks balls.
The rounded thing could give a crap. The air can approach from any direction and it behaves in a reasonably predictable manner.
It will detach on the lee side at some point, but the pointy thing? you might as well be forcing a pipe through the air at a 45 degree angle. It will be a muddy mess in the wind tunnel.
Aircraft solve this by truly living in that sea of air. Other than turbulence, they are not seeing the crosswind relative to the ground track. A wheeled thing on the ground? Not so much.

[freebeard]

However at road speeds, the air is rarely either still or not a crosswind. In a crosswind, the pointy thing sucks balls.

I'd thought of that but hadn't posted. IIRC, motorcycles with bluff noses have a single fence (like the ones NASCAR uses on car's roofs) to act as a wickerbill to the cross winds.

(I think I'll add that to my wooden geodesic motorhome design. )

[y2kbug]

Thanks for the help!

[aerohead]

Originally Posted by skyking

The other factor is relative velocities. A simplification:
We live in a moving sea of air. When the object's velocity is so great that any cross wind is a small fraction, then that pointy nose will work as intended, typically in that transonic flow regime as outlined above.
However at road speeds, the air is rarely either still or not a crosswind. In a crosswind, the pointy thing sucks balls.
The rounded thing could give a crap. The air can approach from any direction and it behaves in a reasonably predictable manner.
It will detach on the lee side at some point, but the pointy thing? you might as well be forcing a pipe through the air at a 45 degree angle. It will be a muddy mess in the wind tunnel.
Aircraft solve this by truly living in that sea of air. Other than turbulence, they are not seeing the crosswind relative to the ground track. A wheeled thing on the ground? Not so much.

From observation, the SAE adopted an annual average wind speed of 7-mph, as the statistical average mean wind speed, and from 'any' direction, and wind tunnel testing of road vehicles includes yawing the vehicle in the test section in order to reflect a 'crosswind-averaged' coefficient of aerodynamic drag.
In Hucho's chapter on Commercial Vehicles, one can see that the 'bulbous' nose on Class-8 and Class-7 semi's is least sensitive to crosswind effects ( Alexander Gustav Eiffel's, 1910, Parisian ice cream cone, falling down a wire from his tower laboratory, ice cream first ).

[Logic]

Originally Posted by y2kbug

explain to me: why a the front of a rounded object is more aerodynamic than the front of a pointed object?

I thought a new post was in order with videos that explain all this.
You should have a way better base for 'getting it' all form those.