Box fan, free spinning in the bed of a pickup - what would that do for drag ?

[Piotrsko]

It looks like a need for freebeards coanda nozzle

[M_a_t_t]

Originally Posted by Cd

I would imagine that to decrease a cars wake by blowing air into it, you would need a massive amount of wind.

Do you mean decrease, as in the size (area) of the wake, or decrease as in the drag from the wake?

[Cd]

"As an additional source of resistance, any chaotic, random turbulent air it's initially embedded within, may end up a little less energetic due to viscous shearing effects, however I don't know that they'd be significant enough to even measure."

In other words, are you saying that the air will have less velocity as it leaves the NACA duct and enters the wake due to viscous shearing ?
What is viscous shearing ? Is that what air smacking into a strong opposing force is ? ( In this case, that opposing force being the air spilling around the back of the car into the wake ? )

If there is a large amount of airflow at the back of a car moving in a certain direction ( the wake vortex ), and a small amount of air is added to it with the intention of 'pushing' it into a smaller wake, ( my NACA ducts ) wouldn't the large amount of air just overpower the incoming air and cause the two to merge ?

In other words, I think that may be what you mean ?

I'm beginning to think the NACA ducts I added may be useless.

But at least I like the look of them !

[Piotrsko]

If you mean you doubt that there is insufficient pressure gradient to flow through the ducts and do something,, then you are probably incorrect. The actual discussion should be will these ducts provide enough flow to stabilize or eliminate the vortex Caused by that fiaring.

[aerohead]

Originally Posted by freebeard

Depends on how you do it.

Kawanishi uses a 3,060-hp General Electric turbine engine for the blown slots on it's Shin Meiwa PS-1 and US-1 STOL aircraft.

[aerohead]

Originally Posted by Cd

"As an additional source of resistance, any chaotic, random turbulent air it's initially embedded within, may end up a little less energetic due to viscous shearing effects, however I don't know that they'd be significant enough to even measure."

In other words, are you saying that the air will have less velocity as it leaves the NACA duct and enters the wake due to viscous shearing ?
What is viscous shearing ? Is that what air smacking into a strong opposing force is ? ( In this case, that opposing force being the air spilling around the back of the car into the wake ? )

If there is a large amount of airflow at the back of a car moving in a certain direction ( the wake vortex ), and a small amount of air is added to it with the intention of 'pushing' it into a smaller wake, ( my NACA ducts ) wouldn't the large amount of air just overpower the incoming air and cause the two to merge ?

In other words, I think that may be what you mean ?

I'm beginning to think the NACA ducts I added may be useless.

But at least I like the look of them !

1) the sentence at the top has to do with flow through the fan in the back of the truck.If the fan was already submerged in turbulence, any turbulent air flowing through the fan would only gain in turbulence,as the fan would be akin to an aerodynamic torture chamber.
2) NACA submerged ducts are designed to harvest laminar flow, and then decelerate it within the diverging nozzle, while increasing static pressure.
3) A duct is a 'pipe' and suffers 'pipe losses' as a function of viscosity and Reynolds number effects.( if you've ever been in a chemistry lab, you may have seen a laboratory glass container of some solution under an automatic stirring machine, which is adding 'heat' to the mixture, via hydrodynamic shearing forces of the viscous solution ).
4) At the back of your car, all air entering the duct is turbulent boundary layer. By definition, it's all turbulence.
5) Kinetic energy in the turbulence can never experience pressure regain ( recovery ), all it's energy will eventually be converted to atmospheric heat, hundreds of feet behind the car. This is the 'friction drag' of your car.
6) The NACA duct is a ' rob Peter to pay Paul' proposition. You gain a 'jet' of energetic flow into the wake, while that same air is simultaneously 'robbed' from the outer flow, jeopardizing the boundary layer at the risk of triggering separation. ( Conservation of mass ).
7) So far, no researcher has been able to make any nozzle work without the addition of an added power source to 'run' the nozzle.
8) To add salt to the wound, you run the risk that, wake air will flow forwards through the duct, attempting to reach the very low pressure near the A-pillars and windshield header.
9) The base pressure may be 'low' with respect to the forward stagnation point, however, it may be a high' pressure with respect to the mid-section of the car.
10) The 'size' of the wake pales in significance to the 'pressure' of the wake. This is where streamlining is made or defeated.
11) Two really good examples of what to do are, the Renault Vesta-II and Mercedes-Benz Bionic Boxfish.