Airflow Analysis by CFD

[200Volts]

OK, don't bust my chops for rough Prius model.
I had a 2 week software tiral from work to learn Solidworks and Cosmos Floworks, make a 3D model of the Prius and do CFD modeling and outputs ( at night after my full time job).

What you are looking at took about 3 hours of processing time (per run) on my 2.4 GHz gaming machine to do the CFD calculations(only) on about 20,000 grid cells on and around my 3D model. Some 3D images show clipping due to memory constraints on my PC.

Calculated pressures are shown in colors (nominal pressure 14.5psi and velocity vectors (75mph oncoming wind) are the arrows(longer arrow means faster flow).

I think this was pretty close to accurate as shown by:
1-Flow is curling up around the back-top of the car, similar to streaks left by dew after a short drive.
2-Windshield flow in front of the driver is about a 45 degree angle, similar to flow seen during rain.
3-Only subtle pressure increases and decreases are shown over most of the body, indicating mostly laminar flow (good aerodynamics).

Suprises are:
a- The rear window-rear spoiler shows a small higher pressure zone (exactly where the BT Tech spoiler goes).
b- A higher pressure zone is located between the rear wheels.
c-The flow curling around the two rear sides-rear window area appear to meet exactly as they exit the back of the car (like a perfect vortex generator).
d-Flow exiting the rear top and and rear bottom seem very symetrical.

Expectations are:
i)- The lowest pressure zone on the top of the car is from the driver area back to rear window ( suprise is the ulta low pressure area exactly where the antennae is located-lowest drag).
ii)air does flow under the front of the car
iii) air flows horizontally around the front bumper area to the sides.

I don't pretend to know it all, but I hope this helps our understanding of what's going on.

[Frank Hudon]

interesting! to say the least, the area of high pressure behind the rear wheel could that be caused by the air exiting thru the wheel opening and the wheel building up in that area? in laminar flow the air flow is tightly on the surface and the air coming out of the wheel opening is unable to break the flow and there by build in that area?

[V8Cobrakid]

cool... so on the ( i think '05 and '06" they put that extra little black piece ont he front to make more air pass over the front? i know the nose of my car isn't as bold as the newer ones.

low air between the rear wheels and the back spoiler is. so extending it helps eliminate that low pressure.. does low pressure create lift?

if it does create lift we are getting it in the back of the car... so the bt spoiler helps bring the back down by giving it more of that ... is it called an air foil?

what would happen if there was a little "skirt" type of thing installed in front of the rear axle? i wonder what that would do....

if i'm completely wrong, please correct me.. lol.. it's late and i'm sleep deprived. I'm eager to learn though.

[robinsod]

Originally posted by 200Volts@Mar 17 2006, 02:11 AM
OK, don't bust my chops for rough Prius model.
I had a 2 week software tiral from work to learn Solidworks and Cosmos Floworks, make a 3D model of the Prius and do CFD modeling and outputs ( at night after my full time job).

What you are looking at took about 3 hours of processing time (per run) on my 2.4 GHz gaming machine to do the CFD calculations(only) on about 20,000 grid cells on and around my 3D model. Some 3D images show clipping due to memory constraints on my PC.

Calculated pressures are shown in colors (nominal pressure 14.5psi and velocity vectors (75mph oncoming wind) are the arrows(longer arrow means faster flow).

I think this was pretty close to accurate as shown by:
1-Flow is curling up around the back-top of the car, similar to streaks left by dew after a short drive.
2-Windshield flow in front of the driver is about a 45 degree angle, similar to flow seen during rain.
3-Only subtle pressure increases and decreases are shown over most of the body, indicating mostly laminar flow (good aerodynamics).

Suprises are:
a- The rear window-rear spoiler shows a small higher pressure zone (exactly where the BT Tech spoiler goes).
b- A higher pressure zone is located between the rear wheels.
c-The flow curling around the two rear sides-rear window area appear to meet exactly as they exit the back of the car (like a perfect vortex generator).
d-Flow exiting the rear top and and rear bottom seem very symetrical.

Expectations are:
i)- The lowest pressure zone on the top of the car is from the driver area back to rear window ( suprise is the ulta low pressure area exactly where the antennae is located-lowest drag).
ii)air does flow under the front of the car
iii) air flows horizontally around the front bumper area to the sides.

I don't pretend to know it all, but I hope this helps our understanding of what's going on.

[snapback]225786[/snapback]

Thanks for sharing your hard work. I learned a lot. Good job.

[TheForce]

From the looks of the pictures it explains how the rear bumber gets very dirty when the rest of the car is almost spotless.

[hdrygas]

Very Nice. Very Nice indeed. Explains why my back up cam is dirty. What about mud flaps. My daughter has access to a quad processor Mac, would that help.

[200Volts]

Hdrygas, Thanks for the offer but the 2 week software trial is over.

[Bill Lumbergh]

Fantastic model! Should you decide to purchase the software, you can do Hummer H2 model very easily. Hint, start with a perfect cube and remove the area over where the hood would be, then suspend it on four slightly smaller cubes to mimic tires. You should be able to run the model after a whole 30 seconds of work, and it should be pretty accurate. :P

[Sufferin' Prius Envy]

Originally posted by V8Cobrakid@Mar 16 2006, 11:42 PM
.. does low pressure create lift?

if it does create lift we are getting it in the back of the car... so the bt spoiler helps bring the back down by giving it more of that ... is it called an air foil?

[snapback]225799[/snapback]

The airfoil design is what give a wing, and a Prius, lift. An airfoil is a shape which is curved on one side and flat on the other side. The air which travels past the curve has a longer distance to travel than the air which passes the flat side. When the airfoil is moved through the air as designed, this causes there to be more molecules of air per square inch of surface area on the flat side and, in relation, less molecules per square inch on the larger curved side. That causes a pressure difference. The bottom of the wing, with its higher pressure is pushing the wing up, and the top, with its lower pressure is sucking the wing up . . . together this is lift.

. . . does low pressure create lift? On a car it depends where the low pressure is located. If it is on top, then it is fighting the effects of gravity and is called lift. If it is on the bottom, the car is being sucked down toward the ground, and is known as “ground-effects” or downforce. This is good for formula-one race cars which need to make sharp turns at high speeds, but not so good for a passenger car because it takes tremendous amounts of power to move a car which is essentially sticking to the ground.
If the low pressure is behind the car, it is actually sucking the car backwards, and is known as drag.

The holy grail of car design would be to swap the low pressure at the back with the high pressure at the front. Your car would then be blown and sucked forward, and, if the car were not a perpetual motion machine, energy consumption would be minuscule.

The BT spoiler helps bring the rear down by “spoiling” the airfoil design at that point, thereby decreasing the lift and exchanging it for more drag. Not a good thing for MPGs, but theoretically better for Prius stability.

[donee]

Originally posted by Sufferin' Prius Envy@Mar 18 2006, 06:35 AM
The airfoil design is what give a wing, and a Prius, lift. An airfoil is a shape which is curved on one side and flat on the other side. The air which travels past the curve has a longer distance to travel than the air which passes the flat side. When the airfoil is moved through the air as designed, this causes there to be more molecules of air per square inch of surface area on the flat side and, in relation,  less molecules per square inch on the larger curved side. That causes a pressure difference.  The bottom of the wing, with its higher pressure  is pushing the wing up,  and the top, with its lower pressure is sucking the wing up . . . together this is lift.

. . . does low pressure create lift? On a car it depends where the low pressure is located.  If it is on top, then it is fighting the effects of gravity and is called lift. If it is on the bottom, the car is being sucked down toward the ground, and is known as “ground-effects” or downforce.  This is good for formula-one race cars which need to make sharp turns at high speeds, but not so good for a passenger car because it takes tremendous amounts of power to move a car which is essentially sticking to the ground.
If the low pressure is behind the car, it is actually sucking the car backwards, and is known as drag.

The holy grail of car design would be to swap the low pressure at the back with the high pressure at the front. Your car would then be blown and sucked forward,  and, if the car were not a perpetual motion machine, energy consumption would be minuscule.

The BT spoiler helps bring the rear down by “spoiling” the airfoil design at that point, thereby decreasing the lift and exchanging it for more drag. Not a good thing for MPGs, but theoretically better for Prius stability.

[snapback]226385[/snapback]

Hi Sufferin,

While that is the common theory of wing lift taught in secondary schools, NASA teaches its incorrect in its extensive K-12 web site (http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html) .

They show with models based on results from CFD analysis that the lift is not caused by low pressure, but by the rotating of the direction of air flow off the top of the wing. The resulting mass of the air thrust downward creates a reaction force of the wing upward they say. As evidence they point out that some of the best wings have are just curved plates, with nearly the same topside as downside surface areas. Also, even symetrical air-foils create lift.

This may help explain while in racing reduction of aerodynmic form drag has not be wildley successful, but increase of downforce has. Air thrust upward goes off into the sky unecumbered creating a great downforce. Air thrust downward hits the road and changes direction suddenly creating drag. Aircraft even with all those wings and control surfaces have have Cd under .02 typically, while the Prius - one of the best cars - is at .26 (http://www.aerodyn.org/Drag/tables.html).

The socalled Kamm (Prof. Kamm was a german aerodynacist in the 1930's)tail which the Prius and BT Spoiler are based on can be designed to minimize drag or spoil lift, depending on the details of the design. Typicially in road vehicles the goal is to reduce drag. This is done by providing some resistance to the flow over the top of the car thus preventing flow delamination and turbulence. Tubulence creates increased drag. Another common way (but not for cars) that area of turbulence is reduced is the dimples on a golf ball. This phenona is difficult to model due to the mathematically kaotic (Kaos Theory) nature of turbulence. Turbulent flow may not be modeled by 200Volts program. Possibly he knows more about this. That red zone on the rear of the Prius in 200Volts models is probably where turbulent flow is occuring.

If one could take air from behind the radiator and with low drag guide it into that red area at the rear of the car, that would probably be the next step in reducing drag.