Window screen grille block?

[jthistle]

.13 is designated for mites and other insects smaller than a flea, and has en effective coverage area of about 34%

Just so I understand the 34% coverage area is equivalent to a free hole area of 66%. So the lower end of the table I posted the 25% free hole area mesh will have a coverage of 75%. Am I correct in interpreting your response?

BTW, that is a trick challenge, and I'll even tell you why. Ive already done it, it made 0 improvement on my 99 Ranger XLT

What coverage mesh did you use for your testing?

This is what I am going to use for my test.


I am going to epoxy a square of plexi-glass with a 3 inch circular cut out in the center to the face of the anemometer. In the corners of the plexi there will be holes so I can bolt the stainless steel mesh test pieces to the plexi.

I will then have a friend with a long arm hold it out the window as far as he can to get away from the turbulence of the car and take measurements as I try to hold speeds.

There will be several sources of error in the test.
1) Air will be deflected from my car and create turbulence in the face of the meter.
2) Ambient wind will conflict with the measurements
3) The meter itself has a measurement error of +3%
4) Holding the meter steady at higher speeds with be difficult. There will likely be a rocking back and forth of several degrees.
5) Air flow around the meter will create a low pressure area behind the meter which may help draw air through the mesh.
6) The speed of the car will vary around the test speed.
Three are probably even more sources of error that I am not thinking of right now.

I am hoping that by measuring uncovered and covered measurements and doing many 10+ measurements for each speed I will be able to average out alot of the error.

[California98Civic]

Originally Posted by justjohn

What has more drag, a solid wheel cover or one with holes in it?

Drag is important, and it is our normal obsession. But I would trade marginal drag increases in my grille blocks if it meant significantly reduced radiator-fan run times.

[jthistle]

When you have a solid grill block does all of the air reflect directly off of the block or do you get a build up of air in front of the block that deflects the incoming air?

[justjohn]

Excellent work mort! That study is definitely more applicable to what we want to know. It would still be be nice to have a single screen with very small openings tested at full highway speeds, but I think it's a pretty safe bet this estimate is reasonably accurate:

Originally Posted by mort

Window screen might have a Cd of about 5 for highway speeds. Or 5 times worse than a flat plate.

I also found it interesting how drastically the Cd drops with more open screens. The cliff notes for people who can't be bothered to look at the study:

At 47mph single screen
Open area/Cd
34% ~4.8
47% ~3.0 and barely changes with speed
57% ~0.8 and barely changes with speed
61% ~1.0 and barely changes with speed
67% ~0.4 and barely changes with speed
77% ~0.2 and barely changes with speed

[justjohn]

Jthistle, look forward to the test. Do you have a rack on top of the car you could mount to perhaps? If you could come up with a way to hold the screen and measuring device completely stead it would help your results a lot I'm sure. Plus your friend is going to have REALLY tired arms by the end of the day, lol.

[jthistle]

One of the issues with these articles we are finding is the air must flow through the mesh whether it is in a clear pipe like the vent hood article or in a rectangular wind tunnel like the dissertation. So because the air cannot go around as the free hole area decreases the drag is going to increase. If you take this to the limit and have a solid sheet your drag will be infinite because no air will pass. In the case of a grill block the air has the option to flow around the obstruction i.e. up and over your car dramatically reducing the drag. If the test setups in either paper had vents in the structure immediately before the mesh that would give the air someplace else to go then we could directly compare.


As for the test setup I do not have a roof rack. I have not come up with a good way to stably mount the meter. A good solid mounting that could be rotated would certainly be useful for testing different angles. Though I am most interested in vertical because that is how the grills on my xB are. Perhaps the handle of the meter could be epoxied into a PVC pipe and at the other end a T with extensions to give leverage for keeping it straight.

That would certainly give me more separation from the vehicle.

[justjohn]

Originally Posted by jthistle

One of the issues with these articles we are finding is the air must flow through the mesh whether it is in a clear pipe like the vent hood article or in a rectangular wind tunnel like the dissertation. So because the air cannot go around as the free hole area decreases the drag is going to increase. If you take this to the limit and have a solid sheet your drag will be infinite because no air will pass. In the case of a grill block the air has the option to flow around the obstruction i.e. up and over your car dramatically reducing the drag. If the test setups in either paper had vents in the structure immediately before the mesh that would give the air someplace else to go then we could directly compare.

Excellent excellent point.

It would be very nice to find some tests where airflow was allowed to go around and some with angled screens since many grill block will be at slight angles to the airflow.

As far as the mounting setup, that would probably help quite a bit but I do foresee one problem. At highway speeds it's going to be quite a bit of pressure. If you triple the length of the lever you'll need 9x the force to hold it in place. You may have to have the apparatus extend well into the car to be able to hold it in place at full speed.

[mort]

Hi jthistle,
I think you misunderstand the value of wind tunnel (or other sources) findings for coefficients. On the front of the car is already some structure, replacing that will alter the drag by almost exactly the ratio of the previous Cd to the replacement's Cd. If the grille had a Cd of 0.2 and the replacement grille block is just a flat plate it has a Cd of about 1, then the additional drag will be about 5 times greater than the old grille. If the replacement has a Cd of 5 it will be 5 times worse than the plate. Now we can't say exactly how much drag the original grille produces without fancy testing, but we can say that other structures will produce more or less drag proportional to the change in Cd.

Coast down testing is a quick and easy way to test, especially grill blocking. I look forward to your test results too.

Here, California98Civic says,

Drag is important, and it is our normal obsession. But I would trade marginal drag increases in my grille blocks if it meant significantly reduced radiator-fan run times.

I know what you mean. We all want to know, and in particular for our own cars, which modifications save fuel.

-mort

[jthistle]

Well it could go behind the front seats and out the front window. That way it would pivot on the door frame and be blocked by the seats. I am limited as to how far it can stick out the window the cable connecting the 2 parts of the meter is only 120mm (3.9 feet).

[justjohn]

Originally Posted by jthistle

Well it could go behind the front seats and out the front window. That way it would pivot on the door frame and be blocked by the seats. I am limited as to how far it can stick out the window the cable connecting the 2 parts of the meter is only 120mm (3.9 feet).

Yeah, that could work pretty well as long as the angle is adjusted for so that the wind is still coming head on.