Side pressures and attached flow

[JulianEdgar]

My Edgarwit air curtains give a measured reduction in drag (using the throttle stop method) and tuft testing shows slightly better flow attachment on the side panels behind the front wheels.

My measured pressures without, and then with, the air curtains were as follows:





...giving this percentage increase in pressures:



But does an increase in pressure actually indicate better flow attachment? After all, more air and faster speed (eg under the car) causes a lower pressure.

I went to all my references to find out about side pressures but this data is almost impossible to find. I do have diagrams for a 1970s Mercedes sedan (and the lines of equal pressure diagram shows lots of variation on the side of the car) but the numbers are too small to read! Lots of CFD images of car pressures just show the side of the car all the same colour - either there's not enough resolution or they're not right, based on the Mercedes data and my measurements.

So, if there is improved flow attachment, do side surface pressures rise or fall?

I decided to do a test on our Mercedes. Rather than set up all the measuring pucks and the EvoScann logger, I used a single puck, static pressure ref provided by a pitot tube, and Magnehelic gauge. That is, total cost of measuring gear under US$100.

Previously with the EvoScann I measured -91Pa in the centre, lower part of the rear door. Today with the Magnehelic, I measured -73Pa (gusty wind, hotter day).

Now, what would happen if I deliberately caused separation in front of this point on the car? I attached a device in front of the area, a device which I was sure would cause separation.



The measured pressure decreased to -103Pa.

So I am fairly confident in saying that when you are setting up air curtains, and possible even wheel faces, the higher the pressures you can measure on the side of the car, the better. That makes trialling different cardboard (etc) designs much more straightforward.

[AeroMcAeroFace]

How much do you think (although of course there is no way of knowing without testing) of the benefit of your air curtains come from the increased surface pressures pulling back less, on the tapered sides of the car and how much do you think is thanks to better flow attachment causing better wake patterns?

I suppose my question is, all other things equal, on a non-tapered car would the resultant drag decrease be the same?

[Ecky]

Finally summer in the southern hemisphere, so we get some more hard data posts from Julian!

Approximately what does this add up to in Cd? Any chance of a coast down test?

[AeroMcAeroFace]

Julian will tell you that coastdown tests are too unreliable, he may have some cD data though.

Originally Posted by JulianEdgar

Yes, fractionally. But the testing was of total drag (not just Cd) and it showed a reduction.

[JulianEdgar]

Originally Posted by AeroMcAeroFace

How much do you think (although of course there is no way of knowing without testing) of the benefit of your air curtains come from the increased surface pressures pulling back less, on the tapered sides of the car and how much do you think is thanks to better flow attachment causing better wake patterns?

I suppose my question is, all other things equal, on a non-tapered car would the resultant drag decrease be the same?

I don't know!

As far as I can find, there is almost no coverage in the tech literature as to the mechanism by which changed side pressures/flow influence drag reduction.

Obviously early separation down the side of the car will enlarge the wake, and via tuft testing, you can see that occurring on some cars (eg from behind the rear wheels). But that doesn't answer the question about cars that have ostensibly attached flow but at lower side pressures.

Aerohead's theory that the pressures of the flow at separation imparts a pressure that's reflected in wake pressure doesn't seem to be supported, although I can see these pressures may influence wake pressure. It seems to me that wake pressures are much more influenced by vortex behaviour, which just leads us back to the question of how changed side pressures influence vortex behaviour.

Perhaps the higher pressures at side separation give less of an abrupt pressure change to the wake, resulting in weaker vortex formations? (But that seems a bit chicken and the egg.)

I've asked my experts (different thread) about the relationship between trailing panel pressures and wake pressures, but only one has got back to me - not a good time of the year to be asking.

So if you find anything in the tech lit, please let me know!

[JulianEdgar]

Originally Posted by Ecky

Finally summer in the southern hemisphere, so we get some more hard data posts from Julian!

Approximately what does this add up to in Cd? Any chance of a coast down test?

I've been doing a lot of aero testing over the last 6 months, so posting quite a lot of hard data.

I am confident that amateurs doing coastdown testing, unless perhaps done only between two very high speeds, does not give valid results.

Refer to SAE papers 950626 - ABCD –An Improved Coast Down Test and Analysis Method and 940420 - A Detailed Drag Study Using the Coastdown Method to see the level of complexity required to get valid coastdown results.

Or just do a series of tests, windows up / windows down in both directions - you should of course see a marked increase in drag with the windows down. I have never been able to detect this via coastdowns. On the other hand, throttle-stop testing sees this change every time.

The throttle-stop testing showed a change in drag with the air curtains of about 5 per cent. I never work from those figure to a proposed change in Cd, as I think such an extrapolation is too unreliable.

[freebeard]

Aerohead's theory that the pressures of the flow at separation imparts a pressure that's reflected in wake pressure doesn't seem to be supported, although I can see these pressures may influence wake pressure.

Do I detect a change in the weather? If a spoiler and front airdam interact, that interaction has to have some pathway.

I once coasted from the top of the Siskiyou Pass to the first Ashland exit (IIRC some 30 miles) On such a course, might not elapsed time to markers give an averaged result? Downside would be the 60-mile roundtrip between passes. ...and the corners.

[JulianEdgar]

Measured pressure at puck (just behind wheelarch spat).

Without spat: -65Pa
With spat: -53 Pa

Again, pressure rose with better flow attachment.

[Vman455]

Originally Posted by AeroMcAeroFace

How much do you think (although of course there is no way of knowing without testing) of the benefit of your air curtains come from the increased surface pressures pulling back less, on the tapered sides of the car and how much do you think is thanks to better flow attachment causing better wake patterns?

I suppose my question is, all other things equal, on a non-tapered car would the resultant drag decrease be the same?

Now that's interesting!

I measured pressures down the side of the car and in the wake with air curtain ducts (front and rear, but a different design from Julian's, made of sheet metal). I found no change on the doors and wake (both directions showed exactly the same pressure with and without ducts), but a +10 Pa difference on the (tapered) bumper cover behind the rear wheels. Fuel economy testing suggests that overall the ducts are reducing drag slightly; I wonder if this is how?

[JulianEdgar]

https://www.youtube.com/watch?v=9DepKM51OaE