Measured aero pressures on Insight

[JulianEdgar]

Originally Posted by cr45

This looks very promising indeed and stagnation pressure is now where it would be expected to be.

I decided to have a look at the sensitivity of pressure to temperature change for a pressure box.

Using Gay-Lussac's law for a closed container then P2/T2 = P1/T1

Hence P2 = (T2/T1) * P1

Lets assume that the atmos pressure is 101325 Pa and the temperature is 10 C.

Also lets assume that the temperature of the air rises by 0.1 C by the time you take your aero measurements.

The new value for the pressure in the box will be

P2 = ((10.1 +273.15) / (10.0 + 273.15)) * 101325 = 101361

That is a difference in pressure of 36 Pa which is very significant considering the magnitude of the pressure differentials you are trying to measure.

That much!

With the reference tank you can get it so that there's very little drift, but you need to first let the car's interior settle a lot in temperature, reset the tank pressure to atmospheric and then quickly do the test. Before then repeating the process for subsequent tests. (I also ended up insulating the tank.)

But I think the pitot tube ref pressure is better in every way - cheaper and easier for people to put together, too.

As I have shown above, the stability of the reading also gives me enough confidence to measure very small changes (eg changes in wake pressures).

I think your suggestion genuinely takes on-road aero pressure measurement to the next level. I am very excited by the possibilities that are now open.

[JulianEdgar]

All explained - using a pitot tube as the reference when measuring panel pressures

https://www.youtube.com/watch?v=WAg2...ature=youtu.be

[Daschicken]

Originally Posted by JulianEdgar

I have some lip-style rear spoiler molding coming to make the proper separation edges (like this)



When it arrives - two to four weeks I imagine - then I'll do a bunch of measurements. My guess is that the separation edges will impact only the pressures near the edges, not generally within the wake.... but we will see!

Quick and easy trip strips! I love it. Thanks for the idea. I'll be looking forward to your results, if I can resist just buying them anyways and trying them out.

[JulianEdgar]

Rob Palin (ex Tesla aerodynamicist) has just given me a fascinating idea with regard to separation.

So I will be doing some more testing today.

But whether I can achieve what he suggests is another matter....

[JulianEdgar]

So that testing was interesting.

Rob's point was that if the boundary layer is thicker, the strength of trailing vortices at the point of separation are likely to be less strong, as the velocity gradient will be reduced.

So I wondered if thickening the boundary layer prior to the separation edge would result in a higher pressure (ie be improved) behind the separation edge.

Test:

• Honda Insight
• 12 degrees C
• light winds
• 80 km/h
• two way average
• 0-120 Pa Magnehelic
• Pitot tube 2.2m above ground as static reference
• Pressure sensor located just inside wake from separation edge
• One one side tested - other side standard

Separation edge alone: -14 Pa. (Note 1: a different separation edge from the one tested yesterday - smaller, reflecting what will actually be used. Note 2: the same pressure as recorded yesterday - new sep edge works as well as yesterday's, and very repeatable data)

I'd thought I would see if I could thicken the boundary layer with the disruption of a line of egg cartons.



Vertical line of egg crate carton plus separation edge, 80cm forward of separation edge: -18Pa

Vertical line of egg crate carton plus separation edge, 50cm forward of separation edge: -17Pa

Vertical line of egg crate carton plus separation edge, 20cm forward of separation edge (as pictured): -33 pa

So the egg cartons didn't have an impact until they were close to the separation edge, whereupon they dramatically decreased pressure (the opposite of what we want).

I then thought I'd try some Airtabs. (You'd expect to them to make things worse if they inject momentum into the boundary layer.)



Three Airtabs plus separation edge, 20cm forward of separation edge: -18 Pa

Three Airtabs in reversed orientation, 20cm forward of separation edge: -21 Pa

So the Airtabs basically made no difference to the measured pressure.

As a reminder, yesterday adding the separation edge increased the measured pressure from -30 to -17, so a dramatic improvement.

[JulianEdgar]

I tested a box cavity via pressure measurements in the wake.



Test:

• Honda Insight
• 14 degrees C
• light, slightly variable winds
• 80 km/h
• two way average
• 0-120 Pa Magnehelic
• Pitot tube 2.2m above ground as static reference
• Pressure sensor LHS, centre and RHS at bumper level

Without box cavity:

Centre: -18 Pa
LHS: -22 Pa
RHS: -20 Pa

Average: -20 Pa

With box cavity:

Centre: -16 Pa
LHS: -15 Pa
RHS: -18 pa

Average: -16 Pa

Now how significant is this? I am not sure. On the one hand you can say that the pressure was increased by 20 per cent - so that's excellent.

On the other hand, looking at the wake area (about 0.85 m^2), the actual force changes like this:

At 80 km/h, without box cavity, calculated wake drag = 1.7kg
At 80 km/h, with box cavity, calculated wake drag = 1.4kg

I doubt if a change of drag force of 300g at 80 km/h is significant - but perhaps it is?

But....

If the 20 per cent change also occurs on cars with big wakes, then it would become quite significant.

and...

This drag force will rise at the square of the speed, so at constant high speed, the drag force change (in absolute terms) of having the extensions will be much greater.

All very interesting.

(And yes, I am now much less skeptical of box cavities!)

[niky]

Was wondering, have you thought of testing the separation edge with the air tabs alone (yes, realizing they also might not have a positive effect there, but still...)

[JulianEdgar]

Originally Posted by niky

Was wondering, have you thought of testing the separation edge with the air tabs alone (yes, realizing they also might not have a positive effect there, but still...)

Yes I did do this:

Three Airtabs plus separation edge, 20cm forward of separation edge: -18 Pa

Is that what you meant?

[niky]

Originally Posted by JulianEdgar

Yes I did do this:

Three Airtabs plus separation edge, 20cm forward of separation edge: -18 Pa

Is that what you meant?

Oh, my bad! What I meant was testing the pressure without the modified edge... on the stock bumper.

[JulianEdgar]

Originally Posted by niky

Oh, my bad! What I meant was testing the pressure without the modified edge... on the stock bumper.

No I probably wouldn't bother doing that. The separation edge works well, so Airtabs would have to improve things over having just the separation edge - and they didn't.

(I am first a car modifier, and so I want to do things that work on my own car.)

Remember all this testing equipment costs less than US$75 total - so anyone can do it to find out anything they want. (And on the car they wish to modify, which is best of all.)