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JulianEdgar · 06-09-2020, 03:15 AM

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!)

06-09-2020, 03:15 AM	#26 (permalink)
JulianEdgar Banned Join Date: Nov 2017 Location: Australia Posts: 2,060 Thanks: 107 Thanked 1,607 Times in 1,137 Posts	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!)