Multi-channel high speed pressure measurement

[JulianEdgar]

Originally Posted by freebeard

It might be interesting to put pucks on either side of [one of] the vertical fins.

I expect to come back to the Insight in great detail later. Just trying to get some 'as-driven' data happening at this stage.

Should be able to do our W212 Mercedes sedan tomorrow, and maybe also our T5 Volkswagen tray-top.

Handling dozens of metres of silicone tube as you move it from car to car is really hard, so I made this today:

[COcyclist]

Originally Posted by JulianEdgar

Nice. Has your wife figured out why her favorite cutting board went missing?

Seriously though, I appreciate the time and effort you are putting into developing these testing methods and your willingness to share them here.

[JulianEdgar]

Went to do the W211 Mercedes and ran into a problem - some of the hoses aren't long enough!

I thought I'd provided plenty of hose for each puck, but on the Merc, the hose to the rear undertray sensor cannot go under the car as there is a big exhaust pipe hanging down. So you have to go over the car and then under, and that takes a few more metres of tube!

I'll have to buy some more silicone tube - comes from China, so unless I can find a local supplier, there will be delay of a few weeks (at least).

[JulianEdgar]

I have just tested my Honda Insight in three different rear-end configurations:

• no spoiler and fins
• with spoiler but no fins
• with spoiler and fins

The car:



The results:



The groups of bars show how many reading locations were taken - so three locations across the spoiler (LHS, middle, RHS), two in most other regions (LHS and RHS) and just one at the centre-forward part of the roof.

As can be seen, the presence of the spoiler alone dramatically increases pressures immediately in front of it, with its influence occurring as far forward as the front of the roof!

In every location on top of the car, the presence of the fins improves the action of the spoiler.

Highest pressure in the wake occurs with no spoiler or fins, but the presence of the fins increases base pressure a little over having only the spoiler.

(Note that my overall drag measurements show the fins + spoiler reduce drag, so the increased pressures on the rear window are more than making up for the reduced pressure in the wake. With the spoiler fitted, the wake is the same size as the standard car - ie the trailing edge of the spoiler is no higher than the standard car.)

[JulianEdgar]

This time just showing averages, with red when equipped with spoiler and fins, and green without.

[Vman455]

Wow, that is a dramatic difference. And nice to see corroboration of what I found on my car: that a spoiler at the very rear can affect flow all the way up the roof (on a fastback, but I wouldn't be surprised if the same is true on a modern notchback).

[JulianEdgar]

Originally Posted by Vman455

Wow, that is a dramatic difference. And nice to see corroboration of what I found on my car: that a spoiler at the very rear can affect flow all the way up the roof (on a fastback, but I wouldn't be surprised if the same is true on a modern notchback).

Yes, it was because of your measurements that I ran that one puck right at the front of the roof. One always reads about flow effects ahead of the change in shape, but these measurements really show that.

[serialk11r]

Wow, I always had a feeling vertical fins did something (OEMs are fond of them), but this is good confirmation. On most cars they're not pretty, but can't argue with the results.

[freebeard]

As fins go these are pretty good. Heated and bent to follow the window's edge might work as well as look better. Kamm-style slots? Rounding the edge and buffing it out would look twice as good, right?

[JulianEdgar]

Originally Posted by freebeard

As fins go these are pretty good. Heated and bent to follow the window's edge might work as well as look better. Kamm-style slots? Rounding the edge and buffing it out would look twice as good, right?

Ah, well if anyone could explain how Kamm style slots actually work, I'd be very interested.

As to the rest - I drive a Honda Insight. Why would I care what people think of my aesthetic taste?