Side pressures and attached flow

[JulianEdgar]

Originally Posted by Vman455

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?

Yes, very interesting - and thanks for reminding me.

But if we look at the angle of that rear bumper taper, and do a forward force triangle diagram from that increase in pressure, it has to be really tiny ie the taper wouldn't be more than about 10 degrees, would it?

But I can certainly see a change in pressure there having an influence on the wake pattern.

I posted my video to Linkedin, and I have some contacts there who are professional aero people - both CFD and wind tunnel. I've asked if they have any ideas about the relationship between side pressures / degree of attachment and resulting drag.

I just always took it on face value that 'improved side flow attachment = lower drag' but like all aspects of car aero, the more you think about it, the less you realise you know.

[AeroMcAeroFace]

That looks to me like there is a reduction in angle of those tufts, it looks like the tufts are being sucked downwards towards the sill more without the curtain.

The curtain is blocking the downwards flow? Or there is the possibility that the wing shape is creating a vortex that acts to "seal" the underside of the car, blocking the flow. It looks like any wingtip vortex would be the correct direction to reduce airflow under the car.

As in moving outwards at the bottom and inwards at the top.

[JulianEdgar]

Dr Adrian Gaylard (Jaguar Land Rover) and Dr Thomas Wolf (Porsche) have now got back to me about the relationship between pressures and flows on the rear part of the car, and base pressure. Adrian also linked to an interesting paper and reference text.

Professor Joe Katz previously also got back to me on this topic.

There's lots to think about, and Adrian and Thomas take significantly different approaches in their explanations. Adrian's explanation also reminds me of something that Rob Palin (Tesla) told me, and I need to look that up again.

I'll post something when I have thought about it some more.

[aerohead]

Originally Posted by JulianEdgar

Yes, very interesting - and thanks for reminding me.

But if we look at the angle of that rear bumper taper, and do a forward force triangle diagram from that increase in pressure, it has to be really tiny ie the taper wouldn't be more than about 10 degrees, would it?

But I can certainly see a change in pressure there having an influence on the wake pattern.

I posted my video to Linkedin, and I have some contacts there who are professional aero people - both CFD and wind tunnel. I've asked if they have any ideas about the relationship between side pressures / degree of attachment and resulting drag.

I just always took it on face value that 'improved side flow attachment = lower drag' but like all aspects of car aero, the more you think about it, the less you realise you know.

1) If flow attachment can be maintained for the entire length of the body, on a body of progressive cross-sectional contraction, by default, the pressure existing at the separation line will be the highest achievable, translating to highest base pressure ( for that length ), lower pressure drag, and lower overall drag.
2) Minimizing separation is the drag reduction.
3) Un-rounding the curvature, but maintaining a streamlined profile, provided a little extra pressure recovery on the sides, with a drag reduction reward.

[orange4boy]

I have a couple of questions about a couple of possible issues with throttle stop testing. In order for this type of testing to be reasonably accurate, both the environment of the test location as well as engine power output would have to be constant over the full period of testing.

As I understand it, fuel injected cars change the air/fuel ratios pretty much constantly with changes in engine temperature, etc and the Insight, in particular, has it's lean burn mode that happens automatically. This seems to make a big difference in throttle position/speed as I have noticed and I think MetroMpg noted as well.

Are you controlling for this in your tests?

The other is atmospheric effects which can change engine power significantly, which I'm sure you are aware of. Barometric pressure can change dramatically in the space of a few hours in extreme cases, so one should probably keep tabs on that especially if tests are done on different days or times of year. Humidity also plays a role in engine power.

Have you taken these into account?

[aerohead]

Originally Posted by orange4boy

I have a couple of questions about a couple of possible issues with throttle stop testing. In order for this type of testing to be reasonably accurate, both the environment of the test location as well as engine power output would have to be constant over the full period of testing.

As I understand it, fuel injected cars change the air/fuel ratios pretty much constantly with changes in engine temperature, etc and the Insight, in particular, has it's lean burn mode that happens automatically. This seems to make a big difference in throttle position/speed as I have noticed and I think MetroMpg noted as well.

Are you controlling for this in your tests?

The other is atmospheric effects which can change engine power significantly, which I'm sure you are aware of. Barometric pressure can change dramatically in the space of a few hours in extreme cases, so one should probably keep tabs on that especially if tests are done on different days or times of year. Humidity also plays a role in engine power.

Have you taken these into account?

1) Technically, we'd never know if the BSFC was drifting around on the engine map.
2) And in the past, it was discovered that 30% of streamlining benefit could be lost if gear-matching wasn't accomplished to keep a constant load on the engine after aero modifications.
3) With modern, EFI, and high sampling rates, compared to carbureted vehicles, it may be that BSFC isn't as unstable as in the past. I couldn't prove it one way or another.
4) Universities competing in the mileage marathons typically have a fuel tank that can be weighed to ascertain the mass of fuel consumed over a measured distance. Thermal volumetric expansion wouldn't enter into the calculus. The only unknown you're solving for is mpg based upon the fuels mass. Easy.

[JulianEdgar]

Originally Posted by orange4boy

I have a couple of questions about a couple of possible issues with throttle stop testing. In order for this type of testing to be reasonably accurate, both the environment of the test location as well as engine power output would have to be constant over the full period of testing.

As I understand it, fuel injected cars change the air/fuel ratios pretty much constantly with changes in engine temperature, etc and the Insight, in particular, has it's lean burn mode that happens automatically. This seems to make a big difference in throttle position/speed as I have noticed and I think MetroMpg noted as well.

Are you controlling for this in your tests?

The other is atmospheric effects which can change engine power significantly, which I'm sure you are aware of. Barometric pressure can change dramatically in the space of a few hours in extreme cases, so one should probably keep tabs on that especially if tests are done on different days or times of year. Humidity also plays a role in engine power.

Have you taken these into account?

Yes of course.

I see a display of air/fuel ratio on my dash the whole time.

My car's lean cruise mode doesn't change during the test runs (if it did change, I'd simple reprogram the MoTeC ECU so it didn't).

I do test runs back to back, with and without the aero changes - normally over a period of 15 minutes or so.

The test runs in the one aero configuration are extremely consistent (typically under 1 per cent variation in top speed).

[JulianEdgar]

Originally Posted by aerohead

1) Technically, we'd never know if the BSFC was drifting around on the engine map.
2) And in the past, it was discovered that 30% of streamlining benefit could be lost if gear-matching wasn't accomplished to keep a constant load on the engine after aero modifications.
3) With modern, EFI, and high sampling rates, compared to carbureted vehicles, it may be that BSFC isn't as unstable as in the past. I couldn't prove it one way or another.
4) Universities competing in the mileage marathons typically have a fuel tank that can be weighed to ascertain the mass of fuel consumed over a measured distance. Thermal volumetric expansion wouldn't enter into the calculus. The only unknown you're solving for is mpg based upon the fuels mass. Easy.

BSFC, fuel consumption and fuel density have nothing to do with throttle-stop testing.

[aerohead]

Originally Posted by JulianEdgar

BSFC, fuel consumption and fuel density have nothing to do with throttle-stop testing.

1) altering the road load at any given throttle setting can cause the engine to perform at a less efficient manner.
2) Unless the engine is loaded ( through gear-matching), it will operate at this diminished efficiency.
3) You could lower drag by 30% and you'd never know it.
4) It's the thermodynamic efficiency of the engine which is called into question.
Just sayin'

[JulianEdgar]

Originally Posted by aerohead

1) altering the road load at any given throttle setting can cause the engine to perform at a less efficient manner.
2) Unless the engine is loaded ( through gear-matching), it will operate at this diminished efficiency.
3) You could lower drag by 30% and you'd never know it.
4) It's the thermodynamic efficiency of the engine which is called into question.
Just sayin'

I don't think you understand very well what is being described by throttle stop testing. We're talking changes in engine rpm of 40 or 50 rpm - basically inconsequential in terms of torque change. (And it's torque we're working with in this case, not power.)

But hey, don't take my word for it. Go and do some testing yourself, eg windows up / windows down and see if the measured drag changes matches what you'd expect in the two different configurations. Practical testing, not just theorising.