Calling All Aerodynamicists - Pressure Differential Query

[Jeo]

Only echoing what others have said but top job Juffa. Keep it coming

[Matty]

Juff, is the gauge you have anywhere near accurate enough to measure surface pressure differentials? (say accurate to 0.1 inch of water)

And are you orienting the tube ends perpendicular to the air flow?

re intercooler - if your temps after the intercooler are exactly the same as the temp of the intake air, your intercooler is 100% efficient. You really need to know the post-turbo temp (pre-intercooler) to make a meaningful comparison.

[Damo]

You don't need a pressure difference across the radiator for there to be flow through it. The flow is due to the motion of the car, not pressure difference. Think of passing a control volume through open air: Is there going to be a pressure difference between air within the volume and the air outside? Of course there won't but there will be airflow through it - the cross-section area of it perpendicular to motion x velocity.

Mind you, there does need to be some change in pressure, density or flow velocity between flow before radiator to flow after as the energy from the coolant is transferred to the air. Whether the change would be measurable I wouldn't know off the top of my head.

[Matty]

You don't need a pressure difference across the radiator for there to be flow through it. The flow is due to the motion of the car, not pressure difference.

That's just flat out wrong.
Of course you need a pressure difference to generate flow across any restriction.

Think of passing a control volume through open air: Is there going to be a pressure difference between air within the volume and the air outside? Of course there won't but there will be airflow through it - the cross-section area of it perpendicular to motion x velocity.

Perhaps, but a control volume represents zero restriction. It is just a special case of the above.

Mind you, there does need to be some change in pressure, density or flow velocity between flow before radiator to flow after as the energy from the coolant is transferred to the air. Whether the change would be measurable I wouldn't know off the top of my head.

Well, you need a pressure difference as said above, but the density and air flow speed will depend on how much heat is dumped into the air stream. It would be measurable or the radiator wouldn't be transferring any heat.

[bruce]

re intercooler - if your temps after the intercooler are exactly the same as the temp of the intake air, your intercooler is 100% efficient. You really need to know the post-turbo temp (pre-intercooler) to make a meaningful comparison.

Yes, according to your stats, the intercooler is working suspiciously too well.

[Juffa]

Juff, is the gauge you have anywhere near accurate enough to measure surface pressure differentials? (say accurate to 0.1 inch of water)

And are you orienting the tube ends perpendicular to the air flow?

re intercooler - if your temps after the intercooler are exactly the same as the temp of the intake air, your intercooler is 100% efficient. You really need to know the post-turbo temp (pre-intercooler) to make a meaningful comparison.

Thanks Matty,
yes, this particular manometer is good up to 830 inH2O, with an accuracy of 1 decimal place, i.e 0.1 inH2O.

I attempted to align the tube ends at 90 degrees to the air flow., but I would'nt be surprise if they moved as the tube is flexible. Am going to look into that.

Thats interesting about the air temp at the pod. I was expecting the air temp under the bonnet to be higher than ambient, hence my confusion. Will have to insert the temp probe after the turbo and before the IC.

J

[greenMachine]

I attempted to align the tube ends at 90 degrees to the air flow., but I would'nt be surprise if they moved as the tube is flexible. Am going to look into that.



J

Try getting a piece of pipe (a few cms long and approx same diameter as your tube), and plug one end. Drill a number of small holes radially through the pipe and then attach your tube. If you can, keep the pipe parallel to the airflow - you are looking for the static pressure, not the pressure of the flowing air. And try and keep your tube as short as possible, long tube might induce some loss of accuracy (just a guess).

Keep us posted!!

[Juffa]

So, had time to play some more last night...

Just to verify some previous results I took more readings at a indicated speed of 60Kmh from the front and rear of the IC/condensor/radiator. But this time I took separate readings using the + pressure port on the manometer, rather than using both ports in differential mode. Came back with 0.4 inH2O front and back. A couple of times the front reading jumped to 0.9 and 1.3, which I suspect may have been the radiator fans kicking in. I'm thinking that I will rig up some kind of indication to tell me when the fans are operating so I can take that into account when taking the readings.

The fun will continue.

J.

[Matty]

Juff, is the gauge you have anywhere near accurate enough to measure surface pressure differentials? (say accurate to 0.1 inch of water)

And are you orienting the tube ends perpendicular to the air flow?

re intercooler - if your temps after the intercooler are exactly the same as the temp of the intake air, your intercooler is 100% efficient. You really need to know the post-turbo temp (pre-intercooler) to make a meaningful comparison.

Thanks Matty,
yes, this particular manometer is good up to 830 inH2O, with an accuracy of 1 decimal place, i.e 0.1 inH2O.
J

I would suspect if the full scale is 830, that it will have fuck-all chance of any accuracy at 0.1 increments - that's 0.01% accuracy...

You would need a gauge with maybe 2" full scale range to be considered accurate at 0.1 increments.

[Hammer]

Juffa, have you had a chance to play around more on this?

[Juffa]

Juffa, have you had a chance to play around more on this?

Just back from leave. Hope to start playing again this weekend.

J