Sealing the hood vent?

[CSBM5]

Originally Posted by Law

Excellent post.

This is what exactly what I was trying to explain and you've done so very eloquently.

It's hard to explain fluid dynamics without getting into fluid viscosity and density, Reynolds numbers, boundary layer theory and potential flow theory...I'm glad it made sense. It's been more than 35 years now, but all that early CFD research from the old days on F-15 V/STOL engine inlets helps a bit. I'm not a car aero engineer, but I've studied automobile aerodynamics off and on since 1982.

Cheers,
Chuck

[Richbot]

Originally Posted by Richbot

Wut

Chuck I think my post above did a better job of explaining personally but that’s just my opinion

[Mvy]

Originally Posted by CSBM5

The hood vent is in the front portion of the region of boundary layer separation and static pressure recovery that all cars experience from about that location on toward the windshield. Someone setup probes on an E90 hood and attempted to measure static pressures years ago now and showed the obvious (well, obvious to a fluid dynamics engineer)...they didn't perform an engineering level test of course since that would require drilling the hood and precisely locating the static pressure probe opening level with the surface, etc.

However, you're going to have a very hard time forcing air to flow out of a vent in the high static pressure zone near the back 1/3 portion of any hood. The faster the car goes, the higher the static pressure in that zone. Hence the reason you see almost all cars have cabin air inlets in this area and also things like "cowl induction" hoods, etc. That dew can emit from the vent is not surprising given water is almost 800 times denser than air. You don't do fluid flow design for a fluid with such dramatically different properties -- in other words, a "system" designed around air will not function in the same manner when you replace the air with water. Similarly, a system designed for water flow will not function in the same way if you replaced the water with mercury. Reynolds numbers rule, and viscosity and density play a large role in fluid behavior.

The M3 vent is not perfectly located in that region as I mentioned, but it's clearly in the region of the hood where the surface curvature has flattened out and static pressure is rising rapidly. At the best location right near the hood to windshield junction, static pressure recovery is near 1.0. BTW, pressure recovery is referring to the static pressure at a location versus stagnation point pressure. The stagnation point is the spot on the car's front where air velocity is zero (right near the center point of the front end), and pressure is at a maximum. In aerodynamic analysis, it's common practice to reference the ratio of local static pressure to stagnation point pressure.

Doesn't this mean that the hood vent really isnt acting as an intake then? Break this down for the layperson please.

[CSBM5]

Originally Posted by Mvy

Doesn't this mean that the hood vent really isnt acting as an intake then? Break this down for the layperson please.

No, the opposite. The M3 intake is located in the front portion of the high static pressure zone near the back of the hood.

If you wanted a "vent", you need to be in the front 1/3 of the hood where you have a much higher surface curvature and therefore velocity (higher velocity = lower surface static pressure). Hence the reason you see radiator venting on the front 1/4th-1/3rd or so of a hood on some cars, Ferraris, F80 M3 CS, etc, etc.

[CSBM5]

Originally Posted by Richbot

Chuck I think my post above did a better job of explaining personally but that’s just my opinion

Why say something in a few words when 700 will do.

[Law]

Originally Posted by Mvy

Doesn't this mean that the hood vent really isnt acting as an intake then? Break this down for the layperson please.

Originally Posted by CSBM5

No, the opposite. The M3 intake is located in the front portion of the high static pressure zone near the back of the hood.

If you wanted a "vent", you need to be in the front 1/3 of the hood where you have a much higher surface curvature and therefore velocity (higher velocity = lower surface static pressure). Hence the reason you see radiator venting on the front 1/4th-1/3rd or so of a hood on some cars, Ferraris, F80 M3 CS, etc, etc.

So basically, if I'm reading this right, Chuck is saying that the shape, position, and curvature (angle) of the vent is one way to determine its function.

On cars like the F80 M3 CS, where you have a "vent" sometimes known as a heat extraction vent, the vent is shaped differently (optimized for surface area) and angled/pointed opposite of the direction of travel, while also positioned in a pressure optimized area to achieve said effect.
Such a design is engineered to remove heat and the said vent (a la F80 M3 CS) would act as an outlet for hot air to escape.

Since the E9x M3 hood "vent" is curved along the hood, positioned by the "high static pressure zone near the back of the hood", and ultimately in the direction of travel (even if ever so slightly), it is indeed, an inlet designed to collect air as part of the intake system.

[Law]

Originally Posted by CSBM5

Why say something in a few words when 700 will do.

[CSBM5]

Originally Posted by Law

So basically, if I'm reading this right, Chuck is saying that the shape, position, and curvature (angle) of the vent is one way to determine its function.

On cars like the F80 M3 CS, where you have a "vent" sometimes known as a heat extraction vent, the vent is shaped differently (optimized for surface area) and angled/pointed opposite of the direction of travel, while also positioned in a pressure optimized area to achieve said effect.
Such a design is engineered to remove heat and the said vent (a la F80 M3 CS) would act as an outlet for hot air to escape.

Since the E9x M3 hood "vent" is curved along the hood, positioned by the "high static pressure zone near the back of the hood", and ultimately in the direction of travel (even if ever so slightly), it is indeed, an inlet designed to collect air as part of the intake system.

Not really.

It's far, far more dependent on the actual location, longitudinally, along the hood as opposed to the vent/intake physical shape.

The "best" place for an intake would be right in the center at the inflection point between the hood and windshield for example. The shape of said intake is only relevant with respect to airflow restriction of the intake path itself (i.e. you would like to have a nicely shaped junction between the duct and the opening of course).

So recapping, location, location, location -- just like in real estate, is the most important when you are dealing with a surface level intake (or vent). Now, if you get *outside* the boundary layer region such that the intake is exposed to the dynamic pressure of the freestream airflow (like a scoop on a drag car), then the whole deal changes. But for now, we're talking about surface vents/intakes.

If you do some research on static pressure plots along a car hood, you'll see very low pressure zone near the front (high surface velocity), and from the mid-point reward toward the windshield a rapidly increasing surface pressure profile. This building static pressure gradient is what causes boundary layer separation in that zone near the back of the hood. The boundary layer becomes reattached again on the lower part of the windshield and the process starts all over again...

[Law]

Originally Posted by CSBM5

Not really.

It's far, far more dependent on the actual location, longitudinally, along the hood as opposed to the vent/intake physical shape.

The "best" place for an intake would be right in the center at the inflection point between the hood and windshield for example. The shape of said intake is only relevant with respect to airflow restriction of the intake path itself (i.e. you would like to have a nicely shaped junction between the duct and the opening of course).

So recapping, location, location, location -- just like in real estate, is the most important when you are dealing with a surface level intake (or vent). Now, if you get *outside* the boundary layer region such that the intake is exposed to the dynamic pressure of the freestream airflow (like a scoop on a drag car), then the whole deal changes. But for now, we're talking about surface vents/intakes.

If you do some research on static pressure plots along a car hood, you'll see very low pressure zone near the front (high surface velocity), and from the mid-point reward toward the windshield a rapidly increasing surface pressure profile. This building static pressure gradient is what causes boundary layer separation in that zone near the back of the hood. The boundary layer becomes reattached again on the lower part of the windshield and the process starts all over again...

Oh, Chuck, now you're losing me (just a little bit).

What I got from your quoted post is that the shape is not as relevant as the location, and that the E9x hood "vent" is not necessarily in the most optimal location?

For the purposes of this thread, I think we're just looking for information that based on all these pressure zones and the static pressure of the location, if the hood "vent" on the E9x is an inlet (like BMW says) or an outlet.
My understanding is that it is an inlet for induction air (even if I might not completely understand the very technical happenings).

Thanks for your insight.

[CSBM5]

Originally Posted by Law

Oh, Chuck, now you're losing me (just a little bit).

What I got from your quoted post is that the shape is not as relevant as the location, and that the E9x hood "vent" is not necessarily in the most optimal location?

For the purposes of this thread, I think we're just looking for information that based on all these pressure zones and the static pressure of the location, if the hood "vent" on the E9x is an inlet (like BMW says) or an outlet.
My understanding is that it is an inlet for induction air (even if I might not completely understand the very technical happenings).

Thanks for your insight.

Yeah, that's right, it's definitely an inlet, and although in an "ok" zone for an inlet, if it could be further back so much the better; however, BMW has obviously designed the totality of the inlets to work as they desired.

Here's an image I found on the web of a race car showing the hood static pressure distribution. In our case, assume the M3 intake is in the range of the brownish to yellow coloring zone (although this car shape is much different than the M3 as it has a very short hood by comparison). It's an ok location. You can see the much higher static pressure zone at the base of the windshield.

[Law]

Originally Posted by CSBM5

Yeah, that's right, it's definitely an inlet, and although in an "ok" zone for an inlet, if it could be further back so much the better; however, BMW has obviously designed the totality of the inlets to work as they desired.

Here's an image I found on the web of a race car showing the hood static pressure distribution. In our case, assume the M3 intake is in the range of the brownish to yellow coloring zone (although this car shape is much different than the M3 as it has a very short hood by comparison). It's an ok location. You can see the much higher static pressure zone at the base of the windshield.

I see, I see.

Thank you for the wonderful insight and knowledge.

[jdamore]

Originally Posted by CSBM5

Yeah, that's right, it's definitely an inlet, and although in an "ok" zone for an inlet, if it could be further back so much the better; however, BMW has obviously designed the totality of the inlets to work as they desired.

Here's an image I found on the web of a race car showing the hood static pressure distribution. In our case, assume the M3 intake is in the range of the brownish to yellow coloring zone (although this car shape is much different than the M3 as it has a very short hood by comparison). It's an ok location. You can see the much higher static pressure zone at the base of the windshield.

So, using your image, if the hood vent is located in the bright green area it would be sucking air out? Am I understanding that correctly?

[CSBM5]

Originally Posted by jdamore

So, using your image, if the hood vent is located in the bright green area it would be sucking air out? Am I understanding that correctly?

More that likely, yes. The further forward you go toward the front, the lower the surface static pressure too.

Take a look at the M3 CS radiator hood venting. Note where it is located with respect to distance from the front edge of the hood -- low static pressure area near the hood front. The front of the radiator is near stagnation point pressure (highest there is) while this hood vent is located in a low pressure zone near the front of the hood = lots of flow through the radiator, intercooler, what have you.

[Richbot]

Yeah but chuck that vent is just there to keep the carbon fiber u brace cool

[Richbot]

Oh and btw I have also done some unscientific thermocouple testing on the passenger side vent with the plug removed

At speeds above 20-25mph, with the probe located centered about 1” below the vent, it gets colder (and eventually settles right around ambient), not hotter. If air wanted to go through the radiator and out that vent it would get hotter at speed. Instead, the temp only goes up at low speed/stopped at lights. Meaning hot air is escaping from a high point as you’d expect when sitting still. But not flowing through and out at any legal speed I measured.

Is that turbulence or something weird going on? Maybe. But that vent is in the same region as the drivers side, it’s just not ducted from the front end Like the airbox

The top 25% of the bottom bumper intake and the kidney grilles are the only frontend air places that are not ducted to exhaust outside the engine bay (or be consumed and spit out the back by the engine)

Trans cooler -> passenger wheel well
Oil cooler, which takes up like 3” of the Lower front, full width -> dumped under the car

The amount of frontal area the radiator (and AC condenser, first!) gets is surprisingly small now that I think about it, and has a lot of fins and engine to get around before it gets to that vent, so, not apples to apples to the ducted intake vent at all, but a datapoint

[roastbeef]

Interesting perspective and I appreciate he discussion. We may not fully agree, and that's ok. It works good for however which way it achieves it.

[roastbeef]

Originally Posted by Richbot

Oh and btw I have also done some unscientific thermocouple testing on the passenger side vent with the plug removed

At speeds above 20-25mph, with the probe located centered about 1” below the vent, it gets colder (and eventually settles right around ambient), not hotter. If air wanted to go through the radiator and out that vent it would get hotter at speed. Instead, the temp only goes up at low speed/stopped at lights. Meaning hot air is escaping from a high point as you’d expect when sitting still. But not flowing through and out at any legal speed I measured.

Is that turbulence or something weird going on? Maybe. But that vent is in the same region as the drivers side, it’s just not ducted from the front end Like the airbox

The top 25% of the bottom bumper intake and the kidney grilles are the only frontend air places that are not ducted to exhaust outside the engine bay (or be consumed and spit out the back by the engine)

Trans cooler -> passenger wheel well
Oil cooler, which takes up like 3” of the Lower front, full width -> dumped under the car

The amount of frontal area the radiator (and AC condenser, first!) gets is surprisingly small now that I think about it, and has a lot of fins and engine to get around before it gets to that vent, so, not apples to apples to the ducted intake vent at all, but a datapoint

Without air being forced out, the passenger side vent isn't going to do much. I'd 100% agree with that. I bet if we put a small gurney flap in front of it, we would see some activity though.

The trackspec vents stick up a little bit, essentially creating their own gurney flaps and allowing for air to escape. When I park the car, the hood is cool to the touch (before it sits and gets warmed up), and the vents are very warm to the touch.

[Richbot]

Imma break the wing off my GT3 and try it out

[Pig Farmer]

When trying solve the heat soak issues on my old 135i (which BTW is nearly impossible), I considered venting the hood. I did a little research and discovered much of what is being discussed in the thread. To find the low pressure zone, I cut a bunch of 4” sections of kite string and tape one end of each piece to the hood with painter’s tape, spaced every 6” in a grid pattern. I’m sure this is not perfectly scientific, but it was interesting to see how the strings at the front of the hood were pulled tight by the air flow (i.e. the low pressure zone), the strings in the middle of the hood hardly moved at all, and strings at the rear of the hood danced around in circles. I figure you could do a similar experiment by taping strings around the vent. You may want to have a friend video the results from the passenger seat. It’s never a good idea to take your eyes off the road at speed.

[Banana Hammock]

I always thought the vent on the hood was an outlet meant to help extract air and increase airflow by means of the Venturi effect.

[1MOREMOD]

Originally Posted by Hurricane Beefaroni

I always thought the vent on the hood was an outlet meant to help extract air and increase airflow by means of the Venturi effect.

Same I opened the other side as well

[Kamen]

I've not seen anythyng coming up out from the inlet, but I am assuming that happens only when the car is moving. The hood is wing-shaped, and when the car is moving the air pressure right above the hood surface drops, which combined with the rammed air from bumper intake creates positive pressure and stuff gets shot out of the hood inlet. That probably happens only when the car is moving, and only when the sum of positive pressure from the bumper inlet and the negative pressure right above the hood(due to turbulence), is bigger than the negative pressure, which the engine creates.