Aerodynamics flow diagram?
12-08-07, 05:58 AM
#51
Actually, the arrows pointing outward is higher pressure and the arrows pointing inward shows lower pressure.
Airflow moving up the windshield combinds with the air above causing higher pressure, then the airflow moves toward the back near the hatch glass which releases some of the pressure.
Thats a low pressure zone.
The air flowing over the hood which is continuously compressed with the airflow above it, reaches that point and then will release some of the pressure.
How much lower the pressure is at the end of the hood line compaired to the pressure in the engine bay is something I would like to know. This would be the only way to know for sure if the air will enter or exit for the looks of it.
That wont work
Airflow moving up the windshield combinds with the air above causing higher pressure, then the airflow moves toward the back near the hatch glass which releases some of the pressure.
Thats a low pressure zone.
The air flowing over the hood which is continuously compressed with the airflow above it, reaches that point and then will release some of the pressure.
How much lower the pressure is at the end of the hood line compaired to the pressure in the engine bay is something I would like to know. This would be the only way to know for sure if the air will enter or exit for the looks of it.
That wont work
12-08-07, 09:29 AM
#53
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It would be nice if those of you who have no clue would phrase your posts as questions rather than statements.
For those of you who still have difficulty with this subject:
When high-velocity air slams into something, it compresses as the velocity is decreased. For example, if you throw a tomato against a wall, it does not compress until it hits the wall and its velocity starts to reduce. As you know from turbocharger theory, as air compresses, it heats up. Also from your turbocharger knowledge, you probably realize that the increase in pressure will increase engine performance even with the resulting increase in temperature (i.e. a non-intercooled turbo will still gain performance vs. no turbo at all).
Therefore, high velocity = low pressure and low temperature, low velocity = high pressure and high temperature. If you just look at any car, try and imagine where the air slams into it, and those will most likely be the high pressure areas. After the air slams into something, it needs to accelerate to catch back up to the rest of the airstream, so its velocity needs to increase, which will decrease pressure. This is why you see low pressure areas (top of the grille and top of the windshield) at the trailing end of high pressure areas (bumper and lower windshield).
Ah, I know where the confusion is coming from...
There are TWO different types of hoods that look the same: Cowl Induction and Cowl Vent (aka Extractor Vent)
Cowl Induction = The rear of the hood has a duct for the engine inlet, allowing it to suck in the compressed, stagnant air from the base of the windshield. Note that the duct is SEALED so that the engine does not suck in air from the engine bay, and so that any high velocity air that flows into the engine bay does not flow out through the duct. As a side note, the cowl induction hoods originally installed on the muscle cars of yesteryear were said to supply "cold air" to the engine, which I find amusing because it is actually a little hotter than ambient due to compressing against the windshield base. However, as I stated above, the increase in pressure, and also possibly the decrease in turbulence, still give better overall performance.
Cowl Vent (Extractor Vent) = The rear of the hood has a duct that allows air to flow out of the engine bay. For this to work, the airflow through the engine bay needs to travel at a high enough velocity to force its way past the high pressure zone at the base of the windshield. Therefore, you will sometimes see some type of ram air inlet that feeds the engine bay. If there is no ram air inlet, then the hood will vent well at low speed, but not so well at high speed when the windshield base pressure zone builds, which is sometimes the goal of this self-regulating system. The hot air flowing over the top of the car can affect the aerodynamics.
LOL, no kidding? I have 2 college degrees and over 20 years of experience in this subject and the advanced stuff is still too complicated for me.
For those of you who still have difficulty with this subject:
When high-velocity air slams into something, it compresses as the velocity is decreased. For example, if you throw a tomato against a wall, it does not compress until it hits the wall and its velocity starts to reduce. As you know from turbocharger theory, as air compresses, it heats up. Also from your turbocharger knowledge, you probably realize that the increase in pressure will increase engine performance even with the resulting increase in temperature (i.e. a non-intercooled turbo will still gain performance vs. no turbo at all).
Therefore, high velocity = low pressure and low temperature, low velocity = high pressure and high temperature. If you just look at any car, try and imagine where the air slams into it, and those will most likely be the high pressure areas. After the air slams into something, it needs to accelerate to catch back up to the rest of the airstream, so its velocity needs to increase, which will decrease pressure. This is why you see low pressure areas (top of the grille and top of the windshield) at the trailing end of high pressure areas (bumper and lower windshield).
There are TWO different types of hoods that look the same: Cowl Induction and Cowl Vent (aka Extractor Vent)
Cowl Induction = The rear of the hood has a duct for the engine inlet, allowing it to suck in the compressed, stagnant air from the base of the windshield. Note that the duct is SEALED so that the engine does not suck in air from the engine bay, and so that any high velocity air that flows into the engine bay does not flow out through the duct. As a side note, the cowl induction hoods originally installed on the muscle cars of yesteryear were said to supply "cold air" to the engine, which I find amusing because it is actually a little hotter than ambient due to compressing against the windshield base. However, as I stated above, the increase in pressure, and also possibly the decrease in turbulence, still give better overall performance.
Cowl Vent (Extractor Vent) = The rear of the hood has a duct that allows air to flow out of the engine bay. For this to work, the airflow through the engine bay needs to travel at a high enough velocity to force its way past the high pressure zone at the base of the windshield. Therefore, you will sometimes see some type of ram air inlet that feeds the engine bay. If there is no ram air inlet, then the hood will vent well at low speed, but not so well at high speed when the windshield base pressure zone builds, which is sometimes the goal of this self-regulating system. The hot air flowing over the top of the car can affect the aerodynamics.
LOL, no kidding? I have 2 college degrees and over 20 years of experience in this subject and the advanced stuff is still too complicated for me.
12-08-07, 10:38 AM
#54
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When high-velocity air slams into something, it compresses as the velocity is decreased. For example, if you throw a tomato against a wall, it does not compress until it hits the wall and its velocity starts to reduce. As you know from turbocharger theory, as air compresses, it heats up. Also from your turbocharger knowledge, you probably realize that the increase in pressure will increase engine performance even with the resulting increase in temperature (i.e. a non-intercooled turbo will still gain performance vs. no turbo at all).
Therefore, high velocity = low pressure and low temperature, low velocity = high pressure and high temperature. If you just look at any car, try and imagine where the air slams into it, and those will most likely be the high pressure areas. After the air slams into something, it needs to accelerate to catch back up to the rest of the airstream, so its velocity needs to increase, which will decrease pressure. This is why you see low pressure areas (top of the grille and top of the windshield) at the trailing end of high pressure areas (bumper and lower windshield).
Therefore, high velocity = low pressure and low temperature, low velocity = high pressure and high temperature. If you just look at any car, try and imagine where the air slams into it, and those will most likely be the high pressure areas. After the air slams into something, it needs to accelerate to catch back up to the rest of the airstream, so its velocity needs to increase, which will decrease pressure. This is why you see low pressure areas (top of the grille and top of the windshield) at the trailing end of high pressure areas (bumper and lower windshield).
Over the hood and across the car the air isn't so much going faster becuse it's expanding, it's going faster because it's being deflected off it's original course by a flow obstruction (car), and the air needs to move faster to get all of it around the obstruction without compressing or expanding.
It's a little easier to think of a flow in a pipe. When the pipe gets smaller, the air will stay at the same density, so to maintain the same mass flow rate the flow must go faster through the narrow section (mass flow = density x cross section x velocity). The car has the effect of acting like a constriction to the flow, just as a smaller pipe does.
12-08-07, 09:33 PM
#55
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http://www.sportrider.com/tech/146_9910_ram/index.html
I think it is easier for most people to understand aerodynamics with compressibility because humans have little tangible experience with incompressible substances. Also, I think it makes more sense to initially teach the full spectrum, and then remove insignificant factors like compressibility and humidity later on to speed up calculations.
Yes, the primary cause of temperature change is from forced convection, but I think that is better left to the classroom rather than an internet forum.
12-09-07, 03:09 PM
#58
I thought the rx7 would attract a lot of smart kinds of people due to it's design. I know I view my whole experience with my RX7 as an experiment.
But we also seem to get a few dumbies in here as well..... alls good. and it does seems like there are a few well informed people...
But we also seem to get a few dumbies in here as well..... alls good. and it does seems like there are a few well informed people...
12-09-07, 03:35 PM
#59
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Some people are attracted to it because it's a cheap, fun sports car, others, were attracted to it because of the design, the weird and the wonderful bits of it.
It was a combination of both that did it for me. I like having an engine fundamentally different than >99% of all vehicles out there and I'm very impressed with lots of the little details of the car.
It was a combination of both that did it for me. I like having an engine fundamentally different than >99% of all vehicles out there and I'm very impressed with lots of the little details of the car.
12-10-07, 02:47 AM
#61
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Wow, after all that we get one more burst of ignorance. 
That diagram shows nothing about the performance of the TMIC. It shows static pressure acting on the body. The dynamic pressure (what you feel when you hold your hand out the window) pushing air into the scoop will always be positive and will be 10-20 times greater in magnitude.
Anyone who's actually measured the intake temps on a stock TMIC knows that it's at low speeds and when stationary that the TMIC sucks, not when moving.
That diagram shows nothing about the performance of the TMIC. It shows static pressure acting on the body. The dynamic pressure (what you feel when you hold your hand out the window) pushing air into the scoop will always be positive and will be 10-20 times greater in magnitude.
Anyone who's actually measured the intake temps on a stock TMIC knows that it's at low speeds and when stationary that the TMIC sucks, not when moving.
12-10-07, 06:28 AM
#62
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since the above posted diagram is a " sales tool" and really doesn't contain active values" hope thats the right wording"
Doesn't anyone have a active diagram not a passive one? I am looking for the scentific data based on the wind tunnels tests. Not some CAD drawing.
Thank you for your time.
Doesn't anyone have a active diagram not a passive one? I am looking for the scentific data based on the wind tunnels tests. Not some CAD drawing.
Thank you for your time.
12-10-07, 07:00 PM
#63
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Wow, after all that we get one more burst of ignorance.
That diagram shows nothing about the performance of the TMIC. It shows static pressure acting on the body. The dynamic pressure (what you feel when you hold your hand out the window) pushing air into the scoop will always be positive and will be 10-20 times greater in magnitude.
Anyone who's actually measured the intake temps on a stock TMIC knows that it's at low speeds and when stationary that the TMIC sucks, not when moving.
That diagram shows nothing about the performance of the TMIC. It shows static pressure acting on the body. The dynamic pressure (what you feel when you hold your hand out the window) pushing air into the scoop will always be positive and will be 10-20 times greater in magnitude.
Anyone who's actually measured the intake temps on a stock TMIC knows that it's at low speeds and when stationary that the TMIC sucks, not when moving.
It sucks because it is small and the outlet duct is inefficient. On the good side, it has low pressure drop and it minimizes the volume of the intake tract which aids throttle response.
since the above posted diagram is a " sales tool" and really doesn't contain active values" hope thats the right wording"
Doesn't anyone have a active diagram not a passive one? I am looking for the scentific data based on the wind tunnels tests. Not some CAD drawing.
Thank you for your time.
Doesn't anyone have a active diagram not a passive one? I am looking for the scentific data based on the wind tunnels tests. Not some CAD drawing.
Thank you for your time.
Mazda R&D Center, 3-1 Shinchi, Fuchu-cho, Aki-gun Hiroshima 730-8670.
Also, keep in mind that this car was designed in 1983, so any technical data would probably be in text vectors and arms, or low-resolution pen plots.
12-11-07, 12:30 AM
#64
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Besides, the stock TMIC would work better if the inlet were located in a high pressure zone.
12-11-07, 06:12 AM
#65
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No need to jump his case when we didn't even discuss dynamic pressure in this thread. Besides, the stock TMIC would work better if the inlet were located in a high pressure zone.
It sucks because it is small and the outlet duct is inefficient. On the good side, it has low pressure drop and it minimizes the volume of the intake tract which aids throttle response.
I have no idea what you could possibly use that for (assuming you even understood it), but try here:
Mazda R&D Center, 3-1 Shinchi, Fuchu-cho, Aki-gun Hiroshima 730-8670.
Also, keep in mind that this car was designed in 1983, so any technical data would probably be in text vectors and arms, or low-resolution pen plots.
It sucks because it is small and the outlet duct is inefficient. On the good side, it has low pressure drop and it minimizes the volume of the intake tract which aids throttle response.
I have no idea what you could possibly use that for (assuming you even understood it), but try here:
Mazda R&D Center, 3-1 Shinchi, Fuchu-cho, Aki-gun Hiroshima 730-8670.
Also, keep in mind that this car was designed in 1983, so any technical data would probably be in text vectors and arms, or low-resolution pen plots.
Aren't you the guy that has a 20B project car and knew Lee with the red RX7. Did you live near Riverview?
12-11-07, 01:10 PM
#66
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I'll probably get chewed out for this...
It shows static pressure acting on the body. The dynamic pressure (what you feel when you hold your hand out the window) pushing air into the scoop will always be positive and will be 10-20 times greater in magnitude.
Anyone who's actually measured the intake temps on a stock TMIC knows that it's at low speeds and when stationary that the TMIC sucks, not when moving.
Anyone who's actually measured the intake temps on a stock TMIC knows that it's at low speeds and when stationary that the TMIC sucks, not when moving.
No need to jump his case when we didn't even discuss dynamic pressure in this thread. Besides, the stock TMIC would work better if the inlet were located in a high pressure zone.
12-11-07, 03:03 PM
#67
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The stock intake snorkle is not in the path of any heavy airflow. The hood is gasketed and then there are the radiator panels which direct the airflow flowing into the front bumper toward the radiator instead of allowing the airflow to go up and over the rad where the snorkle is.
The snorkle is mainly getting cooled air from air leaks within the snorkle area and air from the engine bay.
The snorkle is mainly getting cooled air from air leaks within the snorkle area and air from the engine bay.
12-11-07, 03:42 PM
#68
The purpose of the aero kit pieces underneath are to reduce drag from the wheels. I'd guess that they direct air downward away from the wheels. The top of the wheel is moving forward twice as fast as the car is, which means drag is 8 times as great at the top of the wheels (2 cubed). Not to mention the turbulance from a spinning wheel.
Major sources of car drag are:
the body (duh)
the undercarriage
the radiator/etc.
the wheels
the side mirrors (kinda minor, though)
The body is about half the drag, while everything else is the other half. That's quite a bit considering how most people overlook it.
The vast majority of drag comes from flow seperation, i.e. where the air flow seperates from the car body and vortices form in between. To prevent flow seperation you need a gentle trailing edge, probably less than 7 degrees. 15 degrees is as bad as 40 degrees (where there's a dip in drag), and drag is even higher than that between 15 and 40 (with a peak at 30). So if you can't get the trailing side angle under 15, you might as well just make it 40.
I don't know what's so special about 30 and 40; those are just the numbers found in the test I read. It might be different depending on the car and the speed. The RX-7's hatch has a nice ~10 degree bend where the hatch starts that gently curves downward to keep the tail from being super long.
Major sources of car drag are:
the body (duh)
the undercarriage
the radiator/etc.
the wheels
the side mirrors (kinda minor, though)
The body is about half the drag, while everything else is the other half. That's quite a bit considering how most people overlook it.
The vast majority of drag comes from flow seperation, i.e. where the air flow seperates from the car body and vortices form in between. To prevent flow seperation you need a gentle trailing edge, probably less than 7 degrees. 15 degrees is as bad as 40 degrees (where there's a dip in drag), and drag is even higher than that between 15 and 40 (with a peak at 30). So if you can't get the trailing side angle under 15, you might as well just make it 40.
I don't know what's so special about 30 and 40; those are just the numbers found in the test I read. It might be different depending on the car and the speed. The RX-7's hatch has a nice ~10 degree bend where the hatch starts that gently curves downward to keep the tail from being super long.
Last edited by ericgrau; 12-11-07 at 03:54 PM.
12-11-07, 06:00 PM
#69
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The stock intake snorkle is not in the path of any heavy airflow. The hood is gasketed and then there are the radiator panels which direct the airflow flowing into the front bumper toward the radiator instead of allowing the airflow to go up and over the rad where the snorkle is.
The snorkle is mainly getting cooled air from air leaks within the snorkle area and air from the engine bay.
The snorkle is mainly getting cooled air from air leaks within the snorkle area and air from the engine bay.
12-11-07, 07:25 PM
#70
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the snorkle would be right around one of the biggest low pressure systems on the car, which would lead me to figure that the air density would be far lower than ambient--compare that to where the radiator sits is by contrast a very high pressure zone and so more dense.
I'm personally not going to be re-routing anything anytime soon until after I put the turbo on, then I'll consider plumbing ideas and what not. I was wondering why the Mazda engineers put the snorkle there (by all means, my interpretation could be way off)?
12-11-07, 07:28 PM
#71
I just read this thread all the way through. Quite a tangent.
You cannot compress uncontained air with anything that moves under 200mph. It will simply move somewhere else when you push against it, without getting squeezed at all. You will still feel the pressure when you push on it, just like you can feel the impact from other things that cannot be compressed... like golf *****.
Pressure builds up when you put something directly in the path of the air. That's why when you stick your hand out in the wind you feel something pushing on your hand. Duh. That's also why the arrows point into the nose. Pressure gets lower when the air moves faster past (and not so much against) an object. That's why the arrows point away from the hood, and why a lightweight carbon fiber hood will fly open at high speeds if it's not latched down. And, surprising as it may be, sticking a hood scoop directly in the path of the air forces air into the hood scoop.
You cannot compress uncontained air with anything that moves under 200mph. It will simply move somewhere else when you push against it, without getting squeezed at all. You will still feel the pressure when you push on it, just like you can feel the impact from other things that cannot be compressed... like golf *****.
Pressure builds up when you put something directly in the path of the air. That's why when you stick your hand out in the wind you feel something pushing on your hand. Duh. That's also why the arrows point into the nose. Pressure gets lower when the air moves faster past (and not so much against) an object. That's why the arrows point away from the hood, and why a lightweight carbon fiber hood will fly open at high speeds if it's not latched down. And, surprising as it may be, sticking a hood scoop directly in the path of the air forces air into the hood scoop.
Last edited by ericgrau; 12-11-07 at 07:35 PM.
12-11-07, 08:41 PM
#72
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So either the Mazda engineers did their best to choke the inlet of their flagship sports car, or there is something that you are missing.
Plenty of airflow is coming from there, its by no means getting air as cool as the outside air though.
12-11-07, 09:07 PM
#73
Banned. I got OWNED!!!
That's my single BIGGEST complaint about people like rotaman. He posts out of his *** thinking he's helping when in fact all he is doing is solidifying my opinion of him. He wasn't mis-informed, he misread. Link it up rotaman. Show us where Rarson's wrong.
I'm also curious where you think all this cold air is leaking up from
IIRC there are panels there to keep the air forced into the rad and afterall - the hood is gasketed.
12-12-07, 12:17 AM
#74
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According to the diagram the snorkle would be right around one of the biggest low pressure systems on the car, which would lead me to figure that the air density would be far lower than ambient--compare that to where the radiator sits is by contrast a very high pressure zone and so more dense.
The other thing is that the static pressures shown on the diagram are tiny. Think tenths of an inch of water, where 1"H2O = 0.036psi. By contrast the dynamic pressure at 60mph is 0.125psi. In this case "far lower than ambient" is actually about 0.05% below ambient pressure.
I'm curious as to where you think the air comes from. Rotaman is half right in that while it's not coming from the engine bay, it is essentially leaking into the area where the stock intake is located from the multitude of openings to outside. Common sense really.
12-12-07, 03:13 PM
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The most important thing you're missing here is that the diagram shows pressure on the outside of the body. The intake snorkel is on the inside of the body, where the pressure will be something different and that diagram is irrelevant.
The other thing is that the static pressures shown on the diagram are tiny. Think tenths of an inch of water, where 1"H2O = 0.036psi. By contrast the dynamic pressure at 60mph is 0.125psi. In this case "far lower than ambient" is actually about 0.05% below ambient pressure.
The other thing is that the static pressures shown on the diagram are tiny. Think tenths of an inch of water, where 1"H2O = 0.036psi. By contrast the dynamic pressure at 60mph is 0.125psi. In this case "far lower than ambient" is actually about 0.05% below ambient pressure.