Exhaust Flow and Aero

[SPARKY]

I talked to an engine builder several years ago who had built an intake manifold for a medical supply salesman, for the use of 3 dozen tiny medical cameras for a year.  Instaled them in intake manifolds of several different combinations they built.  But them on the dyno.  My question, What did you learn?

I will never forget his reply--"We learned there was more "stuff" going on in there, moving different directions than one would ever imagine and some of them were fuel dropletts larger than a quarter."

That kind of research was probabaly on the front end of "wet flow" and "charge quality" buzz words that we read about today.

[WOODY@DDLLC]

Woody:
Yeah, ditto what Rex said. I would like to understand what I am seeing. Are the inlet and outlet areas similar? And according to Bernoulli, the zero velocity (blue) areas are the highest pressures, right? The velocities appear to range from 0 mph to about 255mph. The flow appears to be stagnant (highest pressure) in the trough and in front of the inlet but with nominal speeds of about 255mph mid-stream. Then there are other speeds of zero (in the diverging cone area) and 300 mph on the opposite side of that high pressure area. Geeze, a lot is going on in there!  I know Woody, you said CFD is "case specific" and now I finally see the magnitude of what you are saying. You said it was done with a specific mass flow so that would equate to some constant velocity right? The pics we see are at a single speed?

What is interesting about the CFD picture is how it points out how incredibly difficult instrumentation would be. Just look at the widely deviating pressure distribution inside. If you put in only one pressure sensor, and made assumptions from that data point, you would be clueless as to what is really going on! Even a couple of sensors in the wrong spots would be giving you very bad information. That might lead to some really bad decisions on jetting or head-scratching when reading the plugs and looking at scoop pressure in the data log.

I hear the mental gears grinding! 

CFD visuals, like the little cameras, show that there is a lot more going on than meets the eye. [Be careful what you ask for!] But that helps you plan your experiments and where to place the sensors and what to expect them to tell you. Also to see if you have correlation in the model. Computer models are all "perfect" the real world is not! Sorry, there are no exact answers!

In this case the round duct area is equal to the NACA duct inlet area: 1" X 4" = 4 in².

Attached are some pressure plots of the same run. These were looking for the lowest pressure so you will not see the pressure field associated with the low velocity at the top of the transition duct [~2psi range]. You have to look at several resolutions to "find" what you are looking for. Also some flow trajectories colored with velocity. You will note that since we are pulling the air in that airflow at the NACA duct start is reversed! If you tested this duct on a flow bench I would expect to see this. If we start pushing the air in with a 300 mph airstream then I expect this and the downstream flow to change. Or if you did this on a flow bench and blew across the NACA with a leaf blower then we would try to simulate that condition to see if it agreed with the model.

The fun part is just trying to finger all this Subaru out!

[Tman]

Thanks guys, I dug out some NACA Duct info last night after reading a TON of posts on here about them. Stainless, woody and others. I will be using one for my intake.

[Saltfever]

Thanx Woody, for the follow-up note and clarification. Your CFD pics have rammed the point home on how chaotic the pressure distribution really is inside of the scoop.

In Post #167 Superford says "The NACA duct forms a vortex on each wall that will force more outside air to flow into the inlet than normally would, generating a small ram effect. The two vortices improve the pressure recovery and improve air flow through the duct."

I know what vortices looks like and their powerful effect. Somehow I don't see how that effect on a NACA wall is accomplished as clearly as Superford is saying. I'm not doubting him at this point, but since he doesn't post references I would like to look into this some more. Is there any chance you could explore these wall vortices in CFD and give us a pic? 

[WOODY@DDLLC]

Saltfever, there is a NACA duct forum here: http://www.landracing.com/forum/index.php/topic,4062.0.html

There are some diagrams of the vortex that is formed. I will add that to my list of CFD Subaru to do!! 

More eye candy - think of a vortex generator as an external NACA duct! The vortices roll outward instead of inward! 

[Saltfever]

Thanx, Woody.    Some interesting information in the other thread on how sensitive the location is for a NACA duct.

[hotrod]

Intentional vortex generation is used in many aerodynamic environments.

Here is the exact same process used to develop increased lift at the wing root and help maintain attached flow at high angles of attack.

The fuselage chine on the F16 behaves very much like a one sided NACA scoop. As the high pressure air on the  lower side of the chine spills over to the low pressure side as a "sheet" it rolls up into a high energy vortex that trails aft along the top of the wing root. This significantly increases lift at high angles of attack which would cause flow separation on the wing root if the chine did not exist.



Larry

[hotrod]

Here is a head on quartering view of the vortex condensation rolling up over the top of the wing.



Larry

[manifest]

Forgive me for being simple but how would any of this apply to exhaust flow on a vehicle?  If one was to run the exhaust out of a NACA duct would this help air flow and not interupt the boundry layer as much as if you were to just have the collector bologna cut with the fender or ran straight out the rear?

Just thinking and keeping my mind busy here at work.

Zach

[hotrod]

This discussion has gradually moved to more general topics. The discussion of using exhaust flow energy to help manage air flow is pretty much confined to the first few pages of the topic. It has now broadened out to discuss related aerodynamic issues. The NACA duct and its use is highly dependent on the local flow over the cars body and how the boundary layer behaves.

Larry

[superford317]

As I set typing this, I am watching dust devils crossing back and forth over the Iraqi dessert and watch in amazement at the power they have and the size of some of them.
Vortices are generated by almost all aerodynamic devices, no matter in lift or down force.  Vortices are generated by sharp edges; drop offs and where 2 sharp angles meets each other. Vortices can create more lift, generate more down force or simply generate drag and hinder vehicle performance. Vortices can delay boundary layer separation, allow a wing to have a larger operating envelope, allow a thick turbulent boundary layer to become thinner and even cause a detaching boundary layer to become reattached to the vehicle surface.
As with most anything in life, you don’t get anything for free, the same is true for Mother Nature. If you try to increase the down force from a wing you increase the drag, if you increase the speed of an object through the earth’s atmosphere an increase in drag will result and increase the aerodynamic forces on the vehicle surfaces.
Evolution in nature over billions of years has created a lot of surprises we are just starting to unravel and understand. Everything from the placoid scales on shark skin, I made a rather lengthy post about earlier, that decrease drag on sharks while they swim, to vortices that are formed on the wings of bats, bumblebees and many other creatures in nature that allow them to fly.
If air had no viscosity there would be no drag, no lift or no down force. The viscosity of the air is what makes all of this possible.
Bill Lear invented vortex generators; he used them on his jets. Vortex generators can increase takeoff weight, lower takeoff and landing speeds.
Vortices help to suppress boundary layer turbulence and will reduce the thickness of the boundary layer. The core of the vortex is not always circular and the core radius is always changing.
Some of the factors that will influence vortex development on vehicles are the pressure gradient from the outer air stream, the boundary layer on the vehicle surface, the amount of turbulence in the free stream airflow and the turbulence level in the core of the vortex. The vortex generator size has a large effect on the strength of the vortice. 
As the speed of the free stream airflow increases, the vortice will be stretched, decreasing the radius.
A flat bottom vehicle in close ground proximity will generate vortices at the trailing edge of the vehicle where the upper body meets the underbody and if a rear diffuser is ran, vortices will be generated from each corner of the diffuser. If vortices are generated on the vehicle flat underbody, they will increase airfow and increase the amount of down force generated from the underbody, as the vehicle ground clearance is reduced, the vortices will increase the suction on the underbody. As the ground clearance is reduced the suction on the vortex generator will increase, creating stronger vortices but vortice decay will be greater, due to the increase in vortice speed increasing friction and energy loss, turning the lost energy into het.       
We owe our entire human existence to vortices, if it were not for vortices, bumblebees would not be able to fly and there for not pollinate our crops.
Scientists for many years could not understand how a shark could swim so fast or how many of nature’s creatures could even fly, according to scientific research bats and bumblebees should not even be capable of flight. Putting these creatures in smoke chambers with high speed cameras revealed what was going on and what allowed them to fly. Vortices were the answer, that allowed them flight by increasing airflow over there wings.
Weather you realize it or not many people around the earth are affected by vortices in one way or another. Tornadoes and hurricanes are some of the most powerful forces on earth and are examples of vortices and the power they possess, good or bad.
The mushroom cloud formed from a nuclear explosion is a vortice also. High flying jet aircraft leaving visible contrails is due to vortices.
Vortices in NACA submerged air inlets, on wings in down force or in rear diffusers, all aid airflow.
In our case, vortices are the rotation of gasses around a common center.
The earths spinning motion is what gives hurricanes and tornadoes there rotation and the vortices, both in nature and manmade may become visible due to moisture condensation or when dust is caught up in the vortices. Tornadoes develop from the clouds down to the ground and the dust devils develop from the ground up.
As we understand more and more about vortices it has spawned new nomenclature and even different branches of science. A new form of drag has recently sprung up and talked about more in racing, vortex drag, as more and more devices are being used in racing to generate down force, more vortices are being formed and thus an increase in drag.
Vortices are formed off of each wingtip, in lift or down force. Twin vortices are formed in underbody rear diffusers, on each side wall of a NACA duct and even off of the front and the rear of a tire on open body vehicles.
Some of the vortices are good, helping to make boundary layers stick to surfaces longer and some are bad, only parasites that do nothing but create drag.
Vortices can be maintained on vehicle surfaces for a considerable time if surface heating, suction or blowing are used to aid them.
Some of the vortices can trail behind vehicles for quite a ways, lasting until the viscosity of the air takes enough energy away from the vortices to cause them to break down, turning the lost energy into heat.
Jumbo jets form strong wingtip vortices that trail behind the airplane and last for a considerable time, before breaking down. Small airplanes landing behind jumbo jets have crashed form the turbulence the vortices produce; there are strict rules in place that prohibit small airplanes from landing behind jumbo jets for several minutes.
Counter rotating wingtip vortices are produced by wings in lift or wings in down force. A lifting wing has a low pressure area on top and a high pressure area on the bottom, the( HIGH  ) pressure air will travel to and try to mix with the (  LOW  ) pressure air and that is where the vortices are produced, as the air travels over the wing tip and falls over the edge and starts swirling. The same thing happens with a wing in down force, there is a high pressure area on top of the wing and a low pressure area below the wing and the (  HIGH  ) pressure air will try to travel to the area of (  LOW  ) pressure at the wing tip, forming the vortice where the two pressures meet.
As a lifting wing gets close to the ground the wingtip vortices begin to break down thus decreasing induced drag. As the wing drops below one wing span from the surface, ground effects start to take over and become stronger as it gets closer to the ground, the lift to drag ratio can be increased by 50% from ground effect on the wing.   
There are several different types of vortex generators that can be used on vehicles or aerodynamic devices, to create vortices, some being capable of generating multiple vortices at one time. Some are known as half delta, rectangular or wing type, are but a few of them.
The qualities of vortices generated by vortex generators can be influenced by the angle, height, air flow and edge radius of the generator. 
Vortex generators can be broken down into 2 classes. Vortex generators that do not reach above the vehicle boundary layer, known as sub boundary layer vortex generators and vortex generators that reach above the vehicle boundary layer into the free stream airflow. The drag generated by sub boundary layer vortex generators will be substantially lower than the drag from the taller vortex generators.
The strength of the vortex generated by a regular vortex generator will be much more than the vortices generated by a sub boundary layer vortex generator. The strength of the vortices will be reduced over time by the viscosity of the air taking energy away from the vortices and converting it into heat.
Sub boundary layer vortex generators placed on a vehicle surface will be affected by the boundary layer pressure gradients similarly. Over a distance equal to 40 sub boundary layer vortex generator heights the vortice strength was within 3% in a zero pressure gradient and a adverse pressure gradient.
If placed on a highly angled surface, sub boundary layer vortex generators would be better than regular vortex generators. On an angled surface sub boundary layer vortex generators will have a longer life without breaking down.     
 
CAPS IN (  )  are transposition corections PER SF 317 request  ---sparky 11/16/2011

[Seldom Seen Slim]

Shall I archive this Superford post, too?

[WOODY@DDLLC]

Except maybe for this part: 

Counter rotating wingtip vortices are produced by wings in lift or wings in down force. A lifting wing has a low pressure area on top and a high pressure area on the bottom, the low pressure air will travel to and try to mix with the high pressure air and that is where the vortices are produced, as the air travels over the wing tip and falls over the edge and starts swirling. The same thing happens with a wing in down force, there is a high pressure area on top of the wing and a low pressure area below the wing and the low pressure air will try to travel to the area of high pressure at the wing tip, forming the vortice where the two pressures meet.

Air can only flow from high pressure to low pressure - it's a one way street!

http://centennialofflight.gov/essay/Theories_of_Flight/Vortex/TH15G1.htm

[Interested Observer]

If air had no viscosity there would be no drag, no lift or no down force. The viscosity of the air is what makes all of this possible.

This is complete nonsense.  As is the notion of air flowing from low pressure to high pressure.  It is clear that Superford has no understanding of fluid dynamics.

It would better serve the community if all of his postings were put in one place where people that want to read the random, repetitive, inaccurate, and contradictory regurgitations of information gleaned from other sources can do so, and where such information would not contaminate authoritative contributions.

Superford's most useful contribution would be to cite his sources, so anybody who is interested could read the information in its correct context and thus be able to evaluate its authority and applicability.

[Seldom Seen Slim]

Thanks both Woody and IB.  I'm not versed well (at all) in aero stuff, so rely on folks that are (you two, in this case) to vet the information given to help determine what should be saved in an easy-to-find location for use as reference material.  So far, on this subject, we've got one "Heck, no!" and one "Well, most of it".  Any others care to help decide?