Sign into an existing account

https://www.youtube.com/watch?v=ArQE3SB0kyM

Non-technical. Video illustrates principle with paper cup and ping pong ball. Decent video of actual installations.

Suppose you have a pipe with x amount of air flowing through it. Suppose the flow is nice and laminar throughout the pipe, it's velocity distribution will look something like this.

https://files.catbox.moe/ysa5y8.jpg

The flow near the walls will be slower because the drag from the wall's boundary layer and the moving flow slows it down, while it moves faster near the center of the pipe because the greatest mass of air is moving uniformly carrying its momentum forward with no wall to obstruct it.

Now, what happens if we put a vortex in the middle of that pipe?

https://files.catbox.moe/yk9slc.jpg

The vortex itself is occupying the middle of the pipe, as long as the vortex maintains its integrity, the air cannot pass through the vortex it must go around it. Therefore the vortex is acting like a flow restriction, reducing the cross sectional area of the pipe. The shear stress of the vortex squeezes the air against the walls of the pipe, and reduces the cross sectional area of the pipe. In order to maintain continuity, the flow speeds up, and the pressure in the pipe drops.

People think it's some magic ability, but it isn't. Anyone can have CFD eyes, all you have to do is understand how air behaves. There's no magic to it, you just understand.

When air impinges on a moving object, the object is pressed against the air, the pressure increases. Air mass is deflected away, some of the air sticks to the object, most doesn't. Most of the air flows around a thin layer of air that sticks to the object. To really understand air you really need to understand that air sticks, and the air isn't flowing around an object as much as air is flowing around air.

Let me rephrase that, air doesn't flow around objects, air only flows around more air. The shape the air takes is defined by the object doing the deflecting. Assuming a constant and steady motion in still air, the object displaces air, in a steady and consistent way. That steady and consistent wake then becomes the actual object displacing the air around it. The wake then becomes a sort of virtual bodywork.

Recirculation bubbles aren't necessarily a bad thing, they can create flow restrictions which accelerate flow when needed.
https://youtu.be/qhsTQn0uUOQ?t=1244

https://velocimetry.net/psp_principles.htm

Flowvis only shows you the boundary layer, pressure taps and keel probes only show pressure. PSP shows you both, and unlike flowvis, it's not easily seen by others without a special light. Which means it can be hidden from competitors. Unlike keel probes/pressure taps, pitot tube arrays, there is nothing affecting the Aerodynamics of what you are investigating upstream or downstream.

What are the properties of a vortex? To answer this question, it helps to understand the properties of air, and how air seeks stillness. Air is a substance, this substance occupies space, has mass, and inertia however it is fluid, it yields to solid objects with higher density. When you move a solid object through the air, the air being disturbed, seeks to become still once again. It does this by filling in the wake left by the object with itself. If the air tries to fill in the wake but misses the low pressure center, that is it overshoots, the air will keep circling that low pressure center trying to fill in the wake in order to return to equilibrium. When a mass of air is made to overshoot a low pressure center, the resulting spiral motion is known as a vortex.

A vortex is a mass of air circulating and trying to fill in a center of low pressure. The vortex itself does not produce suction, the vortex is a result of low pressure, and high pressure being separated. Meaning the suction already happened, because the pressure was already increased somewhere else. Now, being that angular momentum is conserved with substances with mass and inertia, such as air, the rotational energy of the vortex is transferred to the surrounding air. This can be helpful to keep airflow attached to a surface, as the vortex wall impinges on a solid surface, it basically scrapes away the boundary layer in the direction of rotation combined with the free stream velocity. This entrains surrounding air to fill in the outwashed air, which further entrains air from upstream.

https://files.catbox.moe/r3fg62.jpg

Notice in this image that the vortices at the wing tip are being pulled into the center of the jet. That is because the entire upper surface is a low pressure surface. This means that pressure gradients can be used to steer vortices.

During the blowdown phase, as the exhaust valves open, the gas entering the exhaust ports is just about supersonic, especially at low lift angles. This supersonic flow is susceptible to expansion fans.

https://en.wikipedia.org/wiki/Prandtl%E2%80%93Meyer_expansion_fan

This is why radiused exhaust valve seats and tulip valves with a rounded valve edge on the CC side helps flow. Expansion fans accelerate flow and are isentropic, that means without generating entropy in the way that a normal or oblique shockwave does. Not only does this simplify the calculations, but the expansion fans have very little losses. Typically a shockwave acts as a barrier for air, and causes non-reversible losses. An expansion fan on the other hand eliminates MOST, not ALL losses up to the accelerated velocity of ~Mach 1.4.

The angle needed to achieve M1.4 via expansion fans is ~9-10 degree steps. The air that flows through the seat and the valve face is flowing through a series of angles, if these angles can be kept to 9-10 degrees, one can dramatically lower losses on exhaust blowdown.

This is a lot harder to achieve on the intake valve(s) because the flow there never goes above Mach .55 maybe locally at low lifts with high VE it could get close to M=.6-.7 or the trans-sonic regime. Although for port injection, one also has to consider wet flow(ie fuel) and as a result limits the angles that can be used depending on the engine application. So while a full radius job on the intake will flow more CFM, it won't necessarily make more power, because smooth angles not only allow air to stick, but also any other fluid, such as fuel.

https://i150.photobucket.com/albums/s92/chippievw/F1technical/F1technicalintakeport002_zps9d11215f.jpg

Here you can see a cross section on the valve seats of a Cosworth V-10 F1 engine. Exhaust on the right, intake on the left. You can see the curve on the exhaust port is more pronounced compared to the relatively straight intake port, which only has a slight curve near the valve seat. This turn is necessary to guide airflow across the seat and into the combustion chamber, as well as to allow fuel and some air to separate from the wall to avoid pooling. Of course, at 18,000rpm the air is already mostly turbulent and will mix the crap out of the fuel. However at part throttle and lower rpm, the flow can be so laminar that fuel sticks to the walls and falls out of suspension.

For this reason engines that operate at lower RPMs and are undervalved(ie not enough valve for the combustion chamber), will benefit from 5 discreet angles on the intake side, particularly if the engine is carbureted.

https://www.speednik.com/wp-content/blogs.dir/1/files/2013/05/Valves_2.jpg

While such a setup will flow a bit less than a full radiused valve seat, they typically work better in terms of combustion stability, and can help to limit flow reversion. On the exhaust seats, there is usually no fuel being burned(unless you're doing anti-lag), and you want the flow to leave the CC as fast as possible, doing full radius helps. You don't have much reversion on the exhaust seats, because the exhaust manifold is at a much lower pressure, and the overlap helps maintain the pressure gradient towards the exhaust.

With direct injection, since you don't have to worry about fuel in the intake, the rules change somewhat, and what is beneficial for the exhaust side, is also beneficial for the intake side as fuel is injected directly into the CC. When boost is introduced the similarities and ability to exploit expansion fans starts to work on the intake side because air density has a small but measurable effect on flow velocity. This can leave the air smack dab in the transonic region especially at low lift levels. Since valves open and close, the valves are at low lifts twice, and high lift once.

In order to understand fluid dynamics, I felt it would be helpful to share how air behaves under different circumstances. This will be a multi-part series, where I describe air and why it flows the way it does. I will try to keep things simple and easy to understand, with little to no math. Any equations I present will be explained in simple terms that are intuitive.

Air is capable of creating incredible forces, it keeps planes up in the air, it can crush steel drums, it keeps water as a liquid, how is this possible? This is because air has mass, at sea level the pressure is nearly constant and there is a pressure of ~14.7 pounds over every square inch of area. This pressure is pressing on all objects from all sides, as above so below the saying goes. It is this fact that we use and exploit in order to create aerodynamic forces. Air wants to fill everything with itself, and whenever something moves through the air, the physical object removes air for a split second, and as a result air will try to fill that displacement. This is true of fluids in general.

You may have heard the term "nature abhors a vacuum" this is especially true of air. It is the motion of air attempting to fill that vacuum that creates aerodynamic forces. Then following this logic, air moves as a result of the difference in pressure, the default atmospheric pressure, or what is known as "static pressure" will move towards anything that is less than this standard pressure.

Where it starts to get complicated is the path the air takes to fill that difference in pressure. Logic would tell you that the most efficient path between two points is a straight line, so naturally you would think that air follows the path of least resistance, I.E. a straight line. However this is not the case, air rarely if ever flows in a smooth straight line except under the most carefully controlled conditions. By and large, air takes a curved path, that is its preferred path of motion, why does air do this?

The primary reason is that air has mass, and as a result it is subject to inertial and viscous forces. Thus we have the Reynolds number, which accounts for the inertial and viscous forces in a packet of air and serves as a rough estimate of when a flow becomes turbulent. The Reynolds number is estimated as the density of the fluid times the velocity, times the distance the fluid covers, over the viscosity of the fluid. The smaller the Reynolds number the more likely the flow is to be laminar, linear and smooth. Inversely, the greater the Reynolds number the more likely it is to be turbulent and chaotic.

This means that increasing the size of the air packet, or parcel, increasing the velocity of the flow, or its density will give you a greater reynolds number, in other words a flow more likely to be turbulent and chaotic. Inversely the greater the viscosity of air, the lower the reynolds number. Interestingly, the viscosity of air increases with temperature, and its density decreases. Thus temperature has a big influence in the tendency of air to become chaotic and turbulent, however this relationship is not linear.

Another reason why air likes to take curved paths is a result of the speed air moves at, this is because as mentioned previously air has mass, as a mass of air moves, it displaces air, and as a result lowers the static or default pressure of a given volume. Bernouli tried to capture this in his famous equation. As a result of moving or displaced air in a given volume the surrounding air will move to fill that slightly lower pressure from all sides. The easiest way for something to surround something from all sides is a sphere. However, if the air keeps moving, the leading part of the displaced air will have higher pressure as it butts or collides with the standard or static pressure, and a lower pressure on the trailing side. Thus, the tendency of air to fill that low pressure will be curved.

In the next part, I'll cover more advanced aspects of airflow, including why vortexes happen, thank you for your time.