When designing transportation systems like planes, trains, automobiles, and ships, engineers always look for ways to reduce friction to increase the speed and efficiency of the vehicle they are building. Friction can arise in several forms: static, kinetic, and rolling friction, as well as drag (better known as air resistance in the case of vehicles travelling through the air).

A hyperloop is a new type of proposed transportation system in which frictional forces are minimized through the clever application of some pneumatic and fluid dynamics principles.

First, to reduce friction resulting from wheels rolling on a track, the hyperloop capsule does away with wheels entirely. Instead, it incorporates an air compressor that delivers a supply of pressurized air to a series of air bearings, which maintain a thin layer of pressurized air between the track and the capsule supports, allowing movement with virtually no friction between the capsule and track.

Second, to reduce drag, more pumps are used to draw air out of the tube, reducing the pressure in the tube as well as the density of the air. This results in very low drag, since there are fewer air molecules to impede the motion of the capsule.

It is believed that a hyperloop system, if constructed, could support capsule speeds near 1000 km/h, faster than any common existing mode of transportation, including passenger jets.

Pneumatic Systems

Just like hydraulic systems, pneumatic systems use a fluid to transmit and control pressure, which can be turned into a force to accomplish useful work. While hydraulic systems use an incompressible liquid, pneumatic systems use a compressible gas (usually air).

Pneumatic systems use many of the same components that hydraulic systems do including cylinders, transmission lines, fittings, valves and filters. Instead of a hydraulic pump, a pneumatic system uses an air compressor to transform mechanical energy into a higher fluid pressure. Pneumatic systems also include a type of reservoir, but, because gases need to be fully contained, a pressurized tank is used to store the compressed gas from the compressor until it is needed. In many cases, the compressor and tank are built as a single combined unit, so that they can be easily repositioned in an automotive repair shop, for example.

Because the fluid in a pneumatic system is compressible, pneumatic actuators such as cylinders, are not great for holding a fixed position, since a change in the force on the cylinder’s rod will act to compress or decompress the gas in the cylinder and move the piston. Pneumatic systems can pose a higher physical danger than hydraulic systems due to the energy that is stored in the system. If a tank or transmission is punctured, this energy will be released in an uncontrolled manner than may cause damage or injury. The escaping gas might cause objects to move rapidly. Recall that gases cool when they expand, so a person in the way of a pressurized gas leak would be at risk of a “cold burn” – damage to the skin resulting from contact with a very cold substance. While there are fewer environmental concerns with pneumatic systems they still pose real safety risks.

Pneumatic systems offer many advantages. Because the fluid can be air, leaks do not contaminate nearby surfaces as would be the case with hydraulic fluid, such as oil. Air is readily available so this makes pneumatic systems more portable. Because air can be moved through transmission lines very quickly, pneumatic systems can react more quickly than hydraulic systems in many applications.

Applications of Pneumatic Systems

Pneumatic systems typically have a wider variety of actuators than do hydraulic systems. Especially if the pneumatic system uses air as its fluid, the compressed air can be returned to the surroundings by an actuator, simplifying the design of the system and opening up many new design possibilities. The following examples list some (but not nearly all) common applications of pneumatic systems.

Air brakes are commonly used on large trucks, particularly semi trucks where a tractor connects to a trailer. Using pneumatic air brakes instead of hydraulic brakes makes it much easier to connect to a trailer since there is no risk of brake fluid leaking into the environment or of air leaking into a hydraulic line.

Air tools, such as wrenches, nailers, grinders, hammers and grease guns convert the potential energy stored in a compressed air tank into mechanical energy. These have the advantage that they can be made to be lighter and smaller than a comparable electric tool.

Exercise machines can be built with pneumatic actuators that vary their resistance depending on their internal air pressure. A single machine can be adjusted to give a small or large amount of resistance or anywhere in between, and can be built much lighter than a comparable machine that used weights to offer resistance.

Air bladders can be used to right or raise a damaged ship and recover trucks that have rolled over. The air bladder is placed under the vehicle, and gradually inflated by an air compressor so that it produces an upward force to right the vehicle. This type of recovery has the advantage that the force is distributed over a large area, supporting for example, weak or failed sidewalls.

Paint sprayers use compressed air to draw paint from a reservoir and apply it onto a surface. This allows paint to be applied much more quickly than if it were applied by hand using a brush or roller.

Pneumatic tires use compressed air or nitrogen to remain inflated. Pneumatic tires are lighter and provide a smoother ride with better traction than a solid tire design would offer.

Viscosity

The particles that make up a fluid experience forces of attraction toward each other. These attractive forces are responsible for keeping a raindrop together as it falls through the air, but they are also responsible for a key property of a fluid known as viscosity. Simply put, viscosity is a measure of how difficult it is to get a fluid to flow; the attractive forces between the particles make it difficult for the fluid to flow.

Fluids with high viscosity, like honey or maple syrup, don’t flow as easily as fluids with low viscosity, like water or gasoline. Viscosity is not only dependent on the fluid in question, but also on its temperature. The particles in a fluid at a low temperature will be moving more slowly than particles in a fluid at a high temperature. Since the particles are moving more slowly, the attractive forces they feel toward each other are better able to keep them from moving about too much, making it more difficult for the fluid to take the shape of its container. This difference is obvious to anyone who has tried to pour maple syrup from a jar straight out of the refrigerator.

Laminar and Turbulent Flow

While a fluid is in motion, its particles experience forces of attraction to each other and their surroundings. This means that particles near a surface will find it difficult to move, as they will be attracted to the particles of the surface which are stationary. Particles some distance away from the surface will be able to move more easily. At lower rates of flow, this results in what is known as laminar flow.

Laminar flow is characterized by the straight lines that the fluid’s particles move in. In a pipe, particles near the walls of the pipe will be moving slowly and particles in the middle of the pipe will be moving more quickly. If the flow is laminar, they will all move in straight lines.

If there is an irregularity in the surface in contact with the fluid in motion, its particles will interact with the irregularity and this will disrupt their otherwise smooth and straight-line motion, resulting in chaotic paths for the particles known as eddies. An irregularity can be a physical barrier that is in the way, an opening that the fluid passes by, or even just a patch of pipe surface that is rougher than the rest of the pipe.

The number and size of the eddies will increase with larger irregularities and faster rates of flow. If the rate of flow is high enough, even microscopic imperfections in the surface that is in contact with the fluid in motion will create eddies. This is known as turbulent flow, where there is very little order to the motion of the particles.

If you’ve ever flown through turbulence in an airplane, the chaotic motion of the air particles is obvious, as the air currents jostle the airplane on all sides and affect the lift of the wings and the ability of the pilot to control the flight. Fluids with low viscosity are more susceptible to turbulent flow than fluids with high viscosity. This makes sense, as the particles in a fluid with high viscosity are more strongly attracted to each other, so they might be less disturbed by an irregularity in a surface they are in contact with and recover from any chaotic motion induced by the irregularity more quickly, returning to a smooth and laminar flow.

Applications of Laminar and Turbulent Flow

Turbulent flow wastes energy, as some energy is converted into kinetic energy of the fluid particles in eddies. As a result, engineers seek to reduce or eliminate turbulent flow wherever they can. In general, the closer a system can get to a laminar flow, with few or no eddies, the more efficient it will be.

When designing vehicles, engineers will seek to make the vehicle as streamlined as possible with a smooth shape from front to back, smooth body surfaces, and as few irregularities on the body as possible. Vehicles like pickup trucks generate a lot of turbulent flow due to the irregular shape of the cab; this is one reason that pickup trucks have poorer fuel economy than cars.

 

When building pipelines to transport water, oil, and other fluids, the pipeline diameter must be chosen to be large enough so that the pipeline can sustain the necessary volumetric flow rate (the volume of fluid passing through a point per unit time) while still keeping the velocity of the fluid flowing through the pipe low enough to ensure laminar flow and therefore higher efficiency than would be possible with turbulent flow.

 

Despite laminar flow being desirable in many situations, turbulent flow also has its uses. For example, when constructing a heating or cooling coil, turbulent flow within the coil will result is more efficient heat transfer, as there will be more mixing of the fluid in the coil due to all of the eddies. If the flow in the coil were laminar, it would be difficult for heat to transfer to or from fluid in the middle of the tube making up the coil, so turbulence in this case can improve efficiency.

Bernoulli’s Principle

Daniel Bernoulli was a Swiss scientist who studied fluids in motion. He came to the conclusion that if a fluid is moving slowly, the pressure will be high and if a fluid is moving quickly, the pressure will be low. The speed Bernoulli referred to was the speed of the particles of the fluid (not the volumetric flow rate). A Venturi tube provides a good demonstration of this effect.

Bernoulli’s principle, as it applies to a Venturi tube, at first seems counterintuitive. All of those particles of fluid are being squished into a smaller diameter section of tube, so the pressure there must be higher, right? Wrong. The correct way to think about the fluid dynamics at work in a Venturi tube is to think about the energy in the fluid. Recall that pressure is a kind of potential energy in a fluid. The fluid in the Venturi tube is moving, so it also possesses kinetic energy. The fluid must speed up to travel through the smaller diameter tube, meaning the particles in that section of tube will have higher kinetic energy than the slower moving particles in the larger diameter sections of tube. Since energy is always conserved in a closed system, some of the potential energy resulting from the pressure the fluid is under is transformed into greater kinetic energy. So, when a fluid speeds up, the pressure goes down.

Another way to think about the Venturi tube is by imagining a particle of fluid as it transitions from the larger diameter section of tube to the smaller diameter section of tube. There must be some force acting on the particle to speed it up. There is, it is the pressure differential between the high pressure fluid in the larger diameter section and the low pressure fluid in the smaller diameter section that gives the particles a “push” to speed up . When they reach the transition from back to the final larger diameter section, there is again a pressure differential, but this time it is the higher pressure in the final larger diameter section of tube that acts to slow the particles down as they exit the smaller diameter section.

Summary