“Alice is like Princess Kaguya, except instead of being found in a stalk of bamboo and sending suitors on impossible quests, she was found in a toy store and eats all the candy.” – Fossilmaiden

Fluids are things that flow, like gases and liquids, but not solids.

Fluid flow is generally categorized into two "flow regimes" -- laminar flow, where the fluid flows smoothly and in well-organized manner, and turbulent flow, where the fluid flows in a chaotic manner, tumbling over itself. Sometimes, very slow flow falls into a third regime, called creeping flow. These differences matter a lot because it matters how the fluid flows when you're trying to get it to do something, such as trying to make a chemical reaction happen in it. You may have noticed that water flowing slowly looks smoother than water flowing quickly, and that's an example of the difference between laminar and turbulent flow.

The way to determine flow regime is usually by using what's called the Reynolds Number. This is a dimensionless quantity, which sounds a little weird but that basically means it's functionally similar to a ratio. The Reynolds number relates the speed of the flow, the viscosity of the fluid, and some other things. Laminar flow (and creeping flow) are much simpler to model mathematically, than turbulent flow.

The system of equations that describes the flow of a fluid is known as the Navier-Stokes Equations. Any solution to system of equations is really freaking complicated (and symbolic solutions are impossible) except for the simplest of flow situations. If you really need to solve them, simplify the assumptions and/or throw them into a computer using numerical estimations of differential equations. Otherwise, just use some other estimations for the properties of that bulk flow.

Chemical engineers use chemical reactions to make products on an industrial scale. This often means working with fluids, changing them chemically or physically. Fluids can be used to make chemical reactions happen by doing things like: * passing them over a surface made of a different material, which may be a reactant or a catalyst * flushing the fluid through an obstruction of a different material, which the fluid has to flow through/around. This is called a "bed" of material. If the material just stays in place while the fluid flows around it, then it's called a "packed bed", but if the material can split into pieces and move around with the fluid as the fluid flows through it, then it's called a "fluidized bed".

(This is why it is a known fact that chemical engineers do it in packed beds.)

When a flowing fluid picks up an object (such as a piece of a material bed, or a pebble in a stream), this process is called "entrainment", wherein the fluid "entrains" the object. You are free to imagine this as a train named "The Fluid" smashing into something and continuing on its merry way, except the train isn't running on tracks and can just go any which way it wants as dictated by the Navier-Stokes Equations.

Boundary conditions are a thing in fluid flow. When a fluid flows past a different material, the pattern of flow in the middle is called "bulk flow", and this pattern is different from the flow in the "boundary layer", near the surface of the other material. There isn't an absolute edge between bulk flow and boundary layer, but it's generally defined by whether boundary still has much influence mathematically. Laminar flow boundary layers are thinner and more orderly, while turbulent flow boundary layers are thicker and contain little swirls called "eddies" (singular "eddy"). Flow rate is slower at the boundary and rises to meet the rate of bulk flow as you go further into the fluid. In short, boundary conditions mean that you have to conceptually account for there being a buffer area at the edge of a fluid where the flow properties may be different.

also the Navier-Stokes equations are extraordinarily complex differential equations for which the "trivial" case (involving something like laminar flow through a simple geometry, or such) is barely solvable symbolically, and anything past that, good luck

## Comments

Fluids are things that flow, like gases and liquids, but not solids.

Fluid flow is generally categorized into two "flow regimes" -- laminar flow, where the fluid flows smoothly and in well-organized manner, and turbulent flow, where the fluid flows in a chaotic manner, tumbling over itself. Sometimes, very slow flow falls into a third regime, called creeping flow. These differences matter a lot because it matters how the fluid flows when you're trying to get it to do something, such as trying to make a chemical reaction happen in it. You may have noticed that water flowing slowly looks smoother than water flowing quickly, and that's an example of the difference between laminar and turbulent flow.

The way to determine flow regime is usually by using what's called the Reynolds Number. This is a dimensionless quantity, which sounds a little weird but that basically means it's functionally similar to a ratio. The Reynolds number relates the speed of the flow, the viscosity of the fluid, and some other things. Laminar flow (and creeping flow) are much simpler to model mathematically, than turbulent flow.

The system of equations that describes the flow of a fluid is known as the Navier-Stokes Equations. Any solution to system of equations is really freaking complicated (and symbolic solutions are impossible) except for the simplest of flow situations. If you really need to solve them, simplify the assumptions and/or throw them into a computer using numerical estimations of differential equations. Otherwise, just use some other estimations for the properties of that bulk flow.

Chemical engineers use chemical reactions to make products on an industrial scale. This often means working with fluids, changing them chemically or physically. Fluids can be used to make chemical reactions happen by doing things like:

* passing them over a surface made of a different material, which may be a reactant or a catalyst

* flushing the fluid through an obstruction of a different material, which the fluid has to flow through/around. This is called a "bed" of material. If the material just stays in place while the fluid flows around it, then it's called a "packed bed", but if the material can split into pieces and move around with the fluid as the fluid flows through it, then it's called a "fluidized bed".

(This is why it is a known fact that chemical engineers do it in packed beds.)

When a flowing fluid picks up an object (such as a piece of a material bed, or a pebble in a stream), this process is called "entrainment", wherein the fluid "entrains" the object. You are free to imagine this as a train named "The Fluid" smashing into something and continuing on its merry way, except the train isn't running on tracks and can just go any which way it wants as dictated by the Navier-Stokes Equations.

Boundary conditions are a thing in fluid flow. When a fluid flows past a different material, the pattern of flow in the middle is called "bulk flow", and this pattern is different from the flow in the "boundary layer", near the surface of the other material. There isn't an absolute edge between bulk flow and boundary layer, but it's generally defined by whether boundary still has much influence mathematically. Laminar flow boundary layers are thinner and more orderly, while turbulent flow boundary layers are thicker and contain little swirls called "eddies" (singular "eddy"). Flow rate is slower at the boundary and rises to meet the rate of bulk flow as you go further into the fluid. In short, boundary conditions mean that you have to conceptually account for there being a buffer area at the edge of a fluid where the flow properties may be different.