### #25Fundamentals_Navier-Stokes equation

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(It seems a little bit awkward for me to consider Navier-Stokes equation as fundamentals.....XD")
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Navier-Stokes equation is one of the most important equations in fluid mechanics. From the simulation of weather to the current in blood vessels, every complex problem related to fluid requires it to be solved or approximated. However, because the solution of Navier-Stokes equations are usually very complex or even unsolvable currently, most of the time we will rely on numerical simulation to finish the task. Today we will focus on the physical meaning of Navier-Stokes equation, which is basically the Newtonian second law used to describe the change in flow field.
Navier-Stokes equation可說是流體力學中最重要的方程式之一，舉凡天氣、洋流、飛機感受到的氣流、血管中的血流等，要對他們進行模擬與計算，最基本的統馭方程大概都是Navier-Stokes equation或他的化簡變形版本。但因為這個方程式太過複雜，只有非常化簡的情形可以人工計算，大部分的時候都是仰賴數值模擬。克雷數學研究所(Clay Mathematics Institute,CMI)於2000年公告的七個千禧年大獎難題中，就有一個是關於Navier-Stokes equation。今天我們要來和大家解釋Navier-Stokes equation中每個項所代表的物理意義，本質上他就是牛頓第二運動定律用來描述流場的變化而已。
Firstly, we shall consider the acceleration of a fluid element. Since the location of the fluid element would also change with time, we have to account both the changes with time and the changes with space to fully consider its acceleration. For simplicity, we should consider motion in 1 dimension first. Assume that the fluid element move Δy in time Δt, then its change in velocity could be written as: 首先我們要先考慮一個單位流體(fluid element)的加速度。由於組成流場的單位流體隨時間流逝，他的位置也有改變，因此我們在探討流場的加速度時，除了時間的變化之外也要考慮空間的變化。如果我們只考慮一維的情形，要算出加速度必須知道速度的變化量，所以對一單位流體而言，假設經過Δt他往前移動了Δy，那麼它的速度變化為：
So we can write down the acceleration as:

And the motion in 3 dimensions would then be:

This could be simplified by some vector calculus:

The former term in the RHS is the change in flow field with respect to time while the latter term is the change in flow field with respect to space.

Recall that the Newtonian second law told us "F=ma." If we could write down the acceleration of a mass, we could know how much forces the fluid element is enduring. Here we shall consider body force, pressure force and viscous force.

The simplest term of the three is the body force, which means forces only related to the volume of the fluid element, rather than to the surface area of it. Classic examples include gravitational forces and electrostatic forces. Assume that the fluid has a density of ρ, we could write down the body force as:

Sometimes this term could be ignored all at once if we are discuss a system which limits the motion of fluid in a horizontal plane.

Let's move on to the effect of pressure force. We shall focus on y direction as we just did previously. From the figure shown above, the net pressure force in y direction could be written as:

So this term would become something like this if we consider all 3 dimensions:

The last one would be the viscous force. According to the hypothesis proposed by Newton, viscous force could be calculated as viscosity times the gradient of fluid speed. Therefore, let's first consider the viscous force generated from the change of Vz with respect to y direction:

If we consider the changes in both side of the fluid element, the net viscous force would be:

Then we further consider the changes of Vz with respect to 3 directions, x, y, and z, and we could write down the net viscous force as:

Finally we consider the changes of Vx, Vy and Vz, then we will get:

Add all these stuffs together and we will get our Navier-Stokes equation:

We may use these terms to perform dimensional analysis, or we may compare the effect of these terms to determine which one we could ignore. Please make sure you fully understand the meaning of each terms before you move on. Stay tuned!