From cc391ce3b9867d88d124e147931d33be34e756fc Mon Sep 17 00:00:00 2001 From: Prefetch Date: Mon, 14 Sep 2026 18:11:38 +0200 Subject: Improve knowledge base --- source/know/concept/bernoullis-theorem/index.md | 29 ++++++------------------- 1 file changed, 7 insertions(+), 22 deletions(-) (limited to 'source/know/concept/bernoullis-theorem/index.md') diff --git a/source/know/concept/bernoullis-theorem/index.md b/source/know/concept/bernoullis-theorem/index.md index 867c443..2795f22 100644 --- a/source/know/concept/bernoullis-theorem/index.md +++ b/source/know/concept/bernoullis-theorem/index.md @@ -12,36 +12,21 @@ layout: "concept" For inviscid fluids, **Bernouilli's theorem** states that an increase in flow velocity $$\va{v}$$ is paired with a decrease in pressure $$p$$ and/or potential energy. -For a qualitative argument, look no further than -one of the [Euler equations](/know/concept/euler-equations/), -with a [material derivative](/know/concept/material-derivative/): - -$$\begin{aligned} - \frac{\mathrm{D} \va{v}}{\mathrm{D} t} - = \pdv{\va{v}}{t} + (\va{v} \cdot \nabla) \va{v} - = \va{g} - \frac{\nabla p}{\rho} -\end{aligned}$$ - -Assuming that $$\va{v}$$ is constant in $$t$$, -it becomes clear that a higher $$\va{v}$$ requires a lower $$p$$. - - -## Simple form - -For an incompressible fluid +Quantitatively, for an incompressible fluid with a time-independent velocity field $$\va{v}$$ (i.e. **steady flow**), -Bernoulli's theorem formally states that the -**Bernoulli head** $$H$$ is constant along a streamline: +Bernoulli's theorem states that +the **Bernoulli head** $$H$$ is constant along every streamline: $$\begin{aligned} \boxed{ H - = \frac{1}{2} \va{v}^2 + \Phi + \frac{p}{\rho} + \equiv \frac{1}{2} |\va{v}|^2 + \Phi + \frac{p}{\rho} } \end{aligned}$$ Where $$\Phi$$ is the gravitational potential, such that $$\va{g} = - \nabla \Phi$$. -To prove this theorem, we take the material derivative of $$H$$: +To prove this theorem, we take the +[material derivative](/know/concept/material-derivative/) of $$H$$: $$\begin{aligned} \frac{\mathrm{D} H}{\mathrm{D} t} @@ -50,7 +35,7 @@ $$\begin{aligned} + \frac{1}{\rho} \frac{\mathrm{D} p}{\mathrm{D} t} \end{aligned}$$ -In the first term we insert the Euler equation, +In the first term we insert the [Euler equation](/know/concept/euler-equations/), and in the other two we expand the derivatives: $$\begin{aligned} -- cgit v1.3