1. ## Differentiation Simples

Why is this true?

for $\rho(\eta,t)$

$
\dfrac{\partial}{\partial \eta}(\ln(\rho))=\dfrac{1}{\rho}\cdot\dfrac{\parti al \rho}{\partial\eta}
$

Is this needed to derive

$
\dfrac{D\ln(\phi)}{Dt}=\nabla\cdot\bf U.
$

from the continuity equation

$
\dfrac{\partial\rho}{\partial t}+\nabla\cdot(\rho \bf U)=0.
$

where $\phi(\textbf{x},t)=1/\rho(\textbf{x},t)$ is the specific volume

2. Originally Posted by davefulton
Why is this true?

for $\rho(\eta,t)$

$
\dfrac{\partial}{\partial \eta}(\ln(\rho))=\dfrac{1}{\rho}\cdot\dfrac{\parti al \rho}{\partial\eta}
$

Is this needed to derive

$
\dfrac{D\ln(\phi)}{Dt}=\nabla\cdot\bf U.
$

from the continuity equation

$
\dfrac{\partial\rho}{\partial t}+\nabla\cdot(\rho \bf U)=0.
$

where $\phi(\textbf{x},t)=1/\rho(\textbf{x},t)$ is the specific volume
$
\dfrac{\partial}{\partial \eta}(\ln(\rho))=\dfrac{d \ln (\rho)}{d \rho} \cdot \dfrac{\partial \rho}{\partial\eta}
$

3. Thank you. You have answered many of my questions in the past. You really do live up to your name. Any idea on the second part?