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Telemachus
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Hi there. I'm reading Gurtin's 'the mechanics and thermodynamics of continua', and working some exercises of his book. In the section 21: 'The first law: balance of energy', after the derivation of the balance equation, he uses an identity to rewrite the balance of energy.
The balance of energy can be written:
##\rho \dot \varepsilon=T:D -div(\vec q)+q##
The dot over the epsilon represents the material time derivative, epsilon is the internal energy, rho the mass density, T is the Cauchy stress tensor, D the stretching tensor, and ##\vec q## is the heat flux vector, while the scalar ##q## represents the heat transferred to the region by agencies external to the deforming body (e.g. radiation).
Then, due to a relation previously derived in the book, the given material time derivative is related to the spatial time derivative through:
##\rho \dot \varepsilon=(\rho \varepsilon)'+div (\rho \varepsilon \vec v)##
Then, the balance of energy in the local form can be written:
##(\rho \varepsilon)'=T:D-div(\vec q+\rho \varepsilon \vec v)+q##
The book then asks to provide a physical interpretation for the quantity: ##\vec q+\rho \varepsilon \vec v##.
What I did was to rewrite the localized form in the global form for the divergence:
##\displaystyle -\int_{\cal P(t)} div(\vec q+\rho \varepsilon \vec v) dv=-\int_{\partial \cal P(t)}\vec q \cdot \hat n da-\int_{\partial \cal P(t)} \rho \varepsilon \vec v \cdot \hat n da##
(The differential is over volume, but the field ##\vec v## represents the velocity field)
So, the q term clearly represents the heat influx through the boundary ##\partial \cal P(t)##, and the term containing ##\rho \varepsilon \vec v## I think that represents the inflow of internal energy in ## \cal P(t)## due to convection. Is this right? I'm in doubt because it is not a control volume which we are working with, but the convecting region. So it is not so clear to me that this flux of internal energy due to convection is actually possible, and I was in hope to clarify this through this discussion.
Bye there and thanks in advance.
The balance of energy can be written:
##\rho \dot \varepsilon=T:D -div(\vec q)+q##
The dot over the epsilon represents the material time derivative, epsilon is the internal energy, rho the mass density, T is the Cauchy stress tensor, D the stretching tensor, and ##\vec q## is the heat flux vector, while the scalar ##q## represents the heat transferred to the region by agencies external to the deforming body (e.g. radiation).
Then, due to a relation previously derived in the book, the given material time derivative is related to the spatial time derivative through:
##\rho \dot \varepsilon=(\rho \varepsilon)'+div (\rho \varepsilon \vec v)##
Then, the balance of energy in the local form can be written:
##(\rho \varepsilon)'=T:D-div(\vec q+\rho \varepsilon \vec v)+q##
The book then asks to provide a physical interpretation for the quantity: ##\vec q+\rho \varepsilon \vec v##.
What I did was to rewrite the localized form in the global form for the divergence:
##\displaystyle -\int_{\cal P(t)} div(\vec q+\rho \varepsilon \vec v) dv=-\int_{\partial \cal P(t)}\vec q \cdot \hat n da-\int_{\partial \cal P(t)} \rho \varepsilon \vec v \cdot \hat n da##
(The differential is over volume, but the field ##\vec v## represents the velocity field)
So, the q term clearly represents the heat influx through the boundary ##\partial \cal P(t)##, and the term containing ##\rho \varepsilon \vec v## I think that represents the inflow of internal energy in ## \cal P(t)## due to convection. Is this right? I'm in doubt because it is not a control volume which we are working with, but the convecting region. So it is not so clear to me that this flux of internal energy due to convection is actually possible, and I was in hope to clarify this through this discussion.
Bye there and thanks in advance.
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