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Old   July 15, 2000, 06:43
Default Converting mechanical energy into heat
Marat Hoshim
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I have a basic question concerning the capabilities of CFD. In hydrodynamic brakes for trucks you have some kind of impeller on the drive shaft which runs in a box of oil. The impeller is designed to convert mechanical energy into heat of the oil. 1. Is that conversion process reproduced by the solving NS-equations + energy equation ?

2. If I'm only interested in the velocity field inside the impeller box, is it necessary to solve the energy equation as well ? Or do I get different solution for the velocity field considering the energy equation or not ?

Any comments will be appreciated !

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Old   July 15, 2000, 19:59
Default Re: Converting mechanical energy into heat
John C. Chien
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(1). When the space shuttle returns from the space, it must enter the atmosphere at very high speed. Cfd has been used to simulate the hypersonic flow over the space shuttle, so that heat shield can be designed to withstand the high temperature field produced by the viscous and shock effects. This was done many many years ago. (2). So, you can safely say that cfd can produce the high temperature field as a result of viscous effect. (it is proportional to the F_viscous x V_object. where F_viscous is proportional to V_object squared. So, both high velocity and high viscosity can produce high temperature field.) (3). By the way, it is necessary for the non-professional to solve the "full" set of equations all the time, including the energy equation. Energy equation always converge much faster than other equations.
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Old   July 16, 2000, 10:46
Default Re: Converting mechanical energy into heat
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For imcompressible flow, the mass and momentum conservation equations are decoupled from the energy equation if the coefficient of viscosity and heat conduction can be assumed to be independent of temperature. If in your case, the viscosity and heat conduction coefficients are almost constant for the temperatures that are encountered, then you can ignore the energy equation.
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Old   July 17, 2000, 00:30
Default Re: Converting mechanical energy into heat
Tareq Al-shaalan
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Check the oil viscosity as a function of temperature. If it is strongly a function temperature, then you need to solve the energy equation coupled with momentum and continuity equations. If it is not, then most probably you need only momentum and continuity equations.

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