| Conservation of Energy-7 |
- Rate of work done is assumed positive if the force and velocity vectors are in the same direction. Only surface forces are used for work calculations and not body forces since the gravitational potential energy has already been included in the energy per unit mass of fluid term. Of course, this argument is valid only for gravitational body force term. Need to consider the work done if other types of body forces exist.
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| Conservation of Energy-8 |
- Consider now the work terms on the Control Volume surfaces. Notice the signs of the work terms on the different faces of the CV
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| Conservation of Energy-8a |
- Consider the top (pink), front (blue) and the right (red) face. The force due to the stress is in the direction from left to right. The positive 'v' direction is also from left to right. The dot product is therefore positive as shown.
- The left face (green), however, has force due to stress from right to left and 'v' in the left to right direction, making the work negative. It is not possible to show the bottom and back face which also have negative values.
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