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Basics of Fluid Mechanics, 2014a

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214 CHAPTER 7. ENERGY CONSERVATION<br />

The reference was set to infinity. The gravity force for fluid element in small distance<br />

then is gdzdm. The work this element moving from point 1 to point 2 is<br />

∫ 2<br />

1<br />

gdzdm= g (z 2 − z 1 ) dm (7.91)<br />

The total work or potential is the integral over the whole mass.<br />

7.4.2 Linear Accelerated System<br />

The acceleration can be employed in similar fashion as the gravity force. The linear<br />

acceleration “creates” a conservative force <strong>of</strong> constant force and direction. The “potential”<br />

<strong>of</strong> moving the mass in the field provides the energy. The Force due to the<br />

acceleration <strong>of</strong> the field can be broken into three coordinates. Thus, the element <strong>of</strong> the<br />

potential is<br />

dPE a = a · dldm (7.92)<br />

The total potential for element material<br />

PE a =<br />

∫ (1)<br />

(0)<br />

a · dldm=(a x (x 1 − x 0 ) a y (y 1 − y 0 ) a z (z 1 − z 0 )) dm (7.93)<br />

At the origin (<strong>of</strong> the coordinates) x =0, y =0, and z =0. Using this trick the notion<br />

<strong>of</strong> the a x (x 1 − x 0 ) can be replaced by a x x. The same can be done for the other two<br />

coordinates. The potential <strong>of</strong> unit material is<br />

∫<br />

PE atotal = (a x x + a y y + a z z) ρdV (7.94)<br />

sys<br />

The change <strong>of</strong> the potential with time is<br />

D<br />

Dt PE atotal = D ∫<br />

(a x x + a y y + a z z) dm (7.95)<br />

Dt sys<br />

Equation can be added to the energy equation as<br />

˙Q − Ẇ = D ∫ [E u + U 2<br />

]<br />

Dt sys 2 + a x x + a y y +(a z + g)z ρdV (7.96)<br />

The Reynolds Transport Theorem is used to transferred the calculations to control<br />

volume as<br />

Energy Equation in Linear Accelerated Coordinate<br />

˙Q − Ẇ = d ∫ [E u + U 2<br />

]<br />

dt cv 2 + a x x + a y y +(a z + g)z ρdV<br />

∫<br />

+<br />

(h + U 2<br />

)<br />

cv 2 + a x x + a y y +(a z + g)z U rn ρdA<br />

∫<br />

+ PU bn dA<br />

cv<br />

(7.97)

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