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Basics of Fluid Mechanics, 2014a

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182 CHAPTER 6. MOMENTUM CONSERVATION<br />

velocity <strong>of</strong> the jet is uniform. Neglect<br />

the friction between the liquid (jet) and<br />

the toy and between the air and toy.<br />

Calculate the absolute velocity <strong>of</strong> the<br />

jet exit. Assume that the friction between<br />

the toy and surface (ground) is<br />

relative to the vertical force. The dynamics<br />

friction is μ d .<br />

Solution<br />

x<br />

y<br />

control<br />

volume 2<br />

1<br />

U U j 0<br />

F f<br />

Fig. -6.5. Toy Sled pushed by the liquid<br />

jet in a steady state for example 6.4.<br />

The chosen control volume is attached to the toy and thus steady state is obtained. The<br />

frame <strong>of</strong> reference is moving with the toy velocity, U 0 . The applicable mass conservation<br />

equation for steady state is<br />

A 1 U 1 = A 2 U 2<br />

The momentum equation in the x direction is<br />

∫<br />

∫ ∫ ∫<br />

F f + gρdV − PdA+ τ dA = ρUU rn dV<br />

c.v.<br />

c.v.<br />

c.v.<br />

c.v.<br />

(6.IV.a)<br />

The relative velocity into the control volume is<br />

U 1j =(U j − U 0 )ˆx<br />

The relative velocity out the control volume is<br />

The absolute exit velocity is<br />

U 2j =(U j − U 0 )ŷ<br />

U 2 = U 0ˆx +(U j − U 0 )ŷ<br />

For small volume, the gravity can be neglected also because this term is small<br />

compared to other terms, thus ∫<br />

gρdV ∼ 0<br />

c.v.<br />

The same can be said for air friction as<br />

∫<br />

c.v.<br />

τ dA ∼ 0<br />

The pressure is uniform around the control volume and thus the integral is<br />

∫<br />

PdA=0<br />

c.v.<br />

The control volume was chosen so that the pressure calculation is minimized.

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