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Basics of Fluid Mechanics, 2014a

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8.4. MOMENTUM CONSERVATION 243<br />

The same can be said for τ yx for y<br />

τ yy<br />

direction. The clarity <strong>of</strong> this analysis can<br />

y<br />

τ yx<br />

be improved if additional terms are taken,<br />

τ xx τ xy<br />

yet it turn out that the results will be the dy<br />

same. The normal body force (gravity)<br />

τ xy τ xx<br />

acts through the cubic center <strong>of</strong> gravity.<br />

τ The moment that created by this action<br />

can be neglected (the changes are insignificant).<br />

However, for cases that body force,<br />

such as the magnetic fields, can create<br />

yx τ yy<br />

dx x<br />

torque. For simplicity and generality, it is<br />

assumed that the external body force exerts<br />

a torque G T per unit volume at the tensor.<br />

Fig. -8.6. Diagram to analysis the shear stress<br />

specific location. The body force can exert<br />

torque is due to the fact that the body force is not uniform and hence not act through<br />

the mass center.<br />

Advance material can be skipped<br />

The shear stress in the surface direction potentially can result in the torque due<br />

to the change in the shear stress 12 . For example, τ xx at x can be expended as a linear<br />

function<br />

τ xx = τ xx | y<br />

+ dτ xx<br />

dy ∣ η<br />

y<br />

(8.48)<br />

where η is the local coordinate in the y direction stating at y and “mostly used” between<br />

y

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