According to the conservation law, “Net mass flow across the fluid element is equal to the rate of change of mass inside the element”. But, stating the final equation, “Net mass flow across the fluid element + the rate of change of mass inside the element = 0”. Why is the operation not subtraction?

Question

According to the conservation law, “Net mass flow across the fluid element is equal to the rate of change of mass inside the element”. But, stating the final equation, “Net mass flow across the fluid element + the rate of change of mass inside the element = 0”. Why is the operation not subtraction?
Questioncomputational-fluid-dynamics-questions-answers1.jpg

Questioncomputational-fluid-dynamics-questions-answers1.jpg

Accepted Answer

c) Change in sign is not considered

Answer

a) Irrespective of the law, the sum is always zero

Answer

b) The two terms are always opposite in sign

Answer

d) Rate of change may be increase or decrease

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