Flow rate is calculated using only the parallel velocity

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SUMMARY

Flow rate is determined solely by the parallel component of velocity relative to the area vector, as established through the dot product. The mathematical proof involves demonstrating that any perpendicular component of the velocity vector does not contribute to volumetric flow rate, defined as \dot{V}=\frac{dV}{dt}. Substituting the area vector \vec{A} and the velocity vector \vec{v} into the equation confirms that only the parallel component influences flow rate. This principle is critical for understanding fluid dynamics in various applications.

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  • Understanding of vector mathematics, specifically dot products
  • Familiarity with fluid dynamics concepts, particularly volumetric flow rate
  • Knowledge of calculus, especially differentiation with respect to time
  • Basic principles of conservation of mass in fluid systems
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  • Study the mathematical properties of dot products in vector analysis
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davidbenari
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So flow rate is calculated using only the parallel velocity to the area vector. Why is this? How can I mathematically prove this? Namely, how do I prove any perpendicular component of the velocity vector is not contributing to any volume output? I know this is the result of the dot product; I want to know why the dot product is valid reasoning for this scenario.

thanks.
 
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If the entire velocity were perpendicular, what would be the flow rate?
 
Consider the definition of the volumetric flow rate

\dot{V}=\frac{dV}{dt}

say

V=\vec{A} \cdot \vec{x}

substitution yields

\dot{V}=\vec{A}\cdot\frac{d\vec{x}}{dt}=\vec{A}\cdot\vec{v}

where \vec{v}=\frac{d\vec{x}}{dt} and A materially conserved
 

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