GATE 2023 AE – Question 16
In a two-dimensional potential flow, the doublet is a limit of the superposition of
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Correct answer: (B) a source and a sink of equal strength
Explanation
In two-dimensional potential flow:
- A **source** emits fluid radially from a point and a **sink** absorbs it.
- Place a source of strength $+m$ and a sink of strength $-m$ a small distance $2a$ apart.
- Let $a\to0$ while $m\to\infty$ in such a way that the product $2am=\kappa$ (the doublet strength) stays fixed.
The limit of this pair is a **doublet**. It is therefore the limit of the superposition of **a source and a sink of equal strength** (option B).
(A uniform stream plus a doublet gives the flow past a circular cylinder.)