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\(u = ax + by\), \(v = cx + dy\) where \(a, b, c, d\) are non-zero constants are velocity components of a possible incompressible fluid motion only, if :

  • A.\(a = d\)
  • B.\(a + d = 0\)
  • C.\(b = c\)
  • D.\(b + c = 0\)

Answer: B

The motion is possible only if \(a + d = 0\).

Incompressibility requires the divergence of the velocity to vanish:

\[ \begin{aligned} \frac{\partial u}{\partial x} + \frac{\partial v}{\partial y} &= a + d \\ &= 0 \end{aligned} \]

The constants \(b\) and \(c\) do not enter the divergence; they decide instead whether the flow is irrotational (\(c - b = 0\), i.e. \(b = c\)). So \(b = c\) and \(b + c = 0\) concern vorticity, not incompressibility, and \(a = d\) would give divergence \(2a \neq 0\).

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