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For a steady flow, the velocity field is \(\vec{V} = (-x^{2} + 3y)\hat{i} + (2xy)\hat{j}\). The magnitude of the acceleration of a particle at \((1, -1)\) is

  • A.2
  • B.1
  • C.\(2\sqrt{5}\)
  • D.0

Answer: C

The acceleration magnitude is \(2\sqrt{5}\).

For steady flow the acceleration is purely convective. With \(u = -x^{2} + 3y\) and \(v = 2xy\), at (1, -1): u = -4 and v = -2.

\[ \begin{aligned} a_{x} &= u\frac{\partial u}{\partial x} + v\frac{\partial u}{\partial y} = (-4)(-2) + (-2)(3) = 2 \\ a_{y} &= u\frac{\partial v}{\partial x} + v\frac{\partial v}{\partial y} = (-4)(-2) + (-2)(2) = 4 \end{aligned} \]

So \(|a| = \sqrt{4 + 16} = 2\sqrt{5}\). Zero would be right only if one forgot that steady flow still has convective acceleration; 2 is the x component alone.

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