Railways Technical Ability Fluid Mechanics and Machinery Question Bank Fluid Mechanics

  • question_answer Air (kinematic viscosity \[=15\times {{10}^{-6}}{{\text{m}}^{\text{2}}}\text{/s}\]) with a free stream velocity of 10 m/s flows over a smooth two- dimensional flat plate. If the critical Reynolds number is \[5\times 10,\] what is the maximum distance from the leading edge up to which laminar boundary layer exists?

    A) 30 cm              

    B) 75 cm

    C) 150 cm             

    D) 300 cm

    Correct Answer: B

    Solution :

    \[v=15\times {{10}^{-6}}{{\text{m}}^{\text{2}}}\text{/s,}\] \[{{U}^{\infty }}=10\,\text{m/s}\] \[{{\operatorname{Re}}_{c}}=5\times {{10}^{5}}\]  \[{{\operatorname{Re}}_{c}}=\frac{{{U}_{\infty }}x}{v}\] \[x=\frac{5\times {{10}^{5}}\times 15\times {{10}^{-6}}}{10}=0.75\,m\,\,or\,\,75\,cm\]

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