# Computational Fluid Dynamics Questions and Answers – Boundary Conditions – Wall and Symmetry

This set of Computational Fluid Dynamics Interview Questions and Answers for freshers focuses on “Boundary Conditions – Wall and Symmetry”.

1. For a no-slip condition which of these about velocity components is true near the wall boundary?
a) u=1, v=0, w=0
b) u=0, v=0, w=0
c) u=0, v=1, w=0
d) u=0, v=0, w=1

Explanation: No-slip is the ideal condition with the highest viscosity. Here, the fluid is relatively stationary with the wall and all of the velocity components are zero. Therefore, u=0, v=0, w=0.

2. Which of these represents the temperature of the fluid layer immediately near the wall at a condition analogous to no-slip? Note: Tw is the temperature at the wall.
a) T=-1
b) T=1
c) T=Tw
d) T=0

Explanation: For a condition analogous to no-slip in temperature, the temperature of the fluid layer near the wall is the same as the wall temperature. Therefore, T=Tw.

3. Which of these is true for an impermeable wall?
a) $$\vec{V}=0$$ above the surface
b) $$\vec{V}=0$$ at the surface
c) $$\vec{V}.\vec{n}=0$$ at the surface
d) $$\vec{V}.\vec{n}=0$$ above the surface

Explanation: For an impermeable wall, there can be no mass flow into or out of the wall. Therefore, the velocity at the surface must be completely tangential and its normal component will be zero ($$\vec{V}.\vec{n}=0$$).

4. For inviscid flows, which is correct immediately near the wall?
a) $$\vec{V} ≠ 0$$
b) $$\vec{V} = 0$$
c) $$\vec{V} > 0$$
d) $$\vec{V} \lt 0$$

Explanation: $$\vec{V}$$=0 only for the no-slip condition. For inviscid flows, the flow velocity immediately near the wall is a finite non-zero value. We cannot specify its sign as the sign depends upon flow direction. We can only say $$\vec{V}≠0$$.

5. For no-slip condition, which of these is true regarding the pressure correction equation if the wall is at the bottom?
a) an=0
b) aw=0
c) ae=0
d) as=0

Explanation: When velocities are known near the wall for no-slip condition, pressure correction is unnecessary. As the wall is at the bottom here, we set as=0 to omit pressure correction in the southern side.
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6. What is the shear force of a fluid (velocity $$\vec{u}$$) near the wall for a moving wall (velocity ($$\overrightarrow{u_{wall}}$$))?
a) $$\vec{F}=-\mu\frac{\vec{u}}{\Delta y}\times area$$
b) $$\vec{F}=-\mu\frac{\vec{u}-\overrightarrow{u_{wall}}}{\Delta y}\times area$$
c) $$\vec{F}=-\mu\frac{\vec{u}-\overrightarrow{u_{wall}}}{\Delta y}$$
d) $$\vec{F}=-\mu\frac{\vec{u}}{\Delta y}$$

Explanation: Shear force is a product of shear stress and area. Shear stress is defined by Newton’s law of viscosity. For moving walls, the relative velocity $$\vec{u}-\overrightarrow{u_{wall}}$$ should be taken. Therefore, the shear force is given by $$\vec{F}=-\mu\frac{\vec{u}-\overrightarrow{u_{wall}}}{\Delta y}\times area$$.

7. Which of the following applies to a symmetry boundary?
a) There is no flow and no scalar flux across the boundary
b) There are flow and scalar fluxes across the boundary
c) There is no scalar flux but flow is possible across the boundary
d) There is no flow but scalar flux is possible across the boundary

Explanation: There are 2 conditions for a symmetry boundary: There is no flow across the boundary and there is no scalar flux across the boundary.

8. For a symmetry boundary, which is correct?
a) Vn≠0, τnn=0
b) Vn≠0, τnn≠0
c) Vn=0, τnn≠0
d) Vn=0, τnn=0

Explanation: For a symmetry boundary, there is no flow across the boundary. Therefore, Vn=0. But, the gradients of normal flow are non-zero. So, τnn≠0.

9. A symmetry boundary is treated the same as a wall boundary for this reason.
a) There is flow across this boundary
b) No convection flux across this boundary
c) There is convection flux across this boundary
d) No flow across this boundary

Explanation: A symmetry boundary is treated as a wall boundary (for its no flow across the boundary) with an additional condition that there is no scalar flux across this boundary.

10. Which is true for a symmetry boundary?
a) Diffusive flux is non-zero
b) Diffusive flux is zero
c) Convective flux is zero
d) Convective flux is non-zero

Explanation: The convective flux at a symmetry boundary is always zero. The diffusive flux may or may not be zero depending upon the coincidence of this boundary with the Cartesian coordinates.

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