# Computational Fluid Dynamics Questions and Answers – Boundary Conditions – Inlet and Outlet

This set of Computational Fluid Dynamics Objective Questions & Answers focuses on “Boundary Conditions – Inlet and Outlet”.

1. Usually, in the inlet boundary conditions ___________ are known.
b) diffusive fluxes of properties
c) convective fluxes of properties
d) flow properties

Explanation: Flow properties in the inlet should be known. If the flow properties are not known, it is good to move the boundary as far as possible from the region of interest.

2. The convective fluxes in the inlet boundary layer _________
a) are specified
b) can be calculated
c) should be approximated
d) are not necessary

Explanation: The convective flux of a property is a product of mass flow rate and the flow property. So, it can be easily obtained from the known values.

3. The diffusive fluxes in the inlet boundary conditions _________
a) are specified
b) can be calculated
c) are not necessary
d) should be approximated

Explanation: The diffusive fluxes are usually not known. They should be approximated using the boundary values of the variables and one-sided finite difference approximations for the gradients.

4. An extra grid is used before the physical boundary. What is the use of this grid?
a) To store the inlet boundary values
b) To solve the discretized equation
c) To calculate the inlet boundary values
d) To solve the system analytically

Explanation: The grid extends outside the physical boundary and these nodes are used to store the inlet variables. The discretized equation is solved starting from the first internal cell.

5. _________ is fixed at one inlet node and pressure correction is set to _________ in that node.
a) Gauge pressure, one
b) Absolute pressure, one
c) Gauge pressure, zero
d) Absolute pressure, zero

Explanation: While calculating, the pressure field does not give absolute pressures. A reference pressure should be set before to resolve this problem. To set a reference pressure, absolute pressure is fixed at inlet node and pressure correction is set to zero there.
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6. If we are solving a k-ε model, the values of k and ε _________
a) are either specified or estimated at the inlet
b) are either specified or estimated at the outlet
c) need not be specified or estimated anywhere
d) are not estimated

Explanation: The most accurate solutions can be achieved only if k and ε are specified at the inlet boundary. However, if it is not possible to provide, CFD codes can estimate these values using a formula.

7. Which of these is the best practice regarding outlet boundaries?
a) Outlet boundaries should be at the exact outlet of the geometry
b) Outlet boundaries should be set as close as possible to the inlet boundaries
c) Outlet boundaries should be set as far as possible to the physical geometry
d) Outlet boundaries should be set as close as possible to the physical geometry

Explanation: At the outlet, usually we know only little about the flow properties. For this reason, we should set the outlet boundary conditions as far as possible from the region of interest. Otherwise, error may propagate upstream.

8. Which of these statements is false regarding the pressure correction at boundaries?
a) Pressure reference is set at the inlet boundaries
b) Pressure reference is set at the outlet boundaries
c) The link to the inlet boundary side is suppressed
d) The link to the outlet boundary side is suppressed

Explanation: Pressure reference is set at the inlet boundaries and the link to the inlet side is suppressed. In the outlet boundaries, there is no need for setting another reference pressure but the link to the outlet side is suppressed.

9. The mass flux out should be equal to the mass flux in. This is in accordance with _________
a) energy conservation
b) momentum conservation
c) continuity condition
d) flux conservation

Explanation: The theory of continuity (mass conservation) states that the inlet and outlet mass fluxes should be the same. So, they are always matched by some external calculations.

10. Which of these following is used to match the outlet and inlet flow velocities?
a) uN,j = uN-1,j×$$\frac{M_{in}}{M_{out}}$$
b) uN-1,j = uN,j×$$\frac{M_{in}}{M_{out}}$$
c) uN-1,j = uN,j×$$\frac{M_{out}}{M_{in}}$$
d) uN,j = uN-1,j×$$\frac{M_{out}}{M_{in}}$$

Explanation: To match the flow velocities, velocity in the node immediate west of the outlet is multiplied by the mass ratio $$\frac{M_{in}}{M_{out}}$$. This will help us to maintain continuity.

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