This set of Computational Fluid Dynamics Questions and Answers for Experienced people focuses on “Boundary Conditions – Constant Pressure and Periodic”.

1. When do we use a constant pressure boundary condition?

a) When there is an impermeable boundary

b) When there is constant pressure

c) When we do not know the flow distribution but we know the pressure at the boundaries

d) When we do not know the pressure at the boundaries

View Answer

Explanation: When the flow distribution including velocity are not properly known but the boundary pressures are known, a constant pressure boundary condition is used.

2. While applying the constant pressure boundary condition, which of these is done?

a) Pressure is set to 0

b) Pressure correction is set to 1

c) Pressure correction is set to zero

d) Pressure is set to 1

View Answer

Explanation: When constant pressure boundary is used, pressure is set to a fixed value which is the boundary pressure value and pressure correction is set to zero.

3. Which of these pose a problem in constant pressure boundaries?

a) Velocity

b) Flow direction

c) Density

d) Heat flux direction

View Answer

Explanation: Constant pressure boundaries are used when the velocity is unknown. The main problem in this is the flow direction. It has to be found while solving the flow properties.

4. Which of these conditions is used to find the velocity component of the boundary while using constant pressure boundary condition?

a) Conservation of angular momentum

b) Conservation of linear momentum

c) Energy conservation

d) Continuity

View Answer

Explanation: Velocity components are obtained as part of the solution procedure. The flow directions are obtained by ensuring that continuity is satisfied at every cell.

5. Constant pressure boundary condition makes the pressure correction cell nearest to the boundaries to act as ____________

a) Source or sink of mass

b) Source or sink of heat

c) Convection cell

d) Diffusion cell

View Answer

Explanation: While implementing constant pressure boundary conditions, the nearest pressure correction cell acts as a source or sink of mass. This happens for all pressure boundary cells.

6. The velocity components in the nodes which are not at the boundary are found using ____________

a) energy equation

b) continuity equation

c) equations of state

d) momentum equation

View Answer

Explanation: The velocities in the non-boundary nodes are found using u and v-momentum equations. After this, the boundary side velocity component is obtained using these values.

7. Periodic or cyclic boundary conditions are a type of ____________

a) wall boundary conditions

b) constant pressure boundary conditions

c) inlet boundary conditions

d) symmetry boundary conditions

View Answer

Explanation: Periodic or cyclic boundary conditions are repeated symmetry boundary conditions. The matching of variables in Periodic or cyclic boundary conditions is done in the same way how it is done for symmetry boundary conditions.

8. For cyclic boundary conditions, which of these should be equated?

a) The flow variables

b) Gradient of the flow variables

c) Flux of the flow variables

d) Second derivative of the flow variables

View Answer

Explanation: The flux of the flow variables in the inlet cyclic boundary should be matched with that of the outlet cyclic boundaries to apply a cyclic boundary. For example the flux of Φ

_{1,j}and Φ

_{n-1,j}

9. The pairing operation can be done for all properties except ___________ of a flow.

a) pressure

b) density

c) velocity components

d) temperature

View Answer

Explanation: Pairing can be done for all properties except the velocity components. Velocity components cannot be paired for the inlet and outlet boundary pairs.

10. For which of these flows, periodic or cyclic boundary conditions are applicable?

a) External flow over objects

b) Swirling flow inside a cylindrical furnace

c) Free surface flows

d) Buoyancy driven flows

View Answer

Explanation: While solving swirling flow inside a cylindrical furnace, if cylindrical coordinates are used, we can pair the boundaries and hence periodic or cyclic boundary conditions can be used.

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