This set of Computational Fluid Dynamics Multiple Choice Questions & Answers (MCQs) focuses on “Incompressible Flows – PISO Algorithm”.

1. What does PISO stand for?

a) Pressure Implicit with SIMPLE Operators

b) Pressure Implicit with Settling of Operators

c) Pressure Implicit with Splitting of Operators

d) Pressure Indication with Splitting of Operators

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Explanation: The PISO algorithm is expanded as Pressure Implicit with Splitting of Operators. It was introduced by Issa in the year 1986. This is also an extension of the SIMPLE algorithm with some changes in it.

2. The PISO algorithm is first built for ___________

a) steady compressible flows

b) unsteady compressible flows

c) steady incompressible flows

d) unsteady incompressible flows

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Explanation: This algorithm was actually built for the unsteady compressible flows. It included a non-iterative computation process. This method was then adapted to solve the incompressible flows with an iterative process.

3. How many predictor and corrector steps does the PISO algorithm involve?

a) One predictor and one corrector steps

b) Two predictor and one corrector steps

c) Two predictor and two corrector steps

d) One predictor and two corrector steps

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Explanation: The SIMPLE algorithm has one predictor and one corrector steps. But, the PISO algorithm has an extra corrector step. Therefore, it can be called an extension of the SIMPLE algorithm.

4. Which of these procedures is done in the predictor step of the discretized momentum equation?

a) Initial guess for velocity and the first corrected pressure are obtained

b) First corrected values of pressure and velocity are obtained

c) Initial guesses for pressure and velocity are obtained

d) Initial guess for pressure and the first corrected velocity are obtained

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Explanation: In the first step of the PISO algorithm, an initial guess is made on the pressure and the velocity values. This is corrected later in the following corrector steps. This is exactly the same for the SIMPLE algorithm.

5. The guessed velocities will satisfy the continuity equation when ___________

a) the pressure field is correct

b) the pressure correction is correct

c) the velocity correction is correct

d) the pressure-correction equation satisfies the continuity equation

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Explanation: The momentum equation is solved to get the guessed pressure values. Then the velocities are assumed from this pressure field. This assumed velocity field will not satisfy the continuity equations until the pressure field is correct.

6. The initial corrector step is used to __________

a) get a velocity field that satisfies the momentum equation

b) get a velocity field that satisfies the continuity equation

c) get a pressure field that satisfies the continuity equation

d) get a pressure field that satisfies the momentum equation

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Explanation: The first corrector step of the PISO algorithm results in a velocity field which will satisfy the continuity equation. The equations resulting from this step is the same as the equations used in the SIMPLE algorithm.

7. In which of these steps are the pressure-correction equations obtained in the PISO algorithm?

a) The first predictor step

b) The first corrector step

c) The second predictor step

d) The second corrector step

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Explanation: The pressure-correction equation is obtained by applying the corrected velocities to the continuity equation. This pressure-correction equation is then used to get the first pressure correction value.

8. Which of these equations is solved by the PISO algorithm in the extra corrector step?

a) Pressure-correction equation

b) Pressure equation

c) Continuity equation

d) Momentum equation

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Explanation: The initial pressure guesses are obtained from the momentum equation. This pressure field is used to get the velocity field approximations. This is corrected in the first corrector step using the pressure-correction equation. It is again corrected using the momentum equation in the second corrector step.

9. Which of these is a disadvantage of the PISO algorithm?

a) Additional storage

b) Slow convergence

c) Less consistent

d) Not accurate

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Explanation: The PISO algorithm corrects the pressure and velocity fields twice in the two corrector steps. So, to store the intermediate values and the intermediate equations, additional memory requirements is needed. This is a disadvantage.

10. Which of these characteristics of the PISO algorithm match with the SIMPLE algorithm?

a) No extra equation

b) Extra equation

c) Under-relaxation factor

d) High convergence

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Explanation: The PISO algorithm uses the under-relaxation procedure followed in the SIMPLE algorithm. This is the only algorithm that is modified from the SIMPLE algorithm which uses this under-relaxation factor.

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