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# Computational Fluid Dynamics – Convection-Diffusion Problems – QUICK Scheme

What does QUICK stand for?

a) Quadratic Upstream Interpolation for Convective Kinetics

b) Quadratic Upstream Interval for Convective Kinetics

c) Quadratic Upwind Interval for Convective Kinetics

d) Quadratic Upwind Interpolation for Convective Kinetics

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Answer: aExplanation: QUICK is a higher-order differencing scheme introduced by Brian P. Leonard in his paper in the year 1979. It is the abbreviation of Quadratic Upstream Interpolation for Convective Kinetics.

Which is correct about the QUICK scheme?

a) A two-point upwind biased interpolation

b) A three-point upwind biased interpolation

c) A three-point downwind biased interpolation

d) A two-point downwind biased interpolation

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Answer: bExplanation: QUICK scheme uses a three-point upstream-weighted quadratic interpolation to approximate the cell face values. It uses two immediate neighbours of the face and an extra upstream node (totally, three points).

According to the QUICK scheme, the flow variable (φ) is given by ____

(Note: U, D and C represents the upwind, downwind and the central nodes respectively).

a) ϕ=ϕU+(x−xD)(x−xC)(xD−xU)(xD−xC)(ϕD−ϕU)+(x−xU)(x−xD)(xC−xU)(xC−xD)(ϕC−ϕU)

b) ϕ=ϕU+(x−xD)(x−xC)(xD−xU)(xD−xC)(ϕD−ϕU)+(x−xC)(x−xD)(xC−xU)(xC−xD)(ϕC−ϕU)

c) ϕ=ϕU+(x−xU)(x−xC)(xD−xU)(xD−xC)(ϕD−ϕU)+(x−xC)(x−xD)(xC−xU)(xC−xD)(ϕC−ϕU)

d) ϕ=ϕU+(x−xU)(x−xC)(xD−xU)(xD−xC)(ϕD−ϕU)+(x−xU)(x−xD)(xC−xU)(xC−xD)(ϕC−ϕU)

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Answer: d

Explanation: The scheme should reduce to

ϕ=⎧⎩⎨ϕUϕCϕDifx=xUifx=xCifx=xD

Incorporating these into a formula, the formula for quick scheme is

ϕ=ϕU+(x−xU)(x−xC)(xD−xU)(xD−xC)(ϕD−ϕU)+(x−xU)(x−xD)(xC−xU)(xC−xD)(ϕC−ϕU) .

Consider the stencil.

Assume a uniform grid. What is φ_{e} according to the QUICK scheme?

a) ϕe=ϕP−ϕE2−ϕE+2ϕP+ϕW8

b) ϕe=ϕP+ϕE2−ϕE+2ϕP+ϕW8

c) ϕe=ϕP+ϕE2−ϕE−2ϕP+ϕW8

d) ϕe=ϕP−ϕE2−ϕE−2ϕP+ϕW8

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Answer: c

Explanation: According to the QUICK scheme,

ϕe=ϕW+(xe−xW)(xe−xP)(xE−xW)(xE−xP)(ϕD−ϕW)+(xe−xW)(xe−xE)(xP−xW)(xP−xE)(ϕP−ϕW)

For a uniform grid,

x_{e}-x_{W}=3(x_{e}-x_{P}); x_{E}-x_{W} = 4(x_{e}-x_{P}); x_{E}-x_{P}=2(x_{e}-x_{P});

x_{E}-x_{E} = -(x_{e}-x_{P});x_{P}-x_{W}=2(x_{e}-x_{P});

Applying all these,

ϕe=ϕP+ϕE2−ϕE−2ϕP+ϕW8

What is the order of accuracy of the QUICK scheme?

a) second-order

b) first-order

c) fourth-order

d) third-order

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Answer: aExplanation: As the QUICK scheme is based on a quadratic function, its accuracy in terms of Taylor Series truncation error is third-order. This has a higher order of accuracy than the upwind and second-order upwind schemes.

How many terms does the discretized form of source-free 1-D convection problem modelled using the QUICK scheme has?

a) 3

b) 5

c) 2

d) 4

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Answer: b

Explanation: The discretized form of a source-free 1-D convection problem modelled using the QUICK scheme involves the far upstream and the far downstream nodes too. Therefore, it contains extra terms than the upwind and the second-order upwind schemes. The stencil is

The discretized equation is

a_{P} Φ_{P}+a_{E} Φ_{E}+a_{W} Φ_{W}+a_{EE} Φ_{EE}+a_{WW} Φ_{WW}=0

It contains 5 terms.

What is the first term of the truncation error of the QUICK scheme?

a) 116(Δx)2ϕC”′

b) 116(Δx)3ϕC”′

c) 116(Δx)3ϕivC

d) 116(Δx)2ϕivC

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Answer: c

Explanation: The order of accuracy is 3. Therefore, (Δ x)^{3} should be there in the first term of the truncation error. The truncation error is obtained using the Taylor series. Therefore, this (Δ x)^{3} comes along with Φ_{C}^{iv}.

Which of these is correct about the QUICK scheme?

a) Stable and bounded

b) Stable and unbounded

c) Unstable and bounded

d) Unstable and unbounded

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Answer: dExplanation: The QUICK scheme is not bounded. It involves undershoots and overshoots. The main coefficients (immediate eastern and western coefficients) are not guaranteed to be positive. The coefficients aEE and aWW are negative. Therefore, the solution is not stable.

Consider the stencil.

a) (34ϕP−18ϕW+38ϕE)×max(mw˙,0)−(34ϕW−18ϕWW+38ϕC)×max(−mw˙,0)

b) (34ϕP+18ϕW+38ϕE)×max(mw˙,0)−(34ϕW+18ϕWW+38ϕC)×max(−mw˙,0)

c) (34ϕP−18ϕW−38ϕE)×max(mw˙,0)−(34ϕW−18ϕWW−38ϕC)×max(−mw˙,0)

d) (34ϕP+18ϕW−38ϕE)×max(mw˙,0)−(34ϕW+18ϕWW−38ϕC)×max(−mw˙,0)

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Answer: a

Explanation: Using QUICK scheme, for a flow in the positive x-direction,

ϕe=ϕP+ϕE2−ϕE−2ϕP+ϕW8=34ϕP+38ϕE−18ϕW

In a similar manner, for a flow in the positive x-direction,

ϕw=34ϕP−18ϕW+38ϕE

For a flow in the negative x-direction,

ϕw=34ϕW−18ϕWW+38ϕC

Therefore,

mw˙ϕw=(34ϕP−18ϕW+38ϕE)×max(mw˙,0)−

(34ϕW−18ϕWW+38ϕC)×max(−mw˙,0)

Which of these is correct for a QUICK scheme?

a) False diffusion is zero

b) False diffusion is small

c) False diffusion is big

d) False diffusion is infinity

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Answer: bExplanation: The QUICK scheme involves one downwind node also. So, there will be a false-diffusion in this method. But this false diffusion value is not big as it is an upwind biased scheme (extra nodes in the upstream than the downstream).