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The velocity potential function of a flow field is given by x2 − y2 + constant. Its stream function will be given by
  • a)
    − 2xy + constant
  • b)
    + 2xy + constant
  • c)
    − 2xy + f(x)
  • d)
    − 2xy + f(y)
Correct answer is option 'A'. Can you explain this answer?
Verified Answer
The velocity potential function of a flow field is given by x2 − y2 +...
Φ = x2 − y2 + C
∂ϕ / ∂x = −u and ∂ϕ / ∂y = −v
hence u = −2x and v = +2y
Also
∂ψ / ∂y = u ⟹ ∂ψ / ∂y = −2x ⋯ ①
and
∂ψ / ∂x = −v ⟹ ∂ψ / ∂x = −2y ⋯ ②
Partially integrating both the equations ① and ② we get
ψ = −2xy + f(x) + C1 ⋯ ③ and
ψ = −2xy + g(y) + C2 ⋯ ④
respectively.
Since both the equations ③ and ④ are same hence f(x) and g(y) will be zero and C1 = C2
∴ ψ = −2xy + constant.
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Most Upvoted Answer
The velocity potential function of a flow field is given by x2 − y2 +...
Φ = x2 − y2 + C
∂ϕ / ∂x = −u and ∂ϕ / ∂y = −v
hence u = −2x and v = +2y
Also
∂ψ / ∂y = u ⟹ ∂ψ / ∂y = −2x ⋯ ①
and
∂ψ / ∂x = −v ⟹ ∂ψ / ∂x = −2y ⋯ ②
Partially integrating both the equations ① and ② we get
ψ = −2xy + f(x) + C1 ⋯ ③ and
ψ = −2xy + g(y) + C2 ⋯ ④
respectively.
Since both the equations ③ and ④ are same hence f(x) and g(y) will be zero and C1 = C2
∴ ψ = −2xy + constant.
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Community Answer
The velocity potential function of a flow field is given by x2 − y2 +...
Explanation:

Given: Velocity potential function φ = x^2 - y^2 + constant

Stream function ψ:
The stream function ψ is related to the velocity components u and v of a 2D flow field by the equations:
u = ∂ψ/∂y
v = -∂ψ/∂x

Relation between velocity potential and stream function:
For an incompressible and irrotational flow, the velocity potential φ and the stream function ψ are related by the Laplace equation:
∇^2φ = -∇^2ψ

Find the stream function ψ:
Given φ = x^2 - y^2 + constant
Taking derivatives with respect to x and y:
∂φ/∂x = 2x
∂φ/∂y = -2y
Comparing these derivatives with the definitions of u and v:
u = 2x
v = -2y

Expressing u and v in terms of ψ:
From u = ∂ψ/∂y:
∂ψ/∂y = 2x
Integrating with respect to y:
ψ = 2xy + f(x)
Comparing this with the given options, the correct stream function is:

ψ = -2xy + constant (option A)
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