
Concept explainers
The expression for stream function.
The plot some streamlines of the flow.

Answer to Problem 63EP
The expression for the stream function is ax2y−bxy22+C.
The following figure represents the streamlines of the flow.
Explanation of Solution
Given information:
The incompressible flow filed for which the velocity u component is u=ax2−bxy, where a=0.45 (ft⋅s)−1, b=0.75 (ft⋅s)−1 and the velocity along the x axis is 0. The stream function along the x axis is 0. The range of x is varies from 0 to 3 and the range of y is varies from 0 to 4.
Write the expression for the velocity along x direction.
v=−∂ψ∂x ....... (I)
Here, the stream function along x direction is ∂ψ, the change in distance along x direction is ∂x and the velocity along x direction is v.
Write the expression for the velocity along x direction.
u=∂ψ∂y ...... (II)
Here, the stream function along y direction is ∂ψ, the change in distance along y direction is ∂y and the velocity along y direction is u.
Write the expression for quadric stream function.
y=ax2±√a2x4−2bxψbx ....... (III)
Here, the stream function is ψ, the distance along x direction is x, the distance along y direction is y and the constants are a and b.
Calculation:
Substitute ax2−bxy for u in Equation (II).
ax2−bxy=∂ψ∂y∂ψ∂y=ax2−bxy∂ψ=(ax2−bxy)⋅∂y ...... (IV)
Integrate Equation (IV) with respect to y.
∂ψ=(ax2−bxy)⋅∂y∫∂ψ=∫(ax2−bxy)⋅∂y+f(x)ψ=ax2y−bxy1+12+f(x)ψ=ax2y−bxy22+f(x) ....... (V)
Here, the constant is f(x).
Substitute ax2y−bxy22+f(x) for ψ and 0 for v in Equation (I).
0=−∂(ax2y−bxy22+f(x))∂x−∂(ax2y−bxy22+f(x))∂x=0 ....... (VI)
Differentiate Equation (V) with respect to x.
−∂(ax2y−bxy22+f(x))∂x=0−2ax2−1y+bx1−1y22−∂f(x)∂x=0−2ax1y+by22−∂f(x)∂x=0 ....... (VII)
Substitute 0 for y in Equation (VII).
−2ax1(0)+b(0)22−∂f(x)∂x=0−∂f(x)∂x=0∂f(x)∂x=0 ....... (VIII)
Substitute 0 for y and C for ψ in Equation (V).
ax2(0)−bx(0)22+f(x)=C0−0+f(x)=Cf(x)=C ....... (IX)
Here, the constant is C.
Substitute C for f(x) in Equation (V).
ψ=ax2y−bxy22+C ....... (X)
Substitute C for f(x) in Equation (X).
ψ=ax2y−bxy22+0ψ=ax2y−bxy22
Substitute 0.45 (ft⋅s)−1 for a and 0.75 (ft⋅s)−1 for b in Equation (III).
y=(0.45 (ft⋅s)−1)x2±√(0.45 (ft⋅s)−1)2x4−2(0.75 (ft⋅s)−1)xψ(0.75 (ft⋅s)−1)xy=0.45 (ft⋅s)−1×x2±√0.2025 (ft⋅s)−1×x4−1.50 (ft⋅s)−1×xψ(0.75 (ft⋅s)−1)x..... (XI)
Substitute 1 for x and 1 for y in Equation (XI).
1=0.45 (ft⋅s)−1×(1)2±√0.2025 (ft⋅s)−1×(1)4−1.50 (ft⋅s)−1×(1)ψ(0.75 (ft⋅s)−1)(1)±√0.2025 (ft⋅s)−1×(1)4−1.50 (ft⋅s)−1×(1)ψ=0.75 (ft⋅s)−1−0.45 (ft⋅s)−10.2025 (ft⋅s)−1×(1)4−1.50 (ft⋅s)−1×(1)ψ=±(0.35 (ft⋅s)−1)2.. (XII)
Solve Equation (XII) by taking positive sign.
0.2025 (ft⋅s)−1×(1)4−1.50 (ft⋅s)−1×(1)ψ=±(0.35 (ft⋅s)−1)2−1.50 (ft⋅s)−1×(1)ψ=−0.08ψ=0.05333
Solve Equation (XII) by taking negative sign.
0.2025 (ft⋅s)−1×(1)4−1.50 (ft⋅s)−1×(1)ψ=−(0.35 (ft⋅s)−1)2−1.50 (ft⋅s)−1×(1)ψ=−0.325ψ=0.216666
The following table shows that the value of stream function with respect to value of x and y.
x | y | −ψ | +ψ |
0 | 0 | 0 | 0 |
1 | 1 | 0.216666 | 0.053333 |
2 | 2 | 0.4352 | 0.1052 |
Plot the streamlines on y x plane by using Equation (XII) and the range of x is varies from 0 to 3 and the range of y is varies from 0 to 4.
Figure- (1)
The Figure (1) represents the streamlines of the flow.
Conclusion:
The expression for the stream function is ax2y−bxy22+C.
The following figure represents the streamlines of the flow.
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Chapter 9 Solutions
Fluid Mechanics: Fundamentals and Applications
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