How to Solve the Laplace Equation for Potential Flow Around a Sphere?

  • #1
happyparticle
422
20
Homework Statement
Consider the steady flow pattern produced when an impenetrable rigid spherical obstacle is placed in a uniformly flowing, incompressible, inviscid fluid. Find a solution for the potential flow around the sphere.
Relevant Equations
##\nabla^2 \phi = 0##
I tried to find a solution to the Laplace equation using spherical coordinates and the separable variable method. However, I found equations that I simply don't know how to find a solution. Thus, I tried in cylindrical coordinates with an invariance in ##\theta## but now I'm facing this equation.

##\frac{1}{s} \frac{d}{ds}(s \frac{dS}{ds}) = -k^2 S##

Is there a fairly simple solution for it or should I find another way to do this problem?
 
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  • #2
I am not sure to distinguish S and s in your equation. Let me rewrite it with x for s and y for S
[tex]\frac{1}{x} \frac{d}{dx}(x \frac{dy}{dx}) = -k^2 y[/tex]
Is it the right equation which has constant k with its physical dimension of [1/s] ? It belongs to Sturm -Liouville equation whose solutions are Bessel functions.
 
Last edited:

Related to How to Solve the Laplace Equation for Potential Flow Around a Sphere?

What is potential flow around a sphere?

Potential flow around a sphere refers to the idealized flow of an incompressible, inviscid fluid around a spherical object, where the flow is irrotational and can be described by a potential function. This type of flow neglects viscosity and assumes that the fluid moves in a way that can be represented by a scalar potential function.

How is the velocity field around a sphere in potential flow described?

The velocity field around a sphere in potential flow is described by the gradient of a potential function. For a sphere of radius \( R \) in a uniform flow with velocity \( U \), the potential function \( \Phi \) is given by \( \Phi = -U r \cos \theta + \frac{U R^3 \cos \theta}{2 r^2} \), where \( r \) is the radial distance from the center of the sphere and \( \theta \) is the polar angle. The velocity components can then be derived by taking the gradient of \( \Phi \).

What are the boundary conditions for potential flow around a sphere?

The boundary conditions for potential flow around a sphere include the no-penetration condition at the surface of the sphere and the condition that the flow approaches a uniform flow far from the sphere. Mathematically, this means that at the surface of the sphere, the radial component of the velocity must be zero, and at infinity, the flow velocity must match the uniform flow velocity.

Why is potential flow an idealization and what are its limitations?

Potential flow is an idealization because it assumes the fluid is inviscid (having no viscosity) and incompressible, and that the flow is irrotational. These assumptions simplify the mathematical description of the flow but do not accurately represent real fluids, which have viscosity and can experience rotational effects and boundary layer separation. Consequently, potential flow cannot accurately predict phenomena like drag or boundary layer behavior.

How is the pressure distribution around a sphere in potential flow determined?

The pressure distribution around a sphere in potential flow can be determined using Bernoulli's equation, which relates the pressure and velocity along a streamline. For a sphere in a uniform flow, the pressure at any point on the surface can be found by substituting the velocity components derived from the potential function into Bernoulli's equation. This results in a pressure distribution that varies with the polar angle \( \theta \), showing higher pressure at the stagnation points (front and rear) and lower pressure at the equator.

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