Compare the solution for Creeping flow to a very high Reynolds number flow

To contrast the solution for no inertia, we write down the average velocity for the inviscid (meaning viscous terms are not considered) flow case from a standard reference

[Graphics:../Images/creeping_sphere_gr_182.gif]
[Graphics:../Images/creeping_sphere_gr_183.gif]
[Graphics:../Images/creeping_sphere_gr_184.gif]
[Graphics:../Images/creeping_sphere_gr_185.gif]

[Graphics:../Images/creeping_sphere_gr_186.gif]

The code is as the shading gets darker, the velocity is slower.  We see that coming in the y=0 axis from x=-Infinity, the velocity is decreasing to agree with the: no flow through the surface boundary condition.  Looking at the x=0 axis, coming in from y=∞, the velocity is increasing reflecting the deflection of fluid around the sphere.  This results in a Bernoulli effect with local fluid acceleration.  Below we check some numbers to verify these statements.  

[Graphics:../Images/creeping_sphere_gr_187.gif]
[Graphics:../Images/creeping_sphere_gr_188.gif]
[Graphics:../Images/creeping_sphere_gr_189.gif]
[Graphics:../Images/creeping_sphere_gr_190.gif]
[Graphics:../Images/creeping_sphere_gr_191.gif]
[Graphics:../Images/creeping_sphere_gr_192.gif]

The same question can be asked about the pressure field.  We will find that the pressure is always at or below the far away value for Re=0.  Substitute for the pressure field equation,

Pressure field for high Reynolds number:

In contrast, if a high velocity fluid is impinging on a surface, there will be a pressure increase.  For an inviscid flow, from Bernoulli's equation, we find that dp ~ (U0^2-u^2)/2, where we have already plotted u^2

[Graphics:../Images/creeping_sphere_gr_193.gif]
[Graphics:../Images/creeping_sphere_gr_194.gif]
[Graphics:../Images/creeping_sphere_gr_195.gif]

[Graphics:../Images/creeping_sphere_gr_196.gif]

[Graphics:../Images/creeping_sphere_gr_197.gif]

[Graphics:../Images/creeping_sphere_gr_198.gif]

So that the pressure is highest at y=0, x=1,-1 (i.e. the stagnation point) and decreases out to infinity.  The pressure is lowest, consistent with the Bernoulli effect, at y=1,-1,x=0, and increases out to y=infinity.    

A stream function, which is a function that is always tangent to the velocity field, is another way to view the solution of this problem.  The Re = 0 solution can be used to write the stream function which is, -1/2f[r] [Graphics:../Images/creeping_sphere_gr_199.gif] [Graphics:../Images/creeping_sphere_gr_200.gif].  

[Graphics:../Images/creeping_sphere_gr_201.gif]
[Graphics:../Images/creeping_sphere_gr_202.gif]

We can make a plot of this if we transform from polar to Cartesian coordinates

[Graphics:../Images/creeping_sphere_gr_203.gif]
[Graphics:../Images/creeping_sphere_gr_204.gif]
[Graphics:../Images/creeping_sphere_gr_205.gif]
[Graphics:../Images/creeping_sphere_gr_206.gif]

[Graphics:../Images/creeping_sphere_gr_207.gif]

Note that at y=4, the contours are still deformed, the effect of the sphere is felt very far away!  

Now compare the inviscid flow case:

[Graphics:../Images/creeping_sphere_gr_208.gif]
[Graphics:../Images/creeping_sphere_gr_209.gif]
[Graphics:../Images/creeping_sphere_gr_210.gif]

[Graphics:../Images/creeping_sphere_gr_211.gif]

back to objectives(physical)

back to conclusions

This last plot is a bit messy but shows that inviscid flow is disturbed much less a few diameters from the sphere than Stokes flow.


Converted by Mathematica      August 7, 2000