[ndtrong]

Dear everyone,

I tried to plot the gradient of alpha.water and normal vector of interface using gradient of alpha.water with interFoam solver. And I found a thing that is quite strange for me as shown in the attached file.
Variables are defined as:
gradAlpha = fvc::grad(alpha1)
nAlphaVec = gradAlpha/(mag(gradAlpha) + epsilon)

with epsilon = 1.0e-8

I think that alpha1, gradAlpha and nAlphaVec should have the same contours, however, the contour of nAlphaVec is very different compare to 2 other ones.

Could someone help me to explain what is the reason for that.

I thank you very much