Laplace operator for domain probe---2D axisymmetric tissue-vessel oxygen diffusion model

Hi all,

I have been learning to use COMSOL for some time, but this is my first time to join this community. I have few questions about using Laplace operator in the domain probe function.

I am developing a tissue-oxygen diffusion model: oxygen diffuse from vessel into and within tissue. There is an oxygen consumption term in addition to diffusion term. At the steady state, mean oxygen concentration stays constant in the tissue, and oxygen consumption in the tissue domain should be balanced by diffusion from vessel.

But, when I try to use domain probes to monitor respectively the average diffusion and consumption terms over time, the diffusion term is about 10 times of that consumption term. I would expect that they are the same because of the steady state. I think the consumption term value is correct.

So, my question is how the Laplace operator in the diffusion term should be implemented in the global variable so that it can be used in the domain probe? It is cylindrical coordinates. I used

D*(1/r*d(r*d((mod1.u3),r),r)+d(d((mod1.u3),z),z)).

I have also tried using partial derivative pd(f,r), which produced all 0s.

I think the diffusion term should be calculated correctly in my Transport of Diluted Species model, otherwise, the average oxygen rate won't be zero in the domain probe. When I calculate it individually using probe, it gave me non-sense results.

Another question, in the domain probe, the 'average' option means the volume average, right?

I hope I have stated my problem clearly, if not, please feel free to ask. I would really appreciate your helps and any comments!!

Baochang