I am using single cell diffused interface immersed boundary method for sedimentation of two particles. The objective is to observe very well established DKT phenomenon. I could observe drafting and then kissing which continues without tumbling. The repulsive force model is same as used by Glowinski et al. Drag and lift coefficients are calculated using volume integration technique. Moment coefficient is calculated using staircase approach. The kissing phenomenon is little violent and the particles approach and separate quite a few times before being together.
Usually the kissing motion is damped by the lubrication forces that have to be applied manually as a pair-wise additive potential (I guess this is what you mean by the repulsive force model of Glowinski and al). You may see the article by ten Cate (2002) where this type of approach was used for a single sphere sedimentation. Have you validated the single sphere terminal velocity and near-wall impact/bouncing behavior? How many mesh elements do you have per sphere diameter (usually it should be over 6 at least) ?
I strongly suggest to conduct a good read of my last article in my profile. I studied the problem and draw some interesting conclusions.
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