I have spherical particles in a flow stream at one velocity exiting from a nozzle surrounded by another flow stream at a higher velocity. Any recommendations on modelling the two flows or visualizing the effect the two flows have on the particle(s)? any CFD analyst with this experience or any recommendations for CFD modelling software adept at this type of model?

A simple solution is: 1) obtain the flow field of two-stream mixing problem first, then 2) do the particle tracking. ( release a particle at a location in the central nozzle exit at certain speed and keep track of its position based on the local relative flow conditions). This approach assumes that you have relatively few particles, and the praticles are small. This is one of the extreme case.

I have recently been running simulations of this case. I use a commercial package that models both flow streams and the spherical particles, including complete momentum coupling between the different phases. I believe this treatment is necessary if you are interested in the rate at which the particles accelerate (or decelerate) with the surrounding media. This treatment is also useful for seeing different results for different size particles within the ensemble of the "spray".

Interesting results include the dispersion angle of the particles, gravitational settling and the probability of impingement on nearby walls (both are functions of particle size). I was also interested in seeing the spray evaporate (my particles were liquid droplets).

We can discuss further if you are interested.

How about if the particle is non-spherical? What if the non-spherical geometry is based on two spherical particles joined at a point of tangency (a non-slip condition)? In addition, would it be possible to determine the forces exerted at that "joint"?