The obvious answer is "it depends". However, it's not helpful.
I would certainly separate the work in mathematical modeling and actual numerical simulation. Sometimes it might be a bit tough to draw the line in between, but I think it's usually possible. Thus, by using work in mathematical modeling and numerical simulation does not seem to be redundant and actually captures the right keywords.
However, if you need a very short sentence, using a "blanket" phrase, the actual name of this site computational sciences might work. Since it is pretty much what we, computational scientists, do: we perform numerical simulations of mathematical models, usually describing some practical phenomena.
In more detail:
this involves my own interpretation and sepration of the terms, as well as a particular descriptive example that I am familiar with.
mathematical modeling, I would use this term to address the part of your work that was focused on the analysis, derivation, and improvement of the mathematical description of the model describing some system or a process. This would be also aligned with a Wikipedia definition
numerical simulation, performing a simulation using certain numerical methods, usually (nowadays, all the time) using a computer system. That starts from taking a certain mathematical model, evaluating its feasibility for the numerical simulation (problem setup, problem size, computational resources).
For example, you start from Maxwell's equations (which describe a physical phenomenon on some level) and apply them to specific conditions. In particular, say, a surface electric-field integral-equation formulation for magneto-quasi-statics. The formulation derivation (under certain assumption) is the part of the work corresponding to the mathematical modeling.
Now, to validate this formulation and show its advantages on a practical example (inductance extraction), you solve it numerically (finite differences, finite element method, boundary element method, etc). This part would correspond to the numerical simulation aspect.