Unlike differential viscoelastic models, the simplified viscoelastic model introduces only one additional unknown field , next to velocity and pressure. Considering the fact that the simplified model is aimed at predicting viscoelastic effects at reasonable computational cost, it has been decided to select the interpolation accordingly. Hence, by default, the lowest interpolation functions that still verify the LBB conditions have been selected. In particular, the linear interpolation is selected for the velocity, together with a constant interpolation for the pressure. The additional unknown field is interpolated by means of linear functions. Of course, you have the ability to change these selections, and to consider another interpolation (for example, the mini-element).