When it comes to Abaqus structural solver, picture is clear. There is a standard (implicit) solver as well as an explicit solver. Each of these has its own merits and demerits that we have discussed in previous blog articles. However, in CFD there appears to be only one solver. So…
Is it implicit or explicit!!
Well, if you look at the underlying parameters of the solver, it appears to be hybrid. The solver talks about inner loop and outer loop convergence. That makes user feel that solver is implicit and it requires matrix based calculations. This is true for both steady state as well as transient flows. But then when we talk about transient flow parameters, solver mentions CFL number that primarily governs the step size of transient flow. Higher CFL number results in larger step sizes but beyond a certain value of CFL, the flow may become unstable. This looks more like an explicit scheme where stability plays a role. But then, the transient flow also requires convergence that is not an explicit property. So where do we stand.
The answer is somewhat mixed. The CFD solver of Abaqus is implicit by nature. However, it does not follow all the traits of structural implicit solver. One big difference is that CFD implicit solver is not unconditionally stable. While the explicit structural solver just exits if time increment exceeds its critical value, the CFD solver continues at larger than desired CFL numbers but it may give non-realistic flow results. The other big difference is the physical quantities involved. The structural implicit solver looks at force and displacement residuals for convergence. The CFD solver looks at momentum, pressure and velocity residuals.
SIMULIA has made good efforts in 2018x release of CFD solution on the 3D Experience platform in terms of making the solver fully implicit so that it can handle large time increments. There have been other solver enhancements to improve accuracy and reduce solution time. Wish to know more about SIMULIA CFD techniques! Please get in touch.