ENH: forces: avoid redundant volumetric operations

Problem

It has been observed that employing the forces and forceCoeffs function objects in several simulations causes a slowdown ranging from 2 to 10%.

The culprits found were:

• Calculation of velocity gradient
• Redundant calculations performed for various internal fields

Solution

• It is recommended to cache 'grad(U)' whenever using forces and forcesCoeffs function objects.
• The recent commits have eliminated unnecessary computations for the internal field of devRhoReff.
  • Tests using the simpleCar tutorial ('grad(U)' was cached) produced the following 'profiling' figures:
  • Tests using an industrial case ('grad(U)' was cached) reduced the functionObjects.execute() from ~72[s] to ~23[s].

Meta-data

• EP2065
• linux64ClangDPInt32Opt (clang13)
• linux64GccDPInt32Opt
• linux64GccSPDPInt64Debug
• Alltest: No new error/No change in existing output
• No output change in all operating conditions (e.g. direct force intensity, porosity, compressible flows etc.)