The water MPM material: Kirchhoff stress τ = −J·p(J)·I + μ·J·(C + Cᵀ), an equation-of-state pressure on J
(see waterPressure), viscosity from the velocity gradient C, and no shear stiffness. Its plasticity hook
resets F to J^(1/3)·I every substep (a fluid forgets its shape, only its volume matters) with J clamped to
[WATER_MIN_J, 1]: a particle never stores expansion, so separated spray lands again at rest volume and
the fluid cannot creep into a denser state the pressure no longer sees.
The
waterMPM material: Kirchhoff stress τ = −J·p(J)·I + μ·J·(C + Cᵀ), an equation-of-state pressure on J (see waterPressure), viscosity from the velocity gradient C, and no shear stiffness. Its plasticity hook resets F to J^(1/3)·I every substep (a fluid forgets its shape, only its volume matters) with J clamped to [WATER_MIN_J, 1]: a particle never stores expansion, so separated spray lands again at rest volume and the fluid cannot creep into a denser state the pressure no longer sees.