PLAXIS 2D/3D CONNECT Edition V21 introduced the Fluid model, a new User Defined Soil Model (UDSM) Featured

This constitutive model has been specifically created to simulate the effects of free water on water-retaining structures and quay walls or any other situation involving fluid-structure interaction. Like any other material model in PLAXIS, the fluid is simply modeled as a solid continuum, with the capability to support an indefinite level of hydrostatic pressure and, more importantly, to exhibit negligible shear stresses. The only specific parameter to be defined is the bulk modulus of the fluid (e.g. K = 2.2GPa in case of water) whereas the shear modulus is internally calculated to avoid numerical issues due to the incompressibility constraint, that is Poisson’s ratio approaching 0.5. This model comes in handy when hydrodynamics pressure build-ups need to be taken into serious consideration in the design, specifically when more simplified and straightforward procedures based on the added mass concept, also available in PLAXIS, prove to be not suitable or sufficiently accurate for the purpose.

The Fluid model is an elastic model that can be used to model a body of fluid using active Finite Elements. Modelling fluid using Finite Elements may be necessary in case the stiffness or mass of the water must be taken into account during the calculation. For example, to model confined water in a offshore suction anchor or to model the inertia of water under dynamic loading. 

With the Fluid model the fluid will behave stiff under compression while shear stiffness is very low. The user has to specify the elastic bulk modulus K of the fluid. Internally, the Poisson’s ratio is chosen such that the shear stiffness G is low enough to be negligible without leading to numerical issues, while on the other hand the material is almost incompressible (𝜈 ≈ 0.5).

Usage

The Fluid model is delivered as User Defined Soil Model. In order to use it, follow these steps:

• Create a new Soil and Interfaces material set
• Set the Material model to User-defined
• On the Parameters tabsheet choose the DLL file fluid64.dll
• For the Model in DLL choose the only model available: Fluid

Parameters

With the release of PLAXIS 2D/3D CONNECT Edition V21 (January 2021), this PLAXIS User Defined Soil Model is delivered with the PLAXIS installation. When using the PLAXIS Advanced or PLAXIS Ultimate version and enabling the Geotechnical SELECT Entitlement [GSE] when starting the PLAXIS application, this model becomes available in the Parameters tab when selecting "User-defined" for the Material model.