# Porous Flow Tutorial Page 09. An overview of the PorousFlow architecture

## Introduction

Sometimes PorousFlow can be very frustrating to run because MOOSE keeps spitting back things like

Material property 'PorousFlow_matrix_internal_energy_nodal', requested by 'PorousFlowUnsaturated_EnergyTimeDerivative' is not defined on block caps

You have to create a Material that supplies a property called PorousFlow_matrix_internal_energy_nodal, but which PorousFlow Material do you choose? In multi-component, multi-phase problems these error messages get even more obscure. To debug your input file it is helpful to understand the overall design of PorousFlow.

## The PorousFlow design

Yes, it is actually designed and not just hacked together! The design is fundamentally influenced by the following requirements

1. Multi-component, multi-phase fluids need to be simulated. This means that the Kernels, Materials, etc are all built using C++ std::vectors of components and phases, that are sized at runtime when MOOSE reads your input file. An alternative approach would be to have a separate 1-component, 1-phase module; another module for 2-component, 1-phase; another for 3-component, 1-phase, another for 2-component, 2-phase, etc, as well as all the possible couplings with temperature, mechanics and chemistry. This would be a huge task, made even worse by the exhaustive (and exhausting) testing and documentation. Unfortunately, not only do the std::vectors make the code quite complex, but they necessitate the Joiners (see below). However, to make things easier for users, Actions have been created that cover the common use-case of single-phase fluid flow. These have been used in the tutorial so far, but will be abandoned in Page 10.

2. In many PorousFlow simulations, mass lumping and full upwinding or the Kuzmin-Turek TVD scheme are critical to avoid unphysical results (eg, negative concentrations or saturations) and to vastly improve convergence (to prevent fluid from being withdrawn from a node where there is no fluid). However, MOOSE is not really suited to this approach because the Kernels need all the Material properties (and other things) evaluated at the nodes, rather than at the quadpoints. A special PorousFlowMaterial has been created that allows all other PorousFlow Materials to be evaluated at nodes by specifying the at_nodes input parameter. The required version of each material is added automatically, so the user shouldn't need to set the at_nodes parameter.

3. In contrast to much of the remainder of MOOSE, good nonlinear convergence is only obtained when the Jacobian is built using all the derivative information (a short discussion has already been presented on Page 02). The porous_flow_vars input parameter of the PorousFlowDictator controls which derivatives are entered into the Jacobian. However, sometimes the derivatives are extremely complicated (you will see hundreds of lines of chain-rules if you look at the code) so MOOSE's DerivativeMaterial system is used which allows developers to specify the derivatives if they desire, but will return 0 if none are specified.

4. PorousFlow can run the same simulation using a variety of different nonlinear Variables! For instance, a two-phase simulation can use two porepressure variables, or a single porepressure and a saturation variable. Or a thermally-coupled simulation can use porepressure and temperature, or can use enthalpy as a Variable instead. The reasons for this are: that the choice of Variables can greatly affect the nonlinear convergence; and, when phase changes (boiling of a liquid) occur, a new set of variables is needed, or persistant variables that are not just porepressure and temperature, should be used. PorousFlow provides a set of fundamental Materials that compute porepressures, saturations, temperature and mass fractions from the Variables. Gradients of these quantities, and derivatives with the respect to the Variables are also computed. Those porepressures, saturations, temperature and mass fractions are then Material properties and are used by everything else in PorousFlow.

## Kernels

Often you're not sure which PorousFlow Kernel describes which DE! The DEs are described in the governing equations and at the bottom of that page is a table showing the Kernel names and their mathematical expressions.

## Materials

These form the heart of PorousFlow. Usually the [Materials] block in the input file is the longest and most complicated block.

### Nodal materials

Mentioned above is the at_nodes input parameter. The standard in PorousFlow is that a Material computes a property called property_nodal if at_nodes = true, while it computes the property property_qp if at_nodes = false.

### Derivatives

Almost all PorousFlow Materials compute derivatives, and most of them retrieve derivative information from other Materials and combine then in ghastly chain-rules. The standard is that these are called dproperty_nodal_dvar and dproperty_qp_dvar. Some Materials also compute dproperty_nodal_dgradvar, dgrad_property_qp_dvar, etc

### Joiners

In the input file, you specify things like densities, relative permeabilities, etc, for each phase or component. But most PorousFlow Materials, Kernels, etc, require these to be formatted into C++ std::vectors. This is what the PorousFlowJoiner does. Even single-phase, single-component simulations need Joiners: they just create std::vectors with length 1. These are added automatically by the action system.

### What Material do I need?

The unix grep utility is your friend here! Do the following


cd porous_flow/src/materials
grep matrix_internal *.C


to find the PorousFlowMatrixInternalEnergy Material needed to solve the above MOOSE "undefined" error.