# src/DiffEq/NumberFractionBeta.h file

System of number-fraction beta SDEs.

### Contents

• Reference

This file implements the time integration of a system of stochastic differential equations (SDEs) with linear drift and quadratic diagonal diffusion, whose invariant is the joint beta distribution. The main difference compared to the plain beta SDE (see DiffEq/Beta.h), is that in the number-fraction beta SDE the dependent variable, there are two additional stochastic variables computed from the beta variables.

In a nutshell, the equation integrated governs a set of scalars, , , as

with parameter vectors , , and . This is the same as in DiffEq/Beta.h. Here is an isotropic vector-valued Wiener process with independent increments. The invariant distribution is the joint beta distribution. This system of SDEs consists of N independent equations. For more on the beta SDE, see https://doi.org/10.1080/14685248.2010.510843.

In addition to integrating the above SDE, there are two additional functions of are computed as

These equations compute the instantaneous mixture density, , and instantaneous specific volume, , for equation in the system. These quantities are used in binary mixing of variable-density turbulence between two fluids with constant densities, and . The additional parameters, and are user input parameters and kept constant during integration. Since we compute the above variables, and , and call them mixture density and specific volume, respectively, , governed by the beta SDE is a number (or mole) fraction, hence the name number-fraction beta.

All of this is unpublished, but will be linked in here once published.

## Namespaces

namespace walker
Walker declarations and definitions.

## Classes

template<class Init, class Coefficients>
class walker::NumberFractionBeta
NumberFractionBeta SDE used polymorphically with DiffEq.