# file

VelocityCoeffPolicy.hppVelocity equation coefficients policies.

### Contents

- Reference

This file defines coefficients policy classes for the velocity equation for the fluctuating velocity in variable-density turbulence, defined in DiffEq/Velocity.h.

General requirements on velocity equation coefficients policy classes:

- Must define a
*constructor*, which is used to initialize the SDE coefficient, C0. Required signature:CoeffPolicyName( kw::sde_c0::info::expect::type C0_, kw::sde_c0::info::expect::type& C0, std::array< tk::real, 9 >& dU )

where

- C0_ denotes a real value used to initialize the velocity system.
- The reference C0 is to be initialized based on C0_.
*dU*is an optionally prescribed mean velocity gradient.

Must define the function

*update()*, called from Velocity::advance(), updating the model coefficients. Required signature:void update( char depvar, char dissipation_depvar, const std::map< tk::ctr::Product, tk::real >& moments, const tk::Table<1>& hts, ctr::DepvarType solve, ctr::VelocityVariantType variant, kw::sde_c0::info::expect::type C0, tk::real t, tk::real& eps, std::array< tk::real, 9 >& G ) const

where

*depvar*is the dependent variable of the velocity equation,*dissipation_depvar*is the dependent variable of the coupled dissipation equation,*moments*if the map of computed statistical moments,*hts*is a ctr::Table containing the inverse hydrodynamic timescale,*variant*is the velocity model variant,*solve*is the the lable of the dependent variable to solve for (full variable or fluctuation),*C0*is the Langevin eq constat to use,*t*is the physical time,*eps*is the dissipation rate of turbulent kinetic energy to update, and*G*is the G_{ij} tensor in the Langevin model to update.Must define the static function

*type()*, returning the enum value of the policy option. Example:static ctr::CoeffPolicyType type() noexcept { return ctr::CoeffPolicyType::HYDROTIMESCALE; }

which returns the enum value of the option from the underlying option class, collecting all possible options for coefficients policies.

## Namespaces

- namespace walker
- Walker declarations and definitions.

## Classes

- class walker::VelocityCoeffConstShear
- class walker::VelocityCoeffStationary
- Velocity equation coefficients policy yielding a statistically stationary state.
- class walker::VelocityCoeffHydroTimeScale