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/* Copyright 2019-2020 Neil Zaim, Yinjian Zhao
*
* This file is part of WarpX.
*
* License: BSD-3-Clause-LBNL
*/
#ifndef WARPX_PARTICLE_UTILS_H_
#define WARPX_PARTICLE_UTILS_H_
#include "Particles/WarpXParticleContainer.H"
#include <AMReX_DenseBins.H>
#include <AMReX_Particles.H>
#include <AMReX_BaseFwd.H>
namespace ParticleUtils {
/**
* \brief Find the particles and count the particles that are in each cell. More specifically
* this function returns an amrex::DenseBins object containing an offset array and a permutation
* array which can be used to loop over all the cells in a tile and apply an algorithm to
* particles of a given species present in each cell.
* Note that this does *not* rearrange particle arrays.
*
* @param[in] lev the index of the refinement level.
* @param[in] mfi the MultiFAB iterator.
* @param[in] ptile the particle tile.
*/
amrex::DenseBins<WarpXParticleContainer::ParticleType>
findParticlesInEachCell( int const lev, amrex::MFIter const& mfi,
WarpXParticleContainer::ParticleTileType const& ptile);
/**
* \brief Generate random unit vector in 3 dimensions
* https://mathworld.wolfram.com/SpherePointPicking.html
*
* @param[out] x x-component of resulting random vector
* @param[out] y y-component of resulting random vector
* @param[out] z z-component of resulting random vector
* @param[in] engine the random-engine
*/
AMREX_GPU_HOST_DEVICE AMREX_INLINE
void getRandomVector ( amrex::Real& x, amrex::Real& y, amrex::Real& z,
amrex::RandomEngine const& engine )
{
using std::sqrt;
using std::cos;
using std::sin;
using namespace amrex::literals;
amrex::Real const theta = amrex::Random(engine) * 2.0_rt * MathConst::pi;
z = 2.0_rt * amrex::Random(engine) - 1.0_rt;
amrex::Real const xy = sqrt(1_rt - z*z);
x = xy * cos(theta);
y = xy * sin(theta);
}
/** \brief Function to perform scattering of a particle that results in a
* random velocity vector with given magnitude. This is used in collision events.
*
* @param[in/out] ux, uy, uz colliding particle's velocity
* @param[in] vp velocity magnitude of the colliding particle after collision.
*/
AMREX_GPU_HOST_DEVICE AMREX_INLINE
void RandomizeVelocity ( amrex::ParticleReal& ux, amrex::ParticleReal& uy,
amrex::ParticleReal& uz,
const amrex::ParticleReal vp,
amrex::RandomEngine const& engine )
{
amrex::Real x, y, z;
// generate random unit vector for the new velocity direction
getRandomVector(x, y, z, engine);
// scale new vector to have the desired magnitude
ux = x * vp;
uy = y * vp;
uz = z * vp;
}
}
#endif // WARPX_PARTICLE_UTILS_H_
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