MaxwellBoltzmannDistribution.h

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/*
 * MaxwellBoltzmannDistribution.h
 *
 * @Date: 13.12.2019
 * @Author: F. Gratl
 */
 
#pragma once
 
#include <random>
#include <array>
 
inline std::array<double, 3> maxwellBoltzmannDistributedVelocity(double averageVelocity, size_t dimensions) {
    // we use a constant seed for repeatability.
    // random engine needs static lifetime otherwise it would be recreated for every call.
    static std::default_random_engine randomEngine(42);
    // when adding independent normally distributed values to all velocity components
    // the velocity change is maxwell boltzmann distributed
    std::normal_distribution<double> normalDistribution{0, 1};
    std::array<double, 3> randomVelocity{};
    for (size_t i = 0; i < dimensions; ++i) {
        randomVelocity[i] = averageVelocity * normalDistribution(randomEngine);
    }
    return randomVelocity;
}