PressureProjection.h

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// Copyright Epic Games, Inc. All Rights Reserved.
#pragma once
 
#include "Chaos/ArrayFaceND.h"
#include "Chaos/ArrayND.h"
#include "Chaos/Framework/Parallel.h"
#include "Chaos/UniformGrid.h"
 
namespace Chaos
{
template<class T, int d>
class TPressureProjection
{
  public:
    TPressureProjection() {}
    ~TPressureProjection() {}
 
    void Apply(const TUniformGrid<T, d>& Grid, TArrayFaceND<T, d>& Velocity, const TArrayND<bool, d>& Dirichlet, const TArrayFaceND<bool, d>& Neumann, const T dt)
    {
        TArrayND<T, d> Pressure(Grid), Divergence(Grid);
        // compute divergence
        PhysicsParallelFor(Grid.GetNumCells(), [&](int32 Index) {
            const TVector<int32, d> CellIndex = Grid.GetIndex(Index);
            Divergence(CellIndex) = 0;
            for (int32 Axis = 0; Axis < d; ++Axis)
            {
                Divergence(CellIndex) += (Velocity(MakePair(Axis, CellIndex + TVector<int32, d>::AxisVector(Axis))) - Velocity(MakePair(Axis, CellIndex))) / Grid.Dx()[Axis];
            }
        });
        check(false);
        MPressureRule(Grid, Pressure, Divergence, Dirichlet, Neumann, dt);
        PhysicsParallelFor(Grid.GetNumFaces(), [&](int32 Index) {
            const auto FaceIndex = Grid.GetFaceIndex(Index);
            const auto Axis = FaceIndex.First;
            const auto PrevIndex = FaceIndex.Second - TVector<int32, d>::AxisVector(Axis);
            const auto NextIndex = FaceIndex.Second;
            // Currently we assume that if no boundary is set on the boarder it must be dirichlet
            T PrevPressure = 0;
            T NextPressure = 0;
            if (NextIndex[Axis] < Grid.Counts()[Axis])
            {
                NextPressure = Pressure(NextIndex);
            }
            if (PrevIndex[Axis] >= 0)
            {
                PrevPressure = Pressure(PrevIndex);
            }
            if (!Neumann(FaceIndex))
            {
                Velocity(FaceIndex) -= (NextPressure - PrevPressure) / Grid.Dx()[Axis];
            }
        });
    }
 
  private:
    TFunction<void(const TUniformGrid<T, d>&, TArrayND<T, d>&, const TArrayND<T, d>&, const TArrayND<bool, d>&, const TArrayFaceND<bool, d>&, const T)> MPressureRule;
};
}