DiffusiveKernels.hpp

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// SPDX-License-Identifier: Apache-2.0
//
// Copyright 2026 Caleb Buahin
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

 
#ifndef OPENSWMM_ENGINE_2D_DIFFUSIVE_KERNELS_HPP
#define OPENSWMM_ENGINE_2D_DIFFUSIVE_KERNELS_HPP
 
#include <algorithm>
#include <cmath>
 
#ifndef OPENSWMM_KERNEL_FN
#define OPENSWMM_KERNEL_FN inline
#endif
 
namespace openswmm::twoD::diffusive {

OPENSWMM_KERNEL_FN double faceDischarge(double hf, double slope, double nf,
                                        double s_eps) noexcept {
    if (!(hf > 0.0) || !(nf > 0.0) || slope == 0.0) return 0.0;
    const double h53 = hf * std::cbrt(hf * hf);
    const double sa  = std::fabs(slope);
    const double sr  = std::sqrt((sa > s_eps) ? sa : s_eps);
    return -h53 * slope / (nf * sr);
}

OPENSWMM_KERNEL_FN double cellDiffusiveDt(double alpha, double lchar, double h,
                                          double n, double slope_abs,
                                          double s_eps) noexcept {
    if (!(h > 0.0)) return 1.0e30;
    const double h53 = h * std::cbrt(h * h);
    const double s   = (slope_abs > s_eps) ? slope_abs : s_eps;
    return alpha * lchar * lchar * n * std::sqrt(s) / (4.0 * h53);
}
 
} // namespace openswmm::twoD::diffusive
 
#endif // OPENSWMM_ENGINE_2D_DIFFUSIVE_KERNELS_HPP