stokes_fd_nl_operator.m

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function A = stokes_fd_nl_operator(model, model_data)
%function A = stokes_fd_nl_operator(model, model_data)
% Evaluation of the Frechet-derivative of nonlinear Navier-Stokes problems
% for the approximation of the inf-sup constant.

A = model.operators(model, model_data);

if model.decomp_mode < 2
    
    sim_data = detailed_simulation(model, model_data);
    model.uh = sim_data.uh;
    
    A_nl1 = Fem.Assembly.matrix_part(...
        @Fem.IntegralKernels.stokes_volume_matrix, model, model_data.df_info);
    
    A_nl2 = Fem.Assembly.matrix_part(...
        @Fem.IntegralKernels.stokes_matrix_fd_nl, model, model_data.df_info);
    
    % do not care about dirichlet nodes, as they are deleted later
    
    if model.decomp_mode == 0
        
        A = A + A_nl1 + A_nl2;
    else
        
        for q = 1:length(A)
            A{q} = A{q} + A_nl1{q} + A_nl2{q};
        end
    end
    
end

end