%%%%% Begin file ddz.m
function dudz = ddz(u,alpha) % DG derivative operator
Globals;
du = zeros(2*K,1); du(:) = u(vmapM)-u(vmapP);
uin = -u(1); du(1) = u(1) - uin;            % inflow Dirichlet BC
fluxu = 0.5*nz.*(du- ((1-alpha)*(du.*nz))); % alpha = {0,1} corresponds to 
dudz = Dz*u- LIFT*(Fscale.*fluxu);          % {upwind,central} flux
end
%%%%% End file ddz.m