[Nek5000-users] calculate gradients of fluctuation

Hi nek users, For u=U + u’ where U= mean streamwise velocity and u’= fluctuation of streamwise velocity I know how to calculate du/dx , du/dy and du/dz. But how can I calculate gradients of the fluctuation of streamwise veloctity, i.e. du’/dx , du’/dy , du’/dz ? Thank you.