So, x component does not come into picture. And then when the motion is steady then this becomes this term will be 0. So, that will give us del p by del x equal to mu by rho that will be nu del square u by del y square plus del square u by del z square. So, this becomes for steady case, this becomes the equation of motion and this (( )) for whereas, here p is a function of x and u is a function of y z. (( )) 3 D fully developed flow. So, this is the general form and here again we need to prescribe the wall boundary condition.
To solve this equation if we are solving this as a steady problem then only you need the wall boundary condition and if you want to solve this we need apart from their wall boundary condition one initial boundary, initial condition to take care of the transient part. Now, we will go to simple problems one by one. So, the simplest one in the series is the straight, parallel flow in a straight channel.