Consider the constant -property ,steady laminar flow illustrated in fig 2p-4 A fluid flow through a very long tube in response to an externally imposed pressure gradient along the tube length Because of the infinite length ,it can be assumed that no quantity (other than pressure ) varies in the θ and z directions. Also, under these conditions a fluid particle move solely in the z direction (i.e., v_θ=0=v_r).
Apply the conservation of mass principle to doteed control volume (after first making all assumption) to derive the form of the continuity equation appropriate to this physical situation.