
Preparation 
Just what you need to know ! 
A shell momentum balance is used below to derive a general differential equation that can be then employed to solve several fluid flow problems in rectangular Cartesian coordinates. For this purpose, consider an incompressible fluid in laminar flow under the effects of both pressure and gravity in a system of length L and width W, which is at an angle β to the vertical. End effects are neglected assuming the dimension of the system in the xdirection is relatively very small compared to those in the ydirection (W) and the zdirection (L).
Figure. Differential rectangular slab (shell) of fluid of thickness Δx used in zmomentum balance for flow in rectangular Cartesian coordinates. The yaxis is pointing outward from the plane of the computer screen.
Since the fluid flow is in the zdirection, v_{x} = 0, v_{y} = 0, and only v_{z} exists. For small flow rates, the viscous forces prevent continual acceleration of the fluid. So, v_{z} is independent of z and it is meaningful to postulate that velocity v_{z} = v_{z}(x) and pressure p = p(z). The only nonvanishing components of the stress tensor are τ_{xz} = τ_{zx}, which depend only on x.
Consider now a thin rectangular slab (shell) perpendicular to the xdirection extending a distance W in the ydirection and a distance L in the zdirection. A ‘rate of zmomentum’ balance over this thin shell of thickness Δx in the fluid is of the form:
Rate of zmomentum In − Out + Generation = Accumulation At steadystate, the accumulation term is zero. Momentum can go ‘in’ and ‘out’ of the shell by both the convective and molecular mechanisms. Since v_{z}(x) is the same at both ends of the system, the convective terms cancel out because (ρ v_{z} v_{z} W Δx)_{z = 0} = (ρ v_{z} v_{z} W Δx)_{z = L}. Only the molecular term (L W τ_{xz} ) remains to be considered, whose ‘in’ and ‘out’ directions are taken in the positive direction of the xaxis. Generation of zmomentum occurs by the pressure force acting on the surface [p W Δx] and gravity force acting on the volume [(ρ g cos β) L W Δx].
The different contributions may be listed as follows:
 rate of zmomentum in by viscous transfer across surface at x is (L W τ_{xz} ) _{x}
 rate of zmomentum out by viscous transfer across surface at x + Δx is (L W τ_{xz} ) _{x + Δx}
 rate of zmomentum in by overall bulk fluid motion across surface at z = 0 is (ρ v_{z} v_{z} W Δx ) _{z = 0}
 rate of zmomentum out by overall bulk fluid motion across surface at z = L is (ρ v_{z} v_{z} W Δx ) _{z = L}
 pressure force acting on surface at z = 0 is p_{0} W Δx
 pressure force acting on surface at z = L is − p_{L} W Δx
 gravity force acting in zdirection on volume of rectangular slab is (ρ g cos β) L W Δx
On substituting these contributions into the zmomentum balance, we get
(L W τ_{xz} )  _{x} − (L W τ_{xz} )  _{x+Δx}+ ( p_{ 0} − p_{ L} ) W Δx + (ρ g cos β) L W Δx = 0 

(1) 
Dividing the equation by L W Δx yields
τ_{xz}  _{x+Δx} − τ_{xz}  _{x}
Δx 
= 
p_{ 0} − p_{ L} + ρ g L cos β
L 

(2) 
On taking the limit as Δx → 0, the lefthand side of the above equation is exactly the definition of the derivative. The righthand side may be written in a compact and convenient way by introducing the modified pressure P, which is the sum of the pressure and gravitational terms. The general definition of the modified pressure is P = p + ρ g h , where h is the distance upward (in the direction opposed to gravity) from a reference plane of choice. The advantages of using the modified pressure P are that (i) the components of the gravity vector g need not be calculated; (ii) the solution holds for any flow orientation; and (iii) the fluid may flow as a result of a pressure difference, gravity or both. Here, h is negative since the zaxis points downward, giving h = − z cos β and therefore P = p − ρ g z cos β. Thus, P_{0} = p_{0} at z = 0 and P_{L} = p_{L} − ρ g L cos β at z = L giving p_{0} − p_{L} + ρ g L cos β = P_{0} − P_{L} ≡ ΔP. Thus, equation (2) yields
The firstorder differential equation may be simply integrated to give
Here, C_{1} is an integration constant, which is determined using an appropriate boundary condition based on the flow problem. Equation (4) shows that the momentum flux (or shear stress) distribution is linear in systems in rectangular Cartesian coordinates.Since equations (3) and (4) have been derived without making any assumption about the type of fluid, they are applicable to both Newtonian and nonNewtonian fluids. Some of the axial flow problems in rectangular Cartesian coordinates where these equations may be used as starting points are given below.
Related Problems in Transport Phenomena – Fluid Mechanics :
Transport Phenomena – Fluid Mechanics Problem : Newtonian fluid flow in plane narrow slit
– Determination of shear stress distribution, velocity profile and mass flow rate in slit flow for Newtonian fluid
Transport Phenomena – Fluid Mechanics Problem : Fluid flow in a falling film on an inclined flat surface
– Determination of shear stress distribution, velocity profile and film thickness in falling film
Transport Phenomena – Fluid Mechanics Problem : Power law fluid flow in plane narrow slit
– Determination of shear stress distribution, velocity profile and mass flow rate in slit flow for power law fluid 