In this lecture, we’ll derive the velocity distribution for two examples of laminar flow. First we’ll consider a wide river, by which we mean wide compared with its depth (which we take to be uniform) and we ignore the more complicated flow pattern near the banks. Our second example is smooth flow down a circular pipe. For the wide river, the water flow can be thought of as being in horizontal “sheets”, so all the water at the same depth is moving at the same velocity. As mentioned in the last lecture, the flow can be pictured as like a pile of printer paper left on a sloping desk: it all slides down, assume the bottom sheet stays stuck to the desk, each other sheet moves downhill a little faster than the sheet immediately beneath it. For flow down a circular pipe, the laminar “sheets” are hollow tubes centered on the line down the middle of the pipe. The fastest flowing fluid is right at that central line. For both river and tube flow, the drag force between adjacent small elements of neighboring sheets is given by force per unit area

where now the *z*-direction
means perpendicular to the small element of sheet.

For a river flowing steadily down a gentle incline under
gravity, we’ll assume all the streamlines point in the same direction, the
river is wide and of uniform depth, and the depth is much smaller than the
width. This means almost all the flow is
well away from the edges (the river banks), so we’ll ignore the slowing down
there, and just analyze the flow rate *per
meter of river width*, taking it to be uniform across the river.

The simplest basic question is: given the slope of the land and the depth of the river, what is the total flow rate?

To answer, we need to find the speed of flow* v*(*z*)
as a function of depth (we know the water in contact with the river bed isn’t
flowing at all), and then add the flow contributions from the different depths
(this will be an integral) to find the total flow. The function *v*(*z*) is called the “velocity profile”. We’ll prove it looks something like this:

(For a smoothly flowing river, the downhill ground slope would be imperceptible on this scale.)

But how do we begin to *calculate*
*v*(*z*)?

Recall that (in an earlier lecture) to find how hydrostatic *pressure* varied with depth, we mentally
separated a cylinder of fluid from its surroundings, and applied Newton’s Laws:
it wasn’t moving, so we figured its weight had to be balanced by the sum of the
pressure forces it experienced from the rest of the fluid surrounding it. In fact, its weight was balanced by the
difference between the pressure underneath and that on top.

Taking a cue from that, here we isolate mentally a thin *layer* of the river, like one of those
sheets of printer paper, lying between height *z* above the bed and This layer *is* moving, but* at a steady
speed*, so *the total force on it will
still be zero*. Like the whole river,
this layer isn’t quite horizontal, its weight has a small but nonzero component
dragging it downhill, and this weight component is balanced by the difference
between the viscous force from the faster water above and that from slower
water below.

Bear in mind that the diagram below is at a tiny angle to the horizontal:

Obviously, for the forces to balance, the backward drag on
the thin layer from the slower moving water beneath has to be stronger than the
forward drag from the faster water above, so the *rate of change* of speed with height above the river bed is *decreasing* on going up from the
bed.

Let us find the total force (which must be zero) on *one square meter* of the thin layer of
water between heights *z* and :

First, gravity: if the river is flowing downhill at some small angle , this square meter of the layer (volume , density ) experiences a gravitational force tugging it downstream (taking the small angle approximation, .)

Next, the viscous drag forces: the square meter of layer experiences *two*
viscous forces, one from the slower water below, equal to ,
tending to slow it down, one from the faster water above it, ,
tending to speed it up.

Gravity must balance out the difference between the two viscous forces:

We can already see from this equation that, unlike
the fluid between the plates, *v*(*z*) can’t possibly be linear in *z*the
equation *would not balance* if were the same at *z* and !

Dividing throughout by and by ,

Taking now the limit and recalling the definition of the differential

we find the differential equation

The solution of this equation is easy:

with *C*, *D* constants of integration.

Remember that the velocity *v*(*z*) is *zero* at the bottom of the river, *z* = 0, so the constant *D* must be zero, and can be dropped
immediately. But we’re not throughwe
haven’t found *C*. To do that, we need to go to the top.

What happens to the thin layer of river water *at the very top*the
layer in contact with the air? Assuming
there is negligible wind, there is essentially zero parallel-to-the-surface
force from above.

So the balance of forces equation for the top layer is just

.

We can take this top layer to be as thin as we like, so let’s look what happens in the limit of extreme thinness, . The term then goes to zero, so the other term must as well. Since is constant, this means

*So the velocity
profile function has zero slope at the river surface. *

With this new information, we can finally fix the arbitrary
integration constant *C*.

Now the velocity profile

,

so

,

and gives .

Putting this value for *C* into *v*(*z*) we have the final result:

.

This velocity profile *v*(*z*) is half the top part of a parabola:

Knowing the velocity profile *v*(*z*) enables us to
compute the *total flow* of water in the river. As explained earlier, we’re assuming a wide
river having uniform depth, ignoring the slowdown near the edges of the river,
taking the same *v*(*z*) all the way across. We’ll calculate the flow across *one meter of width* of the river, so the
total flow is our result multiplied by the river’s width.

The flow contribution from a single layer of thickness at height *z*
is cubic meters per second across one meter of
width. The total flow is the sum over
all layers. In the limit of many infinitely
thin layers, that is, ,
the sum becomes an integral, and the total flow rate

in cubic meters per second per meter of width of the river.

It is worth thinking about
what this result means physically. The interesting part is that the flow is
proportional to *h*^{3}, where *h* is the depth of the
river. So, if there’s a storm and the
river is twice as deep as normal, and flowing steadily, the flow rate will be *eight
times* normal.

*Exercise*:
plot on a graph the velocity profiles for two rivers, one of depth *h* and
one 2*h*, having the same values of What is the ratio of the *surface*
velocities of the two rivers? Suppose that one meter below the surface of one
of the rivers, the water is flowing 0.5 m.sec^{-1} slower than it is
flowing at the surface. Would that also
be true of the other river?

The flow rate for smooth flow through a pipe of circular
cross-section can be found by essentially the same method. (This was the flow pattern analyzed by
Poiseuille and used by him to confirm

In the pipe, the flow is fastest in the middle, and the
water in contact with the pipe wall (like that at the river bed) doesn’t flow
at all. The river’s flow pattern was
most naturally analyzed by thinking of flat layers of water, all the water in
one layer having the same speed. What
would be the corresponding picture for flow down a pipe? Here all the fluid *at the same distance from the center* moves down the pipe at the
same speedinstead
of flat layers of fluid, we have concentric hollow cylinders of fluid, one
inside the next, with a tiny rod of the fastest fluid right at the center. This
is again laminar flow, even though this time the “sheets” are rolled into
tubes. The blue circular area on the
cross-section of the pipe shown below represents one of these cylinders of
fluidall
the fluid between *r* and from the central line.

Each of these hollow cylinders of water is pushed along the pipe by the pressure difference between the ends of the pipe. Each feels viscous forces from its two neighboring cylinders: the next bigger one, which surrounds it, tending to slow it down, but the next smaller one (inside it) tending to speed it up. Writing down the differential equation is a little more tricky that for the river, because we must take into account that the two surfaces of the hollow cylinder (inside and outside) have different areas, and . It turns out that the velocity profile is again parabolic: the details are given below.

Suppose the pipe has radius *a*, length *L* and pressure
drop

Let us focus on the fluid in the cylinder between *r* and from the line down the middle, and we’ll take
the cylinder to have *unit length*, for
convenience.

The pressure force maintaining the fluid motion is the difference between pressure x area for the two ends of this one meter long hollow cylinder:

(We’re assuming , since we’ll be taking the limit, so the end area The equality becomes exact in the limit.)

This force exactly balances the difference between the outer surface viscous drag force from the slower surrounding fluid and the inner viscous force from the central faster-moving fluid, very similar to the situation in the previous analysis of river flow.

Using and remembering that the inner and outer surfaces
of the cylinder have slightly different areas, that is positive, but is *negative*,
the force equation is:

Rearranging,

in the limit , remembering the definition of the differential (see the similar analysis above for the river).

This can now be integrated to give

where *C* is a
constant of integration. Dividing both
sides by *r* and integrating again

The constant *C*
must be zero, since physically the fluid velocity is finite at *r* = 0.
The constant *D* is determined
by the requirement that the fluid speed is zero where the fluid is in contact
with the tube, at* r* = *a*.

The fluid velocity is therefore

To find the total flow rate *I* down the pipe, we integrate over the flow in each hollow cylinder
of water:

in cubic meters per second.

Notice the flow rate goes as the *fourth power* of the radius, so doubling the radius results in a *sixteen-fold* increase in flow. *That is
why narrowing of arteries is so serious*.

*Thanks to Linda
Fahlberg-Stojanovska for spotting a sign error in the earlier version.*