# Examples – Fluid Flows

These worked examples have been fielded as homework problems or exam questions.

Worked Example #1

Two pipes of diameters and converge to form a single pipe of diameter . If a liquid flows with a velocity of and in the two pipes, respectively, what will be the flow velocity in the third pipe? Hint: Assume steady, one-dimensional, incompressible flow.

In this case, two pipes merge to produce one pipe. Conserving mass and using average flow properties then

If the fluid is a liquid then the assumption that = = = = constant is justified so

so the flow velocity in the third pipe is

Worked Example #2

Consider the steady flow of a particular gas through a horizontal, converging pipe that has an inlet diameter of 0.22 m and an outlet diameter of 0.16 m. The density of the gas is known to change from kg/m at the inlet to kg/m at the outlet. If the inlet flow velocity of the gas is 5.1 m/s, what is its exit velocity ? Hint: Assume one-dimensional flow.

Conserving mass and using average flow properties we have

where is this case the density must be retained as a variable. In terms of pipe diameters then

and solving for gives

Substituting values gives

Worked Example #3

Air flows at a steady rate through a horizontal pipe of constant circular cross-sectional area. The temperature of the air is constant at C all along the flow direction. At point A, the static pressure is Pa and the mean velocity is m/s. At point B the static pressure is Pa. What is the average flow velocity at B? Assume = 287.057 J kg K.

The requirement for flow velocities suggests the use of the continuity equation, which can be written as

Notice that the density of the air is not given but the pressures and the temperature are, so using the equation of state gives

Likewise

The use of the continuity equation gives

noting that

Worked Example #4

A gas of density 1.7 kg/m flows through a pipe with a volume flow rate of 0.12 m/s. The pipe has an inlet section diameter of 21 cm and and outlet section diameter of 9 cm. Assuming an ideal incompressible fluid flow, then calculate the flow velocities at the inlet and the outlet of the pipe, i.e., and , respectively.

It is reasonable from the information given to assume one-dimensional, steady, incompressible, inviscid flow. The flow rates, flow velocities, and Mach numbers are low enough that compressibility effects can be neglected. Let the inlet be condition 1 and the outlet condition 2. The continuity equation can be used to relate the inlet and outlet conditions, i.e.,

or because the flow is assumed incompressible then just

We see that

and

We can calculate , i.e.,

and also , i.e.,

so that

and

We have now confirmed that the flow velocities are very low, much lower than a Mach number of 0.3, and so the initial assumption of incompressible flow was justified.

Worked Example #5

A flowing liquid enters a pipe of diameter with a velocity . What will the velocity of the liquid be at the exit if the diameter of the pipe progressively reduces to ? Hint: Assume one-dimensional flow.

Conserving mass and using the continuity equation with average flow properties (i.e., one-dimensional flow) gives

Because the fluid is a liquid then the assumption that = constant is justified. So the continuity equation reduces to

In this case

where is the exit velocity. Rearranging gives

Worked Example #6

Consider the flow turning block shown below. A circular jet of water with diameter 10 cm at a velocity ms enters the block. The water is turned back through 180 degrees by the block and then exits through an orifice with an area of 120 cm. First, find the velocity of the water exiting the block. Second, determine the force required to hold the block in place. Hint: Assume one-dimensional flow.

First, apply the conservation of mass to the inlet and outlet flow from the control volume, i.e., for a steady flow then

and in one dimensional form then

or

The fluid is water so constant and so

Rearranging to solve for the exit velocity gives

Substituting the known values gives

Second, from the principle of conservation of momentum the

where on the fluid from the block. In this case then

the minus reminding that the force on the fluid is to the left, i.e., in the negative direction. Mass flow rate is conserved so

where the density of water has been assumed to be 1,000 kg/m, and so

and the reaction force on the block is to the right, i.e., in the positive direction. Therefore, the force needed to restrain the block will be 2196.6 N toward the left.

Worked Example #7

An incompressible gas flow in a pipe is passed through a venturimeter. The diameter of the pipe is = 4.1 inches and the diameter of the throat of the venturimeter = 2.2 inches. The gas flow in the main pipe has a velocity = 14.3 ft/s. If the static pressure difference between the inlet and the throat of the venturi is measured using a U-tube water manometer, then calculate the height between the two columns. Assume 1-dimensional flow and no losses. Also assume that slug/ft and slug/ft.

Assuming steady, 1-dimensional flow applying the continuity equation between inlet and the throat gives

and because constant in this case (the flow is stated to be incompressible) then

Therefore, solving for gives

The application of Bernoulli’s equation gives

There is no change in vertical height of the mean flow in this case, so and

Rearranging for the pressure difference gives

The U-tube manometer will measure this static pressure difference , so using the hydrostatic equation the pressure difference is

making sure the density of water is used. Therefore, equating the latter two equations gives

and solving for leads to

Worked Example #8

A rain barrel with a circular cross-section allows the water to leave at the bottom through a drain valve. The discharge velocity of the flow from the drain varies with the height of the water level above the drain according to . If the initial height of the water level above the drain is 0.5 m, how long will it take to empty the tank? The density of water can be assumed to be 1,000 kg m.

The volume flow rate from the tank through the drain can be expressed as

where is the area of the jet of water from the drain and

Notice that the flow rate depends on the instantaneous height of the water in the tank. This result is actually called Torricelli’s theorem, and it is a special case of using the Bernoulli equation for a fluid that flows out of an orifice based on the height of the fluid above the orifice.

The volume of water in the tank is for any height is

where is the area of the tank. In a small time interval then the decrease in the volume of water in the tank will be

the minus sign denoting that the value of decreases as the tank is emptied. We are told that the initial height of the water in the tank is 0.5 m. Notice that the units in the foregoing equation are in terms of a flow rate.

Now, by continuity considerations (conservation of mass) this flow rate must be equal to the flow rate of fluid coming out of the drain, i.e.

so

Rearranging gives

This is an ordinary differential equation relating the height of the fluid level to time. We now need to solve this equation subject to the boundary conditions given in this question. Separating the variables and integrating gives

where the limits of integration are noted, i.e., at time then and when the tank is considered to be empty. Performing the integration gives

We are given the diameter of the tank and the outlet diameter , so in terms of ratios then

so

Substituting the known values gives

Worked Example #9

A water jet with velocity that exits from a nozzle strikes a vertical plate. The exit area of the nozzle is . Find an expression for the horizontal force on the plate. If the volume flow rate through the nozzle is 0.5 m s and the diameter of the jet is 5 cm, then find the force on the plate in Newtons. The density of water can be assumed to be 1,000 kg m.

The force on the plate is obviously related to the force of the fluid and so to the exit velocity from the nozzle. Using the momentum equation in its one-dimensional form, the the force on the plate from the water jet is just the time rate of change in horizontal momentum, i.e.,

The use of the continuity equation gives

and because we are dealing with water then it is incompressible, so

The value of is

Therefore, the force on the plate is

Worked Example #10

Water flows into a container through both of pipes 1 and 3 and simultaneously exits through pipe 2. The flow rates are adjusted such that the height of water in the container stays constant. The volume flow rate m/s and also m/s, m and m. From this information, determine the value of . Make any assumptions that may be justified.

If the level stays constant then the net mass (or volume) flow the system is a constant in that the mass of water coming in per unit time is equal to the mass (or volume) of water leaving per unit time. In terms of the problem parameters then

where ~m/s. Remember that

so

and so

and solving for , which is what we are asked to find, then

We are given the diameters m and m so

and

so

Worked Example #11

This is an unsteady flow problem because a mass of gas is pumped into a fixed volume and for mass conservation then the density of the gas must increase in time. The general form of the continuity equation is

In this case

also

We will assume uniform mixing and so the density inside the volume uniform so we have

Integrating using separation of variables gives

so

which shows that the density of the gas increases linearly with time. The corresponding pressure can be obtained from the equation of state, i.e., . If the process is isothermal then and so

Worked Example #12

The first term is the time rate of change of the mass of fluid inside the control volume and the second term in the net mass flow rate of fluid out of the control volume. Conservation of mass requires that the net sum be zero.

which is a statement that an equal mass of fluid coming into a control volume per unit time will also leave the control volume in the same time.(b) Steady, incompressible flow. In this case, = constant so that we can write

which is a statement the at the volume flow rate is conserved, i.e., an equal volume of fluid coming into a control volume per unit time will also leave the control volume in the same time.(c) One-dimensional, steady flow. In this case we must again retain the density in the integral but if the flow is one-dimensional then the flow properties can change only in one dimension, so for a fixed control volume with an inlet and and outlet plane defined accordingly, by conserving mass we would end up with

or

This is a common form of the continuity equation and is very useful for problem solving when average flow properties are desired.

Worked Example #13

In words then: Body Forces + Pressure Forces + Viscous Forces = Time rate of change of momentum inside from unsteadiness + Net flow of momentum out of per unit time. It is a vector equation, so three scalar equations rolled into one. This equation is very general and will apply to any type of flow.(a) For steady flows then and the reduced form of the momentum equation will be

(b) For steady, incompressible flows then and constant so

(c) For steady, inviscid flow then and so

(d) For unsteady flows with no body forces then = 0 so

Worked Example #14

The most general form of the energy equation is

which is a statement of energy conservation for a fluid. While this is a very general equation, there are obviously many practical problems in which we can simplify it, i.e., by making certain justifiable assumptions, which commensurately makes the mathematics and solution process easier.

(a) In the case of a steady flow then so

(b) In the case of a steady, inviscid flow, then and all of the terms that have their origin in the viscosity of the fluid can be neglected, i.e.,

(c) In the case of a steady flow with no body forces, heat or work addition, then

Worked Example #15

First approach: From the continuity equation, the flow velocity in the test section will be

The pressures at the inlet (i.e., settling chamber) and the test section of wind tunnel can be related to the flow velocities by means of the Bernoulli’s equation, i.e.,

where is the total pressure. This means that

Therefore, the foregoing equation can be used to determine the velocity in the test section from a measurement of total pressure measured in the contraction section (using a Pitot probe) and the static pressure at the test section (using static pressure taps). However, the density of the air must be known and this is determined be measuring the temperature and pressure of the air in the flow and then calculating the density using the equation of state. This technique is used in many low-speed wind tunnels to measure the flow speed in the test section.

Second approach: From the continuity equation, the flow velocity in the test section will be

The static pressures at the inlet (i.e., settling chamber) and the test section of the wind tunnel can be related to the flow velocities by means of the Bernoulli’s equation, i.e.,

The velocity in the test section can be related to the static pressure drop across sections 1 and 2, i.e., the pressure drop between the contraction section and the test section. From the Bernoulli equation we have

This means that

Solving for (the flow velocity in the test section) we obtain

Worked Example #16

A horizontal water jet with diameter = 4 cm impacts on a vertical plate. By using the conservation laws of fluid dynamics, find the force on the plate, . Make a one-dimensional, steady flow assumption.

The mass flow rate in the jet is

where = 10 m/s and = 0.00126 m.

The horizontal jet hits the plate and is brought to rest in the horizontal direction. To do this, the force on the fluid has to be equal to the time rate of change of momentum in that direction and the force on the plate is in the opposite direction, i.e.,

and substituting numbers gives

Worked Example #17

An incompressible gas flow in a exhaust system encounters a change in the size of the exhaust pipe as it transitions to the muffler. The exit diameter of the muffler pipe is = 4.1 inches and the diameter of the inlet header pipe is = 2.2 inches. The exhaust gas is discharged at a speed of = 14.3 ft/s. If the static pressure difference between the header and the muffler is measured using a U-tube water manometer, then calculate the height between the two columns. Assume 1-dimensional flow and no losses. Also assume that slug/ft and slug/ft.

Assuming steady, 1-dimensional flow applying the continuity equation between inlet (section 1) and the exit (section 2) gives

and because constant in this case (the flow is stated to be incompressible) then

Therefore, solving for the inlet velocity gives

The application of Bernoulli’s equation gives

There is no change in vertical height of the mean flow in this case, so and

Rearranging for the pressure difference gives

The U-tube manometer, as shown, will measure this static pressure difference , so using the hydrostatic equation the pressure difference is

making sure we use the density of water here. Therefore, equating the latter two equations gives

and solving for leads to

Worked Example #18

Gas is flowing at speed through a horizontal section of pipe whose cross-sectional area is m. The gas has a density of kg/m. A venturi meter with a cross-sectional area of m and has been substituted for a section of the larger pipe. The pressure difference between the two sections is Pa. Find: (a) Find the speed of the gas in the original pipe. (b) Find the speed in the throat of the venturi. (c) Find the volume flow rate .

(a) We are told the gas has a density of 1.34 kg/m implying constant density, so the Bernoulli equation can be used, i.e.,

We can assume from the figure that both pressure gages are at the same height so so the Bernoulli equation is now

The continuity equation (constant density gas) is

so

The Bernoulli equation becomes

We are given that m and m so becomes

(b) Using the continuity equation then

(c) The volume flow rate is

so we get

Worked Example #19

(a) We are told to use of the Bernoulli equation ( constant), which is

Rearranging to solve for gives

(b) The need for a force suggests the use of the momentum equation. The momentum of the jet before the wall is where

When the jet hits the wall is in brought to rest in the horizontal direction (it will be deflected in all other directions)

where is the force. Solving for gives

Worked Example #20

Water flows through a converging nozzle. If the pressure at point A at the intake to the nozzle is 19.7 lb/in, then determine the height shown on the U-tube manometer. Notes: Assume atmospheric pressure is 14.7 lb/in. The density of water can be assumed to be 1.93 slugs/ft, and the specific gravity of mercury is 13.6.

Let the height from point A to point B be . Let be the height from the bottom of the U-tube to the lowest point of the mercury. Considering the left leg of the U-tube, then the pressure at the lowest point in the U-tube will be

For the right leg, then the pressure at the same point will be

Equating these latter two equations gives

and rearranging gives

and solving for gives

Now we can substitute the information given. Notice that = 5 lb/in = (5 144) lb/ft = 720.0 lb/ft. Therefore,

Worked Example #21

Water flows through a pipe of circular cross-section with a volume flow rate of 14.2 ft/s encounters a change in the area and height of the pipe as it passes through an elbow-type of coupling. The exit area of the pipe is = 2.4 ft and the area of the inlet is = 4.8 ft. Assume 1-dimensional flow and no losses with slug/ft. (a) Determine the flow velocities at the entrance () and the exit () in units of ft/s. (b) If = 6 inches and = 25 inches, what is the static pressure difference between the inlet and outlet, i.e., the value of , in units of lb/in.

(a) The fluid is water so it is incompressible. The application of the continuity equation in one-dimensional form gives

Therefore, the inlet flow velocity is

and for the outlet flow velocity then we know that

so

It will also be noted that

(b) The Bernoulli equation is

so in this case

We are asked for the pressure difference so using the Bernoulli equation gives

and substituting the known values gives

noting to convert the hydrostatic head (last term) from units of inches to feet to keep the entire equation in consistent base units of lb, ft, and seconds (s). Evaluating the arithmetic gives

noting that 144 lb/in is equal to 1 lb/ft.

Worked Example #22

The 6 cm diameter water jet as shown in the figure below strikes a flat plate containing a hole of 4 cm in diameter. Part of the jet passes through the hole and part is deflected radially symmetrically away from the plate. Assume the density of water is 1,000 kg m. (a) Calculate the mass flow rate of the jet toward the plate; (b) Calculate the mass flow rate of the part of the jet that is deflected; (c) Determine the magnitude and direction of the force required to hold the plate stationary.

(a) Assumptions: Incompressible, steady, no body force, inviscid, and one-dimensional flow. The mass flow rate of the jet towards the plate is

and inserting the values gives

(b) Conservation of mass in the one-dimensional form gives

and so

The mass flow at section 3 and section 4 are equal so

so the mass flow rate of the part of the jet that is deflected is

and

(c) The system is subjected to atmospheric pressure and the net pressure acting on the control volume is zero, Therefore, we get that

and inserting the numbers gives

Therefore, the force applied to the plate is

and so to hold the plate stationary, a force of -980.01 N is required, i.e., to the left.

Worked Example #23

Two plates, set 4 cm apart, are separated by a bed of oil. The bottom plate is stationary but the top plate moves at a speed of 3.5 m/s. Calculate the shear stress in the oil. Assume that the coefficient of dynamic viscosity of the oil is 583.95 Pa s. Note: Pa s is an SI unit of viscosity, and is equivalent to Newton-second per square meter (N s m), which is sometimes referred to as the Poiseuille (Pl).

It is necessary to calculate the shear stress in a fluid, , based on the formation of a velocity gradient. The use of Newton’s formula of viscosity gives

Newtonian flow will give a constant uniform velocity gradient in this case between the upper and lower plates will be

This means that the shear stress per unit depth in the fluid will be

Worked Example #24

Water ( = 1.94 slug ft) flows at a mass flow rate of = 6.0 slug s through a nozzle with an entrance diameter in and and exit diameter of . The water hits an angled bracket, as shown in the figure below, such that exactly half of the mass flow rate of the stream deflects to the left and half deflects to the right. The diameter of the deflected jets is 1.3 in. Assume that the water all stays in a horizontal plane and one-dimensional flow.

- The velocity of the water entering and exiting the nozzle, and .
- The change in pressure between the nozzle entrance and its exit.
- The velocity of the water in the deflected jets.
- The magnitude and direction of the force on the bracket.

1. The velocity of the water entering and exiting the nozzle can be determined using conservation of mass (continuity equation), i.e.,

The entrance area is

The entrance velocity is

and substituting the values gives

The exit jet area is

The exit jet velocity is

and substituting the values gives

2. The change in pressure between the nozzle inlet and its exit can be found using the Bernoulli equation, i.e.,

In this case then

We are asked for the change in pressure so

and substituting the values gives

3. The velocity of the water in the deflected jet can be found using continuity. In this case, have of the mass flow is deflected in each direction so

The deflected jet area is

The deflected jet velocity is

and substituting the values gives

4. The force of the plate from the water can be determined from the time rate of change of momentum of the fluid, a suitable control volume being shown below. To this end, the rate of momentum of the water coming onto the deflector in the downstream (horizontal) direction is . Likewise, the rate of deflected momentum coming out away from the defector is . Notice that the term give the correct horizontal component of momentum.

Therefore, the force on the plate will be the time rate of the net momentum so that

Inserting the numbers gives

which will be applied by the water in the downstream direction away from the nozzle.