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A graphic showing the equations which describe isentropic flow.

As a gas is forced through a tube, the gas molecules are deflected by the walls of the tube. If the speed of the gas is much less than the speed of sound of the gas, the density of the gas remains constant and the velocity of the flow increases. However, as the speed of the flow approaches the speed of sound we must consider compressibility effects on the gas. The density of the gas varies from one location to the next. Considering flow through a tube, as shown in the figure, if the flow is very gradually compressed (area decreases) and then gradually expanded (area increases), the flow conditions return to their original values. We say that such a process is reversible. From a consideration of the second law of thermodynamics, a reversible flow maintains a constant value of entropy. Engineers call this type of flow an isentropic flow; a combination of the Greek word "iso" (same) and entropy.

Isentropic flows occur when the change in flow variables is small and gradual, such as the ideal flow through the nozzle shown above. The generation of sound waves is an isentropic process. A supersonic flow that is turned while the flow area increases is also isentropic. We call this an isentropic expansion because of the area increase. If a supersonic flow is turned abruptly and the flow area decreases, shock waves are generated and the flow is irreversible. The isentropic relations are no longer valid and the flow is governed by the oblique or normal shock relations.

On this slide we have collected many of the important equations which describe an isentropic flow. We begin with the definition of the Mach number since this parameter appears in many of the isentropic flow equations. The Mach number M is the ratio of the speed of the flow v to the speed of sound a.

Eq #1:

M = v / a

The speed of sound, in turn, depends on the density r, the pressure, p, the temperature, T, and the ratio of specific heats gam:

Eq #2:

a = sqrt(gam * p / r) = sqrt (gam * R * T)

where R is the gas constant from the equations of state. If we begin with the entropy equations for a gas, it can be shown that the pressure and density of an isentropic flow are related as follows:

Eq #3:

p / r^gam = constant

We can determine the value of the constant by defining total conditions to be the pressure and density when the flow is brought to rest isentropically. The "t" subscript used in many of these equations stands for "total conditions". (You probably already have some idea of total conditions from experience with Bernoulli's equation).

Eq #3:

p / r^gam = constant = pt / rt^gam

Using the equation of state, we can easily derive the following relations from equation (3):

Eq #4:

p / pt = (r / rt)^gam = (T / Tt)^[gam/(gam-1)]

The dynamic pressure q is defined to be:

Eq #5:

q = (r * v^2) / 2 = (gam * p * M^2) / 2

Using the conservation of mass, momentum, and energy and the definition of total enthalpy in the flow, we can derive the following relations:

Eq #6:

p / pt = [1 + M^2 * (gam-1)/2]^-[gam/(gam-1)]

Eq #7:

T / Tt = [1 + M^2 * (gam-1)/2]^-1

Eq #8:

r / rt = [1 + M^2 * (gam-1)/2]^-[1/(gam-1)]

Then considering the compressible mass flow equation. we can derive:

Eq #9:

A / A* = {[1 + M^2 * (gam-1)/2]^[(gam+1)/(gam-1)/2]}*{[(gam+1)/2]^-[(gam+1)/(gam-1)/2]} / M

The starred conditions occur when the flow is choked and the Mach number is equal to one. Notice the important role that the Mach number plays in all the equations on the right side of this slide. If the Mach number of the flow is determined, all of the other flow relations can be determined. Similarly, determining any flow relation (pressure ratio for example) will fix the Mach number and set all the other flow conditions.

Here is a Java program that solves the equations given on this slide.

Due to IT security concerns, many users are currently experiencing problems running NASA Glenn educational applets. There are security settings that you can adjust that may correct this problem.

You select an input variable by using the choice button labeled Input Variable. Next to the selection, you then type in the value of the selected variable. When you hit the red COMPUTE button, the output values change. Some of the variables (like the area ratio) are double valued. This means that for the same area ratio, there is a subsonic and a supersonic solution. The choice button at the right top selects the solution that is presented. The variable "Wcor/A" is the corrected airflow per unit area function which can be derived from the compressible mass flow. This variable is only a function of the Mach number of the flow. The Mach angle and Prandtl-Meyer angle are also functions of the Mach number. These additional variables are used in the design of high speed inlets, nozzles and ducts.

If you are an experienced user of this calculator, you can use a sleek version of the program which loads faster on your computer and does not include these instructions. You can also download your own copy of the program to run off-line by clicking on this button:

Button to Download a Copy of the Program


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Editor: Tom Benson
NASA Official: Tom Benson
Last Updated: Feb 11 2014

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