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Next: Isentropic Flow Examples Up: The Properties in the Previous: The pressure Mach number   Index

### Relationship Between the Mach Number and Cross Section Area

The equations used in the solution are energy (4.39), second law (4.37), state (4.29), mass (4.26)4.1. Note, equation (4.33) isn't the solution but demonstration of certain properties on the pressure.

The relationship between temperature and the cross section area can be obtained by utilizing the relationship between the pressure and temperature (4.37) and the relationship of pressure and cross section area (4.33). First stage equation (4.39) is combined with equation (4.37) and becomes

Combining equation (4.40) with equation (4.33) yields

The following identify, can be proved as

Using the identity in equation (4.42) changes equation (4.41) into

Equation (4.43) is very important because it relates the geometry (area) with the relative velocity (Mach number). In equation (4.43), the factors and are positive regardless of the values of or . Therefore, the only factor that affects relationship between the cross area and the Mach number is . For the Mach number is varied opposite to the cross section area. In the case of the Mach number increases with the cross section area and vice versa. The special case is when M=1 which requires that . This condition imposes that internal4.2flow has to pass a converting-diverging device to obtain supersonic velocity. This minimum area is referred to as throat.''

Again, the opposite conclusion that when implies that M=1 is not correct because possibility of . In subsonic flow branch, from the mathematical point of view: on one hand, a decrease of the cross section increases the velocity and the Mach number, on the other hand, an increase of the cross section decreases the velocity and Mach number (see Figure (4.5)).

Next: Isentropic Flow Examples Up: The Properties in the Previous: The pressure Mach number   Index
Created by:Genick Bar-Meir, Ph.D.
On: 2007-11-21