**3. Numerical simulation method**

The numerical simulation of aircraft outflow is performed. The outlet section of the inlet duct in the aircraft outflow calculation results is taken as the inlet section in the internal-flow calculation of the compressor. Then, total pressure, total temperature, velocity component, and other gas parameters on the inlet section in the compressor are obtained by interpolation and combination according to the calculation conditions. Finally, the function of the stability margin loss and distortion parameters is established based on the calculation results.

The selected research object is NASA Rotor 67. Rotor 67 is a low-aspect-ratio transonic axial-flow fan rotor designed by NASA Lewis Research Center. The geometry of Rotor 67 and the experimental data obtained using both aerodynamic probes and laser anemometer measurements were reported by Strazisar *et al.* [16]. **Figure 4** shows the meridional view and 3D model of Rotor 67, and **Table 1** lists basic design specifications [17].

### **3.1 Numerical simulation method**

The total pressure distortion, total temperature distortion, and swirl distortion should be combined to study the influences of combined distortions on the stability margin loss. The total pressure distortion, total temperature distortion, and swirl

#### **Figure 4.**

*NASA rotor 67. (a) Geometry definition including stations [16]. (b) 3D model.*


#### **Table 1.**

*Basic specifications of NASA rotor 67 [17].*

distortion in the actual distortion of the outlet section of the inlet duct obtained in aircraft outflow calculation results all exist at the same time and change with the outflow conditions. Therefore, it is impossible to directly obtain the flow conditions with/without the swirl distortion or different swirl distortions under the total pressure and total temperature distortion. The inlet boundary of the rotor in the internal flow calculation is the outlet section of the inlet duct in the outflow calculation. The gas parameters at the outlet section of the inlet duct obtained from the outflow calculation should be interpolated and combined to obtain the internal flow calculation boundary conditions with adjustable distortion parameters (see **Figure 5**).

The total pressure distortion parameter is taken as an example. When the total pressure distortion parameter on the outlet section of the inlet duct in outflow

*The Influences of Combined Distortions on Fan Performance DOI: http://dx.doi.org/10.5772/intechopen.109209*

#### **Figure 5.**

*Interpolation and combination process.*

calculation results is the maximum, the total pressure at each node of the outlet section of the inlet duct under the corresponding working condition is taken as the reference value for pressure interpolations. Similarly, the reference values of the temperature interpolation and velocity-component interpolation can be obtained.

**Table 2** shows the corresponding working conditions of the maximum values of each distortion parameter. The pressure, temperature, velocity components, and other gas parameters at each node are interpolated in turn and then combined to obtain the required rotor inlet boundary conditions of the internal-flow calculation according to the relative distortion degree and the combined distortion type in the designed calculation condition. Besides, the interpolation of calculated grid nodes is required because different calculated grids correspond to the external conditions when the total pressure distortion parameter, total temperature distortion parameter, and swirl distortion parameter are maximum.

## **3.2 Verification of the numerical simulation method**

Calculate the performance of Rotor 67 under undistorted intake air by the CFD method, and compare it with the experiment results of Rotor 67 [16] to verify the numerical simulation method. **Figure 6** shows the comparison between the characteristic maps obtained by the CFD calculation and experiment results. The pressureratio characteristic line of CFD calculation results is lower than that of experiment results under same airflow, and the characteristic line of the efficiency is relatively consistent. The comparison results show that the numerical simulation method used in the work can better fit the actual working conditions, with reliability.


#### **Table 2.**

*Corresponding working conditions of the maximum values of each distortion parameter.*

**Figure 6.** *Comparison of characteristics between CFD results and experiment results. (a) Total pressure ratio. (b) Efficiency.*
