Analysis of Time-Dependent Data

The measured data was collected with the measuring frequency of 1 MHz. 30000 data points were collected, which is a 0.03 s period of time. This is 10.8 rotations of the impeller. Three measurements were made at each measuring point and at each operation condition. The measured data was phase averaged over ten rotations of the impeller, and three different sets of measured data were compared to each other to ensure the regularity of the achieved pressure variations. In this case, phase averaging means that the 10 impeller rotations-

Figure 4. The computational grid of the volute and the diffuser at the 360-degree circumfer­ential position

long measured data were cut in ten 1-rotation-long pieces with the help of the measured rotation speed. Then the measured pressure data points at the same position of the impeller were averaged. The fast Fourier transform (FFT) was made to examine pressure flictuations in the frequency plane.

The time step used in the computational calculations was 1^s. 11600 time steps were calculated, which is about four rotations of the impeller. The data was collected from the last full rotation of the impeller. The data was taken only at every 10th time step. Even when the data was collected from the last rotation of the impeller and from every 10th time step, the amount of achieved data was over 5 Gb. Static pressures were collected from the calculated data at the same location where the measurements were made. A FFT was also made for the calculated data.

4. Results