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Measurement of Velocity in Two-Dimensional Flow Field (Three-Hole Probe)
For two-dimensional flow fields, a three-hole pitot tube can be used to measure the magnitude and direction of the flow velocity. The middle hole of the three-hole probe is the total pressure hole, and the holes on both sides are the direction holes. Before use, the three-hole probe needs to be calibrated in a calibration wind tunnel. It can measure the total pressure, static pressure, and the magnitude and direction of the flow velocity in one measurement, so it is called a two-dimensional composite pressure measuring tube. The common form of the three-hole probe is shown in Figure 1 below.
Due to manufacturing reasons, the holes of the three-hole probe cannot be processed strictly according to the given geometric dimensions, and there are some unavoidable random factors. Therefore, the pressure P2 measured by hole 2 cannot strictly represent P , so each probe must be calibrated before use. The hole sequence of the three-hole probe and the calibration plane angle α during calibration are shown in Figure 2 below.
There are two methods for measuring the flow field with a three-hole aerodynamic probe: the facing method (direct method) and the non-facing method (indirect method). Both methods require calibration data for calculation.
The calculation process of the facing method is as follows:
- Insert the probe into the airflow, make the three-hole plane parallel to the airflow, and record the angle of the direction block;
- Rotate the pressure measuring tube so that P1 = P3, record the angle of the direction block, and the rotation angle at this time is -;
- Record P2 and P1 (P1 = P3) at the same time, and obtain the total pressure P from the total pressure characteristic curve;
- Then obtain the static pressure from the velocity characteristics (using the following formula):
- Measure the airflow temperature to obtain the density, and then calculate the airflow velocity:
;
The calculation steps of the non-facing method are:
Insert the pressure measuring tube into the airflow, try to make the center lines of the two direction holes align with the airflow direction, and record the three-hole pressures P1, P2, and P3;
Calculate the direction calibration coefficient based on the pressures P1, P2, and P3, and obtain the angle α between the airflow and the pressure measuring hole through interpolation of the direction characteristic curve;
Based on α, then use the total pressure characteristic formula to inversely calculate the total pressure P ;
Finally, according to the velocity characteristics, solve for the static pressure P from α, P , and P1/P2 or P1/P2;
Measure the airflow temperature to obtain the density ρ, and combine the total pressure P and static pressure P to solve for the velocity magnitude V.