Experiment 2 - Part 1

1. Why is the calibration grid set with Δα = Δβ ≈ 2° for |α|, |β| ≤ 20° and coarser outside?

• Fine grid spacing (2°) is used in the region where the probe will typically operate, i.e., small incidence angles. This ensures high calibration accuracy in the most relevant range.

• Coarser spacing beyond ±20° avoids wasting time in regions where the probe is either less accurate or less likely to operate, due to flow alignment or physical constraints.

2. What does the fifth-order polynomial capture, and why is such a high degree used?

• The 5th-order polynomial captures the nonlinear relationship between non-dimensional pressure coefficients and flow quantities (V, α, β).

• A high degree is used to achieve sufficient flexibility in fitting complex pressure-to-velocity/angle relationships, especially in a 3D velocity field.

• However, it also risks overfitting, so validation against calibration data is crucial.

3. What are potential sources of error during the calibration phase?

• Probe misalignment (yaw/pitch deviation from true zero).

• Flow unsteadiness in the open jet test rig.

• Mechanical defects in the probe (e.g., bent shaft, blocked port).

• Sampling noise or incorrect averaging in the VI software.

• Thermal drift or barometric variation during measurement.

4. How does the spatial jet survey ensure axial symmetry, and why is this assumption important?

• The grid spans both radial and axial directions (at least twice the nozzle diameter), which helps reveal asymmetries.

• Axial symmetry simplifies the integration over the jet cross-section (ring-wise integration), allowing the use of cylindrical coordinates.

• Without axial symmetry, the velocity field would require full 3D integration, increasing complexity and uncertainty.

5. Why is synchronous acquisition across all channels important in this experiment?

• Synchronization ensures that pressure and temperature readings correspond to the exact same flow state at each moment.

• If channels are sampled at different times, temporal fluctuations could corrupt the correlation between different probe signals or cause integration mismatches in flow rate.

How does the system handle ambient pressure and temperature variations?

• The VI system uses barometric sensors and temperature probes to monitor environmental conditions.

• These values are used to correct the calculated flow properties (especially density, via the ideal gas law), ensuring consistency over the measurement campaign.

What happens if one pressure port is blocked during the test?

• The calibration is no longer valid because the pressure vector space is altered.

• The derived polynomial mapping to (V, α, β) breaks down, leading to erroneous results.

• In practice, the probe must be inspected, cleaned, and recalibrated.

8. How is yaw error minimized in the grid survey, and why is <1° important?

• The probe is carefully aligned with the jet centerline using the traversing tower and visual markers.

• A yaw error <1° ensures that the measured flow angles are not biased, preserving the integrity of angle-sensitive velocity reconstructions.

• Larger errors would distort the vector field and yield incorrect axial/radial distributions.