Experiment 3 - Part 3

Q1: Why did you use both coarse and fine transverse grids in the wake measurements?

A1: Using both coarse and fine grids allowed us to validate the consistency of the wake profiles across different resolutions. The fine grid improves spatial resolution and accuracy, while the coarse grid provides faster scanning and helps identify major wake features. Averaging the two ensures robustness of the resulting profile.

Q2: What does the symmetry of the wake profile indicate?

A2: The symmetry of the wake profile about the centerline (y = 0 mm) suggests proper alignment of the cylinder and airfoil with the flow. It also confirms that the measurements were consistent and not affected by significant setup errors like misalignment or probe tilting.

Q3: What trends did you observe when increasing the Mach number and/or cylinder diameter?

A3:

• Increasing Mach number led to broader and deeper pressure deficits, indicating stronger separation and a more energetic wake.

• Increasing cylinder diameter (from 20 mm to 30 mm) also widened the wake and increased pressure deficit.

• The combination of both (high Mach and large diameter) resulted in the most pronounced wake.

Q4: Why did you normalize the pressure data in Figure 3.2?

A4: Normalization by the maximum absolute pressure allows for a comparison of wake shape independently of dynamic pressure magnitude. This emphasizes geometric and flow-related effects (like separation behavior) rather than absolute pressure values.

Q5: How does the airfoil wake compare to that of the cylinder?

A5: The airfoil wake is much thinner and has a shallower pressure deficit than the cylinder. This indicates lower drag and better flow attachment. The wake of the airfoil also showed little variation between Mach 0.1 and 0.3, suggesting more stable, streamlined flow compared to the bluff body.

Q6: How was the angle of attack determined from the 5-hole probe data?

A6: We used calibration data from Experiment 2, which related pressure differences between the top/bottom and left/right ports to flow angles. Specifically, the pitch pressure differential (ΔPpitch\Delta P_{pitch}ΔPpitch​) was used to calculate the angle of attack via a 5th-degree polynomial fit.

Q7: What effect did grid resolution have on the angle of attack results?

A7: The fine grid provided slightly higher and more consistent angle values than the coarse grid. This is because finer resolution captures pressure gradients more accurately across the probe tip, improving the precision of flow angle estimation.

Q8: Could you have used the 3-hole probe to calculate angle of attack?

A8: Technically yes, but with limitations. The 3HP is less sensitive to pitch angles and is not designed for high-accuracy steady measurements. It’s more suitable for capturing unsteady pressure fluctuations. Also, sideslip angle cannot be measured with a 3HP — only a 5HP can determine both pitch and yaw components simultaneously.

Q9: Why is the wake behind the airfoil largely unaffected by Mach number, unlike the cylinder?

A9: The airfoil’s streamlined geometry maintains attached flow better, reducing sensitivity to increased freestream velocity (within subsonic regime). The cylinder, being a bluff body, causes separation that intensifies with higher velocity due to increased adverse pressure gradients and vortex shedding energy.

Q10: What does the pronounced pressure dip at y=0y = 0y=0 in the airfoil wake represent?

A10: It represents the core of the wake — a narrow region of reduced pressure due to minimal flow separation. This dip confirms the presence of a thin wake, which is typical for streamlined bodies experiencing mostly attached flow.