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Free space correction factor #6

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kaklik opened this issue May 8, 2022 · 4 comments
Open

Free space correction factor #6

kaklik opened this issue May 8, 2022 · 4 comments
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@kaklik
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kaklik commented May 8, 2022

I found this description of a Venturi tube mathematical model:

Venturi Tube Diagram

The important point is that since we are not taking the input pressure from infinity but already from the constricted profile, the TFSLOT will overestimate the measured airspeed. This happens because it uses the already increased velocity at the tube's entry as a reference for ambient airspeed.

This phenomenon indeed occurs. For instance, during the recent flights, we have a value of ASPD_SCALE1 of 0.648327. It seems to work because ASPD_SCALE1 works as a correction factor to V_inf from V_D.

To solve this properly, the TFSLOT model would need to include three parameters for the cross-sectional areas (entry from free flow, cross-section at the first sampling point, and cross-section at the narrowest point).

Alternatively, the construction would need to be changed to take the static pressure from the free flow. This would likely mean taking it from the side of the pitot tube (increasing the risk of clogging again). In this case, only two cross-section parameters would be needed (the external cross-section and the cross-section at the narrowest point).

However, it might be practical to design the model to allow the definition of all three cross-sections, with the option that if the first two are identical, the correction for the external cross-section is skipped, meaning the correction factor would be 1.

@kaklik
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kaklik commented May 8, 2022

In the following case, we consider an A_inf as the effective area of the Venturi tube entry. The continuity equation relates the velocities and cross-sectional areas:

Continuity equation

So, it turns out that the correction factor for the free flow (V_inf) from the velocity measured at the Venturi tube entry (v_1) is:

Correction Factor

Experimentally, we measured that the ratio of the cross-section (A_1 / A_inf) at the first pressure sampling point to the external aerodynamic area of the TFSLOT is 0.648327. The reciprocal of this value is 1.54, which is only slightly larger than the square root of two (1.41). Therefore, the currently used sensor is only slightly more sensitive than a standard Pitot tube.

@kaklik
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kaklik commented Jul 24, 2022

The above equations need to be implemented in this part of the code.

@roman-dvorak roman-dvorak self-assigned this May 2, 2023
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roman-dvorak commented May 3, 2023

The above equations need to be implemented in this part of the code.

This section does not calculate airspeed. It only computes the corresponding pressure corrections for the Pitot tube, which is suitable for resolving negative pressures. For airspeed calculation, it needs to be implemented elsewhere.

@kaklik
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kaklik commented May 3, 2023

The above equations need to be implemented in this part of the code.

This section does not calculate airspeed. It only computes the corresponding pressure corrections for the Pitot tube, which is suitable for resolving negative pressures. For airspeed calculation, it needs to be implemented elsewhere.

The correction can be applied either to the pressure or directly to the velocity. I can't determine which approach is better—correcting the velocity or the pressure. Correcting the velocity likely requires changes in multiple parts of the code, which I understand might be a disadvantage for merging.

On the other hand, it's probably odd to have a Venturi tube sensor and then adjust the measured pressure to fit a calculation designed for a Pitot tube. Given that the pressure is being sampled at reduced diameters in both cases, it would be appropriate to implement a correction for velocity to free flow based on the knowledge of the external cross-section, rather than relying on the ASPD_SCALE1 value, which serves a different purpose. Using the above-delivered equation.

Correction Factor

Therefore, it seems the best solution is likely to introduce a new calculation for airspeed derived from pressure, even if that means more widespread changes in the code.
The purpose of ASPD_SCALE1 should remain to correct uncertainties caused by an unknown effective area for A_inf, which could depend specifically on the mounting option.

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