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Free space correction factor #6
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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: 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: 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 |
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 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. |
I found this description of a Venturi tube mathematical model:
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
of0.648327
. It seems to work becauseASPD_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
.The text was updated successfully, but these errors were encountered: