Passive Pressure Log Spiral DM 7.2 Updates

[bostonblue]

Hello All,

I've been working with the updated log-spiral charts from the latest version of DM 7.2 (2025). I noticed using Equation 4-12, I am not able to replicate the results provided in the far-right column of Table 4-2. I understand that the equation is an approximation but some the values are a ways off, particularly as you increase in friction angle / delta/phi ratio. Does anyone have an idea of where the values in the table came from?

For example, using the equation with a friction angle of 35 and a delta/phi of 0.5 I get a Kp value of 6.3. However, the table provides a value of 8.0.

Even checking against Figure 4-13, for a friction angle of 35, the equation provides a Kp value of 7.6 while the chart shows ~8.0.

What level of accuracy is expected when using these equations/charts/tables?

Thank you!

 

[CarlB]

I don't have access to DM 7.2.
But other source https://www.sciencedirect.com/science/article/pii/S1674775517303955) indicates a Kp should for those conditions should be about 6.5. But it notes other studies using log spiral finds variations, some higher.
So there doesn't appear to be close agreement between methods, especially with higher wall friction. But you'd expect the equation and graphs to match better, for a single theory.

 

[bostonblue]

Should be able to access DM 7.2 for free here:
https://www.wbdg.org/dod/ufc/ufc-3-220-20

Thank you for the other reference, I will take a look. Understandable that there will be variations.