Xylem SonTek FlowTracker2, User Manual

Page: 236 / 308

SonTek – a Xylem brand
FlowTracker2 User’s Manual (January 2019) 235
13.12. Dynamic Pressure (Pressure Sensor)
Dynamic pressure is the kinetic energy per unit volume of fluid and affects the pressure
sensor measurements performed by the FlowTracker2 ADV internal pressure sensor in
flowing water. The extent of the dynamic pressure affecting the pressure sensor
measurements is directly related to the water velocity. The correction for dynamic
pressure on FlowTracker2 pressure sensor measurements is described in this section.
In fluid dynamics, the Total Pressure (P0) is defined as the sum of the Static Pressure
(P) and the Dynamic Pressure (Pd),
Equation 13:1 – Total Pressure
𝑃
0
= 𝑃 + 𝑃
𝑑
The internal pressure sensor of the FlowTracker2 provides an accurate measurement of
the Total Pressure (P
0
). Calculating depth from pressure requires the Static Pressure
(P) to be known. For still or slow moving water the Dynamic Pressure (Pd) is negligible
and thus the Static Pressure (P) and the Total Pressure (P
0
) are very close. Using the
Total Pressure (P
0
) as measured by the pressure sensor instead of the Static Pressure
(P) in these cases provides a good conversion for all practical purposes.
However, for faster flowing water, ignoring the Dynamic Pressures (Pd) is not an option
and would result in a significant error in the pressure-to-depth conversion. Correcting
for Dynamic Pressure is essential to ensure a proper depth computation.
Dynamic Pressure (Pd) is defined as,
Equation 13:2 – Dynamic Pressure
𝑃
𝑑
= 0.5 × 𝑊𝑊𝑑𝑑𝑊𝑊𝑑𝑊𝑊𝑊𝑑𝑊 × 𝑊𝑊𝑑𝑑𝑊𝑊𝑑𝑑𝑑𝑑
2
All parameters required in
Equation 13:2
are available to the FlowTracker2
•
WaterDensity can be approximated or in the case of the FlowTracker2 is precisely
computed from Salinity (user entry), Latitude and Altitude (from the integrated
GPS),
•
WaterSpeed is provided by the ADV (the main function of the FlowTracker2 is to
measure water velocity). To produce a precise WaterSpeed, the AdvWaterSpeed
(measured water speed) is corrected by a constant “head drag coefficient” (a)
which describes the flow disturbance around the pressure sensor introduced by
the ADV head and is different for different head shapes (e.g. 2D or 2D/3D).
Equation 13:3 – Corrected AdvWaterSpeed
𝑊𝑊𝑑𝑑𝑊𝑊𝑑𝑑𝑑𝑑 = 𝐴𝑑𝑣𝑊𝑊𝑑𝑑𝑊𝑊𝑑𝑑𝑑𝑑 × 𝑊