Glidertrim¶
Glidertrim is a simple commandline utility, that, given one or more pairs of dbd/ebd files, and a target density, computes the optimum weight change. The typical application is that during the deployment a test dive is made, and the resulting dbd/edb files are analysed quickly, producing an objective estimate of how much a glider is overweight or underweight, given a target density.
Synopsis¶
glidertrim <glidername> <dbd file> [dbd file] [dbd file]
glidername is the name or identifier of the glider. It is used to write settings in the configuration file ($HOME/.glidertrimrc), so that the settings can be prepared prior to deployment and recorded for later use.
dbd_file is a path to a dbd file and can included wildcards. It is important that the dbd filename is provided only, but the matching ebd file is present in the same directory the dbd file resides in.
Description¶
Glidertrim takes a glider id and one or more dbd (with matching ebd) files as input, and given some configuration settings which the user can change, calculates the ideal weight change.
An example is given below:
$ glidertrim comet comet-2018-136-00-000.dbd
comet-2018-136-00-000.dbd found. Ok
Enter value for target_density (kg/m^3) (current: 1026.000000):
Enter value for mg (kg) (current: 69.500000):
Enter value for Vg (m^3) (current: 0.065000):
Enter value for minlimitdepth (m) (current: 3.000000):
Enter value for maxlimitdepth (m) (current: 55.000000):
Enter value for cond_a (m/S) (current: 1.000000):
Enter value for cond_b (-) (current: 0.000000):
Enter value for buoyancy_engine (shallow|deep) (current: deep):
Enter value for latitude (decimal deg) (current: 54.000000):
Enter value for longitude (decimal deg) (current: 8.000000):
Enter value for calibrate_epsilon (yes|no) (current: no):
Error: 1.1079233e-01 - Cd0=0.1500 Vg=0.0650
Error: 1.0586534e-01 - Cd0=0.1575 Vg=0.0650
:
:
Error: 1.7395719e-03 - Cd0=0.3100 Vg=0.0678
Error: 1.7395722e-03 - Cd0=0.3100 Vg=0.0678
Error: 1.7395718e-03 - Cd0=0.3099 Vg=0.0678
Drag coefficient Cd : 0.309885 (-)
Glider volume Vg : 0.067768 (m^3)
Glider compressibility : 5.000000 (*e-10)
Glider density : 1025.555502 (kg/m^3)
Weight change : 30.122814 (g)
Estimated pitch relationship:
tan(pitch) = T1 * buoyancy(m^3) + T2 * battpos(m) + T3 (tan(pitch0)) + T4 P (kbar)
T1 : 1784.9588582927518
T2 : -23.879872610869665
T3 : 0.09352561010068428
T4 : 0
Press enter to exit
A table with configurable parameters is shown below:
| Parameter (unit) | Description |
|---|---|
| target_density (kg/m^3) | target density |
| mg (kg) | (measured) mass of glider |
| Vg (m^3) | estimate of glider volume |
| minlimitdepth (m) | minimum depth allowed in optimisation routine |
| maxlimitdepth (m) | maximum depth allowed in optimisation routine |
| cond_a (m/S) | scaling factor for correcting conductivity |
| cond_b (-) | offset for correcting conductivity |
| buoyancy_engine (shallow|deep) | sets buoyancy engine used, deep or shallow |
| latitude (decimal deg) | latitude of experiment (used for density) |
| longitude (decimal deg) | longitude of experiment (idem) |
| calibrate_epsilon (yes|no) | whether or not to calibrate for compressibility |
The user is presented with default values and has the option to change the value or simply press enter, which retains the defailt value. After entering the last configuration parameter, the utility optimises for the parasite drag coefficient Cd0, the glider volume Vg, and, if calibrate_epsilon is set to “yes”, the compressibility.
After the optimised values are found, they are displayed. Among the results returned are the glider density and the required weight change to match the glider’s density to the target density.
The results are also shown graphically. The left panel shows the vertical profiles of water velocity (raw and filtered), the glider vertical velocity and the glider speed through water. The right panel shows the in-situ density profile, the target density, and the actual glider density (accounting for the compressibility). The dashed lines refer to the glider’s density with increments of 50 g of weight change.
Estimated pitch relationship¶
The pitch the glider assumes during diving and climbing depends on the total torque exerted on the glider. Components influening the torque balance are the mass, the so-called h-moment (distance between the centres of buoyancy and gravity), the buoyancy drive, pitch battery position and pressure. Based on a linear regression model the contributions to the pitch by the buoyancy change, battery position, and pressure are estimated. The intended purpose is for glider flight simulations to use the correct pitch, when pitch is not set directly, but through a fixed battery position, for example.
The relationship for the pitch is given by
where
\(V_b\) is the buoyancy change in m3
\(b_p\) is the battery position in m, and
\(P\) is the pressure in kbar.