User Forum of Software BASEMENT

Dominik

User

From: Bern

Registered: 2018-09-26

Posts: 19

negativ velocity

Good morning everyone,

I defined a special output for the velocity (node-centred, sms-format) for a certain time step (each 1200 seconds).
The output file shows three columns (x, y and z presumably) with the velocities at the location of the nodes. Some of the velocities show negative values. I assume that the velocity is positive in direction of the flow. Would that be the x-component? When looking in direction of flow does water flowing to the right hand side have a positive sign while the water flowing to the left is negative? Or where does water with a negative velocity flow to?

So far I could not find any related information in the 'User manuel, the 'Tutorials' or the 'Reference Manual'.

Below I printed an example extract from the velocity.sol-file

Cheers
Dominik

Velocity-file:

DATASET
OBJTYPE "mesh2d"
RT_JULIAN 2433282.500000
BEGVEC
VECTYPE 0
ND 46587
NC 91710
NAME "velocity"
TIMEUNITS seconds
TS 0    0x0.000000000000p+0
0.00000e+00     0.00000e+00      0.00000e+00
0.00000e+00     0.00000e+00      0.00000e+00

.
.
.
-2.64011e-01     -5.65659e-02      0.00000e+00
-3.67384e-01     -1.34066e-01      0.00000e+00
1.45027e-01     -5.64088e-02      0.00000e+00
0.00000e+00     0.00000e+00      0.00000e+00
-2.11663e-01     3.15811e-02      0.00000e+00
-3.21055e+00     -4.72234e-01      0.00000e+00
-3.04777e+00     6.36663e-02      0.00000e+00
-2.92383e-03     -2.38865e-04      0.00000e+00
.
.
.

---

Dominik Vogt
MSc Student Institute of Geography University of Berne

Lukas Vonwiller

Developer

Registered: 2014-09-04

Posts: 127

Re: negativ velocity

Hi Dominik

This are the x- and y-components of the depth averaged flow velocity (vx and vy) and hence they can be negative!

To determine the absolute value of the flow velocity: v = sqrt(vx² + vy²)

Cheers, Lukas

Dominik

User

From: Bern

Registered: 2018-09-26

Posts: 19

Re: negativ velocity

Hi Lukas,
thanks a lot for the clarification. Is it possible to determine the flow direction from the velocity?
It looks like the X and Y components can be seen as vectors. However, I'm not sure in which direction they point. Or in other words: How the coordinate system is set in Basement.
e.g. the values (-2.11 for x) and (3.15 for y) would go. Would that be north-west?

---

Dominik Vogt
MSc Student Institute of Geography University of Berne

Matthias Bürgler

Developer

From: Zürich

Registered: 2019-04-04

Posts: 150

Re: negativ velocity

Hi Dominik
It is correct that the x- and y-components of the depth averaged flow velocity are components of the velocity vector u. The flow direction is determined by the direction of the velocity vector in the cartesian x-y-coordinate system. The coordinate system is determined by your mesh and not "set" within BASEMENT. Therefore also the alignment with a geographic coordinate system depends on your mesh.

I suggest to have a look at the equations you are solving with BASEMENT in the Reference Manual, e.g. under Sections "1.1.2  Shallow Water Equations" and "2.2.3  Shallow Water Equations". Figure 2.22 could also be helpful for the understanding.

Best regards
Matthias