implementation of U0/V0/W0

[ericvmueller]

Reading the guide I believe the intent of U0,V0,W0 on WIND is to initialize the flow field to reach a steady state faster when using a VENT to drive the flow:

However, it seem like if this is used in combination with a vent there is an additive effect on the VENT velocity and the flow still takes time to establish. For example, run the lee_waves.fds case. The input is

&WIND U0=5.0,LAPSE_RATE=0.005 /
&SURF ID='WIND',VEL=-5.0,PROFILE='ATMOSPHERIC',PLE=0. /

The inlet flow starts at 5 m/s but then accelerates to 10 m/s

[mcgratta]

I think that this advice is outdated. I suggest you remove it from the User's Guide.

[ericvmueller]

Is there an accepted way to start with a flow field that is matched to a vent velocity - which I think the intent is with that lee_waves.fds case?

[rmcdermo]

Could you use RAMP_UZ? Have a look at the user guide.

[ericvmueller]

I still get an additive effect. In my case I have

&MISC RAMP_UZ='u'/
&RAMP ID='u', X= 0, F=2 /
&RAMP ID='u', X= 1, F=2 /
&SURF ID='INLET', VEL=-2/

I start out with 2 m/s like I want but then things accelerate to 4 m/s

[mcgratta]

Have you looked at INITIAL_SPEED?

[ericvmueller]

I get the same as above. Iooks like INITIAL_SPEED is just another way to define the U0 and V0 global variables

[mcgratta]

I changed the code so that now, INITIAL_SPEED and INITIAL_DIRECTION (new parameter) do nothing except set U and V at T_BEGIN.

[ericvmueller]

Try with this input - I still see the same issue. Unless I'm just not understanding the expected behavior.

initial_speed.txt

[mcgratta]

Yuk. I think FDS is ramping up the specified normal velocity assuming INITIAL_SPEED is the ambient. I'll fix.

[mcgratta]

I remember now the problem with initial velocity. In the past, we would have to subtract off the initial velocity components from calculations of $\partial{u}/\partial{t}$ etc. This became a burden, as this was done throughout the code. It seems that INITIAL_SPEED is no longer working and not used much. To make it work would involve going back to all these subtractions. I don't think it is worth it. I removed it completely.