Oscillation in bottom stress for GLS_MIXING plus SSW_BBL

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m.hadfield
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Location: NIWA

Oscillation in bottom stress for GLS_MIXING plus SSW_BBL

#1 Unread post by m.hadfield »

Hello colleagues

Preface:

I don't know how big a problem the following really is in practice, but I've noticed some curious & spurious behaviour, so I thought I'd let the community know about it.

Short version:

A toy case with oscillating tidal flow shows pronounced, rapid oscillations in bottom stress (also near-bottom speed and TKE) when SSW_BBL and GLS_MIXING are enabled. These oscillations occur as the tidal current speed drops away from its maximum value. They can be avoided by substituting LMD_MIXING for GLS_MIXING or UV_LOGDRAG from SSW_BBL, or by reducing the prescribed bottom orbital velocity.

Long version:

I have attached the necessary header files and the input file, also a time series plot of bottom stress (bustr) and a snapshot of velocity (u).

The toy case involves an oscillating tidal flow (period 12 hours) in a 20m deep, periodic domain. The expected behaviour is that velocity oscillates between about -0.4m/s and +0.4m/s, with a more-or-less logarithmic drop-off towards the bottom. When SSW_BBL and GLS_MIXING are enabled and a reasonable large bottom orbital velocity is imposed (here 0.5m/s) there are rapid oscillations in the near-bottom velocity, bottom stress and TKE when the tidal current is reducing in amplitude. These oscillations develop and die away rapidly. In the velocity snapshot shown here, the bottom velocity at a maximum; at the next time step it is less than 0.

Reducing the bottom orbital velocity to 0.3m/s essentially eliminates the oscillation.

Question:

Has anyone else seen similar behaviour?

Mark
Attachments
Velocity profile snapshot
Velocity profile snapshot
profile_snapshot_u.png (15.19 KiB) Viewed 5058 times
Bottom stress time series
Bottom stress time series
time_series_bustr.png (9.12 KiB) Viewed 5058 times
ana_wwave.h
ANA_WWAVE functional
(10.05 KiB) Downloaded 216 times
ana_sediment.h
ANA_SEDIMENT functional
(9.53 KiB) Downloaded 203 times
ana_grid.h
ANA_GRID functional
(20.08 KiB) Downloaded 198 times
flume.h
Application header file
(678 Bytes) Downloaded 207 times

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m.hadfield
Posts: 521
Joined: Tue Jul 01, 2003 4:12 am
Location: NIWA

Re: Oscillation in bottom stress for GLS_MIXING plus SSW_BBL

#2 Unread post by m.hadfield »

Also attached FYI is the input file (which includes the GLS model coefficients)
Attachments
rom001.in
(5.59 KiB) Downloaded 227 times

jcwarner
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Location: USGS, USA

Re: Oscillation in bottom stress for GLS_MIXING plus SSW_BBL

#3 Unread post by jcwarner »

ok. still need to know how to force this thing?
roms_frc.nc?
was there an ana_smflux?

jcwarner
Posts: 1172
Joined: Wed Dec 31, 2003 6:16 pm
Location: USGS, USA

Re: Oscillation in bottom stress for GLS_MIXING plus SSW_BBL

#4 Unread post by jcwarner »

you still need to set a Pwave_bot value.
i did not see one set in the ana_wwave. try 8 secs or something and see if that helps.
-j

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m.hadfield
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Location: NIWA

Re: Oscillation in bottom stress for GLS_MIXING plus SSW_BBL

#5 Unread post by m.hadfield »

You only need Pwave_bot if SEDIMENT is enabled. The original case where I saw this behaviour did have sediment, but I stripped it out in order to simplify things.

jcwarner
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Joined: Wed Dec 31, 2003 6:16 pm
Location: USGS, USA

Re: Oscillation in bottom stress for GLS_MIXING plus SSW_BBL

#6 Unread post by jcwarner »

um, i think you need pwave no matter if sed is defined or not.
if you activate ssw_bbl, then check the Build/bbl.f90 and see what it needs. if you specify ana_wwave (not sure why there are 2 w's there, wind wave?), then it needs an amplitude and a period. so i still think you need a period. the bbl routines were coded to work with sed on or off.
-j

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m.hadfield
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Location: NIWA

Re: Oscillation in bottom stress for GLS_MIXING plus SSW_BBL

#7 Unread post by m.hadfield »

Um, I think you're right! And setting Pwave_bot to 8s suppresses the oscillation.

Thank you. :oops:

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