open TELEMAC-MASCARET The mathematically superior suite of solvers

Hello,
I understand that the correct quantity of tracer is given by the formula : discharge*(tracer value)* dt

1) Is the discharge used here include tracer discharge also (i.e water discharge + tracer discharge)?

2) At every time step, is it the previous time tracer value applied to find the correct quantity of discharge?

3)"The larger the mesh around the source point, the more it is diluted" - This implies that the concentration of tracer at SOURCE POINT will be MESH SIZE DEPENDENT! .
If the mesh element area around the source point tends to become zero, will the dilution also tend to get reduced?Are there any limitations in this case?

Thanks in advance
Sathya

Hi

1. There is no tracer discharge in telemac. There is a water discharge at the source point and in this discharge you could give a tracer concentration.
2. No as there is no tracer discharge. The water discharge is an imposed value.
3. • Yes, it's mesh size dependent because otherwise you cannot ensure a right mass balance of the tracer.
   • If you reduce the mesh size, you will reduce the initial dilution.
   • I'm not sure about the limitation. Maybe a really small mesh size will create local problems, particularly if you define the velocities at the source point

Hope it's helps

Hello,

To answer question 2, the correct quantity of tracer discharged is as you say discharge*(imposed tracer value)*dt, so the previous value of the tracer is not used. It just appears in the non conservative form of advection equation. On the contrary, when the discharge is negative, the exiting value of tracer is considered to be the local value.

With best regards,

Jean-Michel Hervouet

Hi,

Sorry if this is a straight forward question and has already been answered but I have had some issues for a while trying to get my tracer source concentration in the right units. I think I'm just overthinking it and have confused myself.

In post #16320 on the tracer concentration unit two main points were made:

1- "Units are only for result display. You can specify what you want."

2 - "If your tracer is not an active tracer, like temperature or salinity, it has no importance."

However as I understand from this post and looking at difsou.f the tracer mass is calculated by dt*flowrate*tracer concentration

My specific question is for a bacterial source. I have an input series of cfu/s and flowrate (m3/s) and in my sources file can specify the tracer concentration to be whatever I want. What should I choose?

Is it not important to ensure that the tracer concentration is in compatible units with the flowrate, so cfu/m3 and m3 respectively? Or can I (as mentioned in post #16320 choose what I want; cfu/m3, cfu/l, cfu/100ml etc?

Many thanks
Jonathan

Hello Jonathan

You have the answer to your own question :)
The unit of your concentration is given by yourself. What we call mass of tracer i Telemac (difsou) is just for the case of concentration given in Kg/m3. Actually, if you have a tracer with an input unit of camel/s, you will have a tracer concentration of camel/m3 :)

I hope that this helps

with my best regards

Riadh

Hi Riadh,

Thank you, your help is really appreciated. Just to clarify one last thing.

Are you saying that if I input the tracer time series as cfu/s Telemac recalculates this as cfu/m3 for the output? If so does it use the input flowrate to calculate this?

Would this then give me the same result as if I specified my input time series as cfu/m3 anyway? I.e. does TELEMAC read the units specified in the input file and the subsequent calculation depend on this?

Sorry if I've misunderstood anything, as I said I think I've just confused myself over this.

Many thanks
Jonathan

Hello Jonathan

No Telemac will generate in the result file a tracer with the unit that you have given in your case file (using the keyword NAMES OF TRACERS : 'TRITIUM Bq/l'). This keyword is read upon 32 bits. the last 16 bits are dedicated to the unit of tracer and then will be written in the result file.
However, the subsequent calculation of mass, dosn't depend on that, by default, Telemac assumes that you give everything in international system. The flowrate has to be therefore given in M3/s.

The result will be the same anyway.

with my best regards

Riadh

Hi Riadh,

Okay. I must be missing something. Going back to your example. When you said the following it is the last part I don't understand:

"If you have a tracer input of camel/s, you will have a tracer concentration of camel/m3."

How does TELEMAC calculate camel/m3 when my input is in camel/s? Does it do something like tracer concentration / flowrate (i.e. (camel/s)/(m3/s) == camel/m3).

And if I use camel/s for my tracer time series in the sources file, what units will the output for my tracer be in (regardless of what I have written in my .cas file, as I understand this string is only used for results output)

Sorry if my question isn't very clear

Many thanks
Jonathan

Hello JONATHAN,

Are you clarified all your questions, because i have exactly the same questions but i still don't understand how the real quantity of TRACER is calculated.

Hope that you can help me.

I have checked all the topics in relations with TRACERS SOURCES POINT, but i still confused.

REGARDS.
YASMINA.

Hi Yasmina,

Yes. As explained by riadh the units of tracer concentration are arbitrary and it doesn't matter what they are (temperature, salinity or bacteria concentration in my case).

For example if the input concentration is in mg/100 l, the results will be in mg/100 l. This could just as well be deg C as it has no influence on the calculation of tracer transport.

Calculation of tracer transport is governed by the Advection Diffusion Equation (plus any decay or other processes you might want to include), on which more information is provided in the user manuals. Alternatively you could take a look at the subroutines difsou.f and source_trac.f as a starting point.

Regards
Jonathan