Learning Forum

[Laure]

Hi Fluids community,
I am building a 2D model to represent ambient temperature propagation in water and sediment of a pond. I have defined the air-water BC as "Convection" and hooked air-temperature time series to the "Free Stream Temperature" tap. Also, I assumed the Heat Transfer Coefficient to be 12 W/m2K (please see below a snapshot of the air-water BC window). In my first case, I initialised the 2D model (pond) with an initial temperature equal to 5 C, as I test to easily distinguish the heat transfer from air to water (air temperature started at 26 C). However, I observed that water temperature declined at the air-water boundary (shown in the attached image). Questions:
1) Could please let me know if the BC configuration for this problem is correct?
2) I have changed units form K to C in Fluent and I imported air temperature time series in C as well. Wonder if the reason why the temperature at the air-water boundary is lower than 5 C is because I should import temperature in K units?
Thanks in advance.

Figure 1. Air-water BC settings.

Figure 2. Lower temperature at the air-water boundary after 1 min.

[Rob]

Are you modelling both air and water as separate regions? If so, the contact surface (if not using the multiphase models) would need to be coupled. You're running two separate models with (I suspect) no heat transfer between the pond and air. 

[Laure]

Thank you Rob,
In the conceptual (tank-type) model I am using, the top wall represents the water surface level (please see snapshot below). So, I hooked the air temperature profile to this boundary. I did not create an external mesh representing the air zone. Do I need to include this zone in my mesh?

The lower part of the tank represents the sediment zone where there is no heat transfer with the outer part of the model (air).

The boundary between water and sediment has been configured as Interface. Is this the correct zones connection? If so, should I select "via System Coupling" as the thermal condition for this interface?

I would appreciate your guidance on this.

Figure 1. Model zones and water-sediment interface.

Figure 2. Water-sediment interface configuration. 

[Rob]

If you want a wall between the zones then you really want a coupled wall: so will see wall & wall:shadow. That requires you to have used ShareTopology or have a multibody part (depends on which geometry tool you used). 

System coupling is for Fluent-Mechanical runs or (I assume) Fluent-Fluent. Either way, it's not what you want here. 

I suggest having a look at the various tutorials in the Fluent Help, and the lectures in Learning (book icon left-top) for some guidance.  

[Laure]

Thank you Rob,

I'll have a look to the Fluent documentation for the interface.

Regarding the observed temperature profile, wonder if I need to keep kelvin in the csv file even if I changed the project temperature unit to celcious.

Best regards,

Laureano

[Rob]

Yes, all external input/output values should be in Kelvin regardless of the solver setting. Plots etc in Celcius are fine, it's the solver values that are all SI.