## Fluids

#### Multiphase Flow in Sedimentation Tank (water & sand)

• pcmacapagal
Subscriber

For the Eulerian-Lagrangian model, should I turn on Eulerian in the multi-phase part and turn on DPM besides creating an injection, or should I only turn on DPM and create an injection? To be clear, imagine I want to track the behavior of the particle if it is trapped in the tank bottom. Why and when should I turn on Eulerian?

• DrAmine
Ansys Employee

If you have only one particulate phase and it's volumetric loading is not so high, you might disable Eulerian and just run Single Phase + DPM (it is again multiphase but just deploying Euler-Lagrangian Framework).

• pcmacapagal
Subscriber

Good day, DrAmine,

I am using the Ansys 2023 R1 version for Students. May I ask for your assistance in setting this method into my simulation? I am trying to mimic this process for my thesis, and as an innovative iteration, I will try to install a bottom weir structure to see how many particles the tank can trap and if the weir structure will benefit the enhancement of particle trapping. Attached is my main reference for my thesis. I've highlighted the important parts of the methodology of the study.

Main Reference

Thank you!

• DrAmine
Ansys Employee

Can you describe the process without that an Ansye Employee needs to follow (click on)  the link you posted. Thanks.

• pcmacapagal
Subscriber
1. The trajectories of individual particles through the continuum fluid using the Lagrangian approach are calculated in FLUENT by the discrete phase model (DPM).
2. The segregated solution algorithm was selected. The SST k–ω turbulence model was used to account for turbulence since this model is meant to describe better low Reynolds numbers flows such as the one inside our sedimentation tank
3. The discretization schemes used were simple for the pressure, the PISO for the pressure–velocity coupling, and the second-order upwind for the momentum, the turbulence energy, and the specific dissipation.
4. The water-free surface was modeled as a fixed surface; this plane of symmetry was characterized by zero normal gradients for all variables.
5. As a first step, the fluid mechanics problem was solved in the absence of particles to find the steady-state flow field. The converged solution was defined as the solution for which the normalized residual for all variables was less than 10−6.
6. In addition, the convergence was checked from the outflow rate calculated at each iteration of the run. The convergence was achieved when the flow rate calculated to exit the tank no longer changed.
7. The settling tank was simulated for a specific set of conditions used in the Thessaloniki treatment plant for which the particle size distribution at the inlet and outlet and the total settling efficiency has been experimentally measured. The inlet was specified as a plug flow of water at 0.085 m s−1, whereas the inlet turbulence intensity was set at 4.5%. The outlet was specified as a constant pressure outlet with a turbulence intensity of 6.0%. The water flow rate was 0.6 m3 s−1. Based on this rate, the inlet flow rate of particles was estimated as 0.15 kg s−1 using a measured solids concentration of 250 mg L−1, whereas the primary particle density was 1066 kg m−3.
8. The effectiveness of particle settling is estimated as the percentage of solids settled over the rate of solids introduced from the feed.
• pcmacapagal
Subscriber

I got lost in part 4 up to part 8. I’m not sure how to translate those into the Ansys setup.

• DrAmine
Ansys Employee

They are not modelling a really free surface flow. They are modelling the free surface (air-water interface) as symmetry (free slip wall ) BC.

They are using Trapped BC for the particles and based on the amount of the particle beeing trapped at the wall they calcualte an efficiency.

• pcmacapagal
Subscriber

If that is the case, may I know how to simulate steps 4-8?

• DrAmine
Ansys Employee

As described in my last message. I recommend to go through Standard Fluent tutorials first. Aslo there is some material on the Ansys Learning Hub and Ansys Innovation Course which will be helfpul for you :).

• pcmacapagal
Subscriber

Thank you, Sir!

• pcmacapagal
Subscriber

Good day, DrAmine,

In mimicking the steps of this study, how can I proceed with this step:

“As a first step, the fluid mechanics problem was solved in the absence of particles to find the steady-state flow field. The converged solution was defined as the solution for which the normalized residual for all variables was less than 10^(−6).”

And from that point, when I performed the grid independence test, how should I proceed into tracking about 25,000 particles following these steps:

"The inlet was specified as a plug flow of water at 0.085 m s−1, whereas the inlet turbulence intensity was set at 4.5%. The outlet was specified as a constant pressure outlet with a turbulence intensity of 6.0%. The water flow rate was 0.6 m3 s−1. Based on this rate, the inlet flow rate of particles was estimated as 0.15 kg s−1 using a measured solids concentration of 250 mg L−1, whereas the primary particle density was 1066 kg m−3 .”

Thank you!