Open Channel Flow
I am running a CFD using CFX-Ansys, open channel flow with two species, oil and water. I am new to CFD, and I have several problems. Do you think I could have some help from you Masters?
Open channel flow with underwater gate. Oil on the top and Seawater. I am trying to figure out how much oil mass will be restricted by the underwater gate. I have 3 Domains. The water, the oil on top, and the underwater gate. Inlets on the water and the oil, oil=oil, water=water. A outlet on the other side for both oil and water.
1. Defining crude oil- is this in one of the files that comes with Ansys or do I have to create this material from scratch?
2. How do I determine how much oil has traveled through my outlet? Is this a particle transport problem?
3. Ran the analysis, how do I change the following? I set the boom domain as a wall with no domain interference.
"If the isolated regions do not have the pressure level set either
by the boundary conditions or using a reference pressure equation,
you may encounter severe robustness problems.
This situation may have arisen because a domain interface was not
properly defined during problem setup. Please carefully check
The solver will stop now and write a results file. The isolated
regions can be visualised in CFX Post by making plots of the
variable "Isolated Volumes".
If you are sure that the pressure level is set in each isolated
fluid region then you can force the solver to turn off this check
by setting the expert parameter "check isolated regions = f"."
Your the best,
As for your final question, does the oil float on top of the water? In that case you probably should not model it as multiple domains but as a multiphase fluid. This will allow the oil and water to flow around as they wish, displace each other and form waves and other cool stuff.
Thanks GHORRocks, I did clean up and combine the 2 domains into one domain, which eliminated the "Isolated Doman Error".
Also I read up on mulitphase and multicomponent flow and switched my model from Lagragian to Eularian.
Volume Fractions is gicing me a real head ache, the oil is supposed to enter a vortex at the top of the underwater gate, but instead it is passing underneath. I have defined the bouyancy. For the initialization, the volume fraction of water is set to 1, and oil is set to 0. Hopefully this represents that there is no oil in the open channel at t=0. Also I have defined the outlet boundary as a Avg pressure equal to hydrostatic pressure. There is 9.8% flow back into the domain which is repeatidly blocked by the computer.
I still have no been able to use the model a monitor point that will provide mass information, just density. In fact I can't get mass to output at all.
Any other great suggestions.
You are such a big help. I really appreciate your thoughtful and macro approach.
I defined the element oil, well I figure that having actual oil, and not just water with oil density, will give me the results I am looking for.
Option: Pure Substance
Material Group: User
Material Description: Crude Oil
Therm State: Liquid
Option: General Material
Equation of state
Molar Mass: 900 kg/kmol (this is for vegtable oil)
Density 1.77 Slug/ft^3
Reference State: Automatic
Dynamic Viscosity: checked
Dynamic Viscosity: 4.177 lb*s/ft^2
That's it. I did not define Radiation Properties or Bouyancy Properties. This is a steady state (no thermal) model.
Have you worked through the tutorial example, I think it is free surface flow over a bump? This will show you how to set up a air/water model. You can use this as a template.
This is an multiphase problem, and you can use the freesurface model in CFX,and you can see the tutorial case,get the information how to set the model,this model is the VOF in fluent. good luck.
lilhui, Could you please provide more information about the VOF model? Is it an example? What is the name of the example? Do you know precisely where it can be found?
I am sorry but Ansys Fluent Help is not user friendly on my computer.
Analysis Type: Steady State
Fluid and Particle Definitions:
Option: Dispersed Fluid <- is this correct?
Mean Diameter: 0.015625 [in] sorry i guessed it would be 1/64 of an inch
Option: Dispersed Fluid <-is this correct?
Mean Diameter: 0.3125 [in]
Option: Continuous Fluid
Pressure: Reference Pressure: 830 psf , hydrostatic pressure of water in 13ft depth
Gravity in X: 0
Gravity in y: 0
Gravity in z: -g <- z is positive away from ground
Buoyancy Ref Density: 1.99 slug ft^-3 <- assumed asking for density of principle fluid "Seawater". Is this a correct assumption? Is Buoyancy density different from regular density? Never heard of Buoyancy ref density in any class.
Buoyancy: Ref. Location: Automatic
Tab: Fluid Models
Multiphase: select Homogenous Model
Multiphase: Free Surface model: Standard
Heat Transfer:Homogenous Model: select
Heat Transfer: Option: Isothermal
Heat Transfer: Option: Fluid Temperature: 78F
Turbulance:K-Epsilon <----- from the example, Cfd-Post notifies that reynolds number is not right based on speed of 1.25 knots.
Anything stick out in your mind? I sure am learning alot! :) :p Lol
One last question
So what I did to make a real difference was add velocity profiles to my walls. I set the top of the flume and the bottom as walls instead of openings. THis is valid since the water is standing still and the object is moving.
I can't get the time scale to move past 11 seconds. Can you tell me how to force the time scale to move to 120 sec?
Have you looked at the flow over a bump tutorial example? There are a number of errors in your setup and the example will show you how to set this up.
Forget lihui's comment about Fluent and VOF. Free surface modelling is very different in Fluent and you won't learn much by looking at that.
Looked over the tutorial. And changed a few boundary counditions. I would like to know what you think GHOR Rocks.?
Velocity Cartesian: in Z direction 2.1 ft/s
Wall/ No Slip boundary/ Smooth/ Velocity in Z direction 2.1 ft/s
I had previously said this model was open channel flow. That was incorrect, I am actually pulling an object through the water. SO I modeled the object as stationary and the fluid as moving. Thank you Professor Einstein.
Question about y
Question of the day:
I want "y" to be in feet. Ansys wants y to be in meters.
My equation for the pressure distribution of Hydrostatic pressure calls for
WaterSpecificGravity= 64 [lb ft^-3]
y = depth of water at any point
Obi Wan, you are my only hope!
The Answer is WaterSpecificGravity = 64 [lbf ft^-3]
:) Me Happy!!! Today is MY Birthday!
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