6 Replies Latest reply on Apr 10, 2013 3:15 PM by Kevin Land

    Determining mixing in laminar flow

    Kevin Land

      I have two channels which flow into a single main channel, which has structures in the channel floor to force mixing. The fluids are both aqueous. My question is as follows: once modelled in Flow Simulation, is it possible to determine the mixing efficiency at any cross section along the channel. Is it possible to set one fluid as say value 1 and another at value 0, and then determine the degree of mixing by looking at how closely to a value of 0.5 one gets? Alternatively, is it possible to export data from a cross section of the channel so that calculations can be done externally?


      Many thanks

        • Re: Determining mixing in laminar flow
          Rich Bayless



          take a look at the screen shots in this example:




          shot 'C' shows a cut plot showing volume fraction, which should work for you as an indicator of mixedness.


          There are several ways to retrieve data using Flow Sim, including exporting data from a cross section surface.  Some experimentation and searching of the forum will help you figure out how.



            • Re: Determining mixing in laminar flow
              Kevin Land

              Hi Rich


              Thank you. For this to work, does one need to set up the model using Substance Concentrations, say Water 1 and Water 2, having different volume fractions, but identical fluid properties or is there a better way to do this. Even using modelling without using this approach, the result looks correct (using spheres to visualise the flow), but the cut plot does not work, so I am guessing something like the Substance Concentrations approach is required.


              Thank you

                • Re: Determining mixing in laminar flow
                  Jared Conway

                  the 2 fluids method would be the way to go. otherwise flow doesn't really know that they are different. in real life you'd be able to color them for example. sounds like an interesting potential enhancement for that situation.


                  you could also look at particle studies and tracer studies but i don't think it will give you all the information that you want.

                    • Re: Determining mixing in laminar flow
                      Kevin Land

                      I have tried several of the above suggestions. I have also attached a schematic of the channel with the fluid inlets (left) and resulting flow trajectories. I can make the follwoing comments and observations:


                      1. I think volume/mass fractions do not work since the fraction for each fluid across the channel width will be the same for the entire length of the channel.


                      2. It would appear as if the model works. The outer fluids (small blue spheres) do mix as required and visually this is easy to see.


                      3. However, I still cannot get a quantitative mixing value. Ideally I need to define molar concentrations for each fluid and then look at cut plots etc of the molar concentration change along the channel. Then one would start with fluid 1 having concentration 1 and fluid 2 =0 etc.


                      4. Do you know if it is in some way possible to define a molar concentration for a fluid? I guess if I could export a cross section matrix of the different fluids positions, I could calculate a rough value for mixing, but I am unsure if this is possible.


                      Thanks for the time spent answering already. It is much appreciated.Mixing_b.png

                        • Re: Determining mixing in laminar flow
                          Jared Conway

                          1 flow trajectories is an interesting way to do it.


                          i tried the 3 fluid method and it seemed to work for me. see attached screenshot. standard water down through the middle, water 1 on the left, water 2 on the right. in the screenshot i'm showing the concentration of water 2.


                          since flow doesn't know anything about mols, you'll probably have to use mass or volume fraction to back calculate it.


                          the easiest way to figure things out as you go down the channel would be to create some dummy blocks. use component control to eliminate them from being solved as restrictions and then setup goals on the concentration on the faces. you MIGHT be able to do volume, but you'll have to try it.