Dr Meshさんのブログ | CD-adapco
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Ah, summer holidays, the beach, the sun, brain-matter replaced from thoughts of velocities, pressures and optimizations with pina coladas… Except that I went camping close to a river. Of course, you know what happens when a CFD engineer watches a river for too long? He sees streamlines, algae flowing around bridge pillars in neat flow paths, and immediately spots a leaf following the path of least resistance… and wonders what the residence time of that leaf is. So this all got me thinking about work again.

Wild River image

Before I left for my holidays, I was trying to showcase the recirculation zones in our latest invention. Nothing would portray them as badly as I felt they were. I tried vectors, I tried streamlines, I tried scalars and overlaying various field functions on various derived parts. Nothing would do it. I'm not a Colorful Fancy Designer, I'm Dr Mesh and my speciality is meshing, so I really need to learn about these post-processing things. I read with eagerness the posts from Matthew Godo.

An intern joined our team recently. Fresh out of college, he was tasked with finding best practices for a particularly complex nozzle application. The internship was only to last a few months, but our new colleague was keen on showing the "real" world what he could do. With his new CFD toolset, he set-up the complete analysis with a full-blown-reacting-flow-on-the-real-geometry case - on day three... On day four, he was looking nervously at the screen wondering why his residuals continued to climb and the case did not converge.

I was recently traveling to a User Conference that CD-adapco held in Asia and spent a great deal of time staring out the window of various aircraft. With several hours to contemplate wings, I started thinking about boundary layers and how I have been simulating them. After reading “Boundary Layer Theory” by H. Schlichting, I had to double-check to make sure my designs were modeling the fluid phenomena near the wall correctly. What we design is far from well-known and validated fluid dynamics test cases. In fact, we invent some of the most unconventional products.

The other day, Dr. Design came to me with a new project: “We must find the drag coefficient of a futuristic vehicle concept!!”

As usual the deadline was yesterday. With the CAD in hand, I started to set up my simulation. I had to say, the meshing part was easy. I arrived rapidly to the point of setting up my boundary conditions, but here's where I ran into some doubt. How should I model the rotating tires?? Yes, the vehicle is futuristic, but not electromagnetic yet. I could see a few methods… Which one should I choose?

I am Dr. Mesh and designing is not my cup of tea. Yet somehow, I was roped into, ahem… asked to do a design job. As luck would have it, most of my colleagues were on holiday when my boss ordered, ahem… suggested that I create a more efficient system.

Efficiency is my middle name, but I had to start from a Part that one of my colleagues created earlier. Of course this was one of the aforementioned colleagues, now sunning himself on a beach in Papua New Guinea. I was stuck. I had to know the exact coordinates of a set of faces to continue the project.

It always happens at the last minute. My simulations have been run and converged and are ready to be presented for tomorrow morning's board meeting. Then it happens... I try to create a plot of the temperature variation along a line in my STAR-CCM+ simulation 5 minutes before the end of my day and I find my data is scattered all over the place! There's no way I could possibly present this result! Argh!!!

How Do You Consider Surface Tension Effects Between Particles When Using DEM?

In STAR-CCM+, we can model the effect of the presence of the liquid film on the surface of DEM particles in the approximation of liquid bridge model. 

The capillary force resulting from the surface tension and the pressure difference inside the liquid bridge has known dependence on the wetting angle, liquid surface tension, particle size, etc.

If one assumes particular shape of liquid bridge, the solution of Laplace-Young equation provides the solution for hydrostatic pressure within liquid bridge. This gives the analytical solution for the maximum force needed to separate two particles connected with liquid bridge (lots of literature is available on this, including using liquid bridge model with DEM). Now, you just need to equate the liquid bridge force with STAR-CCM+ expression for linear cohesion force and voilà! - obtain the value of STAR-CCM+ cohesion parameter. Using cohesion model this way should account for the surface tension effect on the bulk flow of wet grains.Liquid Bridge

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Brigid Blaschak
Communications Specialist
Stephen Ferguson
Communications Manager
Dr Mesh
Meshing Guru
Sabine Goodwin
Senior Engineer, Technical Marketing
Prashanth Shankara
Technical Marketing Engineer
Joel Davison
Product Manager, STAR-CCM+
Jean-Claude Ercolanelli
Senior Vice President, Product Management at CD-adapco
Bob Ryan
President Red Cedar Technology