In the serie we will answer some how it works quesitons
How does something work? This question brings us to a deeper understanding of our world and a very interesting journey of curiosity, new questions, surprises, and developments. So, let us ask ourselves this question for some “easy” applications, and go into a deep dive with the help of Simcenter STAR-CCM+.
The first one in the series, we will look at the function of a truck mixer, how the cement/concrete is mixed inside of the truck, and how it is poured.
WHAT IS CONCRETE
First, what is concrete? We are surrounded by buildings, structures, roads, … and what have they in common? The material that they were made of is concrete. One of the most used building materials worldwide. Concrete is a composite material, meaning, it is a mix of some substances, mainly, cement, water, and aggregate (normally sand and/or gravel), forming a slurry fluid, that can be easily poured and brought into a form.
The reaction between cement and water is called hydration. In this process, the mixture will harden and form a hard matrix that binds the materials together into a durable stone-like material. 
Take cement, sand, some gravel, and water, and mix it, you will get some concrete. But wait, in which proportions?
Depending on how much the mixture contains each component, the concrete will get different strength (compressive strength) and based on this resultant strength, the concrete is graded. It can be from M5 to M70, where M stands for mixture and the number for the strength in MPa. See  for the grades and uses of each grade.
And how much water should we add? This is answered by the Water-Cement Ratio, and it is defined by the mass ratio between water and cement. This value should be between 0.4 and 0.6. 
The concrete can be produced on site (we will not talk about it), during transportation, or in a concrete production plant. But how to get the concrete mixture from A to B? This is done using a truck mixer. Its purpose is to transport the concrete to the construction site, where it is needed, and on the way, the ingredients are tumbled with the aim to get a homogenous mixture.
On the way to the construction site, the trommel is rotating so, that the mixture moves down and stays inside of the trommel, creating a homogenous mixture. On the construction site, to offload the concrete, the trommel will rotate in the other direction, pouring the slurry fluid.
DISCLAIMER The simulation is for demonstration purposes only. It does not enforce our best practices of any kind.
Mainly we could create the simulation using a multiphase model, basically using VOF or using DEM. In the recent versions of Simcenter STAR-CCM+ a mesh is not necessary for DEM Simulations. We opted for the Meshfree DEM approach.
The simulation has 3 phases (Cement, Pebbles, and Water). The phases have the same material properties for simplicity. Each phase will be injected by a different phase injector with a different particle diameter. Simcenter STAR-CCM+ offers different injector types, we choose the Lattice Injector because (1) We can control the particle flow rate (~ how many particles are injected) through a field function, and (2) We can inject a large number of particles almost like using the random injector, so the injection process is fast and controllable. To simulate the mixing process, we injected the phases one after another.
As the phases are injected and mixed, they will interact with each other and with the walls. Here we can model the behavior of the phases in each interaction. We opted to use the Linear Cohesion Model, so the particles can connect to each other and form some agglomerations typical for slurry.
Dependent on the value of the Cohesion work, the slurry is more or less “fluid”. Two animations show the results using 0 N/m (the particles were not sticking with each other) and 115 N/m (the particles are cohesive, but still, they have some fluid properties).
While the particles are injected, the trommel rotates in such direction that the particles are forced to go inside of the trommel, and the mixing process happens. On the offload, the trommel moves the other way, and the particles are transported up through the vanes inside, till they leave the trommel, and the mix is poured.