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# Just in Time for Shamrock Shakes: Simulating the Last Straw You Will Ever Need

Mike Gruetzmacher, Mentor Graphics

Guest Post by Mike Gruetzmacher, Mentor Graphics Mechanical Analysis Division

As engineers, we’re a curious lot. When we here at Mentor Graphics watched the video above by a well-known international burger chain, our curiosity was aroused. The video features a new straw design for a milkshake with two different layers. The flow-optimized straw ensures that an optimal mixing of these two milkshakes is achieved for drinking pleasure. To ensure the optimal mix, the openings for the intake area of ​​the straw are optimized. So we thought it would be fun to better understand what’s going on with the use of simulation.

For our simulation, we had to deal with a lot of unknowns, including the exact dimensions of  the cup and the straw, as well as the fluid properties of the milkshakes. But what we were interested in at this point was whether it is possible to use FloEFD frontloading computational fluid dynamics (CFD) simulation to see if the arrangement and design of the holes in the straw influences the mixing ratio. For this purpose, we created a simple cup (Figure 1). We also parametrically defined the dimensions and positioning of the openings on the straw and used estimated non-Newtonian material properties for the milkshakes.

Figure 1.

### Preparing for the Experiment

Getting starting data was fun. We drank a lot of shakes to get a better feel for the flow. Our shake was poured into a measuring cup and put on a scale to measure the volume and mass, which gave us the density and a way to measure the mass flow. We then used the straw to drink the shake in a very scientific manner. We measured how much we can suck at a time. We settled on five seconds for each sip. Ten seconds proved too much as we found our mouths would fill and spill out. This process was done several times to get a good average.

### Analyze, Analyze and Analyze More

We satisfied our thirst by simplifying the model and the problem at hand somewhat. First, we simulated steady-state; therefore, the level of the two shakes remains identical for the whole time. We focused only on the holes. As we can see in the figure and in the animation, the different versions of the holes on the straw also logically show different mixtures at the straw outlet. The lowermost opening at the foot of the straw (shown in Figure 2) seems to be very close to the bottom of the cup so that the suction gap is minimal. Our simulations showed that with a larger gap, the shake would mainly be sucked from the lowest opening. Therefore, this opening seems to be only important for drinking the last bits of the milkshake.

Figure 2.

The first variant in Figure 3 below is the initial model we created. We then enlarged and lowered openings in a handful of variants to see how the flow rate changed. The graphic shows the mass fraction of the two milk shakes. Variant 4 shows the best distribution for the four variants tested here.

Figure 3.

Identifying the best size of holes is only the first step. The next step would be to set up the case for multiple stages of the drinking process, in other words, 1/1, ¾, ½ etc., and a transient simulation. We now know that by using non-Newtonian fluids with FloEFD, we are able to quickly show how the design of the openings on this radical straw design may deliver the mix to the discerning connoisseur’s tastebuds.

Watch a video clip of the Mentor Graphics STRAW in action:

Gruetzmacher started his career as an application engineer for FloEFD at NIKA (now Mentor Graphics). He has held project engineer positions for industrial engineering in the automotive industry and the oil and gas industry among others.For more information, visit Mentor Graphics.