From roar to straw: Simulating Niagara’s power through a straw with Simcenter Flomaster
The perks of being a Simcenter Intern
As a Simcenter software engineering Intern, I use and experiment with various tools (not usually a straw) and software, utilizing the software’s capabilities for real-world and theoretical systems. Thus, I gain a unique perspective and understanding of systems as I delve into various levels of industries, such as Mechanical and Aerospace-specific systems.
Having recently come across a video from ‘xkcd’s What If?’ answering the world’s most sought-after question – ‘What if you funneled Niagara Falls through a straw?’ – I wondered if this could be simulated?
Niagara through a straw?
To sum up the video, such a scenario would be near impossible, or world-ending! This is because, as you funnel such a large flow rate through a narrow cross-sectional area, applying the Bernoulli Principle causes the pressure to drop. This leads to cavitation, where bubbles of steam form within the liquid. As these bubbles start mini-implosions within the liquid, the pipe – or straw in this case – will be gradually damaged by erosion from the implosions. This would eventually lead to structural failure.
Issues we would encounter
To avoid cavitation, the diameter of such a straw would have to be 10 meters, which is much larger than a conventional straw! In addition, even if we attempted to increase the pressure to accelerate the water, it would be very difficult as the speed of sound in water is roughly 1,500 m/s!
Nevertheless, if we could ignore these physical issues, how fast would the water be going through a straw? By his calculations, the water would be going at 25% of the speed of light! At such speeds, cavitation is no longer an issue. However, it would also mean that the water would become plasma, causing nuclear reactions. The water would also have the equivalent power of a star… which is where you can see how such a project would be world-ending!
Now for context, only the key results were mentioned and discussed in the video. So, what if we applied these conditions in a CFD tool to further analyze and validate the results?
CFD to the rescue
This is where Simcenter Flomaster comes in. Its 1D CFD capabilities allow us to model this scenario with ease, yet give us detailed results to fully understand the system behavior.
From the video, we know the following boundary conditions: the Niagara Falls flow rate is 100,000 cfs (cubic feet per second) which is equivalent to 2831680 kg/s, straw diameter of 7 mm, and ambient pressure at outlet (1.01325 bar). The only thing remaining that needs be defined for the model that wasn’t explicitly mentioned was the straw length – hence this is set to 10 cm.
Taking a closer look at the results now gives us an idea of how chaotic this idea would be! Looking at Figure 2, the pressure distribution is shown across the inlet (Niagara Falls), the straw, and towards the outlet. To satisfy the defined mass flow rate, a Total Pressure of 1.76234 x 1012 bar is needed!
To give some perspective, the deepest point on Earth is in the Mariana Trench where the pressure is 1,086 bar, therefore you would need 162 billion Mariana Trenches on top of each other to get the equivalent pressure!
Accuracy of results
Figures 3 and 4 give an additional perspective on how the given flow rate would behave across the system, and at such a flow rate, the speed of the water at the exit of the straw would be 7.37097 x 107 m/s. Converting to the speed of light would be 24.6% of the speed of light. Assuming no simplifications were made from this model to the calculation made in the video, that is a very accurate result from the Simcenter Flomaster system in comparison to the obtained result of 25% of the speed of light from the video!
Strawing conclusions
Unfortunately, we have come to the end of this fascinating rabbit hole, using Simcenter Flomaster to simulate the fluid dynamics of funneling Niagara Falls through a straw. However, this exercise has demonstrated the true power of simulation to explore scenarios—no matter how impossible they are—to further enhance our understanding and provide a way of visualizing ideas.
Just know the next time you sip your drink through a straw, remember that Niagara Falls can only dream of making it through the straw without becoming a new state of matter!
P.S. We did this to test the limits of nature and software… not because we should…but because we could!
