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The hourglass, one of mankind’s earliest forms of timekeeping, is at the forefront of a new, interactive display in the basement of the Engineering Center.

A group of former seniors in the��Paul M. Rady Department of Mechanical Engineering designed a series of hourglass displays for their��Senior Design capstone class this past semester. The project aimed to answer a simple question:��why are hourglasses filled with sand and not water?

But be warned—the team’s Logistics Manager Max Van Cleave says the question isn’t as straightforward as you might think.

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Associate Professor Nathalie Vriend standing in front of the hourglass display in front of her lab window.

“Water is governed by Bernoulli’s Law, which physically states that the flow rate changes as the fluid level changes. The more water you have, the faster it will flow out and vice versa,” said Van Cleave. “Sand is different—it creates force chains that transmit the load to the edges of an hourglass. The friction created keeps the sand particles together and allows the sand to descend at a constant rate.”

There are other factors that play a role, as well. Things like the angle and shape of the hourglass, or even the size of the sand particles can affect the speed in which the sand and water flow down the funnel.

Van Cleave mentioned it was difficult for his team to account for all of these different variables. But he said it was also extremely interesting to see how much complexity and nuance can be packed into something as small and ancient as an hourglass.

“This question we answered—it’s not really a question you even think about until you absolutely have to. Everyone has used an hourglass, but they don’t realize how advanced they are,” Van Cleave said. “There was a lot of physics involved in this project and it was fun to play around with all of those factors.”

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A closer look at the detachable, lightweight aluminum frame the senior design team used to mount their hourglass display.

The project features three motorized hourglass units mounted on a lightweight aluminum frame. Two of them are filled with sand and one is filled with water.

Equipped with wheels, the detachable aluminum frame functions like a cart, allowing the group to transport the display with ease. It’s also fitted with modular components, making it easy to extend or adapt if needed.

With one click of a button on the control panel, users can flip the hourglass displays individually or all at once and observe various flow behaviors firsthand. Project sponsor and Associate Professor��Nathalie Vriend says the display is a great way to demonstrate complex fluid dynamics principles in an understandable way.

“I chose the prompt because I was looking for something nice and educational to put in front of my lab windows,” said Vriend. “There’s some interesting science hidden in these hourglasses. It’s fun, interactive and we can use it to teach high school and middle school students at outreach events.”

Van Cleave is returning to CU Boulder as a part of Rady Mechanical Engineering’s��Bachelor’s-Accelerated Master’s program. Every time he passes Vriend’s lab windows he will see his hard work front and center, like many others already have.

But he says the real fulfillment will come from how others interact with the project.

“It feels great to know that people enjoy it,” Van Cleave said. “It’s hard to boil down granular flow principles into something anybody can engage with, so it’s awesome that people are already learning a thing or two from the display.”

You can try out the hourglass display yourself in the Granular Flow Laboratory located at ECNW 1B90 in the basement of the Engineering Center.