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Physics Dept.

Capstone Research Projects Suggestions

• Why do Cylinder's Filled with Bubbly Fluid Roll More Slowly?
  Recently Kaha'a Rezentes did his senior research investigating the "pop phenomenon" -- that a cylinder filled with soda pop (or any bubbly fluid) rolls more slowly down an incline than a cylinder filled with water. He tried to identify the physical mechanism for the slow down. He ruled out redistribution of the bubbles as a cause. His observations led us to believe that the presence of the bubbles introduced additional frictional forces, both with the cylinder side walls and at the fluid-bubble interface in the cylinder, that result in additional energy loss.

  Additional research is needed to further explore these frictional forces. We'd like to establish their existence by measuring them. We'd like to be able to show that the energy lost to the frictional forces is just the right amount to account for the observed slowing of the rolling cylinder.

  Another hypothesis is that the presence of the bubbles increases energy loss by generating turbulent flow. We might explore this by seeding the fluid with a tracer particle called kallariscope and use a video camera to record the particle paths.

• Numerical study of thermal response of multi-layer structures.
  I would like to build fluid flow sensors made of a thin film evaporated onto a substrate. The substrate could be attached to a flat wall or plate and placed into a fluid flow. When electric current is run through the thin film it is heated to an elevated temperature. Flow past the plate carries heat away from the film; when the flow changes, the heat being carried away changes and the temperature of the film changes. Thus, measuring the temperature of the film tells something about the fluid flow.

  Of course, heat also flows away from the film into the substrate, and that is the subject of this project. An optimal design minimizes the heat flow into the substrate. By doing numerical calculations of the heat flow we can test various materials for the substrate and decide on the best material to use. Also, we can investigate how the size of the device influences the heat transfer.

  Another issue of major concern is the time response of the sensor: how fast can it detect temperature changes? This is governed by its thermal response time and we will want to calculate this for various substrate materials and geometries.

  Working on this project you will learn how to solve the heat transfer equation analytically for simple geometries. For more complicated (but realistic) geometries you will learn how to solve the equations numerically. This will be done either in C++ or by using a Finite Element Analysis software package.

• Patterns in evaporating liquids
  Pour a liquid onto a surface much hotter than its boiling temperature (such as liquid nitrogen on the floor or water onto a very hot frying pan) and you will see the liquid break up into roughly spherical blobs of various sizes. How could we characterize the distribution of sizes of drops? Are all the drops the same size? Is there a preferred size? Or is the distribution more complicated? What does this tell us about the physical process going on? Can we understand why the drops occur the way they do? Using a video camera, we can film evaporating drops on a plate. The images can then be analyzed using software.
• Numerical simulation of sound waves generated by a hemispherical transducer in liquid helium.
  A flat plate of a piezoelectric material can be made to oscillate by applying a sinusoidally varying voltage. If the plate is placed in a liquid, sound waves are generated by the vibrating plate. Imagine bending the plate into a hemisphere. Now the sound waves generated by the vibrations converge as they travel to the center of the hemisphere. Thus, the pressure variation of the sound waves is larger at the center than at the surface, i.e. the sound is "louder". This trick can be used to make large pressure swings in a small volume from relatively small vibrations. How much the pressure amplitude is increased depends on the sound velocity of the liquid. Under the right circumstances in liquid helium the sound velocity varies with the pressure, so there is a differential equation that describes the increase in pressure as the waves travel towards the center. The problem is best solved by numerically simulating the motion of the waves.

  Experiments have been performed in liquid helium with hemispherical transducers, but no one knows what pressures were generated at the center. This calculation would help calibrate some existing measurements and could help answer some persisting questions about cavitation in liquid helium.

• Role of capillary forces in fluid flow through small channels.
  A hot field of research today is microfluidics: the transport of fluid through very small (dimensions less than 1 micron) devices. Using techniques similar to those used to fabricate semiconductors, people can make tiny pumps and channels to transport fluids. (A major area of application is in biology and medicine where people imagine being able to accurately administer very tiny doses of medicine, or perform experiments on very small quantities.) An interesting question is what role capillary forces play in these devices. Capillary forces are illustrated by the well known meniscus that liquids show in narrow diameter tubes, which arise because of the surface tension of the fluid.

  Some things you might do in this project:

  • Estimate size of capillary forces compared to other forces in fluid systems. Determine at what device size they become important.
  • Examine existing designs for microfluidic pumps and estimate effect of capillary forces.
  • Create a design that makes efficient use of capillary forces.
  • Build an experimental prototype that demonstrates the design.

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