Under normal gravity the speed of sound is well known through water and air but not through a mixture of both. Due to the forces of gravity, buoyancy pushes bubbles quickly to the surface so that measuring the speed of sound is fairly inaccurate in a mixture of air and water. This medium is interesting because the basic nature of flow is changed. The speed of sound is very high through water and through air, but slows drastically in a mixture of both to form a roughly U-shaped curve. The bottom of this curve through a bubbly mixture is what we will study and map.

If sound from a signal generator is passed through a container with a bubbly mixture, the oscillations can be read using a high frequency pressure transducer. We propose conducting this experiment in a microgravity environment. In the absence of buoyancy we can create a uniform bubbly mixture. Varying such factors as the pressure, the volume fraction of air and water, the bubble size, and the frequency of the generated sound, we will measure the signals received after they have passed through the medium. Then we can cross correlate these signals from the corresponding signals sent to reveal possible phase shifts. This should reveal the change in behavior of sound at the bottom portion of the curve.

Test Objectives

We expect a slight phase shift (Figure 1) to result between the two signals produced under microgravity and normal gravity. This phase shift should be less than one cycle. Using two experimental set-ups, we plan on obtaining a measurable shift after varying pressure, frequency (perhaps signal amplitude), bubble size, and the water to air ratio. Cross correlating these two signals should reveal the speed of sound through the bubbly medium. This is plausible to measure because in a small box with a mixture of air and water the speed of sound is drastically reduced. Before running the experiment under microgravity, we will conduct the experiment on ground several times to refine our methodology. The aim of our experiment is to add to the information of the speed of sound in a bubbly mixture.

Test Description

The two experimental set-ups will resemble Figure 2. In each a pressurized container will be filled with a measured proportion of air and water. Blender-like blades are located at the top. On the left side is a signal generator (range of 10 - 104 Hz), and on both the left and right sides are high-frequency pressure transducers that will record the data. As we approach the microgravity environment, the blender will be started at a fast speed and then slowed down until it stops. This should distribute the bubbles evenly throughout the water. Then the signal generator will produce several waves at different frequencies. Between the two set-ups and the two test runs we will vary the pressure, the signal frequency, the blade speed (and hence the bubble size), and the volume fraction of air and water. To measure the relative sizes of the produced bubbles we will use a video camera. We will collect all data with a high frequency pressure transducer that will act as a microphone. To analyze the data we will use a program to cross correlate the signal sent and the signal received for each variable. This will tell us how the sound waves are behaving in the tested medium.

In the ground experiments, we expect to see a well-defined curve for high and low water to air ratios. For the middle of this curve - in the bubbly mixture - we expect to see an undefined portion located between the two dashed lines at the bottom of the U-shaped curve in Figure 3 (this portion featured has been extrapolated, not measured). This experiment needs a microgravity environment to map out the bottom portion of this curve because buoyancy forces interfere with such measurements in a normal gravity environment. Our team expects to learn more about the speed of sound waves and how they behave in air/fluid flow in microgravity.


Created February 9, 2002. Last Updated February 11, 2002.gravity@its