In certain applications, vibration measurements must be carried out under water. Some leading examples are sonar sensors for defense applications and ultrasonic medical transducers for diagnostics.
An additional category of measurements to be conducted under water is scaled model characterization, for example used in submarine or marine development. This article outlines the particular changes in the vibrational characteristics of a vibrating specimen when immersed into water to allow later simulation model validation.
A simple metallic beam is selected as a model test object. Vibrations are excited by a lightweight, small Piezo disk, adhered to the sample. The entire vibrational behavior of the sample is analyzed without contact using a 3D laser Doppler scanning vibrometer PSV-500-3D.
The initial measurement is performed in air and then the sample is immersed before measurements are repeated with precisely the same excitation and setup. In this respect, changes created by the immersion can easily be observed.
Polytec provides two model series of interferometer designed to address different fields of application.
The first, the Xtra series, is provided with an infrared laser source. Its wavelength of 1,550 nm is selected because it operates at 10 mW output power and therefore an increased signal-to-noise ratio for certain measurements, for example for low reflectivity targets or surfaces at larger stand off distances.
Operation is eyesafe (laser class 1) because 1,550 nm wavelength light is absorbed very strongly in water and does not reach the retina of the eye. The limitation is obviously that measurements through water are not possible.
The second model series is supplied with a 633 nm red Helium-Neon laser source. The output power at this wavelength must be restricted to 1 mW to remain in a safe laser class 2. At 633 nm, there is a very low attenuation of the laser power in water, which makes it the ideal equipment for vibration measurements underwater.
Three scanning heads are deployed for 3D measurements. Software controls superposition of three laser focus points on the DUT, enabling the extraction of X,Y, and Z vibration vectors at every measurement point.
Polytec's PSV-500-3D Scanning Vibrometer is shown in Figure 1, which is utilized to discern the structural dynamics of the device under test (DUT) in 3D.
Figure 1. Polytec PSV-500-3D Scanning Vibrometer.
The DUT is a basic aluminum beam of 150 mm length and 12 x 12 mm cross section (shown in Figure 2). The vibration excitation is applied using a small Piezo disk (PI Ceramic, PRYY series), only 0.2 mm thick and 10 mm in diameter. This disk is glued to the surface using standard instant glue.
If the electrical conductivity is not too high and the voltages are moderate, this excitation also can be performed underwater. For safety reasons, a thin layer of electrically insulating traditional household glue was applied and the voltage was restricted to 40 V. The DUT is initially placed into an empty glass basin with soft rubber dampers supporting it.
The measurement with the 3D Scanning Vibrometer is made through the glass of the basin’s sidewall. The basin is then conveniently filled with distilled water for the underwater measurement.
Essentially, the entire setup remains the same apart from the presence of water. Figure 3 demonstrates the entire setup with the incident red laser beams and the filled water basin.
Figure 4 shows the chosen resonant deflection shapes.
On the left, the measurements in air are given, and on the right, the equal deflection shape in water. It is easy to see that the same shapes appear but at varying frequencies.
A deeper look at the data outlines the differences as a result of the presence of the surrounding water. Figures 5 and 6 exhibit the average spectra of each measurement, right in water, and left in air.
The differences that happen when the sample is immersed into water are:
- A downshift of the resonance frequency peaks.
- A large increase in damping, seen as a critical broadening of the peaks.
Figures 5 and 6
While the resonance deflection shapes continue to be mostly unchanged, their resonance frequencies are less in water than in air.
The damping is significantly higher in water, which seems reasonable, of course. It is intriguing to note that the longitudinal resonance deflection shape (the lowest line in Figure 4) contains the smallest downshift in frequency and additionally has the smallest increase in damping.
Most likely, this can be explained by the fact that only the small back and front surfaces of the rod need to apply pressure to the surrounding water. The entire (long) surface of the rod at all the transverse resonances has to transport water and a greater effect is therefore created.
To comprehensively cover the investigation, it should be said that the absolute values of the out-of-plane amplitudes must be divided by the index of refraction of water (around 1.3), whereas the in-plane components continue to be unchanged. (This is an estimation for small angles of incidence.)
This correction has a slight impact on the absolute values of the amplitudes. Its impact on the general form of the deflection shapes can be left out in this case.
Correction of the Index of Refraction
The measurements outlined above signify raw values. If absolute magnitudes are the aim of the measurement, the refractive index of water must be included when measuring into water. The calculation is particularly easy and for small angles it must be applied for the out-of-plane component only:
vzw: out-of-plane velocity in water
vza: out-of-plane velocity in air
nw: index of refraction in water (nw = 1.33 @ 20 °C)
vzw = vza / nw
The arrangement of the measurement meant that only the surrounding water’s influence was analyzed as an influencing parameter of the structural dynamics of an aluminum beam. When comparing measurement results in water and in the air, the impact of the surrounding water becomes apparent.
While the resonance deflection shapes are alike, their frequencies shift downwards to lower values and their damping is significantly increased when the sample is immersed into water.
The scanning vibrometer is a simple and quick tool for vibration characterization under water. Utilizing the features of the HeNe source of the PSV-500-3D Scanning Vibrometer, it was proven that measurements under water are possible.
This information has been sourced, reviewed and adapted from materials provided by Polytec.
For more information on this source, please visit Polytec.