**3. Results and discussion**

**Figures 5** and **6a** shows the results with Case 1 bearing arrangement at 500 RPM for each of the three aforementioned propellers. The features seen here are representative of what is discussed in our previous paper [8]. **Figures 5** and **6b** show the result when the bearing design of Case 1 (**Figure 4**) is replaced with Case 2 (**Figure 4**). The result pertains to the force from one of the embedded sensors at 500 RPM, the other channel shows similar trends. In this arrangement, the sensor signature is dominated by the blade passing effects.

**Figure 5.** *Time data of the captured dynamic force from the PVDF with Case 1's design (a) and Case 2's design (b). The two (TBP), bronze two (TBBP) and three (ThBP) blade propeller is spun at 500 RPM when compared to no propeller (NP).*

The time-domain signatures in **Figure 5** is difficult to interpret due to superposition of the frequency components contained in the signal, however, it can be seen that the propeller force varies approximately 2 N in Case 2's result. This change in propeller force is similar in magnitude for the three propeller types. In addition to this, the plain shaft exhibits a large signature at the shaft rate, which is attributed to the shaft rotating in the journal bearing which is coupled to the PVDF film via the thrust bearing.

It can be seen in the frequency domain of **Figure 6** that the most significant peak after the shaft rate A*<sup>f</sup>* <sup>1</sup> *Case* <sup>2</sup> is the second harmonic A*<sup>f</sup>* <sup>2</sup> *Case* <sup>2</sup> for the

*Optimised PVDF Placement Inside an Operating Hydrodynamic Thrust Bearing DOI: http://dx.doi.org/10.5772/intechopen.110153*

**Figure 6.**

*Frequency data of the captured dynamic force from the PVDF with Case 1's design (a) and Case 2's design (b). The two (TBP), bronze two (TBBP) and three (ThBP) blade propeller is spun at 500 RPM when compared to no propeller used (NP).*

two-blade result and the third harmonic A*<sup>f</sup>* <sup>3</sup> *Case* <sup>2</sup> for the three-blade propeller, these correspond to the respective BPFs. The effect of the PPF is eliminated as A*<sup>f</sup>* 2 Case 2 < < A*<sup>f</sup>* 3 Case 2 with reference to the three-blade result; the improvement is less apparent if considering just the form without accounting for the amplitude regarding the two-blade data as the PPF and BBF are superimposed for these arrangements involving a bearing comprising 2 pads and propellers with 2 blades. This is in contrast to when the PVDF is embedded as in Case 1's design. No matter which propeller is

used, the second harmonic of the shaft rate is the most significant, A*<sup>f</sup>* 2 Case 1 > A*<sup>f</sup>* 3 Case 1, due to the two pads sliding over the PVDF film. This artefact is a by-product of the set-up and confounds the true nature of the propeller excitation forces, as revealed in **Figures 5** and **6** (L2, L4).

These trends can be further demonstrated across all test speeds, and more clearly depicted in the frequency domain (**Figure 7**). For the plots shown on the left in **Figure 7**, 10 s of measured data was recorded for each rotational speed tested. The 10 s records were stitched together to produce a spectrogram. For clarity, only the three blade propeller (ThBP) results are shown (similar results were observed for the two blade propeller variants). The spectrogram amplitudes along the second and third harmonics have been extracted and are shown in **Figure 7** and denoted by *Af* <sup>2</sup> and *Af* 3, respectively. This method of data representation clearly evidences that the second harmonic is typically lower or much closer to the third harmonic for the Case 2 arrangement compared to the Case 1 arrangement.

With reference to the Case 1 results depicted in **Figure 7a**, a bifurcation is observed about a 50 Hz centre frequency. (This phenomenon was also observed for the TBP tests not shown here). The bifurcation was not observed in any of the Case 2 tests. The 50 Hz centre frequency is attributed to background Australian supply voltage EMF. The bifurcation is believed to result from the superposition of the strain in the PVDF arising from the hydrodynamic force which moves with rotational speed and the strain caused in the PVDF as a result of it being subjected to the external field. Further investigation is required to understand this phenomenon.

The proposed PVDF arrangement of Case 2 can also be compared to the initial PVDF bearing pressure measurement investigation presented by the authors [25]. In that work the PVDF was embedded in the flat base of the thrust bearing similar to Case 1's configuration presented herein. The vertical test rig arrangement [25] facilitated different rotational speed and different fixed bearing gap investigations in the absence of a fluid immersed propeller. **Figure 8** shows 300 RPM data from that study processed into the frequency domain and compares the result to that of the current study for the Cases 1 and 2 arrangements for the same speed with no propeller (NP). As the sensitivities of the sensors vary, the data has been normalised to the shaft rate. There are two attributes: (1) that the second harmonic is reduced when compared with the original study; (2) that the noise floor is reduced across all frequencies, that warrant more detailed explanation. Firstly, the results obtained by [25] and Case 1 here show an increase in the second harmonic of the shaft rate when compared to newly proposed Case 2 arrangement. The larger amplitude pertaining to the original [25] study compared to Case 1 here is attributed to the increased pressure formed under the tilted pad as a result of the original set-up which forced a fixed film thickness whereas in the current study the bearing can adjust the film thickness to achieve force equilibrium. Secondly, and of potentially greater importance, is that the noise floor across all speeds has been reduced significantly using the Case 2 arrangement. It is thought that this can be attributed again to the new arrangement's reference frame. Specifically, that unlike the arrangements that have the pad moving passed the PVDF, because the PVDF films are mounted on the pads in Case 2 then they effectively experience the cleaner established pressure field above the pad and are not as influenced by the frequency content contained in the upstream and downstream flow and eddies [25] on each side of the pad. Whilst further experimental and numerical work is underway, these results add credence to the utility of the new arrangement.

*Optimised PVDF Placement Inside an Operating Hydrodynamic Thrust Bearing DOI: http://dx.doi.org/10.5772/intechopen.110153*

### **Figure 7.**

*The frequency response of the PVDF sensors across all test speeds for the Case 2's arrangement and Case 1's arrangement (a) with the three-blade propeller (ThBP). The magnitudes at the second and third harmonics are shown in (b) for both test cases.*

**Figure 8.** *Comparison of the [25] frequency content to that of Cases 1 and 2 at 300 RPM with no propeller.*
