# Experimental Verification of the Dynamic Model of Turbine Blades Coupled by a Sealing Strip
## 5 Numerical Results
### Experimental Numerical Comparison  
As stated in Section 5, the results of the numerical simulation may be influenced by the value of contact parameters given as input to the simulation. Different factors can influence contact parameters: temperature [36], surface finishing and therefore the manufacturing process [37; 38], and contact pressure [39]. Several studies can be found in the literature to estimate, either numerically [40; 41] or experimentally [42; 43], contact parameters for solid components under high contact pressure. At present, there are no direct experimental data available on the strip-platform contact. The authors have therefore referred to data coming from direct experiments on the flat-on-flat interface of asymmetric solid dampers, which, in laboratory conditions, were subjected to a contact pressure in the [0.3-0.4] MPa range [44]. An uncertainty band was then applied on all parameters to reflect the uncertainty on the experimental data itself and in light of the fact that the data wasn't derived directly from a strip damper. The final ranges are:  
Figure 9: (**a**) Free (i.e., no strip) numerical FRF compared to IP and OOP numerical full stick FRFs (**b**) Graphical representation of IP and OOP numerical full stick bending modes of vibration of blades-strip system.  
* normal contact stiffness \(k_{n}=[84-252]\) N/\(\upmu\)m,
* tangential contact stiffness \(k_{tx}=k_{ty}=[56-168]\) N/\(\upmu\)m,
* friction coefficient [0.3-0.5].  
The contact stiffness values refer to one [5 \(\times\) 30] mm strip-platform contact patch and is divided uniformly among the 50 contact nodes present on each patch. These values are fed to the numerical tool described in Section 5. It should be noted that contact pressure does influence the contact parameter values. In fact, higher values of contact parameters and, most importantly, a high \(k_{n}/k_{t}\) ratio, are recorded on solid dampers loaded in the [1; 2; 3; 4] MPa range as in [39].  
The results for the IP and OOP modes are shown in Figure 10,b respectively. For each experimental condition (mode of vibration and \(|F_{E}|\) value), eight different numerical calculations have been performed with contact parameters chosen at the extreme of the uncertainty bands mentioned above. This allows for sampling the complete contact parameters space in a classic Design of Experiments framework. The outer envelope of the eight resulting curves is represented as a colored band in Figure 10.  
As shown in Section 3.2, the strip does not influence the IP mode as it tends to detach from the platforms for a large portion of the period of vibration. This hypothesis is confirmed by the numerical simulation, i.e., contact forces go to zero for a significant portion of the period of vibration). The excitation force \(|F_{E}|\) does not modify this behaviour, both in the measured and in the simulated cases. Furthermore, the contact parameters have a very limited influence on the resulting curve, as shown by the very thin band in Figure 10; only the friction coefficient value slightly changes the amplitude of the peak as the strip tends to slide when in contact with the platforms.  
The OOP mode is instead largely influenced by the strip. The low amplitude and rounded shape of the peak encountered experimentally suggested the presence of slip between strip and platforms. This hypothesis is confirmed by the numerical simulation. The choice of contact parameters appears adequate and the width of the uncertainty band is reasonably limited given the large amount of slip caused by the high excitation level (i.e., low centrifugal load).