## 4 Engineering Choices: Guidelines for the Designer  
The previous section showed all the modeling choices the designer of a blade array with strip dampers has to face, and the effects these choices have on the numerical tool results/prediction. It was shown that even if linear and nonlinear state-of-the-art codes are available, an accurate prediction of the blade response with strips and friction is still not assured. On the basis of the study of the influence of different modelling choices, spanning different parameters and modeling steps, this paper aims at offering a set of guidelines in order to raise awareness on the possible sources of errors. Some main guidelines for the designer are summarized below and further condensed in Table 3.
* There are no particular requirements for the blade and strip FE models. \"Standard\" accuracy levels (typical of industrial FE models, i.e., mesh grid size on contact patch \(\approx 1\) mm) are quite sufficient. The mesh size does not have a significant influence on the results in terms of dynamic response. This low sensitivity is easily explained, since the additional compliance offered by a very fine mesh is still orders of magnitude higher than expected values of stiffness at the contact. This is especially true in the case of flexible dampers, as the centrifugal force is limited and, as a result, expected contact stiffness values are moderate [38]. The authors advise designers to adopt, as a good practice, a regular mesh both on the blade and on the strip contact patches. This choice will ensure an easy selection of the contact nodes to be retained in the CB-CMS condensation procedure. * If the mode of interest is a bending mode where the platform of the blade is moving during vibration, the number and location of contact master nodes retained in the CB reduction has critical effects on the numerical prediction. Ensuring a fine discretization of the contact patch is paramount to mimicking the real contact conditions (conforming surfaces and uniform centrifugal pressure application). The asymptotic behavior evidenced in Figure 6b suggests that a convergence study in the linear regime can easily identify the number of contact nodes which will ensure a trustworthy representation of the contact patch at the minimum computational cost. In the present case, the application of 4.5 node columns per cm (in the X strip longest edge direction) and 7 node rows per cm (in the Y strip shortest edge direction) ensured predicted frequency values which did not change even if the contact node density increased further. * Assigning each contact node an equal fraction of the global contact stiffness is a standard practice, typically adopted for rigid dampers. This investigation shows that this uniform division does not ensure an equal equivalent stiffness (i.e., equal stick resonance frequency) if the number of contact nodes varies. This is due to the fact that the contact patch is characterized by a nonuniform kinematics (caused by the damper flexibility). As a result, a portion of the contact patch (in the present case aligned along the strip longest edge), here termed _high mobility region_, will display a higher relative blade-strip displacement. The \"local\" value of the contact springs of the nodes belonging to the _high mobility region_ will strongly influence the response prediction, while the values assigned to nodes located elsewhere will have negligible effect. Locating the shape and position of the high mobility region becomes relevant if the designer is forced to reduce the number of contact nodes (e.g., in the nonlinear calculation) to limit the required computational capabilities. Having knowledge of the shape and position of the _high mobility region_ will enable the designer to reduce the number of contact points in such a way as to provide a conservative estimate of the frequency shift, rather than an overestimation. * The contact stiffness values need to be determined through a direct experimental investigation.