1,720,969 research outputs found
Effects of non-unique friction forces on the dynamic behavior of turbine bladed disks with contact interfaces
Full text linkL'abstract è presente nell'allegato / the abstract is in the attachmen
Determination of periodic response limits among multiple solutions for mechanical systems with wedge dampers
Full text linkWedge dampers are commonly used to utilize the frictional behavior in many engineering fields such as vehicle dynamics and turbo-machinery. However, the presence of non- unique contact forces in the damper interfaces creates an uncertainty that provides different dynamic response amplitudes even for the same input parameters. The maximum limits of the variability range always take the core attention in most of the damper design processes. In this paper, determination of an upper and a lower boundary among multiple steady-state solutions is presented by using a numerical approach. The method is specifically suitable for the mechanical systems with wedge dampers modeled by macro-slip frictional contact elements in the joint interfaces. In the approach proposed, a criterion that determines the periodic response boundaries according to the limit tangential force values is utilized. The method is demonstrated by illustrating several case studies on a lumped parameter system which represents a turbo-machinery application with a symmetric wedge damper pressed against two vibrating adjacent blades. A point-to-point 1D friction model with varying normal force is used in both contact sides. A parametric investigation on the variability range and response limits is performed for different damper configurations. Harmonic Balance Method with Newton’s iteration scheme is used in the numerical solution of the governing equations. The results show that a large variability exists for damper geometries where a strong coupling is present between tangential and normal contact forces. The method proposed successfully captures the limits of the variability range in all cases
On the non-uniqueness of friction forces and the systematic computation of dynamic response boundaries for turbine bladed disks with contacts
Full text linkTurbine bladed disks with friction contacts may have a large scattering of dynamic
response amplitudes in laboratory conditions even for two consecutive tests. The non-repeatability
of experimental studies might directly be related to a physical phenomenon
associated with an uncertainty in contact forces. This observation has also been computationally
shown in many studies with non-unique contact forces and multiple responses
obtained for the same set of inputs. This study presents a numerical aspect and a deeper
insight for understanding the variability observed in the periodic vibration analysis of turbine
bladed disks with friction damping. A novel method based on an optimization algorithm
is proposed to systematically detect the nonlinear dynamic response boundaries.
The main idea of the developed approach is to minimize the system loss factor which ultimately
determines the damping ability of the structure. In the meanwhile, algebraic set of
dynamic balance equations are simultaneously imposed as the nonlinear constraints to be
satisfied. In this way, two cases with the minimum values of the positive and negative loss
factor determine the upper and the lower boundaries, respectively. The method is validated
and demonstrated on a realistic turbine bladed disk with friction interfaces on the
shrouds and on the blade-disk interface. Several case studies are performed on different
cases by using the state of the art 2D friction model with varying normal load. The results
show that the limits of the variability range can be successfully captured by utilizing the
offered optimization algorithm. The great contribution of the study is also discussed with
some accompanying numerical drawbacks
Parametric study for model calibration of a friction-damped turbine blade with multiple test data
Full text linkModel updating using multiple test data is usually a challenging task for frictional structures. The difficulty arises from the limitations of nonlinear models which often overlook the uncertainties inherent in contact interfaces and in actual test conditions. In this paper, we present a parametric study for the model calibration process of a friction-damped turbine blade, addressing the experimentally measured response variability in computational simulations. On the experimental side, a recently developed test setup imitating a turbomachinery application with mid-span dampers is used. This setup allows measuring multiple responses and contact forces under nominally identical macroscale conditions. On the computational side, the same system is modeled in a commercial finite element software, and nonlinear vibration analyses are performed with a specifically developed in-house code. In numerical simulations, the multivalued nature of Coulomb’s law, which stems from the inherent variability range of static friction forces in permanently sticking contacts, is considered to be the main uncertainty. As the system undergoes vibration, this uncertainty propagates into the dynamic behavior, particularly under conditions of partial slip in contacts, thus resulting in response variability. A deterministic approach based on an optimization algorithm is pursued to predict the limits of the variability range. The model is iteratively calibrated to investigate the sensitivity of response limits to contact parameters and assembly misalignment. Through several iterations, we demonstrate how uncertain initial contact conditions can be numerically incorporated into dynamic analyses of friction-damped tur-bine blades. The results show a satisfactory level of accuracy between experiments and computational simulations. This work offers valuable insights for understanding what influences test rig response and provides practical solutions for numerical simulations to improve agreement with experimental results
An experimental and computational comparison of the dynamic response variability in a turbine blade with under-platform dampers
Full text linkRobust design of turbine blades with friction contacts in the presence of multiple response levels
Full text linkDry friction damping is a widely used solution to mitigate vibrations of turbine blades. At design stage, the computation of the forced response of assemblies of bodies in contact (whose relative motion could result in dry friction damping) could give non-unique solutions due to the possibility of having different static equilibria. Infinite possible vibratory levels in a range are hence possible. A desirable condition for designers is to deal with systems whose response boundaries are not far from each other, i.e. with a low scatter in the response. For systems with multiple vibration levels, the notion of robustness (a robust system has a small response scatter, and viceversa), is particularly important.
A robust design is hence needed for such assemblies. Once a design parameter is identified, two possible approaches are possible to accomplish this task.
The so-called manual approach explores a certain number of values of the design parameter belonging to a certain interval, and chooses the most robust configuration among those calculated. This computation could result in a huge effort if the number of considered values of the design parameter is high.
To overcome this issue, a second approach is here proposed. It is based on a Nested Optimization Algorithm (NOA), which consists in two levels of optimization in order to directly find the most robust configuration in the considered range for the design parameter.
In this paper, NOA is applied to a particular test case consisting in a lumped-parameter system simulating three blades with two UPDs interposed among them. Such a system provides the necessary coupling between different contact interfaces necessary to obtain multiple response levels. In addition, it is useful to investigate the mutual interaction among different UPDs.
Results of NOA are presented together with the results of the manual approach in order to give a validation of the double optimization. Dependence of the response scatter from the contact states of the interfaces is also investigated
An Experimental Investigation on the Dynamic Response Variability of a Turbine Blade With Midspan Dampers
Full text linkThis paper addresses two main subjects. First, a novel test setup is described to experi-mentally study the nonlinear dynamic behavior of a turbine blade coupled with two mid-span dampers (MSDs). To this end, a representative turbine blade and midspan friction dampers are originally designed, and they are assembled to a special test rig which has been previously developed at Politecnico di Torino. Second, the variability of the dynamic response is intensively investigated with a purposely defined loading/unloading strategy. To better understand the inherent kinematics of the blade–damper interaction, contact forces are measured through the novel design of the experimental campaign. It is shown that multiple responses, which are obtained in different tests while keeping all user-controlled inputs nominally same, are due to nonunique contact forces that provide different static force equilibria on the damper. This outcome is further supported by the qualitative illustration of hysteresis cycles. This study contributes to the understanding of the response repeatability linked to the nonuniqueness of friction forces
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
- …
