The contribution of NLO and LPM corrections to thermal dilepton emission in heavy ion collisions

Recently lots of efforts have been made to obtain the next to leading order and Landau-Pomeranchuk-Migdal corrections to the thermal dilepton emission rate in perturbative QCD. Here we apply these results to the plasma created in heavy ion collisions and see wether these corrections improve the comparison between theoretical calculations and experimental results for the invariant mass dependence of the dilepton emission rate. In particular, we simulate the quark-gluon plasma produced at RHIC and LHC using a 2+1-dimensional viscous hydro model. We compare our results to STAR experiment and comment on the need for a non-perturbative determination of the dilepton rate at low invariant mass.Comment: 9 pages, 11 figure

Modeling Friction for Turbomachinery Applications: Tuning Techniques and Adequacy Assessment of Heuristic Contact Models

Friction dampers are commonly included into turbine designs to limit the turbine blades resonant vibrations and thus avoid high cycle fatigue failures. In order to effectively predict the effect of friction dampers on the turbine dynamics, friction is included into the simulation through specific mesoscale contact models. These models require knowledge of contact parameters to offer meaningful predictions. Standard single-contact test arrangements may fail to capture the true contact conditions and kinematics of friction dampers, especially for complex multi-interface contacts interested by variable normal loads. Several methodologies have been proposed in the literature: the lack of a “shared” approach in the field pinpoints a true “gap” in the research. Overcoming this difficulty is of primary importance, as it is the one feature that separates a state-of-the-art numerical code from a true design tool. Purpose of this chapter is to illustrate the experimental/numerical tools and methods developed to fill this gap for a common family of friction dampers, called “underplatform dampers” with a curved-flat cross section. Both cylinder-on-flat and flat-on-flat interfaces are addressed. The adequacy of the state-of-the-art contact model is discussed on the basis of a large data set obtained performing an extended experimental campaign on multiple damper samples

Optimal stimulation protocol in a bistable synaptic consolidation model

Consolidation of synaptic changes in response to neural activity is thought to be fundamental for memory maintenance over a timescale of hours. In experiments, synaptic consolidation can be induced by repeatedly stimulating presynaptic neurons. However, the effectiveness of such protocols depends crucially on the repetition frequency of the stimulations and the mechanisms that cause this complex dependence are unknown. Here we propose a simple mathematical model that allows us to systematically study the interaction between the stimulation protocol and synaptic consolidation. We show the existence of optimal stimulation protocols for our model and, similarly to LTP experiments, the repetition frequency of the stimulation plays a crucial role in achieving consolidation. Our results show that the complex dependence of LTP on the stimulation frequency emerges naturally from a model which satisfies only minimal bistability requirements.Comment: 23 pages, 6 figure

Estimation accuracy vs. engineering significance of contact parameters for solid dampers

Abstract All numerical models of friction-damped bladed arrays require knowledge or information of contact-friction parameters. In the literature, these parameters are typically tuned so that the experimental Frequency Response Function (FRF) of a damped blade matches its numerical counterpart. It is well known that there exist multiple combinations of contact parameters capable of satisfying a given experimental-numerical FRF match. A better approach towards a finer tuning could be based on directly measuring contact forces transmitted between blade platforms through the damper: in this case friction coefficients are estimated through tangential over normal force components during those hysteresis segments which are safely identified as being in a slip condition. This has been applied by these authors to rigid bar (solid) dampers. Unfortunately, the four contact stiffness values (left and right damper-platform contact, normal and tangential) are more than the measurements available in the technique presented by these authors. Therefore, the problem is underdetermined. The purpose of this paper is twofold, i.e., to propose an alternative way to estimate contact stiffness values (i.e. thus solving the under-determinacy mentioned above) and to check the effective significance of such estimates from a practical engineering point of view. The contact parameter estimation technique proposed by these authors produces, for each contact parameter, a best-fit value and an uncertainty band. It will be shown that the uncertainty affecting each contact parameter results in an uncertainty on the equivalent damping and stiffness indicators at blade level which is lower than 5%

Competitive time marching solution methods for systems with friction-induced nonlinearities

Finding efficient and accurate solution methods for nonlinear equilibrium equations is a challenging task. This is the case of systems with friction-induced nonlinearities, e.g., friction-damped turbomachinery assemblies and automotive applications such as brakes. In order to tackle this strategic task, several methods have been developed, both in the time and in the frequency domains. Time marching methods are regarded as the most accurate option, but their computational cost becomes prohibitive when friction nonlinearities are present. This poses a problem in all those cases where alternative frequency domain methods cannot be applied effectively, e.g., if transients, non-periodic excitation/solution, or highly nonlinear systems are of interest. The purpose of this paper is to propose three independent methods to make time-marching more competitive. Two of these methods can be applied to any existing direct integration scheme with minimal adjustments, but the computational time cut they introduce is significant. The last method is instead tailored for systems where the inertia force contribution is negligible. All methods are thoroughly validated numerically using a standard Newmark-β integration scheme as a reference

A Minimal Input Engine Friction Model for Power Loss Prediction

The minimization of friction losses in internal combustion engines is a goal of primary importance for the automotive industry, both to improve performance and to comply with increasingly stringent legislative requirements. It is therefore necessary to provide designers with tools for the effective estimation of friction losses from the earliest stages of design. We present a code for the estimation of friction losses in piston assembly that uses semianalytical models that require only strictly necessary geometric and functional inputs for the representation of components. This feature renders the code particularly suitable for the preliminary design phase. Furthermore, models ensure reduced computation times while maintaining excellent predictive capabilities, as demonstrated by the numerical-experimental comparison

on the choice of contact parameters for the forced response calculation of a bladed disk with underplatform dampers

Abstract Underplatform dampers (UPDs) are still in use to reduce the vibration amplitude of turbine blades and to shift the position of resonant frequencies. The dynamics of blades with UPDs is nonlinear and the analysis is challenging from both the experimental and the numerical point of view. A key point in obtaining a predictive numerical tool is the choice of the correct contact parameters (contact stiffness and friction coefficient) that are required as input to the contact model. The paper presents different approaches to choose these parameters: the contact stiffness in normal and tangential direction are both calculated and measured. The calculation is based on the analytical models in literature, the measurements are carried out on a dedicated test rig. The friction coefficient is also measured. Test results of the forced response of the same bladed disk with UPDs are available for each blade, they come from an experimental campaign under controlled excitation and centrifugal force. The forced response of the bladed disk is not used as a mean to tune the contact parameters, but rather as a validation tool: the effect of the different choices of contact parameters in the code is highlighted by the comparison of the calculated and experimental forced response of the bladed disk

Friction damping and forced-response of vibrating structures: an insight into model validation

Dry friction is widely incorporated in turbomachinery, in the form of under-platform dampers, to limit vibrations at resonance and reduce risks of high-cycle fatigue failures. Most of the test rigs that were used to investigate the behavior of under-platform dampers aim at evaluating the damper performance in terms of reduction of forced-response amplitude in blades. This approach could be insufficient to understand local nonlinearities in the contact and the influence of dampers on blade dynamics. A recently developed test rig provides the authors with an unprecedented set of information. It is capable to measure contact forces and relative displacements between dampers and blade in addition to the overall blade dynamic response. This controlled environment, together with an effective model of the blade/dampers system, is used to provide an insight into the subject of model validation. The presented experimental and numerical study of the damper is used to highlight the relevance of an accurate representation of the constraints induced by friction contacts and to discuss the adequacy of state-of-the-art contact models