Volume 2021 (2021): Issue 3 (September 2021)

A review of the existing literature shows that modern pulsed eddy current (PEC) technique for flaw detection in aircraft structure inspection is typically carried out in aperiodic mode. Аt the same time, the unstable characteristic points of the EC signal usually used as informative parameters can restrict the potential of this excitation mode due to significant measurement errors.

This article considers an advanced PEC method of NDT based on the oscillatory mode. To obtain the conditions concerned with different modes of EC probe response oscillations, an equivalent scheme of the “testing object – EC probe” system was developed and analyzed. The frequency and attenuation coefficient of natural oscillations are proposed as the informative parameters of the probe signals. The obtained mathematical model of the probe signals allows for the dependence of proposed signal parameters on the characteristics of the testing object to be evaluated.

Herein, we first develop algorithmic software for determining and analyzing the discrete amplitude and phase characteristics of PEC NDT signals based on the simulation results. The errors of the natural frequency oscillations and the attenuation coefficient determination as well as the optimal time for its determination are analyzed in order to minimize the possible errors. Next, the proposed informative parameters are experimentally investigated using a set of specimens. The obtained results confirm the possibility of the proposed methodology to enhance the inspection procedures related to the electrical conductivity and geometric parameters measurements as well as the detected defect sizing.

Seeking to better understand the physical phenomena underlying detonation wave propagation through small holes (especially the phenomenon of detonation re-initiation or its failure), we investigated the propagation of a detonation wave along a tube filled with a hydrogen-oxygen mixture diluted with argon, in the presence of obstacles with a small orifice hole. Numerical simulations were performed in a two-dimensional domain using adaptive mesh refinement and by solving compressible Euler equations for multiple thermally perfect species with a reactive source term. A premixed mixture of H₂:O₂:Ar at a ratio 2:1:7 at 10.0 kPa and 298 K was used in a 90 mm diameter tube with a detonation wave travelling from one end. We found that a single orifice placed at 200 mm from one end of the tube, with varying diameters of 6, 10, 14, 16, 18, 30, and 50 mm, showed an initial decoupling of the detonation wave into a shockwave and flame front. The detonation wave fails to propagate along the tube for orifice diameters less than λ, while it propagates by different re-initiation pathways for orifice diameters greater than λ, where λ is the cell-width for regular detonation propagation.

This paper presents a numerical simulation for predicting the combustor exit temperature pattern of an aircraft engine, developed using the commercial fluid simulation software Ansys Fluent, which assumes a shape probability density function for the instantaneous chemistry in the conserved scalar combustion model and the standard k-ε model for turbulence. We found the compliance of the radial and circumferential non-uniformities of the exit temperature with the experimental data to be insufficient. To achieve much more accurate result, the mixing intensity was enhanced with respect to the initial calculation due to using the reduced value of the turbulent Schmidt number Sc. Numerical simulation was performed for values of the turbulent Schmidt number from Sc = 0.85 (default) up to Sc = 0.2, with results confirming the reduction of radial and circumferential non-uniformities of exit temperature. However, correlation between radial and circumferential non-uniformities is not admissible for these cases. Therefore, we propose to use a temperature-dependent formulation of the turbulent Schmidt number Sc, accounting for the increase in Sc number with increasing gas temperature. A user defined function (UDF) was used to implement the Sc number temperature dependence in Ansys Fluent. The numerical results for the proposed Schmidt number Sc temperature dependence were found to be in acceptable agreement with the experimental data both for radial and circumferential non-uniformities of the exit temperature pattern.

One of the major problems in the development of algorithms for monitoring the life of aircraft gas turbine engines is that the character of loading in real flight cycles is crucially different from the character of the static and dynamic loading during the testing of samples. This paper proposes a method for taking into account the effect of retentions at maximum stresses and cycle temperatures on the low-cycle fatigue (LCF) of the heat-resistant alloys used in engine parts. Regularities in repeated-static loading (RSL) are used in combination with the method of linear accumulation of damage due to the LCF and RSL, with retentions of a variable length. A non-linear equation is derived for the summation of these damages, the solution of which determines the durability (life) of the part while taking into account the retention duration. The theoretical results were verified by using the experimental characteristics of the GS-6K and EI-437B nickel-based alloys, previously reported by other researchers.

Given the recent surge in interest in UAVs and their potential applications, a great deal of work has lately been done in the field of UAV control. However, UAVs belong to a class of nonlinear systems that are inherently difficult to control. In this study we devised a mathematical model for a PID (proportional integral derivative) control system, designed to control a quadrotor UAV so that it follows a predefined trajectory. After first describing quadrotor flight dynamics, we present the control model adopted in our system (developed in MATLAB Simulink). We then present simulated results for the use of the control system to move a quadrotor UAV to desired locations and along desired trajectories. Positive results of these simulation support the conclusion that a quadrotor UAV spatial orientation control system based on this model will successfully fulfil its task also in real conditions.

This paper proposes a two-point approximation approach to determining aircraft aerodynamic force coefficients, and compares it to the traditional methods. A variational concept is used to conduct aircraft flight trials for the maximal duration of quasi-horizontal flights. The advantages of the described optimization theories are demonstrated, in terms of aviation fuel gas savings. The results of a numerical example are presented.