Analysing dynamic deep stall recovery using a nonlinear frequency approach

Nonlinear Dynamics - Based on bifurcation theory, nonlinear frequency response analysis is a recent development in the field of flight dynamics studies. Here, we consider how this method can be...     

Quasi-active suspension design using magnetorheological dampers

Quasi-active damping is a method of coupled mechanical and control system design using multiple semi-active dampers. By designing the systems such that the desired control force may always be achieved using a combination of the dampers, quasi-active damping seeks to approach levels of vibration isolation achievable through active damping, whilst retaining the desirable attributes of semi-active systems. In this article a design is proposed for a quasi-active, base-isolating suspension system.Control laws are firstly defined in a generalised form, where semi-active dampers are considered as idealised variable viscous dampers. This system is used to demonstrate in detail the principles of quasi-active damping, in particular the necessary interaction between mechanical and control systems. It is shown how such a system can produce a tunable, quasi-active region in the frequency response of very low displacement transmissibility.Quasi-active control laws are then proposed which are specific for use with magnetorheological dampers. These are validated in simulation using a realistic model of the damper dynamics, again producing a quasi-active region in the frequency response. Finally, the robustness of the magnetorheological, quasi-active suspension system is demonstrated.     

Comparing the direct normal form and multiple scales methods through frequency detuning

Nonlinear Dynamics - Approximate analytical methods, such as the multiple scales (MS) and direct normal form (DNF) techniques, have been used extensively for investigating nonlinear mechanical...     

The use of acoustic radiation forces to position particles within fluid droplets

Handling of micrometer sizes particles, such as biological cells or coated beads, plays a relevant role in the field of life science. A number of devices have been presented in the last years, in which acoustic forces generated by coupling the vibration of a solid structure excited by a piezoelectric transducer to the particle suspension are used to collect particles in lines or position them in clumps on a grid. Following the trend of lab-on-a-chip devices, efforts have been made to shrink the size of such systems, aiming at less reagent consumption and shorter reaction times. The majority of these systems consist of closed fluid filled volumes, typically channels. Here the use of an open fluid volume, a droplet, is examined. By exciting resonances into the droplet positioned on a surface, particles can be gathered into a line, two parallel lines or, as the frequency of excitation is increased, into more complex patterns. Such a concentration process will have useful applications in improved detection sensitivity of low concentration particulate solutions.     

Application of the bispectrum for detection of small nonlinearities excited sinusoidally

The nonlinear behaviour of damaged systems excited by vibration or ultrasound offers potential as a technique for damage detection in machine condition monitoring and non-destructive testing applications. The bispectrum, a third-order spectrum, has properties that lend themselves to the measurement of nonlinearities in systems. The properties of interest are insensitivity to Gaussian noise and ability to detect quadratic phase coupling. However, thus far analysis of the statistics of bispectrum estimation has been mainly aimed at stochastic systems. Many applications to vibration and ultrasound involve primarily deterministic, periodic excitations in the presence of stochastic noise. This paper considers the properties of a bispectrum estimate when applied to a system with weak quadratic nonlinearity excited by the superposition of two sinusoids in the presence of additive Gaussian noise. This is compared, using signal-to-noise ratios, to the powerspectrum, with the results validated using numerical data. Also addressed is the effect of quadratic phase coupling on such a system (in the absence of noise).     

Pressure-driven flow in open fluidic channels

Arsenic is an element that exists naturally in many rocks and minerals around the world. It also accumulates in petroleum, shale, oil sands, and coal deposits as a result of biogeochemical processes, and it has been found in fly ash from the combustion of solid biofuels. Arsenic compounds in their organic and inorganic forms pose both a health and an environmental risk, and continue to be a challenge to the energy industry. The environmental fate and removal technologies of arsenic compounds are controlled to a large extent by their surface interactions with inorganic and organic adsorbents. We report thermodynamic binding constants, K(binding), from applying the triple-layer surface complexation model to adsorption isotherm and pH envelope data for dimethylarsinic acid (DMA) and p-arsanilic acid (p-AsA) on hematite and goethite. Ligand exchange reactions were constructed based on the interpretation of ATR-FTIR spectra of DMA and p-AsA surface complexes. Surface coverage of adsorbates was quantified in situ from the spectral component at 840 cm(-1). The best fit to the DMA adsorption data was obtained using outer-sphere complex formation, whereas for p-AsA, the best fit was obtained using two monodentate inner-sphere surface complexes. The significance of the results is discussed in relation to improving modeling tools used by environmental regulators and the energy sector for optimum control of arsenic content in fuels.     

Imaging using air-coupled polymer-membrane capacitive ultrasonic arrays

Polymer-membrane capacitive ultrasonic linear and 2-D arrays have been fabricated for use in air-coupled imaging. By using arrays as receivers, there is a possibility of much faster imaging as the need for physically moving the receiver to scan a sample can be replaced by electronic multiplexing. In order to utilise this, a through-thickness air-coupled image of a composite plate has been made using a 2-D array as a receiver and a comparatively large planar source in air. This was made possible by the use of a chirp drive signal and cross-correlation on the measured waveform. Larger 2-D arrays with an increased number of elements have been simulated using a small scanned single receiver, and excellent imaging potential demonstrated. In addition two array receivers have been used in conjunction with two methods of post-processing, SAFT and ellipse crossing, to locate objects accurately.     

Finite element modeling of a microparticle manipulator

The contactless movement of microparticles and cells to known locations within a fluid volume is of interest in the fields of microtechnology and life sciences. A device which can position such inhomogeneities suspended in a fluid at multiple locations is described and modeled. The device consists of a thin fluid layer contained in a channel etched into a silicon wafer. Waves are excited by a macro-piezoelectric plate with electrodes on the top and bottom surfaces and, as a result, waves propagate into the adjacent fluid. The result is a pressure field throughout the fluidic volume. When an inhomogeneity in a fluid is exposed to an ultrasonic field the acoustic radiation force results; this is found by integrating the pressure over the surface of the particle, retaining second order terms, and taking the time average. Thus, due to the presence of a pressure field in the fluid in which the particles are suspended, a force field is created. The particles are then collected at the locations of the force potential minima. In the device described here, the force field is used to position particles into lines. The locations of the particles are predicted by using a finite element model of the system. The experimental and modeling results, presented here, are in good agreement.     

Real-Time Hybrid Testing of Strut-Braced Wing Under Aerodynamic Loading Using an Electrodynamic Actuator

Experimental Techniques - Real-Time Hybrid Simulation (RTHS) is an experimental framework that allows the testing of components or substructures under realistic, dynamic boundary conditions, by...     

Bifurcations of backbone curves for systems of coupled nonlinear two mass oscillator

This paper considers the dynamic response of coupled, forced and lightly damped nonlinear oscillators with two degree-of-freedom. For these systems, backbone curves define the resonant peaks in the frequency–displacement plane and give valuable information on the prediction of the frequency response of the system. Previously, it has been shown that bifurcations can occur in the backbone curves. In this paper, we present an analytical method enabling the identification of the conditions under which such bifurcations occur. The method, based on second-order nonlinear normal forms, is also able to provide information on the nature of the bifurcations and how they affect the characteristics of the response. This approach is applied to a two-degree-of-freedom mass, spring, damper system with cubic hardening springs. We use the second-order normal form method to transform the system coordinates and identify which parameter values will lead to resonant interactions and bifurcations of the backbone curves. Furthermore, the relationship between the backbone curves and the complex dynamics of the forced system is shown.