A Transfer Function Approach to Structural Vibrations Induced by Thermoacoustic Sources

To decrease NOx emissions from a combustion system, lean premixed combustion in combination with an annular combustor is used. One of the disadvantages is an increase in sound pressure levels in the combustion system, resulting in an increased excitation of the surrounding structure, the liner. This causes fatigue, which limits the life time of the combustor. To model the interaction between flame, acoustics and structure, a transfer function approach is used. In this approach, the components are represented by the frequency dependent linear transfer between their inputs and outputs. For the flame a low pass filter with convective time delay is used as transfer function between velocity perturbations at the burner outlet and the flame as acoustic volume source. The acoustic transfer from volume source to velocity perturbation at the burner outlet is obtained from a harmonic finite element analysis, in which a temperature field from CFD calculations is used. The calculated response is subsequently curve-fitted using a pole-zero model to allow for fast calculations. The finite element model includes the two way coupling between structural vibrations and acoustics, which allows extraction of the vibration levels. The different transfers are finally coupled in one model. Results show frequencies of high acoustic response which are susceptible to thermoacoustic instability. Damping mechanisms and the phase relation between the different components determine stable or unstable behavior and the amplitude of the resulting perturbations. Furthermore there are also frequencies of high structural response. Especially when the two coincide, the risk of structural damage is high, whereas when they move away from each other, the risk decreases

Vibration of the liner in an industrial combustion system due to an acoustic field

The subject of this paper is a numerical study of the properties of the liner of a test rig to be built in the future. The test-rig consists of a flexible tube of square crosssection surrounded by a pressure vessel, also with a square cross-section. At first instance, a two dimensional structural analytical model of the cross-section is made. The influence of the air between liner and pressure vessel and that within the liner on the vibration of the liner is studied using a coupled 2D finite element model. Furthermore the influence of the vibration of the liner on the acoustics of the setup is studied. After this the problem is extended to three dimensions and again the influence of the cavity surrounding the liner is analyzed. Both 2D and 3D results are compared. The cavities are found to substantially influence the structural behavior and therefore they cannot be neglected in predicting the behavior of the liner

Time Scales of CD4+ T Cell Depletion in HIV Infection

Rob De Boer discusses a new study that investigated whether a runaway process of T cell activation/infection would be compatible with the slow time scale of memory CD4+ T cell depletion in humans with chronic HIV infection

The black silicon method II: the effect of mask material and loading on the reactive ion etching of deep silicon trenches

Very deep trenches in Si with smooth controllable profiles are etched using a fluorine-based Reactive Ion Etcher(RIE). The effect of various mask materials and loading on the profile is examined using the Black Silicon Method. It is found that most metal layers have an almost infinite selectivity. When the aspect ratio of the trenches is beyond five, RIE lag is found to be an important effect. Evidence is found that this effect is caused by the bowing of incoming ions by the electrical field

Experimental validation of the interaction between combustion and structural vibration

To decrease NOx emissions from combustion systems, lean premixed combustion is used. A disadvantage is the increase in sound pressure levels in the combustor, resulting in an increased excitation of the surrounding structure: the liner. This causes fatigue, which limits the life time of the combustor. To study this problem experimentally, a test setup has been built consisting of a single burner, 500kW, 5 bar combustion system. The thin structure (liner) is contained in a thick pressure vessel with optical access for a traversing laser vibrometer system to measure the vibration levels and mode shapes of the liner. The acoustic excitation of the liner is measured using pressure sensors measuring the acoustic pressures inside the combustion chamber and in the cooling passage between the liner and the pressure vessel. To validate models, measurements were performed in steps of increasing complexity. Firstly, the structural properties, obtained by modal analysis of the liner outside the pressure vessel, have been compared with a finite element model. Subsequently, results of an acoustic finite element model of the setup have been compared to acoustic measurements on the test rig to validate the acoustic properties of the model, which are made by mounting a well defined acoustic source to the rig. Finally, measured pressures and vibration levels in the presence of combustion are shown

Where next for research on teaching assistants:The case for an international response

In this paper, the guest editors consider the direction of research on teaching assistants (TAs), and how academics can elevate the field within the spheres of education and the social sciences. We begin by unpicking, and endorsing, Giangreco’s idea of applying the ‘Maslow’s Hammer test’ (expressed in this special issue) to manuscripts about studies of TAs and inclusion to journal editorship and peer review processes. The purpose of the test is to address the disproportionate attention paid by researchers to a set of narrow and recurring interests, and to open up new lines of inquiry and discussions about the innovative methodological approaches required to operationalise them. Secondly, we describe a persistent and urgent research gap: the shortage of international data on TAs. Given the pre-eminence of TA deployment as a means to facilitate access to and participation in mainstream education for pupils with special educational needs, we argue that the continued lack of large-scale data on TAs’ characteristics, experiences, practices and impact poses a risk to advancing the global inclusion agenda. Finally, we make a call for expressions of interest in establishing an international research network to help expand, empower and raise the esteem the field of scholarship on TAs

Impaired immune evasion in HIV through intracellular delays and multiple infection of cells

With its high mutation rate, HIV is capable of escape from recognition, suppression and/or killing by CD8(+) cytotoxic T lymphocytes (CTLs). The rate at which escape variants replace each other can give insights into the selective pressure imposed by single CTL clones. We investigate the effects of specific characteristics of the HIV life cycle on the dynamics of immune escape. First, it has been found that cells in HIV-infected patients can carry multiple copies of proviruses. To investigate how this process affects the emergence of immune escape, we develop a mathematical model of HIV dynamics with multiple infections of cells. Increasing the frequency of multiple-infected cells delays the appearance of immune escape variants, slows down the rate at which they replace the wild-type variant and can even prevent escape variants from taking over the quasi-species. Second, we study the effect of the intracellular eclipse phase on the rate of escape and show that escape rates are expected to be slower than previously anticipated. In summary, slow escape rates do not necessarily imply inefficient CTL-mediated killing of HIV-infected cells, but are at least partly a result of the specific characteristics of the viral life cycle

The black silicon method: a universal method for determining the parameter setting of a fluorine-based reactive ion etcher in deep silicon trench etching with profile control

Very deep trenches (up to 200 µm) with high aspect ratios (up to 10) in silicon and polymers are etched using a fluorine-based plasma (SF6/O2/CHF3). Isotropic, positively and negatively (i.e. reverse) tapered as well as fully vertical walls with smooth surfaces are achieved by controlling the plasma chemistry. A convenient way to find the processing conditions needed for a vertical wall is described: the black silicon method. This new procedure is checked for three different reactive ion etchers (RIE), two parallel-plate reactors and a hexode. The influence of the RF power, pressure and gas mixture on the profile will be shown. Scanning electron microscope (SEM) photos are included to demonstrate the black silicon method, the influence of the gases on the profile, and the use of this method in fabricating microelectromechanical systems (MEMS)

The Integration Hypothesis: An Evolutionary Pathway to Benign SIV Infection

Untreated human immunodeficiency virus (HIV) infection in humans is typically characterised by persistent high virus load, failure of the immune response to clear the virus, and fatal disease outcome. Natural hosts of closely related simian immunodeficiency viruses (SIVs)—e.g., sooty mangabeys [1,2]—maintain comparably high persistent virus levels and yet remain healthy