An experimental and numerical study of an oscillating transonic shock wave in a duct

A combined experimental and numerical study of a transonic shock wave in a parallel walled duct subject to downstream pressure perturbations has been conducted. Experiments and simulations have been carried out with a shock strength of M∞ = 1.4 for pressure perturbation frequencies in the range 16-90 Hz. The dynamics of unsteady shock motion and the interaction structure between the unsteady transonic shock wave and the turbulent tunnel floor boundary layer have been investigated. It is found that the (experimentally measured) dynamics of shock motion are generally well predicted by the computational scheme, especially at relatively low (≈ 40 Hz) frequencies. However, at higher frequencies (≈ 90 Hz), some subtle differences between the shock dynamics measured in experiments and those predicted by Computational Fluid Dynamics (CFD) exist. There is evidence from experiments that variations in shock / boundary layer interaction (SBLI) structure caused by shock motion are responsible for a change in the nature of shock dynamics between low and high frequency. In contrast, numerical results at low and high frequencies do not differ significantly and this suggests that the numerical method is not fully capturing the physics of the unsteady flow. Possible reasons for this are considered and a number of areas where CFD is unable to replicate experimental observations are identified. Significantly, CFD predicts changes in SBLI structure due to shock motion that are much too large and this may explain why none of the subtle effects on shock dynamics seen in experiments occur in CFD. Further work developing numerical methods that demonstrate a more realistic sensitivity of SBLI structure to unsteady shock motion is required. Copyright © 2010 by P.J.K. Bruce.

Losses in self-oscillating ballasts for electrode-type compact fluorescent lamps

Commercially available integrated compact fluorescent lamps (CFLs) use self-resonant ballasts on grounds of simplicity and cost. To understand how to improve ballast efficiency, it is necessary to quantify the losses. The losses occurring in these ballasts have been directly measured using a precision mini-calorimeter. In addition, a Pspice model has been used to simulate the performance of an 18 W integrated CFL. The lamp has been represented by a behavioural model and Jiles-Atherton equations were used to model the current transformer core. The total loss is in close agreement with measurements from the mini-calorimeter, confirming the accuracy of the model. The total loss was then disaggregated into component losses by simulation, showing that the output inductor is the primary source of loss, followed by the inverter switches. © 2011 The Institution of Engineering and Technology.

Measurement of liquid film thickness by optical fluorescence and its application to an oscillating piston positive displacement flowmeter

The movement of the circular piston in an oscillating piston positive displacement flowmeter is important in understanding the operation of the flowmeter, and the leakage of liquid past the piston plays a key role in the performance of the meter. The clearances between the piston and the chamber are small, typically less than 60 νm. In order to measure this film thickness a fluorescent dye was added to the water passing through the meter, which was illuminated with UV light. Visible light images were captured with a digital camera and analysed to give a measure of the film thickness with an uncertainty of less than 7%. It is known that this method lacks precision unless careful calibration is undertaken. Methods to achieve this are discussed in the paper. The grey level values for a range of film thicknesses were calibrated in situ with six dye concentrations to select the most appropriate one for the range of liquid film thickness. Data obtained for the oscillating piston flowmeter demonstrate the value of the fluorescence technique. The method is useful, inexpensive and straightforward and can be extended to other applications where measurement of liquid film thickness is required. © 2011 IOP Publishing Ltd.

Modal liquid crystal devices in optical tweezing: 3D control and oscillating potential wells

We investigate the use of liquid crystal (LC) adaptive optics elements to provide full 3 dimensional particle control in an optical tweezer. These devices are suitable for single controllable traps, and so are less versatile than many of the competing technologies which can be used to control multiple particles. However, they have the advantages of simplicity and light efficiency. Furthermore, compared to binary holographic optical traps they have increased positional accuracy. The transmissive LC devices could be retro-fitted to an existing microscope system. An adaptive modal LC lens is used to vary the z-focal position over a range of up to 100 μm and an adaptive LC beam-steering device is used to deflect the beam (and trapped particle) in the x-y plane within an available radius of 10 μm. Furthermore, by modifying the polarisation of the incident light, these LC components also offer the opportunity for the creation of dual optical traps of controllable depth and separation. © 2006 Optical Society of America.

Fabrication of low-cost and high-reflectivity bottom mirrors for Si-based micro-cavity devices

A novel and simple way to prepare high-reflectivity bottom mirrors for Si-based micro-cavity devices is reported. The bottom mirror was deposited in the hole, which was etched from the backside of the sample by ethylenediamine-pyrocatechol-water solution with the buried Sio, layer in the silicon-on-insulator substrate as the etching-stop layer. The high-reflectivity of the bottom mirror deposited in the hole and the narrow hill width at half maximum of the cavity formed by this method both indicate the successful preparation of the bottom mirror for Si-based micro-cavity devices.

Interference and non-interference of sidebands in a quantum dot with oscillating levels

When a quantum dot is suffering an AC gate voltage, the sidebands turn up beside the static levels of the dot. We formularized the conductance and current when the effective coupling between levels in the quantum dot induced by the hybrid terms is included using a bi-unitary transform method, and we investigated the interference of the photon sidebands of deferent levels. The interference occurs if the same sidebands of deferent levels overlap, which is possible only when the static levels lie close to and overlap with each other. The overlap of different photon sidebands leads to a simple non-coherent superposition. (C) 2005 Elsevier Ltd. All rights reserved.