Neutral delay differential equation Kerr cavity model

A neutral delay differential equation (NDDE) model of a Kerr cavity with external coherent injection is developed that can be considered as a generalization of the Ikeda map with second and higher order dispersions being taken into account. It is shown that this model has solutions in the form of dissipative solitons both in the limit, where the model can be reduced to the Lugiato--Lefever equation (LLE), and beyond this limit, where the soliton is eventually destroyed by the Cherenkov radiation. Unlike the standard LLE the NDDE model is able to describe the overlap of multiple resonances associated with different cavity modes

Regenerative memory in time-delayed neuromorphic photonic resonators

We investigate a photonic regenerative memory based upon a neuromorphic oscillator with a delayed self-feedback (autaptic) connection. We disclose the existence of a unique temporal response characteristic of localized structures enabling an ideal support for bits in an optical buffer memory for storage and reshaping of data information. We link our experimental implementation, based upon a nanoscale nonlinear resonant tunneling diode driving a laser, to the paradigm of neuronal activity, the FitzHugh-Nagumo model with delayed feedback. This proof-of-concept photonic regenerative memory might constitute a building block for a new class of neuron-inspired photonic memories that can handle high bit-rate optical signals

Numerical Wind Resource Assessment in Urban Environments

This thesis leads to a framework for micrositing, the process through which the specific location for mounting micro wind turbines in urban environments is determined. It can be used as a guidance on how to model an area of interest, find the optimum location for micro wind turbines installation and calculate the annual energy production, commenting on the accuracy that can be expected from the results. Essentially, it is composed of three parts, each one deals with different set of tasks associated with model development and simulation. The first part investigates the computational practices to the fields of turbulence in urban environments implemented in the open-source CFD library OpenFOAM. It examines the performance of a turbulence model, known as DES, which has not been previously used for external flows in complex urban environments and concludes that this approach offers improved robustness and accuracy over a range of wind conditions. It offers improved prediction of flows in wake regions compared to RANS methods and is less computationally demanding than full LES approaches. The validity of DES implementation is tested using data sets derived from both wind tunnel experiments and field measurements. In the second part, a procedure is developed to identify the optimum location for mounting wind turbines, based on the spatial variations in mean annual wind speed and the corresponding annual energy production (AEP). The procedure utilizes one year of measured wind data for one site to extrapolate (using the `Wind Atlas Methodology') the annual wind speed at the site of interest. Then combining the climate data with the CFD results and the power characteristics of the micro wind turbines, it estimates the mean wind speed and the annual energy yield. Essentially, this methodology leads to the formation of three dimensional fields of the average annual wind speed and the AEP (3d wind maps), which will enable identification of the effects of the complex urban topography on the wind flow, and the potential locations for micro wind turbines installation. The third part examines the accuracy that can be expected from the annual energy production estimation techniques and provides guidelines on the calculations. In particular, it investigates the validity of the standard power curves for the site-specific air density and evaluates their effect on the annual energy production estimations. Differences of the order of 10-3 between the default and the site specific mean air density (ρ), do not change substantially the energy production. However, for higher discrepancies of the order of 10-2 the power output can differ more than 10%. Turbulence affects the wind energy in two ways: through power performance impacts and through effects on turbine loads and fatigue. In the operational range of each turbine, TI increases the output at low wind speeds, while in the transition region to rated power it decreases the power output. In the context of this study, the DES approach was implemented to examine the flow at the De Montfort university campus in Leicester. The 3d wind maps for the mean wind speed and the annual energy production were developed and the optimum locations for micro wind turbines installation were identified. Although the rooftops of the higher buildings have mostly the potential for wind energy applications, the effect of the urban topography on the wind potential is not always apparent. Lower building can occasionally have higher potential for micro wind turbines installation than taller and roofs of the same height and close each other may differ substantially in their predicted energy output. Using the field measurements by two 3d ultrasonic anemometers placed in the campus, the site specific air density and turbulence intensity were considered to correct the energy yield estimations and evaluate their effect on the results

Hybrid RANS-LES simulations for separated flows using dynamic grids

A hybrid RANS-LES approach is used in this thesis to simulate the unsteady aerodynamic flows. Different cases are investigated such as high Reynolds number flows around a circular cylinder, flows over an Aerospatiale A-aerofoil at stall conditions and flows around a flapping wing with mesh deformation. The Dynamic Grid Detached Eddy Simulation (DG-DES) is an in-house solver developed at the University of Sheffield. It is a message passing interface (MPI) code which uses the URANS, DES and DDES techniques with dynamic grid capability. The RANS formulation is used in the code with one equation Spalart-Allmaras (S-A) turbulence model. The S-A turbulence model is used in the frame work of a common hybrid RANS-LES formulation termed as the Detached-Eddy Simulation (DES) and Delayed Detached-Eddy Simulation (DDES). The results presented in this research contain the simulations of the transonic speed steady state flows over the .RAE2822 and the ONERA M6 wing. These simulations are carried out using single and double precision versions of the solver with different simulation techniques. A good comparison of results with the experimental data is achieved. It has also served as a validation of new additions in the code made by the author. These include addition of the inviscid flux calculation schemes (AUSM and HLLC schemes), the turbulence scheme (DDES) and double precision implementation in the solver. A detailed analysis of the A-airfoil at the Reynolds number of 2x106 and angle of attack a = 13.3ø has been carried out using the URANS, DES and DDES schemes. Encouraging results were obtained for different flow parameters including lift coefficient, drag coefficient and modelled stresses in comparison with the experimental data. It was observed that for this particular case, the DES scheme does not function in accordance with its original concept. Due to the thick trailing edge boundary layer, the switching to LES mode is done by the DES within the boundary layer. As per the basic principle of the DES scheme, the whole of the boundary layer is to be treated in RANS mode. This premature switching is known to cause the modelled stress depletion (MSD) in the flow domain. The implementation of DDES solves this irregularity and the LES to RANS switching is delayed to work in accordance with the basic DES principle by treating the whole of the attached boundary layer in the RANS mode. A detailed comparison of the Reynolds stresses is also carried out on the suction side with the experimental data. It is concluded that due to the premature switching from RANS to LES mode, the Reynolds stresses computed by the DES scheme are reduced at the trailing edge of the suction side. However, the DES simulation also predicts the flow separation at the suction side of the trailing edge in accordance with the experimental observations. The Reynolds stresses computed by the DOES scheme are similar to the URANS results. Both the URANS and DOES simulations fail to predict the trailing edge separation. It is argued that despite the premature switching, the DES scheme presents a better flowfield picture as compared with the DOES which is found to be overly dissipated for this particular case. It can be observed that this case may not be a well posed 'natural DES problem' because the flow separation is not very rapid as required by natural DES flows. The results from the DES solution clearly show that a reduced dissipative level for the thick boundary layers near trailing edge presents better quality of the solution, in contrast with RANS and DOES. Modelled and resolved turbulent stresses were calculated using DES and DOES schemes. It is observed that for this particular case the major contribution is from modelled stresses and the resolved stresses are negligible. . The circular cylinder flow with aspect ratio of 2 is simulated at different Reynolds numbers of 1.4x105 , 3.6x106 and 8x106 The comparison of the resolved stresses is carried out with the experimental data and satisfactory results are obtained. The comparison of the modelled and resolved stresses is also carried out to highlight the impact of the resolved and modelled stresses for highly separated flows. A probe point is located two diameters downstream of the circular cylinder at the symmetry plane and instantaneous data for primitive variables is stored to compare the instantaneous results of primitive variables from the DES and ODES schemes. The power spectral density plot at the same point for both the DES and DOES schemes is compared to show the energy content with the size of the eddies (high frequency corresponds to smaller eddies). This shows the energy decay as represented by the Kolmogorov's energy spectrum. Two and three dimensional Delaunay Graph based mesh deformation was incorporated in the respective two and three dimensional versions of the solver DG DES. Initial results from both 2D and 3D solvers are presented. NACA0033 with a flexible tail is simulated using the 2D Delaunay Graph based mesh deformation. The results capture the flow physics well including the vorticity contours during the flapping motion. The computed coefficient of thrust (CT) for the case with the tail thickness b/c=0.S6 x 10-3 at Strouhal number of 0.34 is compared with the experimental data and produces 30% lower values. The MPI version of the DGDES solver is used to simulate the numerical simulation of flow over a NACA0012 wing with the mesh deformation capability. The NACA0012 wing (with a span of 4 times chord length) is simulated for oscillating motion. The wing is fully rigid and this case is essentially 2D oscillating wing. The resultant instantaneous coefficient of thrust is in good agreement with the experimental data

Stabilizing region in dominant pole placement based discrete time PID control of delayed lead processes using random sampling

This is the final version. Available on open access from Elsevier via the DOI in this recordData availability: Data will be made available on request.Handling time delays in industrial process control is a major challenge in the dominant pole placement based design of proportional-integral-derivative (PID) controllers due to variable number of zeros and poles which may arise from the Pade approximation of the exponential delay terms in the characteristic polynomials used for stability analysis. This paper proposes a new concept for designing PID controllers with a derivative filter using dominant pole placement method mapped onto the discrete time domain with a suitable choice of the sampling time to convert the continuous time time-delays into finite number of discrete time poles. Here, the continuous-time plant and the filtered PID controller have been discretized using the pole-zero matching method for handling linear dynamical systems, represented by the first order plus time delay with zero (FOPTDZ) transfer function models of the open-loop system under control. We use a swarm intelligence based global optimization method as a sampler to discover the approximate the pattern of the stabilizable region in the controller parameter as well as the design specification space while also satisfying the analytical conditions for pole placement given as higher order polynomials. Simulations on test-bench plants with open-loop stable, unstable, integrating, low-pass, high-pass characteristics have been presented in order to demonstrate the validity and effectiveness of the proposed control design method.European Regional Development Fund (ERDF