Search Results (5)

Numerical techniques have been developed to study flow structures in the wake behind a tapered circular cylinder via computational fluid dynamics. The Reynolds number, based on the mean diameter of the tapered cylinder, is 4 × 10³; here, the boundary layer on the cylinder surface is laminar before separating into a turbulent wake. In order to model this transient turbulent flow, a large eddy simulation was adopted and vortex-shedding frequencies were determined using the fast Fourier transform. The fundamental behaviors of the cellular distributions of vortex-shedding frequencies, mechanisms of vortex splitting and the vortex cell reorganization were addressed. Two constant-frequency vortex cells were observed in the operating Reynolds number, and the respective Strouhal numbers were validated experimentally. Numerical flow visualizations showed that the spanwise shedding vortices are well aligned, whereas the vortex splitting seems to disconnect vortex lines. The pressure coefficients at specific zones and angular positions of the tapered cylinder were illustrated to explore the correlation of pressure variation with vortex shedding. The results showed that the vortex splitting initiates and completes at boundary-layer separation. Furthermore, numerical techniques are elaborated on for readers to tackle similar problems. Full article

Dark matter may be composed of self-interacting ultralight quantum fields that form macroscopic objects. An example of which includes Q-balls, compact non-topological solitons predicted by a range of theories that are viable dark matter candidates. As the Earth moves through the galaxy, interactions with such objects may leave transient perturbations in terrestrial experiments. Here we propose a new dark matter signature: an asymmetry (and other non-Gaussianities) that may thereby be induced in the noise distributions of precision quantum sensors, such as atomic clocks, magnetometers, and interferometers. Further, we demonstrate that there would be a sizeable annual modulation in these signatures due to the annual variation of the Earth velocity with respect to dark matter halo. As an illustration of our formalism, we apply our method to 6 years of data from the atomic clocks on board GPS satellites and place constraints on couplings for macroscopic dark matter objects with radii $R<{10}^4\mathrm{km}$, the region that is otherwise inaccessible using relatively sparse global networks. Full article

In this presentation, absorption (transient absorption) and emission (steady state and time-resolved fluorescence) spectroscopy were used to study, investigate and characterize the mechanisms of fluorescence quenching and obtain new sensors with which to detect toxic environments: heavy metals from water. For this purpose, new compounds were synthesized in order to have a good fluorescence (high quantum yield), stability and selective sensibility. The study of fluorescence quenching by different metal ions, such as Ni²⁺, Cu²⁺, Co²⁺, Zn²⁺, Fe³⁺, Mn²⁺, Ca²⁺, Pb²⁺, Cr³⁺, Cd²⁺, Sr²⁺, and Mg²⁺, will be conducted in solution and film at different temperatures and variations in time to demonstrate that these samples have good stability and can be used as fluorescence sensors for the selective detection of metal ions. For fundamental study, the theory of dynamic quenching, theory of static quenching and combined dynamic and static quenching were used, and the constants of the process, lifetime in excited state, quantum yield and non-radiative and radiative rate constants were estimated. The lifetime, around 0.0001 s for each of the metal complexes, was calculated by the analysis of the decay with and without oxygen. Emission from singlet oxygen was observed at 1275 nm in all samples, and the lifetime and quantum yield are dependent on the substitution on metal ions. In addition, a new application of the compounds investigated for detection of toxic environments (heavy metals—Fe) was found: a sensor to detect Fe from water. Full article

In the nozzle flapper servo valve, the transient flow force on the flapper is the fundamental reason that affects the pressure stability. The pressure pulsation in the pilot stage causes forced vibration of the flapper, and its deviation will directly influence the control pressure difference, which will make the pressure appear unstable. In order to grasp the principle and characteristics of transient flow force and its influence on pressure stability, a mathematical model of flapper displacement and control pressure is derived. For collecting the dynamic changes of the transient flow force and recording the motion behavior of the flapper, a three-dimensional model of the pilot-stage is established. Numerical simulations of turbulence phenomenon analysis are conducted with a variation of flapper displacement ranging from 5 $\mathsf{\mu }$m to 20 $\mathsf{\mu }$m. It can be concluded that the change trend of the flapper displacement is similar to the steady-state flow force and the transient flow force pulsation amplitude. Under the same structural parameters, the pulsating frequency of the flow force remains basically constant. The fluctuation of the flow force of the pilot stage will cause the pressure of the servo valve control cavity to vibrate to a certain extent, which is a factor that cannot be ignored that affects the output stability of the servo valve. Full article

This paper presents the design of a decoupled linear control strategy for a Dynamic Voltage Restorer (DVR) that utilizes a Matrix Converter (MC) as its core element and obtains the compensation energy directly from the power system. This DVR is intended to cope with power quality problems present in supply system voltages such as balanced and unbalanced variations (sags and swells), and harmonic distortion. The dynamic model of the complete system that includes the Matrix Converter, the input filters and the electrical grid, is performed in the synchronous reference frame ($dq0$), to have constant signals at the fundamental frequency, in order to design the proposed linear control strategy. The coupling in the $dq$ components of the system output signals caused by the Park Transformation, is eliminated by a change of variable proposed for the controller design, giving rise to a decoupled linear control. In this way, the strategy developed makes it possible to establish an adequate transient response for the converter in terms of convergence speed and overshoot magnitude, in addition to ensuring closed-loop system stability under bounded operating conditions. Unlike other proposals that utilize complex modulation strategies to control the MC under adverse conditions at the input terminals, in this case, the ability to generate fully controllable output voltages, regardless of the condition of the input signals, is provided by the designed linear controller. This allows the development of a multifunctional compensator with a simple control that could be of easy implementation. In order to verify the performance of the control strategy developed, and the effectiveness of the proposed DVR to mitigate the power quality problems already mentioned, several case studies are presented. The operational capacity of the MC is demonstrated by the obtained simulation results, which clearly reveals the capability of the DVR to eliminate voltage swells up to 50% and sags less than 50%. The compensation limit reached for sags is 37%. In relation to compensation for unbalanced voltage variations, the DVR manages to reduce the voltage imbalance from 11.11% to 0.37%. Finally, with regard to the operation of the DVR as an active voltage filter, the compensator is capable of reducing a THD of 20% calculated on the supply voltage, to a value of 1.53% measured at the load terminals. In the last two cases, the DVR mitigates disturbances to a level below the criteria established in the IEEE standard for power quality. Results obtained from numerical simulations performed in MATLAB/Simulink serve to validate the proposal, given that for each condition analyzed, the MC had succesfully generated the adequate compensation voltages, thus corroborating the robustness and effectiveness of the control strategy developed in this proposal. Full article