Limit-cycle oscillations in unsteady flows dominated by intermittent leading-edge vortex shedding

High-frequency limit-cycle oscillations of an airfoil at low Reynolds number are studied numerically. This regime is characterized by large apparent-mass effects and intermittent shedding of leading-edge vortices. Under these conditions, leading-edge vortex shedding has been shown to result in favourable consequences such as high lift and efficiencies in propulsion/power extraction, thus motivating this study. The aerodynamic model used in the aeroelastic framework is a potential-flow-based discrete-vortex method, augmented with intermittent leading-edge vortex shedding based on a leading-edge suction parameter reaching a critical value. This model has been validated extensively in the regime under consideration and is computationally cheap in comparison with Navier-Stokes solvers. The structural model used has degrees of freedom in pitch and plunge, and allows for large amplitudes and cubic stiffening. The aeroelastic framework developed in this paper is employed to undertake parametric studies which evaluate the impact of different types of nonlinearity. Structural configurations with pitch-to-plunge frequency ratios close to unity are considered, where the flutter speeds are lowest (ideal for power generation) and reduced frequencies are highest. The range of reduced frequencies studied is two to three times higher than most airfoil studies, a virtually unexplored regime. Aerodynamic nonlinearity resulting from intermittent leading-edge vortex shedding always causes a supercritical Hopf bifurcation, where limit-cycle oscillations occur at freestream velocities greater than the linear flutter speed. The variations in amplitude and frequency of limit-cycle oscillations as functions of aerodynamic and structural parameters are presented through the parametric studies. The excellent accuracy/cost balance offered by the methodology presented in this paper suggests that it could be successfully employed to investigate optimum setups for power harvesting in the low-Reynolds-number regime

Effect of Cold Orifice Diameter and Geometry of Hot end Valves on Performance of Converging Type Ranque Hilsch Vortex Tube

AbstractRefrigeration by vortex tube works on the principle of heat transfer between two layers moving opposite to each other. Various experiments have been performed and has revealed two different approaches one for attaining high cold mass fraction and the other for attaining cold end temperature. The geometry requirement is different for the two approaches. The need is to have a tube to produce higher mass of cold air coming out at low temperature. For this purpose converging type of vortex tube is experimented and the results are promising. The results show increase in cold mass fraction as well as cold end temperature. The overall change in cold end temperature drop was 63% and the COP of the converging tube as compared to straight divergent tube increased by 102%. For conical valve angle of 45° air supply pressure of 5 bars and cold orifice diameter as 7mm the lowest temperature observed was 5°C producing cold mass fraction of about 0.9

Fuzzy Controller Based DTC of SRM Drive Fed by Common High Side Asymmetric Switch Converter

The switched reluctance motor (SRM) is recently gaining huge popularity in electric vehicle (EV) applications due to its control flexibility, simple structure, lower cost and high efficiency than the synchronous and induction motors. Among all the controllers, the direct torque control (DTC) is the most preferred due to its higher efficiency, lower losses and superior control characteristics. In this paper, a 6/4 pole SRM with fuzzy logic based DTC has been proposed for the EV application along with a converter with reduced switch counts to reduce the torque ripples and enhance the performance of the system under steady and transient state conditions. The proposed system is tested and validated under various scenarios that include load torque and speed variations and compared with the vector control method. From, the investigation it has been found that the proposed technique reduces ripples from the system during all the scenarios with a resultant flux of less than 0.5pu

Leading-edge flow criticality as a governing factor in leading-edge-vortex initiation in unsteady airfoil flows

A leading-edge suction parameter (LESP) that is derived from potential flow theory as a measure of suction at the airfoil leading edge is used to study initiation of leading-edge vortex (LEV) formation in this article. The LESP hypothesis is presented, which states that LEV formation in unsteady flows for specified airfoil shape and Reynolds number occurs at a critical constant value of LESP, regardless of motion kinematics. This hypothesis is tested and validated against a large set of data from CFD and experimental studies of flows with LEV formation. The hypothesis is seen to hold except in cases with slow-rate kinematics which evince significant trailing-edge separation (which refers here to separation leading to reversed flow on the aft portion of the upper surface), thereby establishing the envelope of validity. The implication is that the critical LESP value for an airfoil–Reynolds number combination may be calibrated using CFD or experiment for just one motion and then employed to predict LEV initiation for any other (fast-rate) motion. It is also shown that the LESP concept may be used in an inverse mode to generate motion kinematics that would either prevent LEV formation or trigger the same as per aerodynamic requirements

Model reduction in discrete-vortex methods for unsteady airfoil flows

Discrete-vortex methods are a class of low-order methods widely used to study unsteady aerodynamic phenomena. However, these methods demand high computational costs when subject to large number of vortices in the flowfield. This calls for model reduction in discrete-vortex methods. A model-reduction technique is applied to a recently developed discrete-vortex method in which the criticality of the leading-edge suction parameter (LESP) controls the initiation and termination of leading-edge vortices (LEVs). This method, called the LESP-modulated discrete-vortex method (LDVM), has been successfully used in recent work to study unsteady airfoil flows with LEV shedding. In this research, model reduction in the LDVM is achieved by amalgamating suitable pairs of discrete vortices identified through a condition that requires that the velocity at the airfoil leading edge is not affected by amalgamation. The amalgamated vortex is placed at an optimal location to ensure that the bound circulation and the leading-edge suction are conserved. The reduced-order model is able to predict the flow features and the force and moment coefficients in good agreement with the full model while having significantly lower runtimes. Use of physical quantities like leading- edge suction and bound circulation enables the easy implementation of this model-reduction strategy in other computational methods based on discrete-vortex elements

Purification and Characterization of Tannin Acyl Hydrolase from Aspergillus niger ATCC 16620

Tannin acyl hydrolase produced extracellularly by the fungal strain Aspergillus niger ATTC 16620 in solid state fermentation was purified from the cell free culture broth by ammonium sulphate fractionation followed by DEAE–Sephadex A-50 chromatography. SDS-PAGE analysis indicated that the enzyme protein molecular mass was 168 kDa. Enzyme activity was stable up to the temperature of 40 °C and the enzyme activity was optimal at pH=6. Tannase activity was maximal at 0.01 M concentration of the substrate. The addition of metal ions like Zn2+, Mn2+, Cu2+, Ca2+, Mg2+and Fe2+ inhibited the enzyme activity. Only K+ ions enhanced tannase activity, and an activity of 4.31 U/mL was reported here. Enzyme activity was maximal after 15–20 min of incubation time, with an activity of 3.9 U/mL. Km was found to be 1.03 mM and Vmax=4.25 mmol/min. Since the enzyme is active over a wide range of pH and temperature it could find potential use in the food-processing industry

Reduced-order Aeroelastic Model for Limit-cycle Oscillations in Vortex-dominated Unsteady Airfoil Flows

In previous research, Ramesh et al (JFM,2014) developed a low-order discrete vortex method for modeling unsteady airfoil flows with intermittent leading edge vortex (LEV) shedding using a leading edge suction parameter (LESP). LEV shedding is initiated using discrete vortices (DVs) whenever the Leading Edge Suction Parameter (LESP) exceeds a critical value. In subsequent research, the method was successfully employed by Ramesh et al (JFS, 2015) to predict aeroelastic limit-cycle oscillations in airfoil flows dominated by intermittent LEV shedding. When applied to flows that require large number of time steps, the computational cost increases due to the increasing vortex count. In this research, we apply an amalgamation strategy to actively control the DV count, and thereby reduce simulation time. A pair each of LEVs and TEVs are amalgamated at every time step. The ideal pairs for amalgamation are identified based on the requirement that the flowfield in the vicinity of the airfoil is least affected (Spalart, 1988). Instead of placing the amalgamated vortex at the centroid, we place it at an optimal location to ensure that the leading-edge suction and the airfoil bound circulation are conserved. Results of the initial study are promising

Modeling intermittent leading-edge vortex shedding in unsteady airfoil flows with reduced-count discrete vortices

A discrete-vortex method for unsteady airfoil flows with intermittent leading-edge vortex (LEV) shedding was proposed by Ramesh et al. (JFM, 2014). Two novelties were introduced: (i) LEV shedding is initiated using discrete vortices whenever the Leading Edge Suction Parameter (LESP), which is a measure of leading-edge suction, exceeds a critical value, and (ii) the strength of the discrete vortices is determined such that the LESP maintained at the critical value during the shedding process. Although results from this low-order method agree with CFD and experiments, the increasing vortex count with time increases the computational cost. The large number of shed vortices from the TE can be reduced through traditional techniques such as amalgamation and deletion, as they typically convect away from the airfoil and interact only weakly with the airfoil vorticity. The LEV, on the other hand, interacts strongly with the airfoil, and has a large influence on the forces. An approach to reduce the vortex count is desired. Inspired by Wang and Eldredge (TCFD, 2013), we propose a model that has just a single vortex to model an active LEV. The varying strength of this free vortex is determined using our LESP criterion. Results from the method for unsteady airfoil motions are promising

Awareness on Radiation Protection Devices Among Dental Students

Radiation protection refers to the implementation of practices to reduce radiation exposure to patients, workers and the public. The fundamental aim of radiation protection is to reduce risk of harm by ensuring that any dose received is justified and ‘as low as reasonably practicable’ (ALARP). Exposure to Radiation has become common in Dentistry, patients are exposed to radiation by various radiation emitting devices such as OPG, CBCT,CT and IOPA. Apart from patients, dental surgeons and dental assistants are being exposed to radiations on a regular basis. Radiation effects on human cells cause various problems, increased risk of brain cancers, tumors of parotid gland and various problems in the head and neck region. Thus knowing about Radiation protection is important for both patients and Dentists.The Aim of this study is to create awareness and determine the understanding of radiation protection devices and its role in dentistry among dental students.Radiographic examination plays an important part of diagnosis in dental practice.Majority of the dental students within the study were aware of the radiation protection devices and its usage

A New Effective Subject Extraction for Travel Package Suggestions

The new idea suggestion framework is applying in numerous applications.in this task investigate the online travel data of visitors to give customized travel bundle. Be that as it may, conventional suggestion framework cannot giving better travel bundle to sightseers from different geo-realistic areas. Numerous specialized difficulties are accessible for planning and execution of proficient travel bundle suggestion framework. Proposing another model named as traveller region season point model alongside Latent Dirichlet Allocation algorithm which extricates the elements like areas, travel seasons of different scenes. Presenting cocktail approach for better customized travel bundle proposal. Further Extending TAST model with the vacationer connection territory season subject model incorporates relationship among the visitors. In the long run our proposed methodology is effective to give better bundle suggestion for travellers