Engineering topics
Ramachandran, K.
Publications and source records attributed to Ramachandran, K..
Effect of cobalt doping on structural, optical and dielectric properties of TiO{sub 2}
A diluted magnetic semiconductor (DMS) Co-doped TiO{sub 2} is studied here for dielectric properties. Undoped and Co-doped TiO{sub 2} samples were prepared by sol-gel method. The formation of anatase phase has been investigated by X-ray diffraction (XRD). UV-Vis absorption spectra show that the incorporation of Co into the TiO{sub 2} lattice leads to redshift in the optical response, as well as lowering the band gap energy. The defect oriented emissions were seen from photoluminescence (PL) study. The dielectric properties of pure and Co-doped TiO{sub 2} samples were studied in the frequency range of 10 Hz to 10 MHz at different temperatures. Doped sample exhibit low dielectric constant when compared with host system. Both dielectric constant and ac conductivity increases with temperature.
Ethanol gas sensing by Zn-doped CdS/CdTe nanoparticles
Zn-doped CdS/CdTe nanoparticles (NPs) were synthesized and studied here for gas sensing. The crystallographic properties of the samples were studied by X-ray diffraction (XRD), which shows cubic structure for CdS and CdTe NPs. The three longitudinal optical phonon modes at 298, 595 and 895 cm{sup −1} were obtained from Raman spectrum and this also reveals the cubic structure of CdS NPs. The band gap for Zn-doped CdS/CdTe NPs increased slightly when compared with pure sample. The ethanol gas sensing in CdS/CdTe NPs shows an enhancement on Zn substitution.
Study of magneto-optic effect on Mn{sub 1-x}Zn{sub x}Fe{sub 2}O{sub 4} nanoferrofluids
Mn{sub 1-x}Zn{sub x}Fe{sub 2}O{sub 4} nanoparticles (with x varying as 0.1, 0.3, 0.5, 0.7 and 0.9) have been synthesized by co- precipitation method using polyvinyl alcohol as surfactant. Structural analysis and surface morphology of the samples were analysed using X-ray diffraction and scanning electron microscope respectively. The Scherer approximation was used to obtain crystallite sizes and found to decrease from 8.9 nm to 4.3 nm with increase in zinc substitution. The chemical composition of samples obtained through EDAX was comparable with the initial substitutions. Saturation magnetization (M{sub s}), Remanence (M{sub r}) and Coercivity (H{sub c}) were measured using VSM at room temperature and found to decrease with increase in zinc substitution. Verdet constant of the samples are obtained from Faraday rotation for various magnetic fields and it varies from 18 × 10{sup −4} to 3 × 10{sup −4}deg/Gcm with zinc substitution.
Structural, optical, and magnetic properties of Mn and Fe-doped Co{sub 3}O{sub 4} nanoparticles
Mn and Fe-doped Co{sub 3}O{sub 4} nanoparticles were prepared by a simple precipitation method. The synthesized particles were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), UV-Vis absorption spectroscopy, Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and vibrating sample magnetometer (VSM) techniques. XRD analysis showed the cubic structure of Co{sub 3}O{sub 4}. SEM and TEM images confirmed the formation of interconnected nanoparticles. Mn and Fe-doped Co{sub 3}O{sub 4} showed broad absorption in the visible region compared to undoped sample and the band gap values are red shifted. Five Raman active modes were observed from the Raman spectra. FTIR spectra confirmed the spinel structure of Co{sub 3}O{sub 4} and the doping of Mn and Fe shifts the vibrational modes to lower wave number region. The magnetic measurements confirmed that Fe-doped Co{sub 3}O{sub 4} shows a little ferromagnetic behavior compared to undoped and Mn-doped Co{sub 3}O{sub 4}, which could be related to the uncompensated surface spins and the finite size effects.
Free-wake computation of helicopter rotor flowfields in forward flight
A new method has been developed for computing advancing rotor flows. This method uses the Vorticity Embedding technique, which has been developed and validated over the last several years for hovering rotor problems. In this work, the unsteady full potential equation is solved on an Eulerian grid with an embedded vortical velocity field. This vortical velocity accounts for the influence of the wake. Dynamic grid changes that are required to accommodate prescribed blade motion and deformation are included using a novel grid blending method. Free wake computations have been performed on a two-bladed AH-1G rotor at low advance ratios including blade motion. Computed results are compared with experimental data. The sudden variations in airloads due to blade-vortex interactions on the advancing and retreating sides are well captured. The sensitivity of the computed solution to various factors like core size, time step and grids has been investigated. Computed wake geometries and their influence on the aerodynamic loads at these advance ratios are also discussed.
Hover performance analysis of advanced rotor blades
This is an effort aimed at validating recent hover prediction methods. The experimental basis for this validation work is an extensive set of loads, wake and performance data, which were obtained from a pressure instrumented model UH-60 rotor tested at the Sikorsky hover test facility and at Duits-Nederlandse Windtunnel (DNW). This model was equipped with replaceable tips - including a tapered and a BERP-type tip - which permitted studies of the effects of rotor geometry. The central prediction method studied is a free-wake, vortex embedded, full-potential CFD method - called HELIX-I. It is found that the HELIX-I code produces very good comparisons with the data including wake, surface pressure and performance. Comparisons with the measured radial load distributions have permitted an improved understanding of the wake resolution modelling requirements of CFD methods. Since HELIX-I is a combined Eulerian/Lagrangian method, limited comparisons are also made with a Lagrangian boundary element code (called EHPIC) and an Eulerian Navier-Stokes code (called TURNS). In most cases all methods produce good comparisons with the data. It is found that the HELIX-I code provides a good compromise between the speed of boundary integral methods and the comprehensive nature of Navier-Stokes methods.
The appliation of potential CFD methods to helicopter hover flows
Fixed-wing code development is now aimed primarily at the solution of problems dominated by separation--based on the assumptions that the ability to solve such problems implies the ability to solve all other problems and that present inviscid method are already adequate for most other problems. Neither of the above assumptions are correct for rotary wing problems. This is because of the unique and overriding importance of wake modeling to rotor problems and also due to the well-known numerical diffusion problems which convectional Eulerian Computational fluid dynamics (CFD) method encounter when called on to convect strong vortical regions for long distances. The need for accurate wake analyses is probably the most fundamental difference between rotory and fixed-wing aerodynamics. In addition, rotary wing complexity requires a much more intimate relationship between test and analysis than is common in fixed-wing work. With these issues in mind, this paper will review some of our recent experience in using a unique-Eulerian-Lagrangian Computational fluid dynamics (CFC) method for the solution of a critical rotor-wake problem--the prediction of hover performance.
The computation and validation of hovering rotor performance
Recent experience with the HELIX-I code is presented, and its ability to predict the flow and performance of both conventional rotors and the unconventional anhedral parabolic tip rotor utilized on the Super Puma MK2 is described. HELIX-I is a standard full-potential rotor code having the ability to efficiently predict the detailed flow on a rotor blade, including 3D, transonic, and weak viscous effects (using appropriate boundary layer analyses). The resulting code is the first full-potential CFD code with the ability to model free wake convection and the first CFD code of any type to predict hover performance.
Experimental and computational studies of hovering rotor flows
HELIX-I, an essentially standard full-potential CFD helicopter rotor code, is unique in its use of the Clebisch kinematical flow description to specify a freely convecting wake and its capacity for predicting hover performance. A study is presently performed to assess the method's sensitivity to grids and solution-starting techniques. The effects of these parameters on thrust, power, load distribution, and wake geometry are ascertained and compared with an extensive rotor data base. The use of a fairly accurate starting solution yields no obvious advantage over the use of a novel starting method which employs a succession of diminishing artificial flows.
The free-wake computation of rotor-body flows
In this paper a method is described for predicting the compressible, free-wake, flow about a lifting rotor-body configuration. The method is an extension of a unique vorticity embedded full-potential method used to calculate free-wake rotor hover performance. An unusual feature of this method is that it obviates the requirement for multiple grids to treat the rotor-body problem. The approach used to treat the body is similar to that used to include the rotor wake in the full potential calculation. The body is modeled as a structured circulation sheet and the strength of this sheet is determined in an iterative manner. Initially the method is tested to compute the flow past simple isolated bodies like cylinders and spheres. After a comparison of these simple computations with exact solutions this procedure is included into the HELIX-I, free-wake rotor code, to compute the flow around a rotor mounted on a large whirl tower. The effects of the tower on rotor wake geometry and load distribution are presented.
Rotor hover performance prediction using a free-wake, computational fluid dynamics method
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The prediction of loads on the Boeing Helicopters Model 360 rotor
Results are presented from a test/theory correlation investigation involving three rotor-analysis codes, two full potential rotor flow CFD solvers, and data obtained from tests on the Model 360 helicopter's rotor. Attention is given to the problem of reliable higher harmonic loading prediction. It is found that the rotor hover performance and loading experimental data are in excellent agreement with a novel, full potential free-wake computational technology; the need for a multiple tip-vortex wake model's use in predicting vibratory airloads is confirmed.
The free-wake prediction of rotor hover performance using a vortex embedding method
A method is developed to predict the rotor hover performance. This method solves the compressible mass conservation equation much like current full potential codes and can therefore predict the transonic flows on a rotor. However, the newly developed approach also allows for the free convection of shed vorticity and permits the computation of the entire hover wake system. The method uses a vortex embedding scheme in potential flow and has been implemented in a computer code, HELIX -I. To predict power we implement a simple boundary layer and two different induced-drag integration schemes. The induced-drag is obtained from surface pressure integration and an energy flux integral. Comparisons between computations and experiment show good agreement for the prediction of power polars, surface pressure distribution, and tip vortex geometry.
Free wake analysis of helicopter rotor blades in hover using a finite volume technique
A fully compressible method for determining helicopter rotor wake effects is described which computes the wake without requiring external specification of the wake, or separate computations for the wake and blade region. The method is a modification of a compressible finite volume Potential Flow technique, and it has been implemented in a program, HELIX I, for computing compressible rotor flow fields in hover with free wakes. Wake positions in substantial agreement with experiment have been calculated for cases including subsonic and transonic flows, high and low aspect ratios, and two- and four-bladed rotors.
Mean velocity and turbulence measurements in a 90 deg curved duct with thin inlet boundary layer
The experimental database established by this investigation of the flow in a large rectangular turning duct is of benchmark quality. The experimental Reynolds numbers, Deans numbers and boundary layer characteristics are significantly different from previous benchmark curved-duct experimental parameters. This investigation extends the experimental database to higher Reynolds number and thinner entrance boundary layers. The 5% to 10% thick boundary layers, based on duct half-width, results in a large region of near-potential flow in the duct core surrounded by developing boundary layers with large crossflows. The turbulent entrance boundary layer case at R sub ed = 328,000 provides an incompressible flowfield which approaches real turbine blade cascade characteristics. The results of this investigation provide a challenging benchmark database for computational fluid dynamics code development.
The treatment of convected vortices in compressible potential flow
A method is described for incorporating line vortices into the three dimensional compressible potential flow equation. A modified Biot-Savart law is used to compute a vortical velocity field, which is added to the gradient of the potential to form a total velocity. A rapidly converging approximate factorization (AFZ) scheme is then used to compute a potential such that the modified potential flow equation as well as the appropriate boundary conditions, based on total velocity, are satisfied. As part of a coupled iteration procedure, the positions of the line vortices are computed so that they convect with the total flow. The method is used to compute the field due to a single line vortex convecting past a wing. This represents an approximation of the effect of a canard or other lifting surface ahead of the wing, which sheds a tip vortex. It is seen that the flow field is substantially modified by the passage of the vortex. Unlike Euler equation schemes, which are also used to compute these flows, the solutions exhibit no numerical diffusion: The convected vortices retain their initial upstream width. Euler solutions, on the other hand, involve a vorticity which is numerically convected in an Eulerian frame and, unless extensive adaptive grid refinement is used they result in vortices with spread as they convect. Also, the potential flow method requires approximately two orders of magnitude less computing time and much less computer storage than the Euler methods.