Nadeem Ahmed Sheikh

Wear mechanism analysis in milling of Ti-6Al-4V alloy

This article presents an investigation into the wear of cutting tool during milling operation of Ti-6Al-4V using H13A carbide inserts. Wear tests were conducted using machining parameters (feed, speed, and depth of cut) falling in the permissible range recommended by the supplier of the inserts. A wear map was created to identify different regions that characterize the tool wear intensity. The wear map revealed a region of avoidance characterized by higher wear for cutting speed of around 55 m/min and feedrate of 0.15 mm/tooth. The Ti:Al ratio reached values between 4:1 and 6:1 for the cutting parameters that resulted in lower–tool wear rate. However, for higher wear rates, the ratio of Ti:Al did not exhibit any stability across the flank wear land. Scanning electron microscope and energy-dispersive x-ray analysis were performed on the worn inserts to identify the high–tool wear regions and material composition at different locations. Titanium aluminides (TiAl and TiAlN) were found just in the low–tool wear regions, and titanium nitride was found across the avoidance region in the wear map. Microscope and x-ray analysis of inserts in the safety zone clearly revealed a built-up edge on the cutting edge of the tools.

Vortex-Induced Vibrations of a Square Cylinder with Damped Free-End Conditions:

The authors report the results of vortex-induced vibrations of a square cylinder in a wind tunnel. This constitutes a high mass ratio environment. The square cylinder is mounted in the wind tunnel in such a fashion that it only performs rigid body oscillations perpendicular to the flow direction with damped free-end conditions. This physical situation allows a direct evaluation for analytical models relying on simplified 2D assumptions. The results are also compared with two-dimensional fluid-structure (CFD-CSD) numerical simulations. The comparison shows that despite having one-dimensional motion, the analytical model does not predict the VIV region with correctness. Results show that the numerical simulations and experimental results differ from the analytical model for the prediction of reduced velocity corresponding to peak amplitude. Also the analytical reduced velocity envelope is underpredicted compared to both numerical simulations and experimental data despite the structure being lightly damped. The findings are significant as the experimental results for freely oscillating high mass ratio body show differences from the low mass ratio especially in the transition between VIV and galloping regions. However the numerical simulations show comparatively close agreement.

Nuclear magnetic resonance measurements of velocity distributions in an ultrasonically vibrated granular bed

We report the results of nuclear magnetic resonance imaging experiments on granular beds of mustard grains fluidized by vertical vibration at ultrasonic frequencies. The variation of both granular temperature and packing fraction with height was measured within the three-dimensional cell for a range of vibration frequencies, amplitudes and numbers of grains. Small increases in vibration frequency were found—contrary to the predictions of classical ‘hard-sphere’ expressions for the energy flux through a vibrating boundary—to result in dramatic reductions in granular temperature. Numerical simulations of the grain–wall interactions, using experimentally determined Hertzian contact stiffness coefficients, showed that energy flux drops significantly as the vibration period approaches the grain–wall contact time. The experiments thus demonstrate the need for new models for ‘soft-sphere’ boundary conditions at ultrasonic frequencies.

Performance Investigation of Air Velocity Effects on PV Modules under Controlled Conditions

Junction temperature of PV modules is one of the key parameters on which the performance of PV modules depends. In the present work, an experimental investigation was carried out to analyze the effects of air velocity on the performance of two PV modules, that is, monocrystalline silicon and polycrystalline silicon under the controlled conditions of a wind tunnel in the presence of an artificial solar simulator. The parameters investigated include the surface temperature variation, power output, and efficiency of PV modules under varying air velocity from near zero (indoor lab. conditions) to 15 m/s. Additionally, the results were also determined at two different module angular positions: at 0° angle, that is, parallel to air direction and at 10° angle with the direction of coming air to consider the effects of tilt angles. Afterwards, the thermal analysis of the modules was performed using Ansys-Fluent in which junction temperature and heat flux of modules were determined by applying appropriate boundary conditions, such as air velocity, heat flux, and solar radiation. Finally, the numerical results are compared with the experiment in terms of junction temperatures of modules and good agreement was found. Additionally, the results showed that the maximum module temperature drops by 17.2°C and the module efficiency and power output increased from 10 to 12% with increasing air velocity.

Variation of Heat Flux at Lower Frequencies of Vibration in a Vibrated Granular Bed

Granular flows in vibrated bed exhibit various physical phenomena primarily driven by vibrating base. As the vibrating surface is the only source of energy in an otherwise dissipative flow, most of the theoretical models relate the steady state energy input to the RMS velocity of vibration. Here variation of heat flux is studied at varying frequency of vibration while keeping the RMS vibration velocity and the cell loading constant. Using single particle analysis and MD simulations, an extended version of grain-base collision is observed resulting in the reduction of heat flux at lower frequencies (<50 Hz) of vibration. The presented findings are important as most experimental studies are reported at these frequencies of excitation.

Evaluating Energy Flux in Vibrofluidized Granular Bed

Granular flows require sustained input of energy for fluidization. A level of fluidization depends on the amount of heat flux provided to the flow. In general, the dissipation of the grains upon interaction balances the heat inputs and the resultant flow patterns can be described using hydrodynamic models. However, with the increase in packing fraction, the heat fluxes prediction of the cell increases. Here, a comparison is made for the proposed theoretical models against the MD simulations data. It is observed that the variation of packing fraction in the granular cell influences the heat flux at the base. For the elastic grain-base interaction, the predictions vary appreciably compared to MD simulations, suggesting the need to accurately model the velocity distribution of grains for averaging.

Numerical simulation of flow past a square cylinder

A study of the behavior of a square cylinder subjected to an upstream uniform air flow is presented. The present investigation uses commercial CFD software FLUENT 6.2.16 to simulate time-resolved flow field using constitutive relationships suitable for laminar flows. Variation of flow induced lift force coefficient is used to estimate shedding frequency and resultant mean structural vibration for the Reynolds number range of 2000-8000. Simulation results in terms of limit cycle oscillation amplitude are compared with pre-existing experimental data.

Comparison of Constitutive Relationships for Dilute Granular Flow in a Vibrofluidized Cell

We present comparison of two closure models for dissipative flow of granular gas. Initial validation of the models is achieved using MD simulations for a vibrated cell with elastic side walls. With the dissipation at the side walls, convective rolling in the cell is observed. We show that dense inelastic granular gas model exhibits deviations from near elastic model even at low dissipation and dilute conditions. The flow physics shows that the strength of convective roll can be related with the differential dissipations on the side walls. The discrepancy between the two models is significant as we reevaluate the scope of near elastic model in the presence of dissipative boundary conditions.

CFD Analysis of Vortex Shedding in a Circular Cylinder: Flow Induced Vibration Design

Flow past a blunt body, such as a circular cylinder, usually experiences boundary layer separation and very strong flow oscillations in the wake region behind the body at a discrete frequency that is correlated to the Reynolds number of the flow. The periodic nature of the vortex shedding phenomenon can sometimes lead to unwanted structural vibrations. The effect of vibrating instability of a single cylinder is investigated in a uniform flow using the power of computational methods. Fluid structure coupling procedure predicts the fluid forces responsible for structural vibrations. An implicit approach to the solution of the unsteady two-dimensional Navier-Stokes equations is used for computation of flow parameters. Calculations are performed in parallel using a domain re-meshing/deforming technique with efficient communication requirements. Results for the unsteady shedding flow behind a circular cylinder are presented with experimental comparisons, showing the feasibility of accurate, efficient, time-dependent estimation of shedding frequency and resulting vibrations.Copyright