Siva Parameswaran

NUMERICAL AERODYNAMIC SIMULATION OF STEADY AND TRANSIENT FLOWS AROUND TWO-DIMENSIONAL BLUFF BODIES USING THE NONSTAGGERED GRID SYSTEM

The time-averaged Navier-Stokes equations are solved numerically by a finite-volume method and applied to study flow around two-dimensional bluff bodies. The finite-volume equations are formulated in strong conservative form on a general, nonorthogonal grid system. The resulting equations are then solved by an implicit, time marching, pressure-correction based algorithm. If the flow problem has a steady state solution, then it is obtained by taking sufficient time steps until Ike flow field remains unchanged with time. As test cases for the developed methodology, two problems are selected; one has a steady state solution and the other has only a transient solution. Numerical predictions are obtained with the standard k-∊ turbulence model for the steady state, turbulent flow problem. The k-∊ model was able to predict the major, experimentally observed flow characteristics including the small separation bubble near the rear end of the body selected for the steady state test case. For the transient test case...

Atmospheric pressure plasma treatment and breathability of polypropylene nonwoven fabric

Atmospheric pressure plasma treatment is a surface modification technique, which can be used for surface finishing and pretreatment of textiles using a broad range of reactive gases. In this study, atmospheric pressure plasma was created using a mixture of nitrogen and oxygen and was applied to polypropylene spunbond fabric. Physical properties like moisture vapor transport, pore size distribution and tensile strength were evaluated to understand the effect of the plasma treatment on spunbond polypropylene. Chemical composition of the fabric before and after plasma treatment was analyzed by Fourier transform infrared spectroscopy. The spectra showed that oxygen and nitrogen containing groups were generated on the surface of the plasma-treated fabric. Scanning electron microscope was used to observe the surface morphology of the substrate. It is evident from the capillary flow porometer results, pore size increased after plasma treatment resulting in enhanced moisture vapor transport rate. No significant decrease in breaking load was observed after the plasma treatment.

Numerical investigation of the effects of base slant on the wake pattern and drag of three-dimensional bluff bodies with a rear blunt end

Abstract A computational model has been developed to help the automotive design engineer to optimize the body shape with minimum wind tunnel testing. Unsteady, Reynolds-averaged, Navier-Stokes equations have been solved numerically by a finite-volume method and have been applied to study the flow around Ahmeds vehicle-like body. The standard k—e model has been employed to model the turbulence in the flow. The finite volume equations have been formulated in a strong conservative form on a three-dimensional, unstructured grid system. The resulting equations have been solved then by an implicit, time marching pressure-correction based algorithm. The steady state solution has been obtained by taking sufficient time steps until the flow field ceases to change with time within a prescribed tolerance. For the pressure-correction equation, a preconditioned conjugate gradient method has been employed to obtain the solution. Most of the essential features of the flow field around a bluff body in ground proximity, such as the formation of trailing vortices and the reverse flow region resulting from separation, could be well predicted. In addition, the variation of the drag coefficient with the back slant angle agreed reasonably well with the experimentally observed values.

A steady, shock-capturing pressure-based computational procedure for inviscid, two-dimensional transonic flows

The present study describes a method for the prediction of steady, transonic, two-dimensional fluid flow in planar nozzles. The present work is essentially an extension of earlier work by Parameswaran to two dimensions. The steady Euler equations are expressed on a nonorthogonal grid system formed by streamlines and lines parallel to the y axis. Grid-based velocity components are employed to represent the velocity field of the flow. The differential equations are discretized in a finite-volume fashion. A partially staggered grid system is employed to store the variables. The upwind differencing scheme is employed to approximate the convective terms. A modified version of the SIMPLE algorithm is employed to solve the coupled, nonlinear equations. For flows with shocks, a novel procedure is employed to capture the shock within a cell on each stream tube.

Flow structure around a 3D bluff body in ground proximity: A computational study

Abstract A computational model is developed to help the automotive design engineer to optimize the body shape with minimum wind tunnel testing. Unsteady, Reynolds-averaged, Navier-Stokes equations are solved numerically by a finite-volume method and applied to study the flow around GMs vehicle-like body. The standard k-ϵ model is employed to model the turbulence in the flow. The finite volume equations are formulated in a strong conservative form on a three-dimensional, unstructured grid system. The resulting equations are then solved by an implicit, time marching, pressure-correction based algorithm. The steady state solution is obtained by taking sufficient time steps until the flow field ceases to change with time within a prescribed tolerance. For the pressure-correction equation, preconditioned conjugate gradient method is employed to obtain the solution. Most of the essential features of the flow field around a bluff body in ground proximity, such as the formation of trailing vortices and the reverse flow region resulting from separation, were well predicted. In addition, the variation of drag coefficient with Reynolds number per meter faithfully follows the experimentally observed pattern.

Developing an Empirical Relationship to Predict the Influence of Process Parameters on Tensile Strength of Friction Stir Welded AA6061/0–10 wt% ZrB2 In Situ Composite

New families of aluminum matrix composites (AMCs) have been developed over the last decade in search of superior properties. Aluminum reinforced with ZrB2 particles is one such family which has drawn the attention of researchers. Friction stir welding is a relatively new solid state welding which overcomes most of the defects associated with fusion welding of AMCs. An attempt has been made to friction stir weld AA6061/0–10 wt% ZrB2 in situ composite and to develop an empirical relationship to predict the tensile strength of butt joints. Four factors, five levels central composite rotatable design was used to minimize the number of experiments. The factors considered are tool rotational speed, welding speed, axial force and weight percentage of ZrB2. The effect of these factors on tensile strength of the welded joints is analyzed using the developed empirical relationship. The predicted trends are discussed. It is observed that the factors independently influence the tensile strength over the entire range of parameters studied in this work.

A PERFORMANCE COMPARISON OF THE STANDARD k-ϵ MODEL AND A DIFFERENTIAL REYNOLDS STRESS MODEL FOR A BACKWARD-FACING STEP

A differential Reynolds stress model (RSM) based on Launder, Reece, and Rodi (LRR) is formulated with appropriate wall Junctions and applied to predict the backward-facing step problem of Driver and Seegmiller. Numerical prediction obtained with the LRR, with and without “wall reflection” terms in the pressure strain model, are compared with the results of standard k-e model of Launder and Spalding for the step problem. The results demonstrate that both LRR models, i.e., with and without wall reflection terms, are capable of capturing the secondary bubble near the step, as observed in the experiment, whereas the standard k-ϵ model fails to predict the secondary bubble. In addition, the mean velocity profiles obtained with the LRR models agree better with the experimental data than those by the k-ϵ model, particularly inside the recirculating flow region. It also emerges from the present study that, with proper wall functions, LRR model is capable of predicting recirculating flows at least as well as the o...

Application of a Novel Moving-Grid Methodology to Model the Interaction of a Synthetic Jet with a Turbulent Boundary Layer

ABSTRACT This article presents a computational model that employs a novel moving-grid methodology to investigate the interaction of an isolated synthetic jet in a crossflow. This moving-grid methodology can be said to be novel because this scheme conserves space automatically. In the current study, numerical simulation was performed to investigate the transient behavior of a single, two-dimensional synthetic jet that interacts with a turbulent boundary layer. Unsteady, Reynolds-averaged Navier-Stokes equations were solved numerically by a finite-volume method. Results on several phase-averaged velocity profiles agree with the trend of data obtained from experiments set up by NASA Langley Research Center.

Computational predictions of flow over a 2-D building

Abstract Transient, 2-D computations were performed to simulate the flow over the Field Test Facility Building located at Texas Tech University. Texas Tech Building is approximately 9.1 m wide 13.7 m long, and 4.0 m high. The code used to carry out these computations is based on Spalding and Patankars Semi-Implicit Method for Pressure Linked Equations or SIMPLE. A non-uniformly distributed Cartesian grid was used to represent the building geometry and a version of the κ-ϵ model was to simulate turbulence effects. Parameters of interest include building scale, approach velocity, and turbulence profiles. Wind tunnel data were used to define the approach velocity and turbulence profiles. Computations were carried out for a 1 to 100 scale building. The computational results are compared to surface pressure data reported for the Texas Tech Building by researchers from the Civil Engineering Department and to wind tunnel pressure data reported for a 1 to 100 scale model building by researchers at the Japanese Ministry of Construction. In addition, a visual comparison of the predicted large scale vortex structures formed at the leading edge of the roof with flow visualization results for tests of a 1 to 50 scale model building in the Mechanical Engineering Departments water table is presented.

A numerical study of flow characteristics in a helical pipe

Flow characteristics and loss mechanism inside the helical pipe with large-caliber and large-scale Dean number were analyzed in this study. Numerical simulation was carried out for exploring velocity distribution, pressure field, and secondary flow by varying coil parameters such as Dean number, curvature radius, and coil pitch. The velocity gradient in the cross-section increases along the pipe and causes unsteady flow in the pipe. Large pressure differences in the 180° and 315° cross-section generate centrifugal forces on the pipe. The secondary flow is the major factor resulting in flow loss, presented obviously by the streamlines to analyze the effects of pipe parameters on the vortices. The vortex center shifts toward the upper wall with the increase in Dean number and takes a slight deflection with the increase in coil pitch. Meanwhile, a correlation of the flow loss extent inside the pipe as a function of friction factor was presented. The increases in curvature radius and coil pitch can diminish the friction factor to reduce flow losses. The accuracy of the numerical methodology was also validated by conducting corresponding experiments and empirical mathematical analysis. The maximum deviation between the experimental values and the simulated results of the pressure drop is just 2.9%.