Sharif Digital Repository

Being able to optimal design and fabrication of compressors requires understanding of the flow structure through the compressor components. Volutes are commonly the last main component in radial flow compressors. The stream passes through inlet, impeller and diffuser and finally is collected and discharged to the downstream pipeline by a volute. Volute shape has a direct and non-negligible effect on the compressor performance and its stable operating range. This component causes distorted pressure field in the upstream flow passages which could lead to the aero-mechanical forces acting on the impeller. As the flow inside the volute is fully three dimensional and turbulent, better... 

In this paper, the airflow field including the velocity, pressure and turbulence intensity distributions during sneezing of a female subject was simulated using a computational fluid dynamics model of realistic upper airways including both oral and nasal cavities. The effects of variation of reaction of the subject during sneezing were also investigated. That is, the impacts of holding the nose or closing the mouth during sneezing on the pressure and velocity distributions were studied. Few works have studied the sneeze and therefore different aspects of this phenomenon have remained unknown. To cover more possibilities about the inlet condition of trachea in different sneeze scenarios, it... 

This work presents the results of numerical simulation of liquid flow on a static bubble. Velocity and surfactant concentration profiles of the liquid around a bubble exposed in flow field were estimated via unsteady CFD solution. Governing equations are the Navier-Stokes and conservation of mass in the liquid bulk and at the interface. Resulted profiles of surfactant concentration and velocity in the liquid bulk are used to determine the dynamic surface tension data via two adsorption isotherms of Langmuir and Frumkin. Data of dynamic surface tension were validated via experimental data of “bubble in flow field” protocol. Surface concentrations of surfactants were also numerically... 

This study is concerned with evolution of selective withdrawal of a linearly stratified fluid through a line sink at the base of a reservoir with bottom topography in the form of a sill of small height. The problem is investigated theoretically in the linear, inviscid limit using a perturbation technique. The induced flow due to motion of the first few shear waves is studied. It is shown that the effect of a sill on the flow field is confined mostly to the withdrawal layer in the vicinity of the sill. Equations are proposed for the steady withdrawal layer thickness and the critical Froude number in the presence of a sill. © 2003 American Institute of Physics  

Aspects of pitching and plunging motions on unsteady aerodynamic behavior of an aircraft model were studied. Extensive wind tunnel tests were performed on a standard dynamics model, SDM, oscillating in both pitch and plunge modes. Up to now, there has been little or no result on the plunging behavior of an aircraft or missile as a whole and the present experiments can be considered as one of the first attempts to study the compressible flow field over a model undergoing both pitching and plunging motions. The experiments involved measuring normal force and pitching moment of the model at Mach numbers of 0.4, 0.6 and 1.5 and oscillation frequencies of 1.25, 2.77 and 6.00 Hz. The longitudinal... 

Proton exchange membrane fuel cell (PEM-FC) aggregation pressure causes extensive strains in cell segments. The compression of each segment takes place through the cell modeling method. In addition, a very heterogeneous compressive load is produced because of the recurrent channel rib design of the dipole plates, so that while high strains are provided below the rib, the domain continues in its initial uncompressed case under the ducts approximate to it. This leads to significant spatial variations in thermal and electrical connections and contact resistances (both in rib–GDL and membrane–GDL interfaces). Variations in heat, charge, and mass transfer rates within the GDL can affect the... 

Proton exchange membrane fuel cell (PEM-FC) aggregation pressure causes extensive strains in cell segments. The compression of each segment takes place through the cell modeling method. In addition, a very heterogeneous compressive load is produced because of the recurrent channel rib design of the dipole plates, so that while high strains are provided below the rib, the domain continues in its initial uncompressed case under the ducts approximate to it. This leads to significant spatial variations in thermal and electrical connections and contact resistances (both in rib–GDL and membrane–GDL interfaces). Variations in heat, charge, and mass transfer rates within the GDL can affect the... 

In this paper, numerical and experimental investigation of the performance and internal flow field characteristics of the twinentry radial inflow turbine under full and partial admission conditions are presented. The turbine is tested on a turbocharger test facility, which was developed for small and medium size turbochargers. The flow pattern in the volute and impeller of a twin-entry turbine is analyzed using fully three-dimensional viscous program. The computational performance results are compared with the experimental results, and good agreement is found. The flow field at the outlet of the turbine is investigated using a five-hole pressure probe. The tests are performed for both full... 

In this research, the centrifugal compressor of a turbocharger is investigated experimentally and numerically. Performance characteristics of the compressor were obtained experimentally by measurements of rotor speed and flow parameters at the inlet and outlet of the compressor. Three dimensional flow field in the impeller and diffuser was analyzed numerically using a full Navier-Stokes program with SST turbulence model. The performance characteristics of the compressor were obtained numerically, which were then compared with the experimental results. The comparison shows good agreement. Furthermore, the effect of area ratio and tip clearance on the performance parameters and flow field was... 

A series of experiments were carried out to study the unsteady wake phenomenon behind an oscillating airfoil. The airfoil is a section of a wind turbine blade oscillating in pitch about the quarter chord axis at various reduced frequencies, oscillation amplitude and mean angles of attack. Streamwise velocity profiles were obtained by real time and instantaneous measurements at 35 vertically aligned points behind the airfoil via two similar rakes. One of rakes has only static pressure probes and the other is equipped with total pressure ones. An estimation of the variation of linear momentum defect during the oscillation is obtained and has been compared with the corresponding static one. The... 

The dynamics of a two dimensional plane jet injected at the base of a step, parallel to the wall, in backward facing step flow geometry is studied. The objective of this work is to gain insight into the dynamics of the igniter flow field in solid fuel ramjets (SFRJ). The one equation turbulence model of Spalart and Allmaras is used. The system of governing equations is solved with a finite volume method using a structured grid in which the AUSM+ scheme is used to calculate the convective fluxes. It is shown that the wall jet drastically changes the structure of recirculating region of back-step flow. The wall shear stress caused by the wall jet is much greater than that caused by the main... 

This study is concerned with a theoretical and laboratory investigation of selective withdrawal of a linearly stratified fluid through a line sink at the base of a reservoir with a side contraction. The width-averaged equations of motion are solved numerically. The evolution of selective withdrawal and the flow properties at the steady state are studied. Using the numerical results, an equation is proposed for the withdrawal layer thickness, the critical Froude number, and the density thickness. For comparison, experiments were conducted in a flume and particle image velocimetry technique was used to capture the flow field upstream of a contraction during evolution of selective withdrawal.... 

Purpose - One major challenge in turbulent flow applications is to control the recirculation zone behind the backward-facing step (BFS). One simple idea to do so is to modify the original BFS geometry, of course, without causing adverse or undesirable impacts on the original characteristics of the primary stream. The main objective of this work is to examine the solidity of the recirculation zone behind several different geometries which are slightly to moderately different from the original BFS geometry. Design/methodology/approach - The implemented modifications cause complicated irregularities at the boundaries of the domain. The experience shows that the mesh distribution around these... 

The electric-field induced response of a droplet embedded in a quenched polyelectrolyte gel is calculated theoretically. The response comprises the droplet translation and the electric-field induced flow fields within the droplet. The gel is modeled as a soft, and electrically charged porous solid saturated with a salted Newtonian fluid. The droplet is considered an incompressible Newtonian fluid with no free charge. An analytical solution, using the perturbation methodology and linear superposition, is obtained for the leading-order steady response to a DC electric-field. The fluid within the droplet is driven due to hydrodynamic coupling with the electroosmotic flow. The fluid velocity... 

Understanding the mechanism of dispersion within plants is essential for proper operation of natural and constructed treatment wetlands. Plant deflection induced by the current drag modifies the flow and dispersion within an aquatic canopy. In this study, we look at the effect of the deflection angle of the plants on transverse dispersion. Experiments were carried out in a current flume on an array of inclined solid rods representing a deflected emergent canopy. The stem Reynolds number varied between 90 and 360, normalized stem densities between 0.011 and 0.033, and the inclination angles between 0° and 45°. The plume development was recorded using image processing technique. A special... 

Effects of reduced frequency, stop angle, and pause duration have been studied on a thin supercritical airfoil undergoing a pitch-pause-return motion, which is one of the classic maneuvers introduced by the AIAA Fluid Dynamics Technical Committee. Experiments were conducted in a low-speed wind tunnel at both a constant mean angle of attack and an oscillation amplitude with a reduced frequency ranging from 0.01 to 0.12. The desired stop angles of the airfoil were set to occur during the upstroke motion. The unsteady pressure distribution on the airfoil was measured for below, near, and beyond static stall conditions. Results showed that the reduced frequency and stop angle were the dominant... 

High-order accurate solutions of parabolized Navier-Stokes (PNS) schemes are used as basic flow models for stability analysis of hypersonic axisymmetric flows over blunt and sharp cones at Mach 8. Both the PNS and the globally iterated PNS (IPNS) schemes are utilized. The IPNS scheme can provide the basic flow field and stability results comparable with those of the thin-layer Navier-Stokes (TLNS) scheme. As a result, using the fourth-order compact IPNS scheme, a high-order accurate basic flow model suitable for stability analysis and transition prediction can be efficiently provided. The numerical solution of the PNS equations is based on an implicit algorithm with a shock fitting procedure... 

In this paper, the internal ballistics of a solid propellant rocket motor including a pyrogen type igniter is numerically investigated. The aim of the simulation is to calculate the chamber pressure as a function of time, during flame spreading phase. In addition, effects of the igniter flow on the propellant heat-up and ignition are studied in detail using unsteady quasi one-dimensional conservation equations for a working gas, coupled with the transient conduction within a solid propellant. The convective and radiative heat flux from the igniter flow to the solid surface is considered. The flow-field equations are solved using upwind Roe's scheme. The obtained results can be used to design... 

The use of multi-element airfoils has been known as a major approach to boost up the lift of wing without dramatic increase in its drag. In fact, the configuration helps to reduce the chance of flow separation over the airfoil. However, the use of a complicated geometry such as multi-element airfoil would normally cause complexity in flow behavior. The experience has shown that the flow field complexities cannot be properly modeled using standard two-equation k-epsilon turbulence model. Therefore, it is important to improve the accuracy of general turbulence models in specific applications and complex computational domains. In this work, we extend a suitable objective function based on...