Sharif Digital Repository

In this study, factors affecting the noise generation by instability waves in a subsonic jet with acoustic Mach number of 0.5 are investigated using linear stability analysis. The base flow required for instability analysis is obtained by modeling the jet stream based on the k-ϵ turbulence model and using the empirical coefficients suggested by Thies and Tam [1]. The resulting base flow profiles are used to solve the linear instability equation, which governs the pressure perturbation for obtaining the eigenvalues and eigenfunctions. The results of linear instability analysis for phase and amplitude of pressure fluctuations are compared against the existing experimental data, which... 

In this study, factors affecting the noise generation by instability waves in a subsonic jet with acoustic Mach number of 0.5 are investigated using linear stability analysis. The base flow required for instability analysis is obtained by modeling the jet stream based on the k-ϵ turbulence model and using the empirical coefficients suggested by Thies and Tam [1]. The resulting base flow profiles are used to solve the linear instability equation, which governs the pressure perturbation for obtaining the eigenvalues and eigenfunctions. The results of linear instability analysis for phase and amplitude of pressure fluctuations are compared against the existing experimental data, which... 

An appropriate combination of the thin-layer Navier-Stokes (TLNS) and parabolized Navier-Stokes (PNS) solvers is used to accurately and efficiently compute hypersonic transitional/turbulent flowfields of perfect gas and equilibrium air around blunt-body configurations. The TLNS equations are solved in the nose region to provide the initial data plane needed for the solution of the PNS equations. Then the PNS equations are employed to efficiently compute the flowfield for the afterbody region by using a space marching technique. Both the TLNS and the PNS equations are numerically solved by using the implicit non-iterative finite-difference algorithm of Beam and Warming. A shock fitting... 

Turbulent swirling decay pipe flow has been investigated numerically in a vertical straight fixed pipe. The swirling flow is created by means of a rotating honeycomb which produces the solid body rotation at the inlet of the fixed pipe. Since there are no experimental data at the inlet of the fixed pipe; different axi-symmetric approaches may be considered to model the honeycomb effects at the downstream flow. Considering the appropriate approach and using the resulting flow field properties from the exit of the modeled swirl generator which are applied as the inlet boundary condition for the fixed pipe, several high Reynolds turbulence models are used to predict this type of the swirling... 

Despite significant progress in unstructured grid generation and employment, the robust employment of body-conforming coordinate systems promotes the workers to consider it as a major alternative to treat relatively complex flow fields in irregular geometries. Contrary to the laminar flow treatment, there are many ambiguities around treating turbulent flow on body-fitted coordinate system. The ambiguities are mainly originated from the curvature on the boundaries. Considering the past taken efforts in improving the results of treating turbulent flow on curvilinear coordinate system, we have extended a new formulation on body-fitted coordinate system using physical covariant velocities as the... 

More than 80% of desalination plants use the multi-stage flash (MSF) technology in the Persian Gulf and hence may have a severe irreversible impact on the marine environment. In the present study, geometrical, mixing, and turbulence characteristics of thermal-saline jets, similar to MSF effluents, are numerically investigated using the dynamic Smagorinsky sub-grid scale (SGS) model and UNESCO equation of state. For this purpose, two affecting dimensionless parameters are considered: (1) density ratio, which is thermal flux to salinity flux ratio, and (2) salinity Froude number. Examining the effects of density ratio reveals that the jet flow pattern only depends on the density ratio with a... 

A novel functional form for approximating the conditional scalars in turbulent reacting flows is introduced based on the Bernstein polynomial. Multi-scalar measurement data of turbulent premixed and non-premixed flames are used to demonstrate that the new functional form provides an excellent reduced-order model for the conditional scalars. This model order reduction technique can be used to improve the accuracy, reduce the computational cost and enhance the spatial localization of the Conditional Source-term Estimation (CSE) model. CSE is a turbulence-chemistry interaction model similar to the Conditional Moment Closure (CMC) model, except that the conditional scalars are estimated from the... 

In this paper an effective numerical technique is presented to model turbulent motion of a standing surface wave in a tank. The equations of motion for turbulent boundary layers at the solid surfaces are coupled with the potential flow in the bulk of the fluid, and a mixed BEM-finite difference technique is used to obtain the wave and boundary layer characteristics. A mixing-length theory is used for turbulence modeling. The results are compared with previous experimental data. Although the technique is presented for a standing surface wave, it can be easily applied to other free surface problems. Copyright © 2008 by ASME  

A series of experimental and numerical investigations on two tandem cylinders wake have been studied. The velocity profile and turbulence intensity have been acquired by a single one dimensional Hot Wire anemometer. The two cylinders were mounted in a tandem manner in the horizontal mid plane of the working section. The effect of the upstream cylinder diameter, Reynolds number and the distance between the cylinders on the wake profile and turbulence intensity on the downstream cylinder was investigated, while the Reynolds number ranged between1.5× (10)∧4 ∼ 3×(10)∧4. The upstream cylinder diameter (d) was 10, 20 and 25 mm, while the downstream cylinder diameter (D) was 25 mm, corresponding to... 

Forced convective of water in horizontal tubes with conical tube inserts has been studied experimentally. The transient flow regime has been used for the tests. Experimental results are validated with existing well established correlation. The turbulators were placed in two different arrangements: converging conical ring, referred to as CR array and diverging conical ring, DR array. Two correlations for the Nusselt number based on the experiment are introduced for practical use. It is found that the insertion of turbulators has enhanced the Nusselt number for the DR arrangement up to 521%, and for the CR arrangement up to 355%, although using the turbulators cause a significant increase in... 

Achieving more accurate reacting flow numerical solutions apparently demand employing higher-order schemes, utilizing finer grids, and benefiting from more advanced chemistry models. One major objective of this work is to extend an inclusive low-order upwind-biased scheme in the context of finite-volume-element method to predict turbulent reacting flows on coarse grid resolutions very reliably. In this regard, a low-order upwind-biased scheme is suitably extended to approximate the mixture fraction variances at the cell-faces. This scheme implements the reacting flow physics explicitly in deriving the proposed mixture fraction variance expressions. These physical implementations enhance the... 

In this paper, the problem of designing optimal conflict-free maneuvers for planar multiple aircraft encounters is studied. The maneuvers propose suitable heading changes for aircraft in a cooperative manner. The new mathematical approach provides optimal trajectories to resolve a wide variety of conflicts, especially in the presence of high-altitude Clear Air Turbulence (CAT). The proposed approach effectively uses Genetic Algorithms (GA), together with modified webs, to quickly find conflict-resolving maneuvers. Different case studies show the method is fast enough to be used for real-time applications when resolving conflicts involving two aircraft. It is also efficient enough to resolve... 

In this work, we extend a finite-volume pressure-based incompressible algorithm to solve three-dimensional compressible and incompressible turbulent flow regimes. To achieve a hybrid algorithm capable of solving either compressible or incompressible flows, the mass flux components instead of the primitive velocity components are chosen as the primary dependent variables in a SIMPLE-based algorithm. This choice warrants to reduce the nonlinearities arose in treating the system of conservative equations. The use of a new Favre-averaging like technique plays a key role to render this benefit. The developed formulations indicate that there is less demand to interpolate the fluxes at the cell... 

Large-eddy simulations are carried out for a particle-laden vertical turbulent channel flow at Reynolds number of 180 based on friction velocity and channel half-width. To minimize the numerical and aliasing errors, a fourth-order compact finite-volume method in space and a fourth-order Runge-Kutta method in time along with a dynamic Smagorinsky model with explicit filter-grid size ratio 2 have been used to solve the filtered equations of the carrier flow. Heavy, small particle motion is governed by drag, gravitational, and Saffman lift forces in the Lagrangian frame. These particle equations are integrated in time using a second-order Adams-Bashforth method. The effect of subgrid-scale... 

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... 

In this paper, the numerical results of hydrodynamic modeling of primary settling tanks are presented. The flow field is assumed to be incompressible and non-buoyant. The effects of two different types of turbulence model, standard κ - ε and RNG, are compared with each other. The effects of an inlet baffle on the hydrodynamics of settling tanks are also studied. Results are obtained for the primary settling tank of the city of Sarnia, Ontario, Canada. The effects of the existence and position of another interior baffle in the settling tanks are also studied. Results in the different parts are compared with experimental and numerical data and showed good agreement. Comparison between two... 

An effective numerical technique is presented to model turbulent motion of a standing surface wave in a tank. The equations of motion for turbulent boundary layers at the solid surfaces are coupled with the potential flow in the bulk of the fluid, and a mixed BEM-finite difference technique is used to obtain the wave and boundary layer characteristics such as bed shear stress. A mixing-length theory is used for turbulence modeling. Although the technique is presented for a standing surface wave, it can be easily applied to other free surface problems. Copyright © 2005 by ASME  

The accuracy of the large-eddy simulation (LES) of turbulent flows can be increased by using high-order numerical schemes in space and time, due to a decrease in numerical errors. This work investigates a high-order compact finite-volume scheme suitable for LES. The explicit fourth-order Runge-Kutta (RK) scheme for time marching and fourth-order compact schemes for spatial derivatives using a cell-averaged approach are implemented. Different subgrid-scale models and the effect of explicit filtering in a fully turbulent channel flow are studied. In this flow, the fourth-order compact finite-volume method in space, and fourth-order RK in time in conjunction with the dynamic Smagorinsky model... 

In this study the developing turbulent swirling pipe flow is investigated both numerically and analytically. Governing equations are derived accompanying the boundary layer assumptions. Uniform and solid body rotation (SBR) distributions are taken into account for the axial and tangential velocities at the inlet of the pipe, respectively. Beyond the boundary layers, the flow pattern is considered to be the potential flow. Making use of the fourth-order Runge-Kutta scheme, the numerical solution of the differential equations is obtained. Further more, by simplifying the governing equations for large Rossby number, the analytical solution is performed. The results of numerical and analytical...