摘要:

A computational finite-volume scheme, based on the structured-grid multi-block approach and incorporating a new variant from the group of low-Reynolds-number k — ϵ models, is applied to the transitional flow in a 3D turbine cascade. The turbulence model is designed, by reference to a particular one-equation model, to give the correct variation of turbulence length scale as the wall is approached. Its performance characteristics are first demonstrated through calculations for a transitional flat-plate boundary layer, prior to its application to the 3D turbine cascade. The characteristics of the cascade flow are examined in detail, especially in terms of the structure of the passage vortex and losses arising from turbulence. The latter are shown to be over-estimated relative to the experimental levels due to a premature onset of transition in the blade's boundary layers, especially near the end wall.

ISSN:1061-8562    

DOI:10.1080/10618569908940812

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

We consider a classical aerodynamic shape optimization problem subject to the compressible Euler flow equations. The gradient of the cost functional with respect to the shape variables is derived with the adjoint method at the continuous level. The Hessian (second order derivative of Ihe cost functional with respect to the shape variables) is approximated also at the continuous level, as first introduced by Arian and Ta'asan [1], The approximation of the Hessian is used to approximate the Newton step which is essential to accelerate the numerical solution of the optimization problem. The design space is discretized in the maximum dimension, i.e., the location of each point on the intersection of the computational mesh with the airfoil is taken to be an independent design variable. We give numerical examples for 86 design variables in two different flow speeds and achieve an order of magnitude reduction in the cost functional at a computational effort of a full solution of the analysis partial differential equation (PDE).

ISSN:1061-8562    

DOI:10.1080/10618569908940813

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

This paper presents a numerical method to identify the friction coefficient in natural free surface flows, based on the optimal control theory. A Lagrangian operator is introduced whose partial derivatives provide state equations, adjoint equations and the optimality condition. A stable finite element method is used for space discretization, and a solution algorithm along with a continuation procedure is proposed and tested for steady and transient shallow-water flows. This algorithm seems robust and converges for realistic initial trials as well as for a large number of parameters.

ISSN:1061-8562    

DOI:10.1080/10618569908940814

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The flow field generated by the impingement of a della-wing vortex on a plate is examined computationally. The flow is simulated by solving the unsteady, three-dimensional Navier-Slokes equations on an overset grid system using a time accurate, implicit Beam and Warming algorithm. Comparison of the computed solutions for two levels of mesh resolution indicates that no additional flow features appear with grid refinement. Both the mean and unsteady flow structures are examined. Over the delta wing the (low exhibits a spiral vortex breakdown induced by the plate. Underneath the plate a highly unsteady, large-scale (owl-type) stall region is formed and results in the shedding of hairpin-like vortical structures. On the top surface of the plate a shallow separation region also exists outboard of the vortex impingement location. These separated flow features result from the spanwise variation in effective angle of attack created by the incoming vortex system. Also present over the upper surface is a mean longitudinal vonical structure arising from the passage of segments of the spiral filament. The frequency of the surface pressure fluctuations at a point on the plate leading edge that corresponds to the spiral mode of breakdown is found to be in agreement with experimental measurements. The mutual interaction between the breakdown and the stalled flow is explored. A pronounced sensitivity of breakdown location to the degree of obstruction created by the plate separation is found. This feedback effect might suggest possible flow control strategies.

ISSN:1061-8562    

DOI:10.1080/10618569908940815

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Differential second-moment (Reynolds-stress) turbulence closure models (DSM) have long been expected to replace the currently popular two-equation k — e and similar eddy viscosity models (EVM) as the industrial standard for Computational Fluid Dynamics (CFD). Yet, despite almost three decades of development and indisputable progress, only a few commercial CFD vendors offer DSM as a modelling option. Even fewer industrial users recognize the natural superiority of the DSM. These models, used and researched mainly within academic community, are still viewed as a development target rather than as a proven and mature technique for solving complex flow phenomena. This paper gives an overview of the rationale for employing more advanced models for the computation of complex flows and transport processes. It also discusses reasons for their slow adoption by the CFD community. Physical arguments are briefly given; these illustrate a higher degree of exactness inherent in the second-moment closure approach. The superiority of these models is demonstrated by a series of computational examples, provided by author's co-workers who used either the same or very similar computational methods and model(s). Examples include several nonequilibrium flows, attached and with separation and reattachment, flow impingement and stagnation, longitudinal vortices, secondary motion, swirl, system rotation. The modelling of molecular effects, both near and away from a solid wall and associated laminar-lo-turbulent and reverse transition are also discussed in view of the need for an advanced closure approach particularly when wall phenomena are in focus. Numerical aspects associated with the application of second-moment closure are then discussed, together with current practice used to overcome numerical problems and to reconcile the need for advanced models with unavoidably increasing computational challenge. Several examples related to the automotive industry illustrate the applicability of DSM to real complex flows which have industrial relevance.

ISSN:1061-8562    

DOI:10.1080/10618569908940816

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor