4/2001 Computational Fluid Dynamics

Contents:

• D.G.Gregory-Smith and S.C.Crossland, Prediction of Turbomachinery Flow Physics from CFD - Review of Recent Computations of APPACET Test Cases - abstract
• S.Kang and Ch.Hirsch, Numerical Simulation and Theoretical Analysis of the 3D Viscous Flow in Centrifugal Impellers - abstract
• S.V.Yershov and A.V.Rusanov, Numerical Simulation of 3D Viscous Turbomachinery Flow with High-Resolution ENO Scheme and Modern Turbulence Model - abstract
• F.Magagnato, The Modeling of Unsteady Turbulent Flows in Turbomachines - abstract
• V.G.Solodov, The Gas Dynamics of the Exhaust Diffusers: Computational Aspects - abstract
• S.Dykas, Numerical Calculation of the Steam Condensing Flow - abstract
• H.Thermann, S.Schmidt, C.Weiss and R.Niehuis, A CFD Package for the 3D Navier-Stokes Computation of Unsteady Flows in Turbomachines - abstract
• P.Doerffer, K.Namiesnik and F.Magagnato, Flow Simulation at Shock Wave Triple Point - abstract
• E.Tuliszka-Sznitko, C.Y.Soong, E.Serre and P.Bontoux, Instability of the Non-isothermal Flow between a Rotating and a Stationary Disks - abstract
• R.Rzadkowski and V.Gnesin, 3D Inviscid Flutter of Rotor Blades and Stator-rotor Stage - abstract
• E.Dick, J.Vierendeels, S.Serbruyns and J.Vande Voorde, Performance Prediction of Centrifugal Pumps with CFD-tools - abstract
• M.Sedlar, Numerical Investigation of Flow in Mixed-flow Pump with Volute - abstract
• A.N.Kochevsky, Investigation of Swirling Flow in Diffusers Installed at the Exit of an Axial-flow Pump - abstract

• A.Januszajtis, Scientists in Old Gdansk: Part III

Abstracts:

• D.G.Gregory-Smith and S.C.Crossland, Prediction of Turbomachinery Flow Physics from CFD - Review of Recent Computations of APPACET Test Cases

  In order to maintain a competitive edge, the turbomachinery industry has to rely increasingly on design and analysis methods based on numerical simulation of flow. The European funded APPACET was set up to study the application of turbulence modelling and the simulation of unsteady interactions to provide guidelines for the application of CFD to design. This paper shows the results of computations of four of the test cases studied by the project. The importance of grid refinement has been clearly demonstrated, but no evidence was found that one family of turbulence models could be definitely better than the other. Compared to steady results, unsteady time-averaged solutions have not shown any major improvement in accuracy. However unsteady flow phenomena generating losses within each blade row have been captured and partly explained.

• S.Kang and Ch.Hirsch, Numerical Simulation and Theoretical Analysis of the 3D Viscous Flow in Centrifugal Impellers

  This paper investigates the three-dimensional viscous flow in centrifugal impellers through theoretical analysis and numerical simulations, which is a summary of the authors' recent work. A quantitative evaluation of the different contributions to the streamwise vorticity is performed, namely, the passage vortices along the endwalls due to the flow turning; a passage vortex generated by the Coriolis forces proportional to the local loading and mainly active in the radial parts of the impeller; blade surface vortices due to the meridional curvature. In the numerical simulation the NASA Large Scale Centrifugal Compressor (LSCC) impeller with vaneless diffuser is computed at three flow rates. An advanced Navier-Stokes solver, EURANUS/TURBO is applied with an algebraic turbulence model of Badwin-Lomax and a linear k-ω model for closure, for different meshes. An in-depth validation has been performed based on the measured data. An excellent agreement is obtained for most of the data over a wide region of the flow passage. Structures of the 3D flow in the blade passage and the tip region, and their variations with flow rate as well, are analyzed based on the numerical results.

• S.V.Yershov and A.V.Rusanov, Numerical Simulation of 3D Viscous Turbomachinery Flow with High-Resolution ENO Scheme and Modern Turbulence Model

  This paper presents the basic principles of construction of numerical models for 3D viscous turbulent flows through multi stage turbomachines. The great attention is given to such properties of the methods as accuracy, linear and non-linear stability, robustness and computational efficiency. It is shown that these properties can be guaranteed if the implicit Godunov's type ENO scheme is used. A 3D code FlowER has been developed within this concept. Using the code the numerical results are obtained for flows through high loaded compressor cascades, a turbine stage, a low-pressure multi stage turbine and a centrifugal compressor stage. The results of optimisation of a low-pressure turbine last stage are presented.

• F.Magagnato, The Modeling of Unsteady Turbulent Flows in Turbomachines

  The suitability of existing models for the simulation of flow through turbomachines is investigated and compared with a recently proposed adaptive turbulence model. Discussed are the improvements in accuracy that can be achieved by using non-linear turbulence models and unsteady calculations. The adaptive turbulence model is based on two equation turbulence modeling. It uses the temporal and spatial scales of the flow field to automatically adapt itself to the unresolved turbulent fluctuations. At its asymptotic limits it reduces either to a Direct Numerical Simulation - when the turbulent scales are in the order of the Kolmogorov micro scale - or to a standard two equation model - when the fluctuations are not resolved at all. In order to compare the quality of the presented models two cases have been considered: the flow past a cylinder and a subsonic as well as transonic flow past the VKI turbine blade. Calculations have been performed for each case using all the models and the results have been compared with measurements. The unsteady calculations gave better agreement with the experimental data demonstrating the superiority over steady state calculations for turbomachines.

• V.G.Solodov, The Gas Dynamics of the Exhaust Diffusers: Computational Aspects

  Some recent advances in computational gas dynamics of exhaust diffusers are presented. The compressor/turbine axial-radial exhaust diffusers and outlet devices of compressor turbine units, exhaust hoods for power steam turbines are considered. The peculiarities of gas dynamics of the above mentioned devices are discussed. The formulations of problems for CFD analysis of these devices are discussed including the coupling procedure for joint operation of the last stage and exhaust hood. The numerical method and the implementation of a differential turbulence model are described. The new approach to the description of computational domain and parallel numerical solution of transport equations based on the unstructured set of structured grid blocks is also presented.

• S.Dykas, Numerical Calculation of the Steam Condensing Flow

  Considering the flow in the last stages of LP (low pressure) steam turbine the strong non-linearity of the thermal parameters of state and possibility of the two-phase flow have to be taken into account in the numerical calculation of the flow field. In this paper numerical calculations of the steam condensing flow for the turbine geometry are presented. The steam properties are described here on the basis of the IAPWS'97 formulation. The classical nucleation theory of Volmer and Frenkel was adapted for modelling of condensing flow. The droplet growth model of Gyarmathy is implemented. The calculations are based on the time dependent 3D Euler equations, which are coupled to three additional mass conservation equations for the liquid phase and are solved in conservative form.

• H.Thermann, S.Schmidt, C.Weiss and R.Niehuis, A CFD Package for the 3D Navier-Stokes Computation of Unsteady Flows in Turbomachines

  A 3D Navier-Stokes package for the time-accurate computation of unsteady flows in turbomachines with emphasis on wide applicability, portability and efficiency is presented. The package consists of three components: the elliptic grid generator FRAME, the parallelized implicit Reynolds- and Favre-averaged Navier-Stokes solver PANTA and the post-processor TREAT especially designed for unsteady flow phenomena. The applicability of the package covers both rotor/stator interaction and blade flutter phenomena in multirow and multipassage 2D, Quasi3D and general 3D configurations in a wide range of flow velocities (subsonic, transonic). For turbulent computations either a Low-Reynolds-Number k-ω or k-ε turbulence model is available. Additionally, an algebraic transition model can be chosen from a variety of different models to enhance the accuracy of prediction for transitional flow phenomena. A description of the underlying algorithms and numerical methods as well as the main features and characteristics of each of the three components is given. Furthermore, selected examples of typical turbomachinery applications are shown to demonstrate these features.

• P.Doerffer, K.Namiesnik and F.Magagnato, Flow Simulation at Shock Wave Triple Point

  The paper presents supersonic flow simulation results concerning the λ-foot formation in the divergent nozzle. The SPARC code was used and the vicinity of the triple point was analysed. Special boundary conditions have been used in order to obtain supersonic inlet velocity with shock wave in the divergent nozzle. It was proved that the condition of pressure equality on both sides of shear layer following the triple point for flow parameter of interest, does not hold.

• E.Tuliszka-Sznitko, C.Y.Soong, E.Serre and P.Bontoux, Instability of the Non-isothermal Flow between a Rotating and a Stationary Disks

  Both theoretical linear stability analysis and the direct numerical simulation are performed to study the transition flow between rotating discs. This paper reports about the three-dimensional spiral and annular patterns computed with a high-order (spectral) numerical method in the Bo"dewadt layer of a cylindrical rotor-stator cavity. The characteristic parameters of these boundary layer instabilities are compared with the theoretical results and interpreted in terms of type I and type II generic instabilities. The absolute instability regions are theoretically identified and the critical Reynolds numbers of the convective/absolute transition in both layers are given. The absolute or convective nature of the flows is determined by examining the branch-points singularities of the dispersion relation.

• R.Rzadkowski and V.Gnesin, 3D Inviscid Flutter of Rotor Blades and Stator-rotor Stage

  Numerical calculations of the 3D transonic flow of an ideal gas through turbomachinery rotor blade row and stator, rotor blades moving relatively one to another with taking into account the blades oscillations are presented. The approach is based on the solution of the coupled aerodynamic-structure problem for the 3D flow through the turbine stage in which fluid and dynamic equations are integrated simultaneously in time. An ideal gas flow through the mutually moving stator and rotor blades with periodicity on the whole annulus is described by the unsteady Euler conservation equations, which are integrated using the explicit monotonous finite-volume difference scheme of Godunov-Kolgan and moving hybrid H-H grid.

  The structure analysis uses the modal approach and 3D finite element model of a blade. There has been performed the calculation for the last stage of the steam turbine with rotor blades of 0.765 m.

• E.Dick, J.Vierendeels, S.Serbruyns and J.Vande Voorde, Performance Prediction of Centrifugal Pumps with CFD-tools

  The CFD-code FLUENT, version 5.4, has been used for the flow analysis of two test pumps of end-suction volute type: one of low specific speed and one of medium specific speed. For both, head as function of flow rate for constant rotational speed is known from experiments. FLUENT provides three calculation methods for analysis of turbomachinery flows: the Multiple Reference Frame method (MRF), the Mixing Plane method (MP) and the Sliding Mesh method (SM). In all three methods, the flow in the rotor is calculated in a rotating reference frame, while the flow in the stator is calculated in an absolute reference frame. In the MRF and MP methods steady flow equations are solved, while in the SM method, unsteady flow equations are solved. The SM method does not introduce physical approximations. The steady methods approximate the unsteady interaction between rotor and stator. The cost of the unsteady method is, however, typically 30 to 50 times higher than the cost of the steady methods. It is found that the MRF and MP methods lead to completely erroneous flow field predictions for flows far away from the best efficiency point. This makes the steady methods useless for general performance prediction.

• M.Sedlar, Numerical Investigation of Flow in Mixed-flow Pump with Volute

  This work deals with modelling of flow in a complete mixed-flow pump with volute, including the tip leakage flows, using both the quasi-steady and true transient models of rotor/stator interaction. The CFX-TASCflow CFD package from AEA Technology is applied to calculate flows for a wide range of flow rates from about 0.2 to 1.4 Q_opt. Fairly detailed flow structures have been predicted based on the flow rates, especially the impeller inlet recirculation and separations on the suction side of the blades for the suboptimal rates of flow, as well as strong secondary flows and separations in the volute for off-design conditions. The rotor/stator interaction influence on flow phenomena both in the impeller and volute have been investigated with very interesting results, providing a good insight into the dynamics of flow close to the volute tongue. Based on the computational results, pump performance curves (Q-H, Q-Η_h and Q-P, Η_hbeing the hydraulic efficiency) have been obtained. The data from this numerical investigation have been used to improve the inlet part of the impeller blades, especially close to the tip. The geometry modifications have resulted in reducing cavitation in the impeller as well as the noise at suboptimal rates of flow.

• A.N.Kochevsky, Investigation of Swirling Flow in Diffusers Installed at the Exit of an Axial-flow Pump

  The swirling flow in discharge diffusers with a converging rotating hub and diverging casing is modeled by reduced Reynolds equations, and a single-sweep method is used for their solution. The flow was probed at the inlet and outlet section of these diffusers under various pump capacities and, therefore, various swirl rates. The correspondence of results in the operating range of the pump was satisfactory concerning the velocity distributions in the outlet section as well as concerning the coefficient of energy losses. The characteristic curves of the pump with different internal angles of discharge diffusers are presented and the optimal internal angle is obtained.