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- Bredberg, Jonas, 1971, et al.
(författare)
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Low-Reynolds Number Turbulence Models:An Approach for Reducing Mesh Sensitivity
- 2004
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Ingår i: Journal of Fluids Engineering, Transactions of the ASME. - : ASME International. - 1528-901X .- 0098-2202. ; 126:1, s. 14-21
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Tidskriftsartikel (refereegranskat)abstract
- This study presents a new near-wall treatment for low-Reynolds number (LRN) turbulence models that maintains accuracy in 'coarse' mesh predictions. The method is based on a thorough examination of approximations made when integrating the discretized equations in the near-wall region. A number of modifications are proposed that counteract errors introduced when an LRN-model is used on meshes for which the first interior node is located at y+≈5. Here the methodology is applied to the k-ω turbulence model by Bredberg et al., although similar corrections are relevant for all LRN models. The modified model gives asymptotically, in the sense of mesh refinement, identical results to the baseline model. For coarser meshes (y+≤10), the present method improves numerical stability with less mesh-dependency than the non-modified model. Results are included for fully developed channel flow, a backward-facing step flow and heat transfer in a periodic rib-roughened channel.
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- Bredberg, Jonas, 1971
(författare)
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Turbulence Modelling for Internal Cooling of Gas-Turbine Blades
- 2002
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Numerical simulations of geometrical configurations similar to those present in the internal cooling ducts within gas turbine blades have been performed. The flow within these channels are characterized by heat transfer enhancement ribs, sharp bends, rotation and buoyancy effects. On the basis of investigations on rib-roughened channel it is concluded that the frequently employed two-equation turbulence models (.kappa -.epsilon., .kappa.- .omega.) cannot predict heat transfer in separated regions with a correct Reynolds number dependency. Extensions to non-linear models, such as EARSM, do not alters this inaccurate tendency. The importance of the length-scale determining equation for this behaviour is discussed, however without any solution given. A low-Reynolds number (LRN) .kappa.- .omega. turbulence model, with improved heat transfer predictions, is proposed. The new model includes cross-diffusion terms which enhances free-shear flow predictability. A new method to reduce the mesh sensitivity for LRN turbulence models is proposed. Within the concept of finite volume codes it is shown that through a carefully treatment of the integrations for the first interior node, minor additions results in a significant reduced grid dependency for non-near-wall sensitive parameter. The latter modification in conjunction with the new .kappa.- .omega. turbulence model results in an accurate and robust method for simulating large and complex geometries within the frame of internal cooling of turbine blades.
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- Hellström, Fredrik, 1971-
(författare)
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Numerical computations of the unsteady flow in a radial turbine
- 2008
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Non-pulsatile and pulsatile flow in bent pipes and radial turbine has been assessed with numerical simulations. The flow field in a single bent pipe has been computed with different turbulence modelling approaches. A comparison with measured data shows that Implicit Large Eddy Simulation (ILES) gives the best agreement in terms of mean flow quantities. All computations with the different turbulence models qualitatively capture the so called Dean vortices. The Dean vortices are a pair of counter-rotating vortices that are created in the bend, due to inertial effects in combination with a radial pressure gradient. The pulsatile flow in a double bent pipe has also been considered. In the first bend, the Dean vortices are formed and in the second bend a swirling motion is created, which will together with the Dean vortices create a complex flow field downstream of the second bend. The strength of these structures will vary with the amplitude of the axial flow. For pulsatile flow, a phase shift between the velocity and the pressure occurs and the phase shift is not constant during the pulse depending on the balance between the different terms in the Navier- Stokes equations. The performance of a radial turbocharger turbine working under both non-pulsatile and pulsatile flow conditions has also been investigated by using ILES. To assess the effect of pulsatile inflow conditions on the turbine performance, three different cases have been considered with different frequencies and amplitude of the mass flow pulse and different rotational speeds of the turbine wheel. The results show that the turbine cannot be treated as being quasi-stationary; for example, the shaft power varies with varying frequency of the pulses for the same amplitude of mass flow. The pulsatile flow also implies that the incidence angle of the flow into the turbine wheel varies during the pulse. For the worst case, the relative incidence angle varies from approximately −80° to +60°. A phase shift between the pressure and the mass flow at the inlet and the shaft torque also occurs. This phase shift increases with increasing frequency, which affects the accuracy of the results from 1-D models based on turbine maps measured under non-pulsatile conditions. For a turbocharger working under internal combustion engine conditions, the flow into the turbine is pulsatile and there are also unsteady secondary flow components, depending on the geometry of the exhaust manifold situated upstream of the turbine. Therefore, the effects of different perturbations at the inflow conditions on the turbine performance have been assessed. For the different cases both turbulent fluctuations and different secondary flow structures are added to the inlet velocity. The results show that a non-disturbed inlet flow gives the best performance, while an inflow condition with a certain large scale eddy in combination with turbulence has the largest negative effect on the shaft power output.
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