Revision as of 09:33, 26 July 2012

Flow in a 3D diffuser

Confined flows

Underlying Flow Regime 4-16

Description

Introduction/motivation

Configurations involving three-dimensional boundary-layer separation are among the most frequently encountered flow geometries in practice. Accordingly, the methods for simulating them have to be appropriately validated using detailed and reliable reference databases. However, the large majority of the experimental benchmarks being used for validating computational methods and turbulence models relate to two-dimensional internal flow configurations, e.g. the flow in a 2-D diffuser (e.g. Obi et al., 1993), flow over a backward-facing step and a forward-facing step, or flow over fences, ribs, 2-D hills and 2-D humps mounted on the bottom wall of a plane channel. In these examples it is assumed that the influence of the side walls (according to Bradshaw and Wong, 1972, the minimum aspect ratio — representing the ratio of the channel height to channel width — should be 1:10 in order to eliminate the influence of the side walls) is not felt at the channel midplane. Consequently, within a computational framework, the spanwise direction can be regarded as homogeneous which allows the application of periodic boundary conditions (even 2D computations when using the RANS approach). By doing so, the three‐dimensional nature of the flow is completely missed: considerable secondary motion across the inlet section of the channel induced by the Reynolds stress anisotropy — which is, as generally known, beyond the reach of the eddy-viscosity RANS model group, complex 3-D separation patterns spreading over duct corners (corner separation and corner reattachment), etc.

These circumstances were the prime motivation for the recent experimental study of the flow in a three-dimensional diffuser conducted by Cherry et al. (2008, 2009). Such a diffuser configuration is also of a high practical relevance. It mimics a diffuser situated between a compressor and the combustor chamber in a jet engine. Its task is to decelerate the flow discharging from compressor over a very short distance to the velocity field of the combustor section. Typically a uniform inlet profile over the diffuser outlet is desirable. Such a flow situation is associated by a strong pressure increase.

Review of UFR studies and choice of test case

Here some information about the objectives for investigating the flow in a 3D diffuser and an overview about the works relevant to this flow are given.

Detailed investigations of the flow separating in a three-dimensional diffuser were until recently practically non-existing. The diffuser configurations investigated intensively in the past relate mostly to two- dimensional symmetric (e.g. Xu et al., 1997) and asymmetric plane diffuser geometries, see e.g. the experimental works by Obi et al. (1993), Buice and Eaton (1996, 2000) and Gullman-Strand et al. (2004). Let us shortly recall that the "Obi-diffuser" characterized by an expansion ratio of 4.7 and an opening angle of 10° (in some earlier experimental works this value was regarded as a lower limit below which separation does not take place; this information can be of help when evaluating the computational models applied to the flow in a 3D diffuser) was the test case of the 8^th ERCOFTAC SIG15 Workshop on "Refined Turbulence Modelling", Hellsten and Rautaheimo (1999). The first study on the flow in a 3D diffuser aiming at providing a comprehensive database for turbulence model validation was provided by the Stanford University group led by John Eaton, Cherry et al. (2008, 2009). The objectives were to design a simple but rigorous test for 3D flow separation simulations with well-defined inflow and boundary conditions, to provide the fully 3D mean flow field and to examine the sensitivity of the flow pattern to small geometric changes. The measurements were performed in a recirculating water (ρ=1000 kg/m³ and μ=0.001 Pas) channel using the method of magnetic resonance velocimetry (MRV). Two three-dimensional diffusers with the same fully-developed channel inlet flow but slightly different expansion geometries were considered: the upper-wall expansion angle is reduced from 11.3° (diffuser 1) to 9° (diffuser 2) and the side- wall expansion angle is increased from 2.56° (diffuser 1) to 4° (diffuser 2), Cherry et al. (2008). See Fig. 2 and the section “Test case description” of the present contribution for the exact geometry and dimensions of the diffusers. Both diffuser flows are characterized by a three-dimensional boundary-layer separation, but the size and shape of the separation bubble exhibit a high degree of sensitivity to the geometry of the diffuser.

Figure 2: Detailed diffuser design: geometry and dimensions. From Cherry et al. (2009)

All these features represent a big challenge for computational models. Therefore, a comparative computational study on both diffuser configurations by using different turbulence statistical (RANS — Reynolds‐Averaged Navier‐Stokes) and SGS (SGS — Subgrid‐Scale models within the LES framework; LES — Large-Eddy Simulation) models was pursued in the framework of the 13^th and 14^th ERCOFTAC SIG15 Workshops on Refined Turbulence Modelling, Steiner et al. (2009) and Jakirlić et al. (2010b). In addition to different RANS models, the LES and LES-related methods (different seamless and zonal hybrid LES/RANS - HLR - models; DES — Detached Eddy Simulation) were comparatively assessed (visit www.ercoftac.org; under SIG15); the comparative analysis of selected results is presented in the section "Evaluation" of the present contribution.

Relevant studies

The flow in a 3D diffuser represents a very interesting benchmark characterized by a complex, truly three-dimensional separation pattern associated with the corner separation and the corner reattachment, both phenomena encountered frequently in various engineering applications. However, there are neither experimental nor computational studies performed in the past that one relevant to such a flow configuration. All relevant computational studies performed recently are related to previously described "Stanford 3D Diffuser".

Contributed by: Suad Jakirlić — Technische Universität Darmstadt