Converging-diverging transonic diffuser

Confined flows

Underlying Flow Regime 4-19

Description

Introduction

Compressible flow phenomena due to high speed and their interaction with turbulence are a very interesting scientific area of fluid mechanics. Flows with compressible characteristics are present in many internal and external mechanical design configurations such as external flows related to aeronautics, and confined/internal flows in inlets and around compressor blades in turbomachinery. Regarding the confined flows, a very interesting area of research is the experimental and computational study of compressible flows inside transonic diffusers, which is the topic of the current UFR.

The flow features that are present in transonic diffusers and characterize the flow field development can be found in supersonic inlets of air-breathing systems of missiles and aircrafts. As reported by Bogar et al. (1982), the flow patterns in the converging-diverging diffusers, occur in supercritical and subcritical inlets, in transonic airfoil flows, and in transonic compressor rotor passages. Studies of these patterns are very helpful for the design and optimization of propulsion systems such as ramjets. The basic characteristic of a transonic converging-diverging diffuser flow is the formation of a shock-wave in the ?throat? of the diffuser. The position and the intensity of the shock-wave depends on the diffuser geometry and also strongly on the inlet and outlet boundary conditions. Additionally, there is a strong interaction between the formed shock-wave and the boundary layer development inside the diffuser. Downstream of the shock-wave there is a subsonic region with adverse pressure gradient and hence a strong possibility of boundary layer separation in the diverging part of the diffuser, depending on the inlet and outlet conditions. All the above make the current UFR very interesting for developing and testing various turbulence modelling and simulation techniques (RANS and or LES). However, there is limited information in the literature regarding the behavior of the turbulent correlations and turbulent energy budgets such as turbulent production, destruction, pressure strain- correlation and Reynolds-stress quantities distributions in the shock-wave and the wall regions.

The accurate modelling of the converging-diverging transonic diffuser flow is a considerable challenge for turbulence models. The models must be capable of predicting correctly the complex flow phenomena. Additionally, the turbulence models must be able to take into account all the mean density variations that become important due to the compressibility at high speeds and properly correlate them with the mean flow characteristics of the transonic/supersonic flow.

The test case investigated in the current contribution under the framework of the described UFR is the Sajben converging-diverging diffuser. Two flow field developments are examined, one with a weak and one with a strong shock-wave formed at the transonic diffuser throat. A characteristic sketch of the Sajben converging-diverging diffuser and the relative position of the two formed shock-waves are shown in fig.3.

Figure 3: View of the Sajben converging-diverging transonic diffuser

Review of UFR studies and choice of test case

Provide a brief review of past studies of this UFR which have included test case comparisons of experimental measurements with CFD results. Identify your chosen study (or studies) on which the document will focus. State the test-case underlying the study and briefly explain how well this represents the UFR? Give reasons for this choice (e.g a well constructed test case, a recognised international comparison exercise, accurate measurements, good quality control, a rich variety of turbulence or physical models assessed etc.) . If possible, the study should be taken from established data bases. Indicate whether of not the experiments have been designed for the purpose of CFD validation (desirable but not mandatory)?

Contributed by: Z. Vlahostergios, K. Yakinthos — Aristotle University of Thessaloniki, Greece