Pascal Bader

Steady and Unsteady CFD Calculation of the Laminar-to-Turbulent Transition in a Turning Mid Turbine Frame With Embedded Design

In order to meet the weight requirements of future jet engines S-shaped intermediate turbine ducts can be supplied with turning load carrying struts in order to save the first vane row of the subsequent low pressure rotor.In such a duct large flow structures emanating from the outlet of the transonic high pressure stage are transported towards the low pressure rotor and are superimposed by secondary effects generated by the turning struts within the duct. To reduce the fluctuations behind the S-shaped duct and to homogenize the flow, the duct can be equipped with additional splitters. Such an embedded design has the advantage of a more homogenized flow entering the low pressure rotor, thus improving efficiency.It is crucial for the overall efficiency to optimize the design of such an intermediate turbine duct. To understand all flow effects it is helpful to perform CFD calculations which do not neglect laminar-to-turbulent transition. Therefore this paper presents steady and unsteady CFD results of an S-shaped turbine duct with splitters considering boundary layer transition using Menter’s γ-ReΘ model. The differences between steady and unsteady simulation and the influence of transition are discussed.Copyright

Flow Evolution Through a Turning Mid Turbine Frame With Embedded Design

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Flow evolution through a Turning Mid Turbine Frame with embedded design

The paper discusses the time-averaged flow of a new-concept turbine transition duct placed in a two-stage counter-rotating test turbine. As a possible architecture for the turbine transition duct o...

An Investigation of Innovative Experimental and Numerical Techniques to Detect Boundary Layer Transition

Different methods to detect boundary layer transition are investigated within the scope of this paper. Laminar and turbulent boundary layers exhibit a significantly different behavior, not only regarding skin friction but also for heat-transfer which affects the blade cooling design. The present work presents a novel and non-intrusive measurement technique to detect the transition, based on acoustic concepts. The reliability of the technique was investigated by means of boundary layer measurements over a flat plate in subsonic flow conditions. After a preliminary assessment with a conventional Preston tube, a row of microphones were installed along the plate to correlate transition pressure fluctuations. To provide a comprehensive representation of the experiment, dedicated measurements with a fast response aerodynamic pressure probe were performed to determine the turbulence intensity and the dissipation rate upstream of the flat plate. The experimental results were systematically compared with calculations performed with three different computational fluid dynamics solvers (ANSYS-Fluent, ANSYS-CFX, OpenFOAM) and using both the and the transition models. Results show a fair agreement between CFD (computational fluid dynamics) predictions and the acoustic technique, suggesting that this latter might represent an interesting alternative option for transition measurements.

Measurement and simulation of a turbulent boundary layer exposed to acceleration along a flat plate

Flow in turbomachines is generally highly turbulent. The boundary layers, however, often exhibit laminar-to-turbulent transition. But also relaminarization of the turbulent flow may occur. It is therefore important for the designer to understand the process of boundary layer transition in both directions and to determine the position of transition onset and the length of the transitional region. In the last decades several transition models have been developed to enable modern CFD codes to predict the transition and relaminarization processes. This has the advantage that the boundary layer behavior can be analyzed in advance, which enables the design of blades which trigger a ”suitable” boundary layer. But in order to use CFD for designing tasks, the code must predict accurately and reliably the transition and relaminarization processes within the boundary layer. Therefore in this work, the γ-ReΘ transition model is tested regarding relaminarization prediction. An in-house flat plate test case is analyzed where extensive measurement results are available. Since the test case shows flow pulsations caused by a separation bubble, different cases are investigated where either the full range of the measured velocity fluctuations or only the fluctuations above the integral subscale are prescribed as boundary conditions. Both boundary conditions are combined with either a steady or unsteady simulation where the latter allows to consider the velocity fluctuations additionally. Aim of this variation is to understand the influence of inlet turbulence boundary condition of the predictions of the transition model regarding relaminarization.

On the Setup of a Test Bench for Predicting Laminar-to-Turbulent Transition on a Flat Plate

At turbomachinery relevant flow conditions the boundary layers are often transitional with laminar-to-turbulent transition occurring. The characteristics of the main flow can depend highly on the state of the boundary layer. Therefore it can be vitally important for the designer to understand the process of laminar-to-turbulent transition and to determine the position and length of the transitional region. In this paper the flow over a flat plate is experimentally studied in order to investigate and better understand transitional flow. Preston tube measurements as well as a thermographic camera system were performed for two different inlet velocities in order to determine the position of the transitional zone. The results of the experiment are compared to numerical flow solutions using a common transition model to determine its capability. The simulation has been performed with the two commercial codes CFX and Fluent by Ansys and an in-house code called LINARS. As a result of this study, a better understanding of the experimental and numerical methods for determining transition shall be given.