Validation of Aerodynamic Non-conformance Definitions

Abstract: Non-conformances are effects related to the difference between the nominal design of an aircraft engine component and the finished manufactured product. At the aerothermodynamics department at Volvo Aero, a number of definitions are used to classify the non-conformances and their impact on the engine performance.

The main objective of this thesis has been to validate the defined definitions limits for local non-conformances (bumps) positioned on the outlet guide vanes of a turbine rear frame, using CFD, and derive a correlation for calculation the drag coefficient of the bumps. The project was divided into two parts; a flat plate analysis and a real geometry analysis.

The definition of a local non-conformance is based on the height of the bumps in relation to the boundary layer thickness at that location. The flow over a flat plate has been studied with and without bumps at a wide range of Reynolds numbers to see how different bump sizes affects the shape and size of the boundary layer. The added drag to the plate due to the presence of the bumps has been calculated and compared to the bump-free cases to see if a correlation was possible to derive.

From the flat plate simulations it was found that the lower limit of 10 % and the upper limit of 99 % of the defined borders are valid. The lower limit can however be rectified due to an increase of just 11 % of the boundary layer thickness for bumps with a height of 40 %. A correlation was derived that calculates the drag coefficient of the bumps with an error of ± 5 % between the correlation calculation and the CFD results.

The real geometries that were analyzed were representative of the regular vanes and mount vanes of a turbine rear frame. The boundary layer thickness has been calculated for both nominal vanes and for vanes with non-conformances (bumps) to determine the effect of the bumps on the boundary layer and if it’s possible to compare the results with the flat plate.

The boundary layer thickness on the suction peak was found to be 3.1 mm on the regular vane and 3.3 mm on the mount vane. However, the method used for calculating the boundary layer thickness was found to be unstable when the flow over the vane separates. The only cases that are separation free are the 1 mm bumps, which are located at a height of 32 % of the nominal boundary layer thickness on the regular vane and 30 % on the mount vane. The increase in boundary layer thickness differs from the flat plate results and a detailed analysis on how the thickness is calculated needs to be performed. The correlation was tested on the 1 mm bumps and the drag coefficient calculated to be 0.285 on the regular vane and 0.275 on the mount vane. This can be compared to a drag coefficient of 0.25 calculated at the department using a similar geometry and another method. However, the correlation needs to be compared with other real geometry bump sizes to be considered fully validated.