Unsteadiness is one of the main characteristics in turbomachinery flows. Local unsteady changes in static pressure must exist within a turbo-machine in order for that machine to exchange energy with the fluid. The primary reason for unsteady effects lies in the interaction between moving and stationary blade rows. The industrial design process of aero-engines and gas turbines is still based on Reynolds-averaged Navier-Stokes (RANS) techniques where the coupling of blade rows is carried out by mixing-planes. However, this methodology does not cover deterministic unsteadiness in an adequate way. For standard aero-optimization, detailed unsteadiness is not essential to the designer of turbomachines but rather its effect on the time averaged solution. The time averaged deterministic unsteadiness can be expressed in terms of deterministic stresses. The present paper presents two different modeling strategies for deterministic stresses that constitute an improvement of the conventional steady mixing-plane approach. Whilst one of the presented models operates with deterministic flux terms based on preliminary unsteady simulations, the other one, a novel transport model for deterministic stress, is a stand-alone approach based on empirical correlations and a wide range of numerical experiments. A 4.5 stage transonic compressor is analyzed regarding blade row interaction effects and their impact on the time averaged solution. The two models are applied to the compressor and their solutions are compared to conventional mixing-plane, time accurate and experimental data. The results for the speedline, the wake shapes, the radial distributions and the rotor blade loadings show that the deterministic stress models strongly improve the RANS solution towards the time accurate and the experimental methods.

The unsteady interaction between blade rows is very important in highly loaded compressors because of its influence on operating performance. One important effect in this context is the impact of a rotor bow shock on the wake of an upstream stator blade. A new transport model is proposed which introduces such deterministic unsteady effects in a steady solution environment. Deterministic stresses are added to the stationary RANS equations by means of an additional source term. The presented approach combines the advantages of time-accurate and stationary simulation procedures, i.e. physical accuracy and computational efficiency. A generic cascade of flat plates and a transonic stator-rotor configuration are investigated numerically using time-accurate methods in order to analyze the wake-shock interactions. The results are compared with steady mixing-plane solutions to point out their shortcomings regarding unsteady effects and to illustrate the demands of a deterministic stress approach. The model is then calibrated for the generic cascade before it is applied to the real three-dimensional compressor stage.

Automated CFD-based optimization procedures have become an essential part of modern aerodynamic compressor design. Although time-accurate CFD provides a higher physical accuracy, due to limited resources still mainly steady state CFD is used. With a constantly growing computing power the question arises, whether it is worth it increasing the computing effort per evaluation using more accurate CFD codes, in order to improve the optimization results. This work investigates how the results of an automated aerodynamic compressor optimization depend on the simulation procedure used to calculate the flow solutions during the optimization. Two configurations of a counter-rotating fan stage with different axial inter-blade spacing have been optimised using a Q3D approach for the midspan airfoil sections. The configurations were chosen, as to represent the two possibilities of low and high unsteady flow interaction between the blade rows. In each case two automated optimizations have been performed. One based on a steady simulation procedure (RANS), the other on a time-accurate (URANS). In addition, the configuration with low axial spacing has been optimized using a RANS procedure with a determinsitic stress model (RANS-DS). A dependency of the optimization results on the CFD method used has been observed for cases showing high unsteady interaction between the blade rows. The best optimization results were obtained using a time-accurate URANS CFD-solver. A comparison between RANS and RANS-DS showed an advantage of using RANS-DS.

In this paper the influence of a non-symmetrical sidewall-contour on the flow field of an axial compressor is investigated. In earlier studies, carried out at the DLR Institute of Propulsion Technology in Cologne, it has been shown that it is possible to influence secondary flow regimes by a non-symmetrical sidewall contour. With the help of the contour a vortex is generated that serves as an aerodynamic barrier and thus reduces the flow transport orthogonal to the main flow direction. In this paper the described method of flow manipulation is investigated for the compressor rotor at the hub wall. It turns out that despite the unsteady inflow conditions the vortex is stably generated and works as an aerodynamic separator. The rotor hub region increases its flow capacity. A higher pressure ratio is achieved at a highly loaded operating point. The efficiency at this point remains unchanged. List of Symbols and Abbreviations m M-relative mass flow, y +-dimensionless wall distance, η-isentropic efficiency, π-total pressure ratio, ω-vorticity , BPrVar-back pressure variation, MVDRmeridional velocity density rate, OP-operating point, PR-pressure ratio, TVD-Total Variation Diminishing