Due to operation at low Reynolds numbers, low pressure turbines of aircraft engines mostly show large laminar boundary layers and transitional separation bubbles which considerably change their viscous losses when interacting with impinging wakes. The change of loss depends on several wake parameters, among others on wake passing frequency and wake orientation. In the present work, these parameters are expressed in terms of Strouhal number and flow coefficient and their influence is investigated by means of unsteady Reynolds-averaged Navier-Stokes (RANS) simulations. Different combinations of both wake parameters which are typical of aircraft engine conditions, are prescribed upstream of a high lift turbine cascade, while the Reynolds number and Mach number are kept constant. The solver TRACE by DLR and MTU Aero Engines together with the γ − Re Θ transition model by Langtry and Menter has been used. Further, the wake profile is representative for upstream turbine profiles and is prescribed by a correlation framework which has been calibrated in previous work. A newly developed quasi-unsteady wake model (QUWM) is applied in order to model the effects of periodically passing wakes in steady state simulations involving mixing plane interfaces. It is shown that the gap between unsteady and steady state simulations is narrowed significantly by the QUWM while still maintaining quick turnaround times that are crucial in industrial flow solver applications.
As low pressure turbine blade rows can operate at fairly low Reynolds numbers, long laminar boundary layers and transitional separation bubbles are not unusual in conventional blade design. Upstream wakes interact with the boundary layer in multistage low pressure turbine applications in a way that can alter the engine performance. In order to provide quick turnaround times, industrial applications mainly involve steady state computations which do not accurately model the unsteady wake effects. In this study, a low-pressure turbine cascade with periodic unsteady inflow as well as a two-stage low pressure turbine are computed at various Reynolds numbers using unsteady and steady RANS methods in the flow solver TRACE by DLR and MTU Aero Engines AG. A quasi-unsteady wake model working together with the γ – ReΘt model by Langtry and Menter is applied to the steady state simulation in another step in order to improve the prediction accuracy of low cost simulations utilizing mixing plane interfaces. The steady state results with and without wake model are compared to the time-resolved reference solution and the computation time is evaluated in order to show that the additional model is able to improve steady state mixing plane simulation results without sacrificing the low computational effort provided by the status quo.
Depending on the manufacturing process and the operating conditions, airfoil surface may show many different roughness characteristics, together with a significant influence at the boundary layer development. In this paper, only the influence at the laminar-turbulent transition is considered and modeled by an extension of the γ-ReΘ transition model of Langtry and Menter. Essentially, an additional transport equation for a roughness amplification scalar is used to modify the transition onset and development, as already presented by Dassler et al. in 2012. In the meantime, the calibrated functional relationship for Argr has been released for the publication for the first time in this paper. In addition, the model performance will be demonstrated and discussed on test cases with increasing complexity, including turbomachinery cascades and rigs. As an input to the model, the equivalent sand-grain roughness is required. In this way, the versatile roughness characteristics of the investigated surface are reduced to only one parameter. The model has been implemented into the CFD software package TRACE of DLR Institute of Propulsion Technology. Only steady flow test cases have been investigated and validated. The transition intermittency is coupled to the two-equation turbulence model of Wilcox. In this model, the roughness influence at the fully turbulent boundary layers is also captured by the variation of the boundary condition for the specific turbulence dissipation rate ω.
Due to relative motion between rotors and stators in aircraft engines, periodic wakes are present in downstream blade rows, which exert significant influence at flow loss and engine efficiency. To quantify and reproduce this influence, a low-pressure turbine cascade is computed using steady and unsteady RANS methods in the flow solver TRACE by DLR and MTU Aero Engines. A thorough grid study is carried out and various aspects of grid resolution requirements are investigated for the setups and considered performance metrics respectively. A steady state transition model extension that has been developed and published by the authors is applied to the cascade flow at a number of operating points and validated with experimental data while being compared to the unsteady results as well as the steady state results without the wake effect extension. Further, a variation of wake-related parameters is carried out while discussing the effects modeled in the unsteady setup as well as the ability of the two steady state setups (with and without wake extension) to capture the trends identified by the unsteady results. A sufficiently accurate reproduction by the wake model extension enables steady simulations of the inherently unsteady effects in the aerodynamic design of the turbine, which results in an enormous saving of computational time and effort.
In external and internal fluid flow analysis using numerical methods, most attention is paid to the properties of the flow assuming absolute rigidity of the solid bodies involved. However, this is often not the case for water flow or other fluids with high density. The pressure forces cause the geometry to deform which in turn changes the flow properties around it. Thus, a one-way and two-way Fluid-Structure Interaction (FSI) coupling is proposed and compared to a CFD analysis of a windsurfing fin in order to quantify the differences in performance data as well as the properties of the flow. This leads to information about the necessity of the use of FSI in comparison to regular CFD analysis and gives indication of the value of the enhanced results of the deformable analysis applied to water flow around an elastically deformable hydrofoil under different angles of attack. The performance data and flow property evaluation is done in ANSYS Fluent using the k-ω SST and k-ε model with a y+ of 1 and 35 respectively in order to be able to compare the behavior of both turbulence models. It is found that the overall lift coefficient in general is lower and that the flow is less turbulent because of softer transition due to the deformed geometry reducing drag forces. It is also found that the deformation of the tip of the hydrofoil leads to vertical lift forces. For the FSI analysis, one-way and two-way coupling were incorporated leading to the ability to compare results. It has been found that one-way coupling is sufficient as long as there is no stall present at any time.
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