Virtually all turbomachinery flows involve turbulent phenomena, including transition from laminar to turbulent states.Thus, the adequate representation of transition and turbulence is crucial for the predictive accuracy of the computational method used in the turbomachinery design process.
The full or even partial resolution of turbulent structures by Direct Numerical Simulation (DNS) and Large-Eddy Simulation (LES), respectively, - see (Link, Numerical Test Rig) – is currently outside of the realm of industrial applications for Reynolds numbers typically found in turbomachinery aerodynamics. Hence, the solution of the Reynolds-Averaged Navier-Stokes (RANS) equations forms the backbone for the numerical simulation of flows of industrial relevance. However, modelling assumptions of some kind have to be made, where transition and turbulence are addressed separately. While turbulence modelling is based on statistics, transition modelling heavily relies on empirical correlations.
Flows in multistage turbomachinery are inherently unsteady. However, while unsteady simulations are of ever-increasing importance, steady RANS computations will remain the workhorse of industrial CFD in the near future. Solving the RANS equations, a closure is needed for the Reynolds stresses. A variety of RANS methods exist, ranging from simple algebraic models up to Differential Reynolds Stress Models (DRSM). To date, however, transport-equation Eddy-Viscosity Models (EVM) remain the backbone of turbulent viscous computations in the industrial design process. These models, however, have difficulties predicting e.g. pressure-induced separation and recirculation, secondary flows, shock-induced separation and shock/boundary-layer interaction.
Several approaches for turbulence representation are followed up in TRACE:
Even though the flow in a turbomachine is turbulent most of the time, it is very important to accurately predict where transition from laminar to turbulent flow takes place. The state of a boundary layer - laminar, transitional or turbulent - has a great impact on its properties, such as frictional losses or heat transfer in the fluid. Depending on the geometry of the component and the flow conditions, this transition process can happen in different places and follow different patterns or modes.
TRACE can use two different models to predict transition: An algebraic model (Multimode model), which uses integral values of the boundary layer to predict the different transition modes and a local, transport-equation-based model (ɣ-ReӨ model) which uses local values of the flow. The models are coupled to the source mechanisms of the turbulence model and thus allow transport of turbulent quantities even in areas where flow is predicted to be laminar. Extensions for the ɣ-ReӨ model in combination with two-equation-models are continuously developed by our university partner (Institute of Turbomachinery and Fluid Dynamics, Hannover). These extend the model with prediction capability of transition due to cross flow effects, on end walls or due to wakes of upstream blade rows.