On the Prediction of Separation-Induced Transition by Coupling Delayed Detached-Eddy Simulation with γ-Transition Model

Abstract

Powerful hybrid RANS/LES methods are gaining more attention for the prediction of transitional turbomachinery flow although the combination of these two worlds (hybrid RANS/LES and transition) is not well considered yet. Therefore, we propose a coupling of DDES and the γ-transition model. The intention behind this coupling is explained and illustrated with two test cases, namely the academic flat plate boundary layer test case with adverse pressure gradient and the T106C turbine cascade, which represent transitional flow with separation-induced transition. The fundamental behavior of the coupled DDES-γ is assessed by the flat plate test case. Further reaching analysis is done with the turbomachinery test case T106C. Both test cases helped to understand the model coupling and yield promising results for the DDES-γ model, while we showed, that the fully-turbulent DDES failed to capture relevant features of the transitional flow for low free-stream turbulence.

Transitional Delayed Detached-Eddy Simulation for a Compressor Cascade: A Critical Assessment

“…Details can be found in [25], which also contains a comprehensive comparison with the original grid-based approach. A further analysis of different sub-grid length scale approaches for transitional flows is shown in [16]. In the following, we use Δ SLS = Δ SLA [25].…”

“…with 𝑃 𝑘,mod,trans = 𝛾𝑃 𝑘,mod and 𝐷 𝑘,mod,trans = max(0.1, 𝛾)𝐷 𝑘,mod . The additional production term 𝑃 lim 𝑘 was intended for RANS simulations to accelerate the transition process in separated shear layers [26], but needs to be eliminated for a coupled DDES-𝛾 model, as shown in [16]. Merging the destruction term of the fully turbulent DDES (cf.…”

“…We analyzed the behavior of DDES-𝛾 for separation-induced transition with canonical and turbomachinery test cases in [16]. Before we assess the compressor cascade in Sec.…”

“…Since the DDES model is mainly designed for massively separated flows, the most obvious transition type to consider is separation-induced transition. Therefore, we introduced a way to couple the DDES with the 𝛾-transition model (DDES-𝛾) and presented an assessment of the model combination for separation-induced transition [16]. Besides separation-induced transition, Mayle [17] determines bypass transition and wake-induced transition as main types occurring in turbomachinery flows, while natural transition can be neglected due to increased free-stream turbulence intensity (FSTI).…”

“…After assessing the model in cases featuring only separation-induced transition (cf. [16]), it is the next logical step, to consider cases including bypass transition. While wake-induced transition is important especially when simulating multi-row and multi-stage configurations, this will be focus of future work.…”