Abstract
Accurate predictions of flow separation are important for aerospace design, flight accident avoidance, and the development of fluid mechanics. However, the complexity of the separation process makes accurate predictions challenging for all known Reynolds-averaged Navier–Stokes (RANS) methods, and the underlying mechanism of action remains unclear. This paper analyzes the specific reasons for the defective predictions of the turbulence models applied to separated flows, explores the physical properties that impact the predictions, and investigates their specific mechanisms. Taking the Menter SST and the Speziale-Sarkar–Gatski/Launder–Reece–Rodi (SSG/LRR)-ω models as representatives, three typical separated flow cases are calculated. The performance differences between the two turbulence models applied to the different separated flow calculations are then compared. Refine the vital physical properties and analyze their calculation from the basic assumptions, modeling ideas, and construction of the turbulence models. The numerical results show that the underestimation of Reynolds stress is a significant factor in the unsatisfactory prediction of separation. In the SST model, Bradshaw’s assumption imposes the turbulent energy equilibrium condition in all regions and the eddy–viscosity coefficient is underestimated, which leads to advanced separation and lagging reattachment. In the SSG/LRR-ω model, the fidelity with which the pressure–strain term is modeled is a profound factor affecting the calculation accuracy.
Highlights
Flow separation is a significant and complex problem in fluid dynamics
It would be of great benefit to the development of turbulence models to analyze the fundamental reasons for the performance differences between Reynolds–stress models (RSMs) and eddy– viscosity models (EVMs)
The separation onset calculated by the SST model is located at 75% of the chord length, whereas the SSG/LRR-ω model gives a location of 77% of the chord length and the SA model gives a location of 79% of the chord length
Summary
Flow separation is a significant and complex problem in fluid dynamics. For aircraft, excessive separation will cause lift reduction, drag increment, and even stall, which affects both flight stability and structural safety. RSMs reflect the physical mechanism of turbulence more directly and accurately They outperform EVMs when applied to separated flows, rotating flows, and corner secondary flows. Computations on the NASA Common Research Model revealed that the SST model produces large wing–root corner separation bubbles, contrary to experimental evidence, whereas the WilcoxRSM-w2006 and SSG/LRR-RSM-w2012-SD models yield very small bubbles. This is because RSMs can predict the difference among the normal stresses, but two-equation models cannot. It is hoped that this contribution will enable improvements to be made in the future
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