Transition Models for Turbomachinery Boundary Layer Flows: A Review

Abstract

Current models for transition in turbomachinery boundary layer flows are reviewed. The basic physical mechanisms of transition processes and the way these processes are expressed by model ingredients are discussed. The fundamentals of models are described as far as possible, with a common structure of the equations and with emphasis on the similarities between the models. Tests of models reported in the literature are summarized and our own test is added. A conclusion on the performance of models is formulated.Keywords: turbomachinery flows; transition models; boundary layers; bypass transition; separationinduced transition; wake-induced transition; Reynolds-averaged Navier-Stokes; intermittency; laminar kinetic energy Transition MechanismsWith the objective of modelling with Reynolds-averaged Navier-Stokes (RANS) or URANS (unsteady or time-accurate RANS) description of a flow, generally, four types of transition from laminar state to turbulent state in turbomachinery boundary layer flows are distinguished [1,2]. These are: natural transition in attached boundary layer state, bypass transition in attached boundary layer state under a statistically steady mean flow, separation-induced transition in the free shear layer formed by separation of a boundary layer under a statistically steady mean flow, and wake-induced transition due to periodically unsteady impact of wakes on boundary layers in attached or separated state. Despite the vast amount of studies on these transition types during the last 4 or 5 decades, understanding of the mechanisms is still not complete, although large progress has been made in the last decade thanks to direct numerical simulation (DNS) and large-eddy simulation (LES). Hereafter, we summarise the mean mechanisms, taking into account the inherent limitations of modelling with Reynolds-averaged Navier-Stokes description of a flow. This means that secondary effects, which mostly are the least understood and sometimes still are a matter of controversy, are not discussed, because, anyhow these features cannot be represented by RANS or URANS. Natural TransitionUnder a statistically steady mean flow of low turbulence level, transition in an attached boundary layer is initiated by 2D viscosity-dependent Tollmien-Schlichting instability waves, followed by a 3D instability, leading to formation of spanwise periodic hairpin vortices, which, farther downstream, cause breakdown of the laminar layer with generation of turbulent spots. These spots finally merge resulting in the formation of a turbulent boundary layer [1,2]. This type of transition is called natural transition. It is a rather slow process and, under a very low mean-flow turbulence level, as in external Int.

Numerical Study of the Effect of Winglets on the Performance of a Straight Blade Darrieus Water Turbine

“…It adopts the value of 1 in fully turbulent flow and 0 in laminar flow. In the locations of the boundary layer where turbulent structures start to be visible, intermittency begins to deviate from zero [51]. Together with an empirical transport equation for the transition momentum-thickness Reynolds number, the transition features of boundary layer are modeled [43].…”

“…Energies 2017, 10, x FOR PEER REVIEW 18 of 24 intermittency begins to deviate from zero [51]. Together with an empirical transport equation for the transition momentum-thickness Reynolds number, the transition features of boundary layer are modeled [43].…”

On the study of transitional low-Reynolds number flows over airfoils operating at high angles of attack and their prediction using transitional turbulence models

“…Piotrowski et al [229,213] compared the PUIM [172,173] and the γ model of Lodefier & Dick [9] with the γ − Re θ model for wake-induced transition on the N3-60 cascade, which showed satisfactory results. Dick & Kubacki [167] compared the γ model, the k − k L − ω, Kubacki et al's model [182] and Kubacki & Dick's model [191] for the same case. They showed that the γ model performed best, since wakeinduced transition is dominated by the bypass transition, a conclusion similar to the one drawn by Pacciani et al [186].…”

Transition turbulence model calibration for wind turbine airfoil characterization through the use of a Micro-Genetic Algorithm

“…The causes leading to the onset of the transition have not yet been fully clarified. However, four types of transition mechanisms have been recognized as the main factors responsible for the first order effects [37]. From an engineering point of view these explanations are more than sufficient and allow for a good comprehension and modeling of the phenomenon.…”

“…From an engineering point of view these explanations are more than sufficient and allow for a good comprehension and modeling of the phenomenon. The types of mechanisms are the natural transition, the bypass transition, the separation-induced transition and the wake induced transition [37,38]. It is obvious that an accurate transition model must take into account all the aforementioned mechanisms.…”