Abstr:Laminar-turbulent boundary layer transition
Underlying Flow Regime 3-04
Laminar-turbulent transition in boundary layers influences performance of many technical devices. The location of the onset and the extension of transition are of major importance since they determine drag and lift forces and heat fluxes that are crucial for an overall efficiency and performance of a variety of machinery and devices. One the most common examples of the machinery where the laminar-turbulent transition is of particular importance is turbomachinery and gas and aero-engine turbines in particular. Despite a technical maturity of gas turbines the research, optimisation and development of this technology still continues, as increasing the engine’s performance by a fraction of a percent or improving the turbine cooling in face of ever-increasing turbine inlet temperature provides enormous economic benefits. Hence the understanding of the laminar-turbulent transition in gas turbine cascades plays very important role in their optimisation (Mayle,1991).
In general there are three important transition regimes. The first is called natural transition. This transition regime can appear in practice only if free stream turbulence is very low that happens only occasionally in technical applications. Such a mode of transition begins with a weak instability in the laminar boundary layer as described first by Tollmien and Schlichting (see Schlichting, 1979) and proceeds through various stages of amplified instability to fully turbulent flow. The linear stability theory plays an important role in research and understanding of this transition regime.
The second mode, frequently called “bypass” transition following Morkovin (1969), is caused by interaction of the vortex structures in the free stream and the boundary layer and completely bypasses the Tollmien-Schlichting waves. This is a common mode of transition in the case of turbomachinery flows.
The third mode, called “separated-flow” transition following Mayle (1991), occurs in a separated boundary layer and may or may not involve the mechanism of Tollmien-Schlichting waves. This mode of transition appears in the boundary layers with strong adverse pressure gradient particularly in the compressors and low-pressure turbines (Howell and Hodson, 2001). The pressure gradient, apart from turbulence intensity, is one of the most important parameters influencing laminar-turbulent transition. The influence of the pressure gradient on the transition is presented among others by Abu-Ghannam & Shaw (1980) and Gostelow et al. (1994). Experimental study of the transition under favourable pressure gradient was in turn showed by Volino & Simon (1995, 1997).
In technical applications due to high turbulence level of the incoming flow the bypass transition is the dominant mode and hence its modelling is crucial for practice.
The LES and DNS developing rapidly during last decade still cannot be applied in practical industrial cases due to limited computer resources. However, the LES and DNS is already widely used to generate test cases in simpler geometry to verify and validate transition models. As it was already mentioned the linear methods cannot be applied to bypass transition hence the RANS methods with appropriately modelled transitional boundary layer remain the only presently applicable engineering tool to study the transitional flows. Existing turbulence models for laminar-turbulent transition boundary layer, as reviewed by Savill (1993,1996) and recently by Menter et al. (2002) are highly empirical and require experimental data for the proper calibration. Following the results of the TRANSPRETURB European Network on transition prediction the two following approaches to model bypass transition in the industrial applications can be pointed out: low-Reynolds number turbulence models (Jones & Launder, 1972, Priddin, 1975, Rodi & Scheuerer 1984, Hadzic 1999) and experimental correlations that relate the free stream turbulence intensity to the transition Reynolds number (Mayle, 1991, Abu-Ghannam & Shaw, 1980). According to Menter et al. (2002) the ability of a low-Reynolds turbulence model to predict transition seems to be coincidental, as the calibration of the damping functions is based on the viscous sublayer behaviour and not on transition from laminar to turbulent flow. Industry favours to use experimental correlations which are usually linked with a two-equation turbulence models by modification of the turbulent production term based on an intermittency equation (Suzen & Huang, 2000) or by the more complex conditioned equations (Steelant & Dick, 1996). However, Menter et al. (2002) mentioned some significant numerical stability problems related to the experimental correlation approach and proposed a new method which is based on the transport equation for the intermittency model which can be used to trigger transition locally.
Most of experimental and numerical works on the laminar-turbulent transition done in the past concerned the so called steady transition which is understood as a steadiness of the mean parameters of the free stream. However, in turbomachinery applications due to rotor-stator interaction an unsteady transition is of the fundamental importance in which the transition is governed by unsteady periodic or large scale vortex structures passing in the free stream. The review of fundamental studies of the laminar-turbulent transition induced by periodically passing wakes was presented by Alfredsson & Matsubara (1996). Direct numerical simulation (DNS) of the unsteady transition induced by the wake of periodically passing cylinders was performed by Wu et al. (1999) and compared with the experimental work of Liu & Rodi (1991). This study showed that the concept of puffs is relevant in passing wake-induced bypass transition. The puffs generated at the inlet have tendency to elongate and decay but due to an interaction with certain types of free-stream vortices these structures can be amplified and evolve into turbulent spots. Visualizations from the numerical study have been compared to liquid crystal experiments of Zhong et al. (1998) showing good agreement of geometrical characteristics of the simulated and measured puffs prior to breakdown as well as the matured turbulent spots.
The present document is focused on the steady bypass transition on the turbine blade profile N3-60. The work was performed within the TRANSPRETURB Thematic Network on Transition (EC Contract ERBICT 20-CT98-005) and the research grant funded by Polish State Committee for Scientific Research No. 7T07A007-15. The experimental data concerning steady transition as well as wake induced transition are available in the data base on the website of the TRANSPRETURB Thematic Network http://tajfun.imc.pcz.czest.pl/transpret/. The numerical simulations of the steady transition will be described and compared with experimental results in the present document. The unsteady transition induced by passing cylinders is also currently studied numerically and will be available soon.
Contributors: Andrzej Boguslawski - Technical University of Czestochowa