Large-eddy simulation of the interaction of wall jets with external stream Iftekhar Z. Naqavi a,⇑ , Paul G. Tucker a , Yan Liu b a Whittle Laboratory, Department of Engineering, Cambridge University, Cambridge, UK b Department of Energy and Power Engineering, Dalian University of Technology, Dalian 116024, China article info Article history: Received 17 September 2012 Received in revised form 8 October 2014 Accepted 14 October 2014 Available online 8 November 2014 Keywords: Wall jet Turbulent boundary layer Cutback trailing edge Film cooling Boundary layer wall jet interaction abstract Large eddy simulations are performed for a wall jet with an external stream. The external stream is in the form of a heated boundary layer. This is separated from a cold wall jet by a thin plate. The Reynolds num- ber based on the displacement thickness, for the incoming boundary layer is 2776. A series of jet velocity ratios in the range M ¼ U j =U 1 ¼ 0:30—2:30, is considered. The wall jet and outer stream velocities are U j and U 1 , respectively. The jets with M 6 1:0 develop von-Karman type shed vortices in the wake region. The higher velocity ratio jets with M > 1:0 undergo Kelvin–Helmholtz instability and develop closely spaced counter-clockwise rolling structures. These structures determine the mean flow field behaviour and near wall heat transfer. At any given streamwise location adiabatic film-cooling effectiveness for M < 1:0 increases rapidly with increasing M. For M > 1:0 it decays slowly with further increase in M. For M < 1:0 heat transfer from the hot outer stream to the wall depends on two factors; mean wall nor- mal velocity and wall normal turbulent heat flux. For M > 1:0 only a wall normal turbulent heat flux is responsible for heat transfer to the wall. The scaling behaviour shows that the near wall flow scales with wall parameters for all values of M. However, scaling in the outer region is highly dependent on M. The flow develops towards a boundary layer in the farfield for M < 1:0 and towards a wall jet for the highest velocity ratio M ¼ 2:30. Ó 2014 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/3.0/). 1. Introduction Plane, two dimensional wall jets have been studied extensively (Launder and Rodi, 1983; Schneider and Goldstein, 1994; Eriksson et al., 1998; Dejoan and Leschziner, 2005; Ahlman et al., 2007). Wall jets have a complex behaviour. The near wall region, called the inner layer, acts more like a turbulent boundary layer flow. The region away from the wall, or outer layer, acts like a free shear layer. They are also an idealised model for the outflow region of impinging jets and some meteorological phenomena (Lin and Savory, 2010). In most practical situations wall jets usually have an external stream. Bradshaw and Gee (1962) and Verhoff (1963) made early fundamental studies on wall jets with external streams. They showed that for thin incoming boundary layers with no wake, the jet shear layer absorbs the boundary layer in a short distance. How- ever, the presence of an external stream results in the involvement of several parameters. These include, the ratio of the wall jet bulk velocity, U j , and the external stream velocity U 1 , i.e. M ¼ U j =U 1 . Also, there are the thickness of the wake plate separating the two streams, incoming turbulence levels and the direction of incoming flows. These parameters determine the evolution of the wall jet. They can be controlled to produce the desired effects in wall jets, depending on their application. The two major applications of wall jets with external streams are cutback trailing edge (TE) film cooling in gas turbines and the control of the boundary layers over high lift bodies, for example, Coanda jets (Nishino et al., 2010; Dunham, 1968). In both of these cases, wall jets interact with the external stream. However, the desired outcome of the interactions are completely opposite. In the case of TE film cooling a cold stream is introduced as a wall jet along the trailing edge. The objective is to keep the external hot stream (combustion gases) away from the wall, and, hence to avoid the mixing of the two streams as far downstream as possible. For the Coanda jet, to prevent the boundary layer separation a strong mixing of two streams is required. M is usually around 1.5 or less for TE film cooling and M > 2:0 for Coanda jet based flow control (Nishino et al., 2010). In the case of TE film cooling, a major focus is the measurement and prediction of film-cooling effectiveness. Martini and Schulz (2004) performed measurements showing the importance of incoming turbulence. Recent large eddy simulations (LES) of TE http://dx.doi.org/10.1016/j.ijheatfluidflow.2014.10.014 0142-727X/Ó 2014 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/3.0/). ⇑ Corresponding author. International Journal of Heat and Fluid Flow 50 (2014) 431–444 Contents lists available at ScienceDirect International Journal of Heat and Fluid Flow journal homepage: www.elsevier.com/locate/ijhff