DIRECT NUMERICAL SIMULATIONS OF NON-ISOTHERMAL AND REACTING WALL-JETS Z. Pouransari, G. Velter, D. Ahlman, G. Brethouwer and A.V. Johansson Linn´ e Flow Centre, KTH Mechanics SE-100 44 Stockholm, Sweden ABSTRACT Direct numerical simulations of plane compressible tur- bulent non-isothermal wall-jets are performed and compared to an isothermal jet. The study concerns a cold jet in a warm coflow and a warm jet in a cold coflow. The influ- ence of the varying density on the flow and scalar mixing are studied. Although the domain length is somewhat lim- ited in the simulations, the growth rate and the turbulence statistics indicate approximate self-similarity in the fully tur- bulent region. The use of van Driest scaling leads to a collapse of all mean velocity profiles in the near wall region. However, taking into account the varying density by using semi-local scaling of turbulent stresses and fluctuations does not completely eliminate differences between the statistics of the cold, isothermal and warm jet. The temperature and passive scalar dissipation time scales are similar in all cases. A direct numerical simulation of a simple reaction in a turbulent plane wall-jet with a slight coflow is also per- formed. At the jet inlet the fuel is added whereas the oxidizer is added in the coflow. The reaction time scale is finite and of the same order as the mixing time scale. As the jet propagates downstream and becomes turbulent the reaction occurs mainly in the upper shear layer, but further down- stream also in the inner layer due to the turbulent mixing. INTRODUCTION The plane wall-jet is an interesting case because the tur- bulence has different properties in the inner layer near the wall and the outer layer, and the flow can display self- similarity. Recently, Ahlman et al. (2007) studied an isothermal wall-jet by direct numerical simulations (DNS) and found similarities between the inner layer and a zero- pressure gradient boundary layer, and between the outer layer and a free shear layer. The flow statistics in the near-wall and outer region at different positions collapsed by applying inner and outer scaling respectively, which in- dicated self-similar development of the jet. We have continued the work by Ahlman et al. (2007) and carried out fully compressible DNS of non-isothermal wall-jets: a cold jet in a warm coflow and a warm jet in a cold coflow (Ahlman et al., 2009). Previous studies have mainly examined compressible effects in relatively high Mach number flows, see e.g., Coleman et al. (1995). Studies of shear flows with significant density gradient and low Mach numbers are relatively scarce. In the present study, we there- fore study non-isothermal turbulent wall-jets with low Mach numbers. The aim of the investigation is to study how the wall-jet development is influenced by the varying density. Properties of the non-isothermal jets are compared to results obtained in an isothermal jet. Proper scaling approaches in the respective inner and outer layers are investigated. The influence of the varying density on the mixing and trans- port of scalars is also studied, and the self-similarity of the velocity and scalar fields is evaluated. We have also taken the next step and studied a simple reaction in turbulent wall-jet. The flow is compressible and a single step reaction between an oxidizer and a fuel species is solved. At the inlet fuel and oxidizer enter the domain separately in a non-premixed manner. The reaction is tem- perature independent and does not release heat. Since the flow is uncoupled from the reactions, the influence of turbu- lent mixing on the reactions can be studied in the absence of temperature effects. DIRECT NUMERICAL SIMULATIONS Figure 1 shows the flow geometry and coordinate system of the plane turbulent wall-jet. The inlet of the jet has a Wall Jet inlet Coflow y x h z y x Figure 1: The plane wall-jet computational domain and co- ordinate system. height h. The flow at the inlet is parallel to the wall. No-slip conditions are used for the velocity at the bottom wall and periodic boundary conditions are used in spanwise direction. A slight coflow is used to convect persistent vortices, which can develop above the jet flow in the initial period, out the domain. At the top of the domain there is a small flow into the domain to account for the entrainment. Sixth International Symposium on Turbulence and Shear Flow Phenomena Seoul, Korea, 22-24 June 2009 947