Large Eddy Simulation of Impinging Jets

University dissertation from Stockholm : KTH

Abstract: This thesis deals with Large Eddy Simulation (LES) of impinging air jets. The impinging jet configuration features heated circular jets impinging onto a flat plate. The problem addressed here is of generic nature, with applications in many engineering devices, such as cooling of components in gas turbines, in cars and electronic devices. The flow is inherently unsteady and contains relatively slowly varying coherent structures. Therefore, LES is the method of choice when the Reynolds number is large enough to exclude Direct Numerical Simulations (DNS).The present LES model is a basic model without explicit Sub-Grid-Scale (SGS) modeling and without explicit filtering. Instead, the numerical scheme is used to account for the necessary amount of dissipation. By using the computational grid as a filter the cutoff wavenumber depends explicitly on the grid spacing. The underlying computational grid is staggered and constructed in a Cartesian coordinate system. Heat transfer is modeled by the transport equation for a passive scalar. This is possible due to the negligible influence of buoyancy which implies constant density throughout the flow field. The present method provides accurate results for simple geometries in an efficient manner.A great variety of inlet conditions have been considered in order to elucidate how the dynamics of the flow and heat transfer are affected. The considered studies include top-hat and mollified mean velocity profiles subjected to random and sinusoidal perturbations and top-hat profiles superimposed with solid body rotation. It has been found that the shape of the mean inlet velocity profile has a decisive influence on the development of the flow and scalar fields, whereas the characteristics of the imposed artificial disturbances (under consideration) have somewhat weaker effect. In order to obtain results unequivocally comparable to experimental data on turbulent impinging jets both space and time correlations of the inflow data must be considered, so also the spectral content. This is particularly important if the region of interest is close to the velocity inlet, i.e. for small nozzle-to-plate spacings. Within this work mainly small nozzle-toplate spacings are considered (within the range of 0.25 and 4 nozzle diameters), which emphasizes the importance of the inflow conditions. Thus, additional to the basic methods also turbulent inflow conditions, acquired from a precursor pipe simulation, have been examined. Both for swirling and non-swirling flows. This method emulates fully developed turbulent pipe flow conditions and is the best in the sense of being well defined, but it demands a great deal of computing power and is also rather inflexibility. In case of the basic randomly perturbed methods the top-hat approach has been found to produce results in closest agreement with those originating from turbulent inlet conditions.In the present simulations the growth of individual instability modes is clearly detected. The character of the instability is strongly influenced by the imposed boundary conditions. Due to the lack of correlation random superimposed fluctuations have only a weak influence on the developing flow field. The shape of the mean profile, on the other hand, influences both the growth rate and the frequency of the dominant modes. The top-hat profile yields a higher natural frequency than the mollified. Furthermore, for the top-hat profile coalescence of pairs of vortices takes place within the shear-layer of the axial jet, whereas for the mollified profile (for the considered degree of mollification) it takes place within the wall jet. This indicates that the transition process is delayed for smoother profiles.The amount of wall heat transfer is directly influenced by the character of the convective vortical structures. For the mollified cases wall heat transfer originates predominantly from the dynamics of discrete coherent structures. The influence from eddy structures is low and hence Reynolds analogy is applicable, at least in regions of attached flow. The top-hat and the turbulent inflow conditions yield a higher rate of incoherent small scale structures. This strongly affects the character of wall heat transfer. Also the applied level of swirl at the velocity inlet has significant influence on the rate of heat transfer. The turbulence level increases with swirl, which is positive for heat transfer, and so also the spreading of the jet. The latter effect has a negative influence on wall heat transfer, particularly in the center most regions. This however depends also on the details of the inflow data.