Coherent thermal structures in a turbulent channel flow

The thermal patterns that arise in a channel flow are analyzed experimentally. The experimental measurements are carried on by means of a non steady application of the Heated Thin Foil technique and by scanning the wall temperature by an infrared camera. The data processing is made by means of algorithms similar to the ones used in Particle Image Velocimetry. Different test conditions were performed and a comparison between them is presented, showing temperature maps, velocity histograms and autocorrelation coefficients. 1. Introduction In a turbulent boundary layer the kinetic energy of the free stream is converted in turbulent fluctuations and then, on account of the viscous effects, dissipated in internal energy. In the absence of particular stabilizing effects this procedure is continuous, i.e. a turbulent boundary layer is self-sustained. The study of coherent and periodic structures is of fundamental importance in the understanding of the dynamics of turbulent boundary layers and, in particular, of the mixing in wall flows. A great part of the published investigations, that deals with coherent structures, are performed at low Reynolds numbers, where the range of turbulent length scales is limited, thus making easier both the numerical and experimental testing [1]. Various studies have shown that the near wall region has a complex structure due to the bursting phenomenon. Indeed the bursting is the major responsible for the production of kinetic energy in the inner layer and, hence, for the increase of the diffusive phenomena. Flow visualization experiments have shown that the bursts manifest themselves as strong perturbation of the wall region accompanied by intensive motion of the fluid away from the wall. Such motion seems to be a result of a powerful interaction of large-scale coherent structures with the elongated structures existing nearby the wall. The large-scale structures of the external layer generate fluctuations of pressure that propagate into the boundary layer towards the wall. These pressure fluctuations cause a deformation of the elongated structures near the wall that first break up and, then, eject low velocity fluid from the wall region, which is replaced by high velocity fluid from the external region. Where bursting occurs, the width of the viscous sublayer decreases by increasing the transport efficiency of passive scalars and, in particular, of internal energy. Whilst in external flows many different types of coherent structures exist, in channel flows the major part of the structures is streamwise elongated (streaks). The formation of these structures is due to the action of streamwise aligned vortices that cause a transfer of low momentum fluid from the wall to the center of the channel and vice versa for high momentum fluid. The presence of these structures influences the turbulent statistics and, in particular, the spanwise correlation of the streamwise velocity component [2]. A passive scalar, and especially internal energy (or, what is the same in the present conditions, temperature), has a behavior practically identical to that of the velocity filed. It is thus expected that the thermal structures and statistics should behave in a very similar way. Really by using both numerical [3] and experimental methods [4] previous authors have already found thermal structures