Simultaneous measurement of turbulent momentum and mass fluxes in channel flow of viscoelastic fluid

In this experimental study, mass transfer characteristics in a turbulent channel flow of viscoelastic fluid was investigated by simultaneous particle image velocimetry and planar laser-induced fluorescence measurements. Instantaneous dye concentration field with fluctuating velocity vector showed spatial phase modulation in the streamwise velocity fluctuation and the dye concentration for the wallnormal velocity fluctuation. In addition, it was observed that hierarchically large-scale wavy motions with inclination transfer mass. A spatial spectral analysis allowed us to understand a higher wall-normal turbulent mass flux of low wave-number modes in the log-layer takes important role in comparison with the Reynolds shear stress. The existence of this characteristic wavy motion explains why the thinner dye concentration layer enhanced the magnitude of dye concentration fluctuation. INTRODUCTION The experimental report of turbulent channel flow, a canonical flow, can be used to benchmark and fine tune numerical simulations in initial step, and so indirectly contributes to the advancement of practical applications. The experimental report needs to include not only turbulence statistics but also instantaneous velocity and/or scalar fields as one of temporal-spatial information. For this thing, there is important to obtain the qualitative nature of the turbulence structure such as the streaky structure and the quantitative characteristics of its frequency and/or wave number calculated by the spectra analysis. Great performance of the numerical simulation according to the guideline of these facts provided from the experiment leads to fulfill practical demand: a precise estimation of the wall shear stress or heat and mass transfer coefficients determined by the turbulent mixing and diffusion. Whereas a comprehensive experimental database in channel flow of Newtonian fluid is enriched by a lot of effort (e.g., Adrian et al. (2000); del Álamo & Jiménez (2003); Gnielinski (1976); Kays (2012)), this usage is unfortunately restricted for nonNewtonian fluid, especially viscoelastic fluid. Virk (1975) proposed a three-layer model of the mean velocity profile of a viscoelastic fluid flow, but this model was still not enough to adopt all cases in the experiment. This matter arose from a lack of understanding of the modulated turbulence structure appearing in this flow, and so past studies (e.g., Ptasinski et al. (2003); White & Mungal (2008)) have been mainly concentrated on an acquisition of the velocity field and turbulence statistics to get a clue. As a result, its overall picture has been gradually described. The scalar transfer closely associated with this structure is also profoundly important to underlie the calculation of transfer coefficients. Li et al. (2004) have reported the mean temperature profile by the fine-wire thermocouple for a turbulent channel flow of the viscoelastic fluid. They revealed an intensive temperature fluctuation in the log-layer with a high temperature gradient as well. As the next step it is reasonable to consider a spatial information of the scalar field in experimental report. Somandepalli et al. (2010) and Fu et al. (2014) have reported the spatial distribution of mass transport by simultaneous particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) measurements for a channel flow and for a turbulent boundary layer flow, respectively. In their studies, the injection of a viscoelastic solution including dye into the Newtonian fluid flow from one sidewall has made the complicated flow situation of coexistence of both fluids. On the one hand, it is required to provide the experimental report for the instantaneous scalar field in a turbulent channel flow of the viscoelastic fluid used in the initial step of the benchmark test. However, as far as we are aware of, there is no experimental report to deal with this. The present work performs simultaneous PIV and PLIF measurements in a turbulent channel flow of viscoelastic fluid in order to acquire the spatial scalar field. The authors have already reported fundamental turbulent statistics on velocity and concentration fluctuations (Matsumoto et al. (2017)). Following the previous result, Reynolds shear stress and wall-normal turbulent mass flux in a spectral description, which can evaluate the spatial scale of the turbu-