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  • [July 5, 2017]

    Jianjun Tao’s Group makes progress on the study of sand storm

  • As turbulent boundary-layer flows, sand storms have strong transportation ability and lead to inhalable particulate pollutions and jeopardize public health. Since the Reynolds numbers of sand storms are extremely high, most previous wind tunnel experiments and numerical simulations focused on the stationary process of particle transport and used simplified flow fields in which the influences of turbulence especially large scale coherent structures were neglected. In addition, the interaction between PM10 (particles with size less than 10 μm) and the atmospheric surface layer of hundreds meters height provides another challenging aspect.

    Prof. Jianjun Tao’s Group in the College of Engineering has cooperated with the Key Laboratory of Mechanics on Disaster and Environment in Western China, Ministry of Education of China, Lanzhou University, and just published a letter on Physics of Fluids “Very large scale motions and PM10 concentration in a high-Re Boundary layer”  29: 061701 (2017) co-authored with Professor Xiaojing Zheng, reporting the following results.


    Fig.1 A sand storm (left figure) and the Qingtu Lake Observation Array (QLOA) site (right figure), which is devoid of vegetation and located on the dry lake bed of the Qingtu Lake in Western China.

    1. The low-speed very large scale motions (VLSMs) are recurrent with a frequency dominating the streamwise velocity power spectra and modulate the streamwise distribution of PM10 concentration.
    2. As sketched in Fig. 2(d), a VLSM may be composed of several periods, reflecting to some degree its 3D meandering features, and a typical period includes a strong u (streamwise velocity perturbation) half period and a strong w (vertical velocity perturbation) half period. Considering that the PM10 emission is dominated by the shear stress on the surface and a larger streamwise velocity corresponds to a larger shear stress in the near-surface region, a positive correlation between u and c (PM10 concentration perturbation) is expected and confirmed in Fig. 2(a).
    3. In the logarithmic layer, the low-speed VLSMs correspond to weak shear stress on the bottom surface and hence decrease the PM10 upward flux. In the higher region, the high-speed VLSMs enhance both the streamwise and the upward transportation of PM10 because of the positive correlation between the velocity and the particle concentration perturbations.

    FIG. 2. The contour maps of correlation coefficient between the band-pass filtered (a) streamwise velocity perturbations u and PM10 perturbations c, (b) u and vertical velocity perturbations w, and (c) c and w with the cut-off frequencies of 0.03 Hz and 0.001 Hz. The temporal evolutions of the high-speed and the low-speed large scale perturbations are sketched in (d).

    The previous experiments and numerical simulations revealed that the streamwise and the vertical velocity perturbations u and w have a negative correlation, which is confirmed again by the present measurements as shown in Fig.2(b). Considering the positive c-u correlation in the near wall region, a ‘surprising’ switch phenomenon is illustrated in Fig.2(c) that the negative c-w correlation turns to be positive with the increase of the height because of the passive transportation manner of PM10 above the logarithmic layer.

    The first author of this letter is Dr. Guohua Wang at the Department of Mechanics and Engineering Science, Peking University, and the project is financially supported by the National Natural Science Foundation of China and the NSF Outstanding Young Researcher Foundation.