پراكنش هواويزها در يك جريان تلاطمي

Dispersion of suspended aerosols in a turbulent flow

در اين پژوهش با نگاهي جديد رفتار هواويزهاي معلق در يك شاره متلاطم بررسي مي شود. در اين راه حركت متلاطم يك شاره در يك مجراي مكعب‌مستطيلي با شرايط مرزي غيرلغزشي روي ديواره هاي آن به روش شبيه‌سازي عددي مستقيم شبيه‌سازي شد. در ادامه مسيرهاي حركت دو مجموعه هواويزها با عددهاي استوكس 5 و 25 با رويكرد لاگرانژي مسيريابي شدند. با توجه به ابعاد و جرم هواويزهاي مورد بررسي، تنها نيروي موثر در حركت هواويزها، نيروي كشندگي شاره است و از نيروي براوني صرف‌نظر مي شود. با بررسي شدت شارهاي گوناگون هواويزها مشاهده شد كه شار تلاطم زدا و شار پخش تلاطمي فرايندهاي اصلي پراكنش هواويزهاي معلق در حركت متلاطم يك شاره است و شكل نمايه غلظت نيز از تقابل اين دو شار حاصل شده است. همچنين ديده شد كه شدت شار تلاطم زداي هواويزهاي كوچك‌تر، از شدت شار هواويزهاي بزرگ‌تر بيشتر است. اين در حالي است كه براي شار پخش تلاطمي، وارون اين نامساوي مشاهده مي شود. مقايسه نمايه هاي سرعت هواويزها و شاره حامل نيز نشان مي دهد كه هواويزهاي ناحيه نزديك ديوار از شاره حامل شان سرعت بيشتري دارند. اين در حالي است كه سرعت هواويزها در ناحيه مركزي مجرا از سرعت شاره حامل كمتر است. تفاوت سرعت هواويزها و شاره حامل از مهاجرت عرضي هواويزها يا همان شار تلاطم زدا ناشي مي شود.

In this study, the dispersion mechanisms of aerosols suspended in a turbulent plane channel flow is investigated using a novel numerical approach. A turbulent channel flow is simulated by a Direct Numerical Simulation (DNS) method, for which no-slip boundary conditions are assumed at the top and bottom walls, while periodicity conditions are applied on the other sides. DNS, in particular, allows a detailed analysis of the near wall region, where most of the particle transfer mechanisms take place. Hence, it is found the best simulating method for detailed analyzing the dispersion mechanisms compared to the other available methods. The simulation procedure of the turbulent flow is continued as along as enough, 14000 time units, when fully developed turbulent condition are achieved. The aerosols with two Stokes number, 15 and 25, are then introduced in the simulated turbulent channel flow, and tracked by a Lagrangian approach. The drag force compared to the effect of Brownian motion is a dominant force due to the aerosols size. The initial concentration of suspended aerosols is also assumed considerably low, so that the simulations conducted under the one-way coupling condition. Besides, the collisions of aerosols with the walls are assumed elastically. The particle tracking was continued throughout the fluid simulation time to obtain the all reliable interesting statistics. Comparison of the particle flux intensities indicates that turbophoretic and turbulent diffusion fluxes are the dominant dispersion mechanisms. In other words, the free-flight flux can be neglected in comparison with the other fluxes in the wall region. The steady-state concentration distribution is not uniform across the channel, primarily due to the opposing actions of the turbophoretic and turbulent diffusion flux. Turbulent diffusion flux separated the aerosols from the core and gathered them in the near wall region, while the turbophoretic flux migrate the particles from the near wall to the wall region. It was also observed that the turbophoretic flux for smaller aerosols is more efficient than that of larger ones. However, the opposite was observed for the turbulent diffusions flux. The smaller particles were less gathered in the near wall region due to a stronger turbulent diffusion flux and more migrated to the wall region due to stronger turbophoretic flux. We also investigated the cross channel fluid and particles velocity profiles. It was shown that the aerosol velocity components lag the fluid velocities in the near wall, but lead it in the core region. This is due to the transverse migration of aerosols across the channel.