关于新冠肺炎中流体力学问题的思考.pdf

NI RESEARCH GROUP 1 NI RESEARCH GROUP 2 Fluid Mechanics Applications: 1. Airborne transmission 2. Ventilation machine 3. Masks 4. Fogging machine 5. Hand wash NI RESEARCH GROUP Fluid Mechanics Problems: 1. Instability 2. Convection 3. Multiphase flow 4. Unsteady flow 3 NI RESEARCH GROUP 4 Two software packages were used for CFD, Fluent (Fluent) and Airpak (Fluent). Droplet nuclei were simulated as passive scalars, and the deposition effect was therefore neglected. NI RESEARCH GROUP 5 NI RESEARCH GROUP 6 Airborne transmission is possible. NI RESEARCH GROUP 7 NI RESEARCH GROUP 8 1944 NI RESEARCH GROUP 9 2009 NI RESEARCH GROUP 10 NI RESEARCH GROUP 11 2013 NI RESEARCH GROUP 12 Problems COVID-19 NI RESEARCH GROUP 13 View area? Spatial Resolution? Sneeze or cough? NI RESEARCH GROUP 14 Infection rate ! " NI RESEARCH GROUP 15 ! Numbers "(', () "# , % *+ An oral fluid average virus RNA load of 7 × 106 copies per milliliter (maximum of 2.35 × 109 copies per milliliter) (Wölfel et al.Nature 2020) A 50-μm-diameter droplet contains at least one virion is ∼37%. For a 10-μm droplet, this probability drops to 0.37%, and the probability that it contains more than one virion. NI RESEARCH GROUP '= 16 C(L,D) Multiphase turbulent puff NI RESEARCH GROUP 1. Stokes number effect Preferential concentration 2. Size-dependent segregation 3. Convection Aliseda et al. 2002 17 C(D)~exp(-t) Stokes drag: t=1.13 min for 12 um t=0.046 min for 21 um t=10.0 min for 4 um NI RESEARCH GROUP 18 Multiphase flow with interfacial mass transfer Evaporation and dispersion of respiratory droplets from coughing Kelvin eﬀect Equilibrium size !" !# NI RESEARCH GROUP 19 Large RH Small RH NI RESEARCH GROUP 20 Mass transfer rate Problem: 1. Ignore particle-turbulence interaction 2. Ignore humidity distribution 3. One-way coupling Liu, Wei, Li, Oo Indoor Air 2017 NI RESEARCH GROUP 21 C(D) 1. Stokes number effect Preferential concentration Multiphase turbulent puff Aliseda et al. 2002 2. Size-dependent segregation 3. Convection+interfacial mass and heat transfer+humidity field NI RESEARCH GROUP 22 Equations NI RESEARCH GROUP 23 ! Numbers ,(') NI RESEARCH GROUP (= "(', () *+ "# , % # 24 Weber number !" = $% &%'(/* !" ≫ 1, breakup !" < 1, do not break &% = 10 m/s, ( = 100 01, !" = 0.14 &% = 10 m/s, ( = 1 mm, !" = 1.4 NI RESEARCH GROUP 25 What has been done NI RESEARCH GROUP 26 Fiegel, Clarke, Edwards, 2006 NI RESEARCH GROUP 27 Kelvin-Helmhotz instability: turbulent gas core in annual-droplet flow, the potential flow assumption may not be applied. Tatterson et al. 1977 does not work well. Mamoru Ishii, Purdue University, Department of Nuclear Engineering !" = NI RESEARCH GROUP = 8/&' 28 NI RESEARCH GROUP 29 where # is the average film thickness !" (Ishii and Grolmes) $% = NI RESEARCH GROUP = 8/)* 30 '(/* #~%& Large flow velocity , small D gas volumetric flux (superficial velocity) Dry cough, large !" , small D NI RESEARCH GROUP 31 NI RESEARCH GROUP 32 Absolute value !" = 16 m/s &' = 2 cm &)* = 3.8 mm !" = 16 m/s &' = 2 mm &)* = 0.8 mm !" = 16 m/s &' = 200 µm &)* = 178 µm !" = 30 m/s &' = 2 cm &)* = 1.7 mm !" = 30 m/s &' = 2 mm &)* = 0.36 mm !" = 30 m/s &' = 200 µm &)* = 77 µm It is difficult to produce droplets smaller than 100 µm Annular flow gas entrainment model may not be adequate NI RESEARCH GROUP 33 NI RESEARCH GROUP 34 NI RESEARCH GROUP 35 respiratory tract lining fluid (RTLF) Bake et al. 2019 NI RESEARCH GROUP 36 1. Steady-state solution for annular flow, but flow is accelerating when one coughs or sneezes 2. Viscoelastic mucus layer NI RESEARCH GROUP 37 Conclusion Numbers ! ,(') "# , % # "(', () *+ - NI RESEARCH GROUP (= . 38