Will I get wetter if I walk through the rain or if I run?

November 22, 2021 Troy Baumgardner

Answers to Enduring Questions

We have all been in this situation and, at least briefly, asked ourselves one of the enduring questions, “Will I get wetter if I walk through the rain or if I run?”  I pondered this question, and the possibility of CFD answering it while watching it rain outside my office more than 10 years ago but never got around to trying to answer it until now.  Part of the delay in answering this question was with the software I was using at the time.  To use CFD to answer this question the software must show a solid body, a human body, in this case, moving through a fluid volume. 


Figure 1:  Human body, with region for overset mesh, in an air domain

It also must be able to show particles of water, or at least particles with the density of water, moving through the fluid volume. 


Figure 2: Air Domain with water particle injection from the top surface (particle size shown at 25x)

The CFD package I was using 10 years ago had limitations.  It could solve transient solid body motion.  It could also solve discrete particle motion through an air volume.  I could not, however, separate the particles which hit the human body from those which hit the ground.  The move to Ansys Fluent made this possible and added several opportunities for improving the analysis. 

Setting up the analysis requires defining the solid body motion speed and parameters to quantify rainfall, both how they enter the domain and how they exit.  To define solid motion, the person moves through the domain at three different speeds, 0.5, 2, and 5 m/s relating to walking, jogging, and sprinting.  The rain is defined as starting from a particle injection surface with randomized starting points and staggered positions on the top surface of the domain.  Particles are injected moving downward at 9 m/s downward as if they have reached terminal velocity.  The diameter of particles varies from 0.2mm to 2mm, which is a common range of raindrop diameters.  The total mass of particles is held constant at 1 Kg/s where anything over 2 Kg/s is “flash flood” range so 1 Kg/s is a steady rain of small to medium size raindrops.  

Both the bottom surface of the domain and the surface of the human body are defined as escape surfaces as if the particles are immediately absorbed by whichever surface they contact.  The graph below shows the distance the person traveled vs the mass of accumulated water that hits the person.  Walking yields the highest mass of absorbed water, above 0.18 kg.  Jogging, 4 times walking speed, reduces wet mass below 0.04 kg and absorbed water at sprinting speed, 10 times walking speed, is even further reduced.  Moving faster through the rain reduces how wet you will get. 


Figure 3: Surface Velocity through a Zero-Depth Entrance and Exit Region


So, does this answer the enduring question?  Significant simplifications were employed including eliminating effects of water-shedding, saturation, and splashing from footfalls.  The answer also may change under different rain conditions or with wind added.  The results also don’t account for the increased risk of slipping and falling at higher movement speeds.  This is one answer, under one set of conditions with several significant assumptions.  It is a significant result that provides opportunities for future study.    

Ansys Fluent is a powerful and flexible tool that can help solve many enduring questions.  If you have an application where not all your questions are being answered, contact Rand Simulation to discuss how CFD can help.

About the Author

Troy Baumgardner

Simulation Specialist for Fluid and Heat Transfer Applications<br><br> Interests: spending time with my family, cooking, mountain biking, reading

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