# This Old CFD House: Part V

### John Wilson

Posted Aug 9, 2011

Welcome to Part V of a multi-part series, where in this installment Travis takes a look at his lawn mower.

FloEFD Lawn mower Model

As I’m not a lawn mower designer, I’m not familiar with all the details that are required for an analysis, so I had to do some research on typical RPM (~4000), and estimate the airflow resistance for the grass.  Once I had analyzed the airflow, I seeded the solution with particles of grass to see where they went.  I used fairly big particles for the grass, as my grass was quite long when I had my issues.  Here are the results of my analysis.

FloEFD results: Air streamlines around lawn mower blade tips

First, you can see from the above image how the blades cause a lot of swirling air, similar to the airflow around helicopter blades.  We can also look at the velocity contours and velocity vectors inside the lawn mower (shown below).  You can clearly see that at the blade tips the velocity is highest.  This is because the blade is spinning, and the rotational velocity multiplied by the radial distance provides the velocity at that spot, so in general the airspeed will be highest near the blade tips.

FloEFD results: Velocity vectors and contours

Next, I injected the particles at the outer edge of the blade, as only that first bit of the blade will do any cutting (you aren’t walking fast enough for a long piece of grass to make it towards the center of the blade).  From this animation, you can see that the grass particles hit the top of the lawn mower “dome”, than impact a second time along the outer “rim” before going back into the grass.  What you can also see, is that the particles slow down a lot after being impacted by the lawn mower blade.  I think this is the key, in that the slower grass clipping is moving, the more likely it will be to stick/accrete to the top of the dome or to the side rim.  Once some grass starts to stick, the surface becomes rougher, and this promotes more grass to stick, leading to a buildup of grass on the interior.

After sanding some rust off to find the model number of my lawn mower, I was able to dig up a user manual online (a huge surprise).  In it, I found that there are “kick plates” at 4 spots along the top outer part of the “dome”.  From my results, it made perfect sense why those were there.  To try and push down the clippings back into the grass or back into the lawn mower blade, before they could impact the top of the dome and collect.  So I emptied the gas tank and did some searching on my lawn mower, and after removing a BUNCH of hardened, blackened grass, I did discover my kick plates.  I’m not sure if it’s mowing better now because those kick plates are exposed, or that my grass has never been let to grow for more than a week or two.  It still does seem to leave clumps of grass instead of evenly dispersing fine mulch though, so I do think some design improvements could be made, but I’m not sure if this company wants to hear how they could improve their 1992 model.

One other discovery during my research was that most mulching lawn mower blades have a wing type shape in the interior to create suction to make the grass stand straight up, kind of like that suck-vac haircut machine in the original Wayne’s World movie.  A problem with this though, is this suction can also cause pebbles and other debris to fly up and hit the top of the dome, leading to erosion and holes.  This would also be a perfect analysis opportunity for FloEFD.  As I got my CAD for free on a website, it didn’t have this airfoil shape so it will have to wait for a future installment of my blog.  Still, I think this was an enlightening exercise into another household application where CFD can be used to gain better insight and improve a design.