One of my first jobs as a new home owner was to mow the lawn. As it happens, we had a friend with a lawn mower that was no longer using it, so he gave it to us. When I went to pick it up, he was using words like, 1992, and “never maintained”, so I knew it wasn’t going to be a super awesome lawn mower. Still, I love to get something for free, and a lawn mower is a couple hundred dollars, so I was very happy with his gift … until I used it for the first time. Granted, with all the renovations at the house and moving in, the lawn was overdue for a cut. Plus it was just after the rainy season here in the bay area, so the grass was thick, long and wet. How long? In some spots it was taller than the lawn mower wheels. The issue I had with this 1992 lawn mower was it was a mulcher. I’ve never used this type of lawn mower before, but the idea sounded great. You cut the grass, and the clippings are put back to the lawn so you don’t lose the nutrients. Sometimes an idea is better in theory though. It’s not that I had to pull start this mower every couple feet that left me with such a bad opinion of my new used lawn mower. It’s all the dead grass that resulted. You see, it seemed to get clogged up with the wet grass, and once it started to stall, it would cough up a bunch of wet grass phlegm clumps all over the place. On the sidewalk, driveway, my car, … but mostly back into the lawn. The thing was, these clumps were wet and heavy and big, so they sunk to the bottom of the lawn, and even after raking, many were missed causing dead patches all over my lawn. I decided to do some analysis on this lawn mower, to see if there was a design flaw with this mulching type of lawn mower. Luckily, I found an already built, free 3D CAD file of a generic mulching lawn mower on one of the CAD user community websites (3dcontentcentral.com). I used our FloEFD software, which is our general purpose, CAD embedded CFD tool. I had the CAD geometry so my time commitment to this analysis was greatly reduced. Any designer wanting to do an analysis in FloEFD would be in similar shoes, having 3D CAD model and wanting to use that model without having to do any importing/geometry clean up or creating a “fluid” part. All these steps are avoided when using FloEFD.
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.
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.
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.