In the first place NOTHING IS
THROWN REARWARD. Can't make that point strongly enough. It is just sprayed up and around and side to side. The engines CATCH UP
before it falls back to the runway.
The spray deflectors (they are not called splash guards) do not prevent water or other solids from being thrown, they just keep the trajectory down.
In the second place, imagine the passing of a non-powered wheel from the runway's point of view
. It is just laying there with a coating of water or slush just laying there on top it. Along comes a torus-shaped object, the nosewheel, rolling through that slush. The wheel, with its share of the airplane weight is going to press itself all the way down to the pavement. To do that it is going to squeeze all the water out from between its surface and the pavement. Okay not ALL
the water, the pavement will still be wet, as will the tire, but let's say all but a very thin film of water is going to have to get out of the way.
Now picture an infinitely small plane (the geometry kind) transverse to the direction of travel of the AIRplane. This extends upward from the surface of the pavement to as high as you care, and as far out to either side as you'd like to consider. The water at this plane is going to react to the passing of the wheel.
The first contact, wheel-to-water will come at the surface of the water or slush, and that corresponding point on the tire tread. It is not rolling, it is pressing down. When it presses all the way through the coating of water (because the wheel has rolled forward that far) it is going to press itself to the surface of the pavement then rise up again WITHOUT rotating. That is what rolling wheels do on a surface - they do not rotate against the surface.
So what does the water experience? It gets squeezed - right straight down toward the ground. Since it cannot be compressed it pressurizes and squirts out the gap at the point of least resistance. This is NOT back toward the rear of the plane! It is forward mostly, and to the sides a bit less.
Yes, some of the water will cling to the tread and be thrown by centrifugal force in every direction around the rotation of the tire but the huge majority of the mass and weight of the water or slush will be squirted out FORWARD or TO
THE SIDES by the passing of the wheel.
Loose rocks or other solids have different physics and present different results but the spray deflectors on the MD
-80 are 99.99% there for water and slush.
The third picture in reply #9 is a good illustration of how one can be deceived by appearances. It looks like the nosewheel is spraying water very far astern but that is an illusion. It is only throwing it up forward, and to the sides. The long stream aft of the nosewheel is us looking BACK IN
TIME. The wheel passed that point a second ago and you are seing, if you will, a succession of individual splashes, spaced so close together that they look like a stream from a firehose. But they are not. It is just water droplets arcing up a few feet and falling right back to the ground within a few feet of where they started.
The splash-area gets progressively larger as you go aft because that is what splashes do. They start at the point of initial perturbation and they radiate outward until their energy is spent. These are splashes of water caused by the pressing-down and lifting up of a point on the surface of a tire tread and it makes no difference that the wheel was rotating. If you could press a non-rotating wheel down through the water, lift it up, move it forward a tiny increment, do it again and keep repeating very rapidly the result would look exactly the same. The spray is higher back near the tail only because the water has had time to arc that high. A few feet farther aft, out of this picture it is going to start falling back to the runway.
But it was not sprayed aft. It was sprayed up in just about every direction except aft. Aft just caught up with it.
Happiness is not seeing another trite Ste. Maarten photo all week long.