By Tom Granberg

 

Background

 

As anyone would have experienced ever shooting at Rio Salado, the wind conditions are tricky, and ever present. Personally I have never seen a calm day shooting at that range. Given the various berms, structures and other  terrain features and obstacles, I was interested in looking at the flow across the range. 
So I decided to run some Computational Fluid Dynamics across it. I've done the left side and the right side separately, since the structures are different. I have also ran the center where there is a blockage that have some effect on it on the positions directly next to it.

The weather specific condition for this CFD run was as follows, and what I find typical at Rio Salado.
Temperature set at 90f. The density at 1.2 (Standard for most wind simulations) it reality it's probably closer to 1.1 given the altitude. Atmospheric, or barometric pressure was set at 101 Kilo-Pascals, or 14.69psi (standard pressure) At our range it's probably 11-12psi, but for the purpose of this test, I've rand it with standard numbers. 
Density and temperature have bigger effect on wind deviation of the trajectory of a projectile at our shooting ranges.

Results

Tail wind on the left side

On the left there is a step down to the parking lot, then there is one angled roof over the shooting area. Then a gentle slope before there is berm running across. I only included one berm in the simulation, as I am doing this to simulate the 25m competition, if it was at 50m or 75m, I would have to include the other berms as well. But at those distances, especially on the left, there is other berms associated with the silhouette racks.

I ran the wind speed at 5m/s, a hair over 10mph. As you can see from the numbers, the wind gradient is pronounced, meaning the wind speeds above the trajectory is generally much higher.

The average speed across the range at the shooting level is only 2.8m/s or 6.2mph. So when you look at the tall flags on the flagpole they would see 10mph while your pellet would see 6.2mph average. 

There is also a bump in the speed over the berm in an upward direction.

Tail wind on the right side

On the right there is a step down to the parking lot, then there is one angled roof over the shooting area plus a taller canopy behind. Then the same gentle slope before there is berm running across.
I ran the same wind speed at 5m/s. The wind gradient is worse, meaning the wind speeds above the trajectory is higher relatively speaking than on the left side.
The average speed across the range at the shooting level is only 2.9m/s or 6.7mph, so slightly higher average wind speeds.

However it is much more unpredictable as there is a huge upswing speed differential immediately after the roofline, and a sharp downdraft vector. Where on the left side the downdraft area is pronounced, it's wind speeds peak at around 2.8m/s . On the right side it peaks at a whopping 4.3m/s. that's 9.8mph on the right vs. 6.2mph on the left side. And to make it worse that peak happens much closer to the muzzle.
 

Tail wind in the middle

Here we have the shed which is going to block some of the wind coming from the rear.

Again I ran with the same numbers. It's similar to the profile on the left, however it's slightly worse in terms of downdraft, even if the peak wind speeds are much lower in front of the muzzle, the downdraft lasts all the way to around the half way mark to the 25m, or about 14m out (13yds) 

There is also some turbulence here, that having the wind go backwards and sideways in both directions. Which can make for a tricky wind read. I would think the two closest shooting positions to center would be as bad or worse in some cases than the far right.

 

Trajectory behavior on all - Tail wind

 

The interesting piece is dip increase above the berm, in goes to 3.5m/s or 7.8mph, and the angular deflection is upwards as you can see by the arrows.
 

There is also a distinct downdraft in front of the shooting position, worse on the right side in terms of shear wind velocity, and pretty bad in the center. It will deflect the trajectory down a little and induce a bit of pressure differential drift to the right (given a clockwise twist barrel)

This have the unfortunate effect that if you are having a tail wind slightly from the left rear (7,8 o"clock) you will be seeing much more drift to the right than you would expect. In such a situation, due to the slight bump across the berm, you would also see an increase in upward deflection, more than just a result of the pressure differential created by the side wind alone. This upward bump from the berm might be slightly negated by the downdraft when wind is from the 7, 8 o"clock direction. But if it's from the 4, 5 o"clock direction, there is a problem with the downshift of the trajectory due to the pressure difference pushing it down. When you have that down-wash over the structures, you can see a severe and sudden drop of the POI if the wind swing just briefly from 6 o"clock to anything towards the right (4 or 5).

The opposite is true if it swings from 6 to let's say 8, then you go from a slightly low center POI to an immediate high POI.