Point of clarity, but you've mentioned three companies. Dimond, TriRig, and Ventum :-)
That said, if you have any questions about the TriRig Omni, I'm happy to help answer them.

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TriRig.com

I really don't know the answer to your questions, but the Dimond Marquise is a second gen design that was improved from the original using windtunnel data and testing with rider-on testing being the important metric. AFAIK, the Ventum was designed without windtunnel testing, has been taken to the windtunnel with less than stellar results after production.

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Dimond Bikes Superfan

Sorry ....Edited ... 3 Companies...

Assuming this Nick... why Lotus shape vs Beam bike.... want understand the fundamentals.

The principal design goal with Omni's shape - what we call the Monofoil - was to guide clean air from the front of the head tube (or really, the front brake cover) back cleanly to the rear wheel, without interruption. The entire frame is basically just one continuous leading edge, which fairs the rear wheel. You can draw an aero chord from the front tip of the front brake cover to the back edge of a disc wheel's tire, with zero additional leading edges. (For what it's worth, the ratio of that chord is something like 30-to-1). We validated the concept in CFD with many rounds of testing, as well as real-world wind tunnel testing BEFORE we locked our 3D data and went to production. You can read all about it at http://www.tririg.com/...ge=windtunnel_report It's worth noting that our transition from aerobar to frame is exceptionally clean - we found this to be very important on our first trip to the wind tunnel. Keeping all the shapes smooth and blended into one other proved very important to keeping a cohesive Monofoil that actually worked aerodynamically.

Interactions with the front wheel are quite a bit simpler than they are when a down tube is in play. Most down tubes try to hug the trailing edge of the rear wheel as much as possible, but always part ways with it somewhere, and you wind up with aero inefficiencies there, with air going in multiple directions simultaneously, causing drag. The Omni frame deliberately moves upward right behind the head tube, creating a little more free space to make way for the air coming off the front wheel, which is traveling in a nearly vertical direction right there. Additionally, the fork blades are very wide-set, and very deep aero chords themselves, so they don't disturb the air much, or interact with the air coming off the wheels, much. The result is that you can pick any front wheel/tire combo you like, and you won't ruin the aerodynamics of the frame. Omni is pretty insensitive to front wheel choice.

Regarding side/cross winds, you can already read about Omni's raw aero performance in the link above. But one thing we don't go into in depth in that report is stability, and notably perceived stability, in cross winds. But if you look at pictures of a rider on Omni, you can see that the bike adds very little surface area for the wind to see from a crosswind perspective, since the rider's legs shield much of the actual side-area of the frame. What we expected was that this would provide additional perceived stability, and we are happy to report that our riders have corroborated this. We frequently hear unsolicited feedback that Omni feels extremely stable in high crosswinds, much more so than other designs which add side area with deep down tubes, or other frame area directly behind the front wheel.

Is that helpful? Let me know if you have any further questions.

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TriRig.com

How do a rider's legs interact with air coming off the fork flowing over the frame? Are they far enough away that they are outside that airstream entirely?

start here: https://www.slowtwitch.com/Tech/Triathlon_Bikes_in_the_Age_of_Peak_Aero_6429.html

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Brian Stover USAT LII
Accelerate3 Coaching
Insta

Airflow from off of the fork is pretty clean by the time it comes back to the rider's legs. The fork blades are very efficient as an airfoil.

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TriRig.com

Good stuff....You mention the improved stability from the legs blocking the biggest part of the frame section under the seat post...

There is mention in recent tests (that Brian S referred to) that the bikes with large down tube Sections like Felt IA, P5x and perhaps Dimond get a sail effect at certain broad and close hauled wind angles. With Omni, is this effect/benefit negated by legs blocking this section? Does the continuous airfoil design that results in clean none turbulent air flows trump sail effects of latter bikes?

It's important to distinguish what we're talking about. When talking about a "sail" effect, you're talking about decreasing drag at higher yaw angles due to deep airfoils. You can see in the drag charts, Omni does exhibit decreasing yaw up to about 10 degrees (the stall angle), after which drag increases. That's what's often described as a "sail effect." 10 degrees is also a good stall angle, as about 90%+ of your riding is done from 0-10 degrees.

When I talk about side force, it's not that the legs are exactly "blocking" the frame - it's that viewed from very high yaw angles (say, 30+), the legs and frame take up much of the same "frontal area" (in quotes, since it's not actually the front of the bike, but the "front" from that high-yaw view). So the frame and legs aren't separate targets for side force, they're in the same spot. This matters for big gusts of wind, or any very high-yaw situation where the rider feels side force. Have you ever been out on a very windy day and felt like you were being pushed sideways? We get those days frequently in Colorado, and riders see them often at Ironman Hawaii, especially on the Northern 1/3 of the bike course. In these situations, you want the wind to see as little surface area as possible, since that surface effectively acts like a parachute, catching the wind and pushing you sideways. Omni's design helps reduce that area, making the ride more stable.

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TriRig.com

just to be clear, the Ventum performed "sub-par" in ONE test. And by "sub-par" I mean the ENTIRE spread from best to worst among those bikes tested was worth less than ~90s over an Ironman.

So is it fair to say that in the case of a beam bike, the leading edge of THAT System better be exceptionally clean (nose, bars, brakes etc) and have sufficient Chord on the Down Tube foil to bring flows back together (including vertically channeled air off of front wheel) .... does this make the rear wheel choice/design more critical in that it is the last component of the system to FIX turbulent flows?

It's my understanding that a disc wheel helps with that flow of air more on something like a Dimond than a traditional frame.

I run H3+ front and rear on my Dimond knowing that I'm giving up a few seconds here or there versus a disc. I did so because I got a slamming deal on the wheelset plus I don't have to worry about fussing around with different valve stem lengths for tubes to compensate for the small disc valve stem opening on a disc.

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Favorite Gear: Dimond | Cadex | Desoto Sport | Hoka One One

I couldn't really say. For Omni, we came up with a design theory, and that theory was validated via CFD and wind tunnel studies. Without actually doing the studies for other types of bike designs, I couldn’t answer definitively how air works in those cases.

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TriRig.com