Flow Through a Venturi

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Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

In the spring of 1978 the AMA (American Motorcyclist Association) issued Tech Bulletin 78-1, requiring the use of intake restrictors on 750cc roadrace bikes—mainly the widely sold Yamaha TZ750. This change was made in response to the warning from rider Kenny Roberts that engine power was fast becoming great enough to break traction at the high speeds being achieved on the Daytona banking.

Bulletin 78-1 required all the air flowing into each of the engine’s four cylinders to pass through a 23mm circular restriction. This reduction of intake cross-section by 55 percent (from the ID of the stock 34mm VM Mikuni carburetors) was expected to be effective.

Roberts won the 1978 Daytona 200, lapping the field (including previous winner Johnny Cecotto) with those 23mm restrictors in place. Yamaha engineers had created a flow device that would pass a race-winning amount of airflow through the AMA-legal 23mm restriction, and would fit into the short 29mm length available in the carburetors, downstream of the throttle slide.

Here’s how they did it: Knowing the length available was very limited, they made the entry to the 23mm restriction as short as possible—a simple radiused bellmouth. This avoided the formation of the so-called “vena contracta”—a contracted flow just downstream from a restriction, caused by the fact that air rushing to enter the bellmouth comes from all directions. The momentum of air approaching from the sides carries it toward the center, causing the flow contraction.

Having accelerated the flow to the higher speed at the throat of the restriction, they now had to recover that kinetic energy back into pressure. With only the bellmouth and the 23mm restriction, the result would be formation of a free jet surrounded by still air. The high velocity in the jet would be wasted in turbulence generated by friction with and entrainment of the surrounding still air.

So downstream from the 23mm restriction they placed a smooth widening taper, its angle chosen to keep the airflow attached to the inner surface of the taper. Make the taper too steep and the flow will separate from the surface to again form a free jet that would waste the energy of the high-speed flow. The smooth widening taper is called a diffuser, and its purpose is to gradually slow the airflow, transforming its energy from velocity to pressure.

A venturi converts speed into pressure. (Cycle World Archives/)

Such an airflow device, which accelerates airflow to a high speed at its throat then smoothly converts the energy of that high speed back into pressure, is called a “venturi tube” or just “a venturi.” Kenny Roberts enjoyed high horsepower during his 1978 Daytona ride because of the efficiency with which such a tube can 1) accelerate a flow through a small orifice, and then 2) efficiently recover most of that energy of velocity (kinetic energy) as the energy of pressure.

Giovanni Battista Venturi (1746-1822) described such a device around 1800, and measured the pressures at entry, at the minimum diameter, and at the exit of such a tube as air was made to flow through it. At rest, the pressure of air is the sum of random collisions of gas molecules, going in all directions, impacting the walls of the vessel containing it. The sum of such impacts is pressure. When allowed to accelerate through an orifice, some of that random molecular energy appears as a velocity through that orifice, and the pressure there drops because that part of molecular energy has been subtracted from the previous random motion. If the energy of that high velocity is now recovered by gradually decelerating the flow, that energy is transformed back into pressure.

Older riders will recall that a venturi is a part of every carburetor. In that role, air moving through the carburetor’s venturi falls in pressure, and that partial vacuum is used to lift fuel from the carburetor’s float bowl, into the airstream at the point of restriction and highest velocity. That high velocity at the throat of the venturi did an excellent job of breaking up and evaporating the entering fuel droplets. But carburetors are ancient history now.

During the 1978 season I was able to have a look at other people’s ideas of how to get air through the restrictor.

1. The simplest was to make a sheet metal washer with a 23mm hole through it, and drop it into the rubber carb manifold before mounting the carburetor. This design works poorly for two reasons: Without a bellmouth to smoothly guide air into the restrictor, a vena contracta forms that further restricts the flow cross-section. And then, with no diffuser downstream of the restrictor, the outside of the resulting high-speed jet turbulently entrains the still air surrounding it, turning its energy into random motions and, eventually, into heat. (Such entrainment and turbulence are what generate the typical roaring sound of a jet engine.) Bikes using this simple restrictor were slow droners.
2. Another used most of the available length to make a smoothly tapering cone <i>upstream</i> of the restriction, with no diffuser downstream to recover pressure. This did a pretty good job of not forming a vena contracta, but a lousy job of pressure recovery. Performance was poor.
3. Yet another also-ran was a double bellmouth—a rounded shape entering the restriction, and a similar rounded outlet. This one did a good job of guiding the flow into the restriction without forming a vena contracta, but its exit angle was too large to keep the flow attached to its inside surface. The result was what the F1 people call “diffuser stall”—the airflow was unable to follow the too-rapid expansion of the exit, so it separated from the surface to form a loss-making free jet. Bikes using this design were slow.

Now let’s turn Yamaha’s restrictor inside out and see what we get. We cut through the design along its centerline, and then change places between the two halves. Now, instead of a bellmouth, we have a hemisphere, and downstream, instead of a diffuser, we have a long tail cone, tapering slowly enough to keep airflow attached to it rather than separating.

What we have here is no longer a venturi. It is a streamlined body.

It is rounded at the front to guide the air smoothly around itself, and tapers down to a point at the rear at an angle not steep enough to cause the airflow to detach from it.

If we look at subsonic aircraft, this is what we see—a rounded nose like that of a Boeing 747 or a B-29 bomber, with a tapering tail. In the case of practical, load-carrying aircraft, the taper is postponed to the aft end of a long cylindrical cargo or passenger section.

Why not a pointed nose? Because the extra surface area of such a nose generates extra skin friction. Pointed noses are for trans-sonic and supersonic aircraft, as part of their management of shock waves. They look techie but have no functional value at subsonic speeds.

Why the tapering tail? The longer we can keep the airflow attached to the aircraft’s surface, the more benefit we get from its pressure, which acts on the tapering tail as a wet hand does, squeezing a bar of soap until it shoots out of our grip. But if the taper is too steep, the flow separates from it, creating a low-pressure wake of random turbulence. The higher the pressure against the front of the moving object and the lower the pressure in the wake behind it, the greater the drag.

The purpose of streamlining is to transfer as little as possible of the energy of the moving vehicle to the air around it. Ideally, the passage of the vehicle would displace the air, then put every molecule back where it had been, with its original energy.

That parallels the job of a venturi—to transform the energy of pressure into the kinetic energy of high velocity at the throat, and then back again to its original values—with zero loss, at the exit from the diffuser.

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