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CFD Improves Irrigation Components
Posted Mon January 20, 2003 @09:47AM
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Application By Mark Ensworth
Engineering Manager
Rain Bird Golf Division
Azusa, California

Engineers at Rain Bird's Golf Division used computer simulation to reduce the pressure loss in golf course irrigation components by more than 50%. Swing joints are the component that connects the sprinkler to the waterline in an irrigation system. There are several thousand of them in a typical golf course, most having three 90 bends that generate considerable pressure losses. Rain Bird engineers had the idea of reducing energy consumption by improving elbow design. Recognizing that the time and money involved in the build and test approach would be high, they used computational fluid dynamics (CFD) to simulate flow through the elbow. They discovered that increasing the cross-sectional area of each elbow and softening its corners substantially reduced pressure losses.

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Rain Bird is one of the leading irrigation equipment manufacturers in the world. Founded in 1933, Rain Bird has over 4,000 product offerings and has been awarded over 130 patents. Rain Bird has manufacturing facilities in the United States in California, Arizona and also in France and Mexico. Rain Bird golf course irrigation products are used in 130 countries, and they can be found at four out of the top five courses on both the Golf Digest and Golf Magazine Top 100 lists. The companys golf course irrigation products include rotors, impact sprinklers, central control systems, satellite controllers, weather stations, valves and swing joints.

Swing joints help minimize head damage

The basic purpose of a swing joint is to connect the water supply to the sprinkler head. The swing joint also provides the sprinkler head with some flexibility in case it is run over by a golf cart or some other piece of heavy maintenance equipment. A swing joint is comprised of three different 90-degree elbows that allow the sprinkler head to rotate in a complete three-dimensional range.

This 3-D range allows the sprinkler head to rotate or swing out of the way when an external force is applied, avoiding damage to either the sprinkler head or to the piping structure to which the sprinkler head is attached. This 3-D range also allows the irrigation installer to easily articulate the sprinkler head so it is flush with the surrounding grade.

Issue of pressure loss

Because they can dramatically reduce the cost of maintenance and replacement parts, swing joints are used in most golf courses to protect every sprinkler head. Many golf courses in the West, where irrigation needs are high, have more than 2000 swing joints connected to a central irrigation system. The only disadvantage of the traditional swing joint is the fact that the three 90 bends found in each one introduce a significant pressure loss. This pressure loss shows up as additional power required to run the central pumping stations found at nearly every golf course. Rain Bird engineers recently initiated a project with the goal of substantially reducing these losses.

The first course of action considered by the engineers was developing a series of prototypes of different elbow designs in an effort to find one with a lower pressure loss. There were several problems inherent in this approach. First of all, they would have to build a prototype of each design that they wanted to evaluate. This would have been a very time-consuming and expensive process. The other problem is that testing would easily determine the pressure loss of the proposed elbow designs but provide little or no information on why the design was behaving as it was. As a result, engineers would obtain little new information from each test and would have to proceed mainly on instinct.

CFD simulation

Rain Bird engineers then considered the use of CFD to address this problem. The CFD software they used was FLUENT from Fluent Inc., Lebanon, NH. A CFD simulation provides fluid velocity, pressure and species concentration values throughout the solution domain for problems with complex geometries and boundary conditions. In conducting the analysis, a researcher may change the geometry of the system or the boundary conditions such as inlet velocity, concentration, temperature, etc., and view the effect on fluid flow patterns or concentration distributions. CFD also can facilitate detailed parametric studies that can significantly reduce the amount of experimentation necessary to fully characterize a design and thus reduce design cycle time and cost.

Engineers began by simulating the company's and competitors' existing elbow designs. The simulation results included diagrams of flow through the elbow that showed pressure at each point. These diagrams showed that pressure loss and turbulence in the elbow was clearly concentrated in the inside corner region. Existing designs at this time all had a sharp inside corner in order to minimize molding expense. But the analysis results showed that after passing this corner, water was directed immediately to the opposite outside wall of the elbow where it was congested, creating a high-pressure zone. Conversely, there was very little flow or pressure on the half of the pipe nearest the inside wall. In effect, water was flowing only in the outside half of the pipe, reducing its effective diameter by 50% and greatly increasing pressure loss.

Developing an improved design

With just a few iterations of analysis, Rain Bird engineers had gained a clear understanding of the problem and were well on their way to solving it. The most significant challenge that they faced from this point was developing a design that not only reduced pressure losses but would still be relatively easy to mold, in order to keep manufacturing costs as low as possible. To address this objective, manufacturing engineers with extensive injection molding experience participated in the design process from this phase onward. Understanding the problem as well as they did, engineers fairly quickly iterated towards a significant design change that had a major impact on pressure losses. First, the cross-sectional area of each elbow was increased by reducing the corner radius of the outer wall. Second, the sharp inside corner of the elbow was softened by increasing its corner radius.

These changes reduced pressure loss by over 50% by doubling the effective diameter of the elbow without changing its external dimensions. Analysis of the new design showed that the water flow now followed the larger diameter internal wall around the corner rather than rushing to the opposite wall. The result is that installing the new swing joint throughout a golf course can substantially reduce the amount of power required to drive the central pumping system. Another alternative, for golf courses that are satisfied with their existing power consumption, is to use smaller and less expensive swing joints without increasing pressure drop. For example, a Rain Bird Eagle 900 rotor, which normally requires a 1-1/2 inch swing joint, can now be supplied by a Rain Bird 1-1/4 inch diameter swing joint. The price of the 1-1/4 inch swing joint is about $8.00 less than the 1-1/2 inch swing joint. The savings on a large golf course would be on the order of $16,000.

Other swing joint features

Rain Bird swing joints are available with an integrated 315 psi service tee that saves time and labor costs by eliminating the need for pre-assembly at the job site. The combination also reduces the number of parts to order. The service tee connection is ACME threaded and double 0-ring sealed to insure long life and easy disassembly. Rain Bird has also introduced a modified ACME outlet designed to improve safety by losing seal engagement before losing thread engagement during rotor removal. This provides a visual indication that the system is still under pressure. Color-coding and distinct size markings reduce cost by eliminating errors and improving installation efficiency with quick size identification at the job site. Threaded inlets are oversized, making hand tightening and blind installations easier. This also reduces the risk of potential damage caused by over-tightening with a wrench. An available triple-top outlet configuration allows adjustments when turf height changes or rotors settle. It also provides greater flexibility in setting the rotor to the grade.

This application provides an excellent example of how CFD can improve product designs by precisely determining the influence of fluid flow. With a relatively low expenditure, Rain Bird engineers were able make a significant product improvement that provides an important differentiator from its competitors. Based on this success, engineers are looking for other applications in which CFD can help them deliver added value to customers.

original design
The sharp inside corner of the original elbow caused the flow to be constricted towards the outer wall.

modified design
The modified design reduced the sharpness of the inside corner resulting in a more uniform flow field.

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