© Photo | Prof. Dr.-Ing. Wolfgang Elmendorf

“Measurements have the last word”

Prof. Wolfgang Elmendorf on the potential and limitations of measurements and calculations for product data.

Where do product data come from, and how dependable are they?

Both depend on the approach taken by the business. There have been cases in which data was regularly extrapolated for years on the basis of measurements made only once. Of course that’s not a dependable solution, but it’s also an exception. The classical approach is to determine the data on test stands, because in the end, measurements have the last word when it comes to reliability.

What requirements have to be fulfilled by such test stands?

First, test stands have to be set up according to standards; the primary standard applicable to fans is DIN EN ISO 5801. Test stands that comply with this standard provide a suitable basis for comparisons. Second, an organization’s quality management specifies the processes for performing and analyzing measurements; in today’s industrial landscape that is governed for the most part by DIN ISO 9001. Then the processes that lead to the results are also reproducible.

In your experience, does that really lead to comparable data?

As a rule, yes. If the comparison result deviates significantly, then the setup conditions were not complied with completely — often because they’re unknown. So the setup has to be clearly defined and documented.

How realistically can the operating conditions even be represented in tests and calculations?

The question to begin with is: What do I need the results for? If I want to compute complex fan geometries, then with some experience I know that I can make certain simplifications that will only have a marginal effect on the results.

Prof. Dr.-Ing. Wolfgang Elmendorf

Prof. Dr.-Ing. Wolfgang Elmendorf heads the laboratory for flow machines at Heilbronn University. His work centers around the development of fan test stands, experimental fluid dynamic analyses, theoretical fluid dynamics calculations, and the optimization of fluid flow systems.

For example, there are many fans in which the blades are not spaced evenly for aeroacoustic reasons. That has no effect on the air performance. So if I’m interested in ventilation results in a computation, I can also space the blades evenly. But if I’m interested in aeroacoustic results, then I have to take the complexity into account. The hard and fast rule is: A fan never works alone. It’s always in an environment. To assess the true operating behavior, I need to know the characteristics of both the fan and the system.

And then come results that are close to reality?

It may happen that not all factors can be taken into account in a computation. But these shortcomings can also arise in measurements since you can’t always make a one-to-one simulation of the original customer situation. If the customer wants to know the exact behavior, the only way to do so is usually to test the fan in the device.

The experimental setup for every measurement has to be clearly defined and documented.“

In cooperation with the customer, the operating behavior in a special installation situation can be predicted very reliably in this way. But it can be computed only with an unreasonable amount of effort.

What is better, measuring or computing?

Both together! In some fields, you can make better progress with measurements and in others with computation. For example, better computing methods allow for faster product cycles. The reliability of modern computational methods is outstanding and continues to improve. But without validation from measurements they aren’t productive. And at the latest, the customer will demand this verification.

What should one watch out for when comparing measurements and computations?

With a computation you will not exactly match the measurement. Often you’ll systematically over- or underestimate parameters such as pressure increase or efficiency — and then notice that the results only undergo a parallel shift. So if I achieve improvements in the computation by changing geometric parameters, I can transfer those one-to-one to the measurements. In this way, computation can identify factors I can use to make improvements — much faster than with measurements.

How much can such results improve the efficiency of equipment?

In ventilation applications, aerodynamic optimization plays a crucial role. There is still potential for improvement here, even though it’s getting smaller all the time. You have to focus on the efficiency chain. The impeller is a major factor, but all the other components play a role as well. The impeller can be viewed in terms of various physical causes of efficiency losses such as friction and static pressure. The efficiency can be increased simply by improving the inlet nozzle or the outlet area, with effects on both the aerodynamic performance and on the aeroacoustic side. Geometric details also offer potential. However, the result of an analysis can sometimes be that it’s impossible to make full use of the aerodynamic potential, for example because of required space limitations.

Do analysis methods also offer potential?

Mathematical optimization strategies are being applied with increasing frequency. They involve taking the results of fluid dynamics calculations and using mathematical methods to derive the optimum configuration of the parameters. That’s very laborious since it’s a multi-variable problem. And such methods typically also require validation by experiment in the end.

How important are measurement results to customers?

The customer needs the measurements but also needs help in interpreting and implementing them. It has to be a matter of course for manufacturers to provide clarification in their statements by means of standards-compliant recording of measured quantities and clear identification of parameters. Open dealings between the parties are called for. Customers have be clear about what they want. That sounds simple but is actually astoundingly complicated because when they design their products, customers often don’t know the requirements of their own customers yet.

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