BDC Methods To Support Your Engine Development:
Engine Type Excitations
Using the proprietary techniques in EzVIBES®, BDC can perform a vibration test on your IBD that simulates the type of traveling wave excitation that it would experience in the engine. We also can use the results to simulate the effect of irregularly spaced stators.
Precise Measurements at Really High Frequencies
Small errors in locating the measurement points on each blade cause large errors in the measured vibratory response – especially in high frequency modes. BDC makes physically meaningful measurements at very high frequencies: we have demonstrated good repeatability at frequencies in excess of 80 kHz. Importantly, our measurements have excellent reproducibility because of our automated method of identifying blade features and locating points.
BDC has proprietary software that determines the Vibratory DNA® of an IBD from a sample of vibratory response data. The Vibratory DNA are the parameters that uniquely characterize the IBDs vibratory response. Once the Vibratory DNA is determined it can be used to predict the vibratory response of the IBD under different conditions, e.g. for a different engine order excitation or the unsteady response it would have to variably spaced stators.
One of the tools used at BDC to extract Vibratory DNA is called EzID™. EzID determines the parameters in a reduced order structural model that best fits the model to the test data, Figure 1.
EzID identifies the structural properties of the IBD as well as information about the vibration test. It determines:
- The nodal diameter map of the IBD
- Each blade’s frequency ratio
- The forces acting on each blade
- The damping level
In many instances these parameters adequately characterize the vibratory response of the IBD. When needed, more sophisticated versions of EzID are available to analyze more complex modes.
Modeling and Simulation
The parameters identified by EzID can be used as input to our proprietary reduced order structural model, FMM, that can be used to predict the vibratory response of the test IBD.
FMM™, the Fundamental Mistuning Model, is numerically efficient. For example, it can compute the frequency response of an IBD in 1/300th of a second. As a result, FMM can be used to calculate the sensitivity of the IBD to small changes in the frequencies of the blades. For example, for one of the tested IBDs it was found that the vibratory response of one of the blades would really take off if its frequency was lowered by a few percent. This is something that you need to be aware of and avoid when you are doing a new IBD design (Figure 2).
Bench to Engine Testing
You often learn things from bench tests than you can’t learn in the engine and you can use that information to get more out of your expensive engine test. During a bench test, you can measure as many points as you like on each blade so you can identify and differentiate neighboring modes. The bench data is a lot cleaner and you can get amazing frequency resolution. We can use that data to develop a reduced order model of the IBD and identify its structural properties. We know the engine order of the excitation that the IBD will experience in the engine. If we estimate the damping in the engine then we can predict the relative peak amplitudes that the blades of the IBD will have in the engine tests, Figure 3.
Once we have the engine test data, we can identify engine force levels, engine damping, and centrifugal effects. We can then bench test another IBD, use EzID to identify its reduced order structural model and then use the engine forces, damping and centrifugal effects to predict what its vibratory response would have been if it had been tested in the engine. As a result, the bench tests become a ‘virtual engine test’. This allows us to bench test other early production IBDs and determine how they would vibrate in an engine before they are, in fact, put in an engine.
Compare with Design Intent
The designer of an IBD develops a nominal geometry that is analyzed with finite element to determine the theoretical nodal diameter plot for the IBD. The finite element analysis assumes that every sector is identical so that it can calculate the IBD’s natural frequencies as a function of the number of nodal diameters. Figure 4 shows the theoretical nodal diameter map for higher order bending modes for a representative IBD design.
The shape of the nodal diameter plot determines if an IBD design will be sensitive to mistuning or not. So, a key question is: Do the nodal diameter plots of the IBDs as manufactured look like the one calculated using FEA and the nominal geometry?
To illustrate this concept, BDC’s EzID was used to identify the real nodal diameter maps of 30 IBDs from vibration test data. The real nodal diameter maps (in grey) are compared with the theoretical nodal diameter map (in red) in Figure 5. In this example, the real diameter maps (grey) looked quite similar to the map predicted using finite element and the nominal geometry (red) except for several of the higher frequency IBDs. The effect of these differences on vibratory response can be assessed using BDC’s modeling and simulation tools.
It is not unusual for blades to start breaking from high cycle fatigue after years of service. Typically, blades fail in only a few engines and you need to know the root cause of the failure in order to fix the problem. Often the search for root cause leads to an engine vibration test. Typically, the vibratory stresses in the engine that’s tested are within limits – so why did the original part fail?
BDC can fit a reduced order structural model to your engine test data. We can use the model to simulate other blade arrangements and determine sensitivity to mistuning. If the part that failed is an IBD we can bench test other IBDs. We can help you determine what combination of IBD to IBD differences and blade to blade differences could lead to unusually high vibratory response. As a result, our unique tools can help you identify the root cause of the failures and also develop strategies for managing the problem.