Daniel Peter Rohe

Coupling of a Bladed Hub to the Tower of the Ampair 600 Wind Turbine Using the Transmission Simulator Method

This paper presents an example of the transmission simulator method of experimental dynamic substructuring combining two substructures of the Substructures Focus Group’s test bed, the Ampair 600 Wind Turbine. The two substructures of interest are the hub-and-blade assembly and the tower assembly that remains after the hub is removed. The hub-and-blade substructure was developed from elastic modes of a free-free test of the hub and blades, and rigid body modes were constructed from measured mass properties. Elastic and rigid body modes were extracted from experimental data for the tower substructure. A bladeless hub was attached to the tower to serve as the transmission simulator for this substructure. Modes up to the second bending mode of the blades and tower were extracted. Substructuring calculations were then performed using the transmission simulator method, and a model of the full test bed was derived. The combined model was compared to truth data from a test on the full turbine.

Coupling Experimental and Analytical Substructures with a Continuous Connection Using the Transmission Simulator Method

The transmission simulator method of experimental dynamic substructuring has the capability to couple substructures with continuous connections. A hardware example with continuous connections is presented in which the method is used to couple an experimental substructure with a finite element substructure to predict full system response. The predicted response is compared with frequency response functions measured on the full system hardware. The experimental substructure captures the motion of a component packed in foam. This is coupled to a finite element model of a cylindrical metal case which contains the foam and is attached through a flange to a plate and beam structure.

Extending the Frequency Band for Fixed Base Modal Analysis on a Vibration Slip Table

In previous work, a modal test of a large beam like structure on a vibration slip table was analytically constrained to fixed base providing estimates of the first three bending modes active in the direction of slip table motion. This work extends the frequency band of the method to extract the first ten fixed base modes of the test article. All ten fixed base modal frequencies are within two percent of the truth test fixed base modes. When compared to the truth test, the estimated damping of the lower modes has large error, but at higher frequencies the estimated damping converges on the truth value.

Physical Vibration Simulation of an Acoustic Environment with Six Shakers on an Industrial Structure

A previous study in the UK demonstrated that vibration response on a scaled-down model of a missile structure in a wind tunnel could be replicated in a laboratory setting with multiple shakers using an approach dubbed as impedance matching. Here we demonstrate on a full scale industrial structure that the random vibration induced from a laboratory acoustic environment can be nearly replicated at 37 internal accelerometers using six shakers. The voltage input to the shaker amplifiers is calculated using a regularized inverse of the square of the amplitude of the frequency response function matrix and the power spectral density responses of the 37 internal accelerometers. No cross power spectral density responses are utilized. The structure has hundreds of modes and the simulation is performed out to 4000 Hz.

Efficient Method of Measuring Effective Mass of a System

Effective mass is a system property that is used in the aerospace industry in predicting the forces of an elastic payload on the delivery system in a specific direction. Effective mass is usually calculated with the finite element model. Experimental effective mass can be used to validate calculated effective mass from a finite element model. Measuring the effective mass of a system, however, has been difficult and has been attempted by putting force sensors between the payload and a test base. A much more tractable method amenable to a payload mounted on a slip table is provided here. The method measures a driving point and a base frequency response function and uses a recently developed method to constrain the base response and calculate the effective mass. This theory will be demonstrated with an analytical system and example hardware.

Dynamic Measurements on Miniature Springs for Flaw and Damage Detection

Small components are becoming increasingly prevalent in today’s society. Springs are a commonly found piece-part in many mechanisms, and as these components become smaller, so do the springs inside of them. Because of their size, small manufacturing defects or other damage to the spring may become significant: a tiny gouge might end up being a significant portion of the cross-sectional area of the wire. However, their small size also makes it difficult to detect such flaws and defects in an efficient manner. This work aims to investigate the effectiveness of using dynamic measurements to detect damage to a miniature spring. Due to their small size, traditional instrumentation cannot be used to take measurements on the spring. Instead, the non-contact Laser Doppler Vibrometry technique is investigated. Natural frequencies and operating shapes are measured for a number of springs. These results are compared against springs that have been intentionally flawed to determine if the change in dynamic properties is a reasonable metric for damage detection.

A Color-Coded Complex Mode Indicator Function for Selecting a Final Mode Set

Many test articles exhibit slight nonlinearities which result in natural frequencies shifting between data from different references. This shifting can confound mode fitting algorithms because a single mode can appear as multiple modes when the data from multiple references are combined in a single data set. For this reason, modal test engineers at Sandia National Laboratories often fit data from each reference separately. However, this creates complexity when selecting a final set of modes, because a given mode may be fit from a number of reference data sets. The color-coded complex mode indicator function was developed as a tool that could be used to reduce a complex data set into a manageable figure that displays the number of modes in a given frequency range and also the reference that best excites the mode. The tool is wrapped in a graphical user interface that allows the test engineer to easily iterate on the selected set of modes, visualize the MAC matrix, quickly resynthesize data to check fits, and export the modes to a report-ready table. This tool has proven valuable, and has been used on very complex modal tests with hundreds of response channels and a handful of reference locations.

Using High-Resolution Measurements to Update Finite Element Substructure Models

Many methods have been proposed for updating finite element matrices using experimentally derived modal parameters. By using these methods, a finite element model can be made to exactly match the experiment. These techniques have not achieved widespread use in finite element modeling because they introduce non-physical matrices. Recently, Scanning Laser Doppler Vibrometery (SLDV) has enabled finer measurement point resolution and more accurate measurement point placement with no mass loading compared to traditional accelerometer or roving hammer tests. Therefore, it is worth reinvestigating these updating procedures with high-resolution data inputs to determine if they are able to produce finite element models that are suitable for substructuring. A rough finite element model of an Ampair Wind Turbine Blade was created, and a SLDV measurement was performed that measured three-dimensional data at every node on one surface of the blade. This data was used to update the finite element model so that it exactly matched test data. A simple substructuring example of fixing the base of the blade was performed and compared to previously measured fixed-base data.

Strategies for Testing Large Aerospace Structures with 3D SLDV

The 3D Scanning Laser Doppler Vibrometer (3D SLDV) has the ability to scan a large number of points with high accuracy compared to traditional roving hammer or accelerometer tests. The 3D SLDV has disadvantages, however, in that it requires line-of-sight from three scanning laser heads to the point being measured. This means that multiple scans can become necessary to measure large or complex parts, and internal components cannot typically be measured. In the past, large aerospace structures tested at Sandia National Laboratories typically have used a handful of accelerometer stations and instrumented internal components to characterize these test articles. This work describes two case studies that explore the advantages and difficulties in using a 3D SLDV to measure the same test articles with a much higher resolution scan of the exterior. This work proposes strategies for combining a large number of accelerometer channels with a high resolution laser scan. It explores the use of mirrors and laser head mounts to enable efficient re-alignment of the lasers with the test article when many scans are necessary, and it discusses the difficulties and pitfalls inherent with performing such a test.

Modal Testing of a Nose Cone Using Three-Dimensional Scanning Laser Doppler Vibrometry

The Structural Dynamics department at Sandia National Laboratories has acquired a 3D Scanning Laser Doppler Vibrometer system for making vibration and modal test measurements. This paper presents the results of testing performed to examine the capabilities and limitations of that system. The test article under consideration was a conical part with two different surface materials which allowed the examination of the effect of angle of incidence and surface reflectivity on the measurement. The system was operated in both 1D and 3D modes, and the results from the 1D scan were compared to a scan performed with a previous generation system to evaluate the improvements between the generations. Data from the laser systems were exported to standard curve fitting software, and modes were fit to the data.