Patrick S. Hunter

Fluid-structure interactions in compressible cavity flows

Experiments were performed to understand the complex fluid-structure interactions that occur during aircraft internal store carriage. A cylindrical store was installed in a rectangular cavity having a length-to-depth ratio of 3.33 and a length-to-width ratio of 1. The Mach number ranged from 0.6 to 2.5 and the incoming boundary layer was turbulent. Fast-response pressure measurements provided aeroacoustic loading in the cavity, while triaxial accelerometers provided simultaneous store response. Despite occupying only 6% of the cavity volume, the store significantly altered the cavity acoustics. The store responded to the cavity flow at its natural structural frequencies, and it exhibited a directionally dependent response to cavity resonance. Specifically, cavity tones excited the store in the streamwise and wall-normal directions consistently, whereas a spanwise response was observed only occasionally. The streamwise and wall-normal responses were attributed to the longitudinal pressure waves and shear layer vortices known to occur during cavity resonance. Although the spanwise response to cavity tones was limited, broadband pressure fluctuations resulted in significant spanwise accelerations at store natural frequencies. The largest vibrations occurred when a cavity tone matched a structural natural frequency, although energy was transferred more efficiently to natural frequencies having predominantly streamwise and wall-normal motions.

Simultaneous Vibration and Acoustic Measurements of a Store in Compressible Open Cavity Flow.

To understand the complex fluid-structure interactions that occur during internal store carriage within a cavity, an experimental program has been developed to simultaneously measure the acoustic loading and store vibrations. A cylindrical store was installed in a cavity having a length-to-depth ratio of 3.33 and a length-to-width ratio of 1. Experiments were conducted at a freestream Mach number of 0.80 and the incoming boundary layer thickness was about 40% of the cavity depth. Fast-response pressures provided a measure of the aeroacoustic loading in the cavity, while triaxial accelerometers and laser Doppler vibrometry (LDV) were used for simultaneous store vibration measurements. Overall, the LDV and accelerometer data were in good agreement, but the LDV offered the advantage of increased spatial resolution, while the accelerometers were able to provide three-dimensional data. The simultaneous measurements demonstrated that cavity modes were able to excite the store, but with a directional dependence. Modal hammer tests were used to measure the natural frequencies of the store. The largest store accelerations were observed to occur along the spanwise direction at frequencies equal to the natural spanwise frequencies of the store.

Structural Dynamics Testing and Analysis for Design Evaluation and Monitoring of Heliostats

Heliostat vibrations can degrade optical pointing accuracy while fatiguing the structural components. This paper reports the use of structural dynamic measurements for design evaluation and monitoring of heliostat vibrations. A heliostat located at the National Solar Thermal Testing Facility (NSTTF) at Sandia Labs in Albuquerque, New Mexico, has been instrumented to measure its modes of vibration, strain and displacements under wind loading. The information gained from these tests will be used to evaluate and improve structural models that predict the motions/deformations of the heliostat due to gravitational and dynamic wind loadings. These deformations can cause optical errors and motions that degrade the performance of the heliostat. The main contributions of this work include: (1) demonstration of the role of structural dynamic tests (also known as modal tests) to provide a characterization of the important dynamics of the heliostat structure as they relate to durability and optical accuracy, (2) the use of structural dynamic tests to provide data to evaluate and improve the accuracy of computer-based design models, and (3) the selection of sensors and data-processing techniques that are appropriate for long-term monitoring of heliostat motions.Copyright

Modal Analysis of a Heliostat for Concentrating Solar Power

A heliostat is a structure whose function is to reflect sunlight to a target collector. Heliostat vibrations can degrade optical pointing accuracy and fatigue the structural components. This paper reports on an experimental and analytical program with a goal to improve understanding of the response to wind loading on heliostats. A modal test was performed on a heliostat located at the National Solar Thermal Testing Facility (NSTTF) at Sandia Labs in Albuquerque, New Mexico. Modal tests were performed with artificial and natural wind excitation. Strain and displacements were also measured under wind loading. The information gained from these tests has been used to evaluate and improve structural models that predict the deformations of the heliostat due to gravitational and dynamic wind loadings. The paper will provide an up-to-date summary of model validation work, evaluation of suitable sensors, and development of data-processing methods for long-term deformation monitoring.

Excitation Methods for a 60 kW Vertical Axis Wind Turbine

A simple modal test to determine the first tower bending mode of a 60 kW (82 feet tall) vertical axis wind turbine was performed. The minimal response instrumentation included accelerometers mounted only at easily accessible locations part way up the tower and strain gages near the tower base. The turbine was excited in the parked condition with step relaxation, random human excitation, and wind excitation. The resulting modal parameters from the various excitation methods are compared.

Examples of Hybrid Dynamic Models Combining Experimental and Finite Element Substructures

Substructuring methods have been used for many years to reduce the size of FE dynamic models and maintain satisfactory response for a limited bandwidth of interest. However, experimental substructures have been used only in a very limited number of cases, generally where there was only a single connection point idealized to six connection degrees of freedom. This is because it is difficult to experimentally characterize the moments and rotations at multiple connection points. Mayes and Allen developed a method to practically characterize continuity and equilibrium at multiple connection points through the instrumentation of a flexible fixture. The instrumented fixture is transformed into a force and response sensor that is expressed in terms of the modal connection forces and modal connection responses of the fixture. The sensor is called the transmission simulator. Two previous results of the method for coupling R&D pieces of hardware are given. The primary emphasis of this paper is on a set of representative system hardware, for which two designs of a transmission simulator are compared with the view to discover design characteristics that optimize the sensor effectiveness. Frequency response functions (FRF) from the assembled system hardware are measured and designated as the truth against which to compare. The accuracy of the experimental substructure is evaluated by attaching it to a finite element substructure and predicting full system response FRFs which are compared against the truth FRFs. The result is that the transmission simulator design is relatively robust to the stiffness chosen, although the stiff transmission simulator appears to be a slightly better choice in the structure evaluated in this work. Substructure modes tend to maintain the shapes of the bare transmission simulator if it is stiff. A one piece transmission simulator design with no joints is easier to model accurately, which is of value in this methodology.

A Simpler Formulation for Effective Mass Calculated from Experimental Free Mode Shapes of a Test Article on a Fixture.

Effective mass for a particular mode in a particular direction is classically calculated using a combination of fixed base mode shapes, the mass matrix, and a rigid body mode shape from a finite element model. Recently, an experimental method was developed to calculate effective mass using free experimental mode shapes of a structure on a fixture (the base) along with the measured mass of the fixture and of the test article. The method required three steps. The first step involved constraining all the free modes of the fixture except one rigid body mode in the direction of interest. The second step involved calculating pseudo-modal participation factors for this case. The third step involved constraining the final fixture rigid body degree of freedom and utilizing the constraint matrices with pseudo-modal participation factors to obtain the estimate of the standard modal participation factors which can be converted to effective mass. This work provides a simpler formulation. After the constraint in step one above, the effective masses are calculated directly from the mass normalized mode shapes of the fixture. In most cases this method gives the same answer as the original approach, within experimental error. In some instances, it appears more robust with low signal to noise ratios. It also provides better physical insight as to which modes have significant effective mass in a particular direction. The new approach is illustrated by experimental example.

Predicting Assembly Effective Mass from Two Component Effective Mass Models

Effective mass models are powerful tools that allow for a convenient means to calculate the energy associated with vibration response of a structure to a base input acceleration in a particular direction. This is useful for hardware qualification activities and margin assessment. Traditionally, these models are generated from purely analytical means such as a finite element model. However, experimental methods have recently been introduced as an intriguing alternative, particularly for applications where no finite element model is available. In this work, an effective mass modal model of a cable-connector assembly is desired, and neither component has a finite element model. Moreover, there can be multiple cable-connector combinations making analytical modeling as well as explicit testing of each combination impractical. This work develops the capability to combine an experimentally derived connector effective mass model with a simplified and easily extensible analytical cable model. The experimental connector effective mass model is generated through specialized modal testing. The simplified cable model is a Timoshenko beam finite element model whose properties are empirically derived from pinned-pinned cable modal data. The modeled length of the cable is appropriately adjusted for each configuration. Finally, the cable and connector component models can be combined to form the final assembly modal effective mass model for a given translational direction. This method lends itself to developing catalogues of connector and cable data, which can then be easily combined to form any number of assembly configurations without having to explicitly test/model them.

Converting a Slip Table Random Vibration Test to a Fixed Base Modal Analysis

Validation of finite element models using experimental data with unknown boundary conditions proves to be a significant obstacle. For this reason, the boundary conditions of an experiment are often limited to simple approximations such as free or mass loaded. This restriction means that vibration testing and modal analysis testing have typically required separate tests since vibration testing is often conducted on a shaker table with unknown boundary conditions. If modal parameters can be estimated while the test object is attached to a shaker table, it could eliminate the need for a separate modal test and result in a significant time and cost savings. This research focuses on a method to extract fixed base modal parameters for model validation from driven base experimental data. The feasibility of this method was studied on an Unholtz-Dickie T4000 shaker and slip table using a mock payload and compared with results from traditional modal analysis testing methods.

Preliminary Validation of a Complex Aerospace Structure

A series of modal tests were performed on a complex aerospace structure, consisting of a shell structure with joints and discrete payloads, in order to validate a finite element model of the structure. Modal tests have been performed on individual assemblies followed by model updating using the measured modal data. The final configuration has placed all assemblies together as a complete unit which includes a multitude of joints and interfaces. Frequency response functions (FRFs) were chosen as the validation metric.