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This work presents preliminary results on aero-mechanical jitter of a hemispherical optical turret. A simplified geometry with a hemispherical shell and optics-holding canister was designed to reduce degrees of freedom and provide better insight into fundamental physics. Modal analysis of the turret and mounting plate to the wind tunnel, performed...
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... modes were solved with finite element analysis using Ansys Mechanical. A zero displacement boundary condition was applied around the edge of the plate, where it is bolted to the wind tunnel. The leading modes are shown in Fig. 4. A significant takeaway is that most of the motion of the model mirror is associated with the deformation of the mounting plate, with negligible deformation of the turret itself. Further, the modes of the plate with the canister mounted look very similar to the modes of the plate without the canister. For the 1/4" plate the first mode ...
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... so we could see which parts were modeled correctly. This way, we could pinpoint which part or connection was not properly modeled. For the impact tests, twelve PCB accelerometers were placed around the plate as shown in Fig. 5, where the canister is facing sensor 3. The accelerometers were placed close to the peaks of the vibration modes shown in Fig. 4, with sensors 11 and 12 placed at the edge of the plate to monitor the boundary motion and to see how close to a fixed boundary condition it is. The turret and plate were installed in the wind tunnel and tapped with a plastic mallet repeatedly in different locations on the plate for a total of about 10 taps, waiting for the ringing to ...
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... a spatial plane shown in Fig. 7 for the first three peak frequencies from Fig. 6. Constant zero values were added to the edge of the plate where there were no accelerometers and values were interpolated between all of the point measurements to create a spatial map of the Fourier mode. We can see that the modes look quite similar to those shown in Fig. ...
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... decomposition was done using the Matlab code from Ref. 29. The real component of the first three SPOD modes for the first three peak frequencies are shown in Fig. 8. Here we see that the SPOD modes very closely match the FEA mode shapes shown in Fig. 4 This modal analysis was important to ensure that the finite element model of the turret and plate matches the measured data from the experiment. Material properties between the different components were matched as best as possible. The weights of the components are within 3% of the weight estimated in ...
Citations
... I n many cases in experiments, only sparse sensor data is available, when a full field measurement is desired. For the case of the work that inspired the analysis done here, we are interested in aerodynamic forcing on a hemisphere [1]. However, to calculate forcing we need pressure over an area, thus knowledge of the full pressure field is required. ...
Reconstructing detailed flow fields from sparse sensor data is a critical challenge in aerodynamic studies. This work evaluates the performance of several reconstruction methods, including Proper Orthogonal Decomposition (POD), Linear Stochastic Estimation (LSE), a combined LSE-POD method, and a method based on Shallow Decoder Networks (SDN). These methods were tested to reconstruct surface pressure fields around a hemispherical turret. The study incorporates data-driven sensor placement using QR factorization to optimize sensor locations and compares results to randomly placed sensors. The reconstruction methods were validated using high-resolution experimental pressure fluctuation data obtained from a hemispherical model in subsonic flow. Results demonstrate that the LSE method achieves the highest accuracy among the methods tested, while the SDN-based method proved to be the most robust to noise and to varying flow speed conditions.QR-based sensor placement was shown to improve reconstruction performance for all methods, particularly in regions with sparse dynamic activity.
... We see that despite the change in surface pressure fluctuation between the oscillating and stationary turrets, topological changes in the wake are relatively small, even for large lateral displacements of the protruding beam director (on the order of one percent of the aperture diameter). Ongoing research continues to explore approaches aimed at mitigating aero-mechanical jitter [104]. ...
This paper serves as part II of a two-part tutorial on “aero-optical effects.” In part I, we provide introductory material with an emphasis on system-level considerations, particularly for those who are new to the field of aero-optics. In part II, we move on to survey several sources of aberrations. For example, we cover foundational sources like boundary layers and shear layers, as well as miscellaneous sources like mechanical contamination, shock waves, and aero acoustics. Throughout part II, we emphasize drivers for system-level performance, which appropriately builds on the system-level considerations covered in part I. This emphasis will inform future efforts looking to develop airborne-laser systems flying at subsonic, supersonic, and hypersonic speeds.
