Technology
Nugget—TMT Aerodynamic Studies: Simulating the Wind Mountaintops are windy places. Nevertheless, they are the best places for observatories. The TMT project plans to build a huge structure, comparable to an oil rig, on the top of a mountain. We also need to align this structure to a precision of a few nanometers, and then keep it so. Can we do it? Yes, we are convinced we can. Here is why. For several years, we have been conducting computational fluid dynamic (CFD) simulations of the potential mountaintops and various enclosures considered for TMT. The earliest aerodynamics models reflected the precursor designs: CELT, GSMT, and VLOT. An extensive wind-measurement campaign was carried out with the Gemini South telescope, producing a large amount of pressure and velocity data, mostly from the 8-meter primary mirror. Two series of wind tunnel measurements were done to investigate the flow field in and around generic spherical enclosures. Since the TMT reference design was established in September 2004, its performance under various wind loads has been scrutinized extensively. A wind model based on all the measurements, CFD calculations, and wind tunnel data was developed. The model estimates the pressure and velocity profile of the turbulence (specifically, its power spectral densities) at a range of critical locations inside the enclosure: at the secondary mirror, along the telescope tube, and on the primary mirror. [The pressure bandwidth and RMS value of the von Karman-type turbulence is predicted as a function of external mean wind velocity and the telescope azimuth and elevation angles, as well as from the effective area of dome venting.] However, knowing the enemy is only half of the battle. We also have to develop and test our strategy for defeating the wind as much as possible. Besides minimizing the wind cross section of the telescope, our most effective weapon is the mount control system. Its encoders detect telescope motion and deformation due to wind, with help from a dedicated wavefront sensor aimed at a natural guide star. Based on this information, the control system continuously adjusts the pointing of the telescope and guides it to minimize the effect of wind buffeting. We test our control strategy via computer, where an integrated telescope model simulates the combined behavior of the telescope structure under wind loading, optics, and all the control systems. The wind problem may be exacerbated by potential coupling to the local thermal environment, most notably in relation to the “seeing” just above the primary mirror, which is the result of the uneven atmospheric boundary layer above the mirror. Another coupled factor is dome seeing, which covers the optical effects of the non-isothermal air turbulence inside the enclosure and around the observing slit. To estimate the cumulative effect of these couplings, dome- and mirror-seeing models were developed, based on thermal CFD and optical models of the enclosure and the boundary layer above the primary mirror. Mitigating local seeing requires maintaining a steady flushing of even-temperature air across the enclosure and the primary mirror, which in turn results in constant low-level wind buffeting. Preliminary simulations show that under reasonable environmental and operating conditions, these two processes dominate distinct external wind speed regimes, meaning that we can mitigate their adverse effects independently of each other. Although we are continuously refining the models used for predicting the wind- and thermal-related performance of the TMT, preliminary results from our simulations indicate that the system can already maintain its very tight error budget for image jitter through the 70th percentile of the standard wind conditions that the telescope can expect to encounter.
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