Thermal modeling environment for TMT

Abstract

In a previous study we had presented a summary of the TMT Aero-Thermal modeling effort to support thermal seeing and dynamic loading estimates. In this paper a summary of the current status of Computational Fluid Dynamics (CFD) simulations for TMT is presented, with the focus shifted in particular towards the synergy between CFD and the TMT Finite Element Analysis (FEA) structural and optical models, so that the thermal and consequent optical deformations of the telescope can be calculated. To minimize thermal deformations and mirror seeing the TMT enclosure will be air conditioned during day-time to the expected night-time ambient temperature. Transient simulations with closed shutter were performed to investigate the optimum cooling configuration and power requirements for the standard telescope parking position. A complete model of the observatory on Mauna Kea was used to calculate night-time air temperature inside the enclosure (along with velocity and pressure) for a matrix of given telescope orientations and enclosure configurations. Generated records of temperature variations inside the air volume of the optical paths are also fed into the TMT thermal seeing model. The temperature and heat transfer coefficient outputs from both models are used as input surface boundary conditions in the telescope structure and optics FEA models. The results are parameterized so that sequential records several days long can be generated and used by the FEA model to estimate the observing spatial and temporal temperature range of the structure and optics. Keywords : Extremely Large Telescopes, Thermal Modeling, Computational Fluid Dynamics INTROD UCTIONThermal Management is a key aspect of Wave-front Control Architecture. Approximately 30% of the total seeing limited error budget allocation has aero-thermal causes, with 80% of that attributed to thermal seeing and deformations. As tools to minimize thermal effects TMT uses air-conditioning during day-time, passive ventilation during night-time and heat release control in various forms (sub-system cooling, proper location of heat sources, coatings of desired emissivity). The optimization of the configuration and operation of these tools is done by modeling and is beyond the scope of this paper. However, once these tools have been incorporated in the design, they create a more stable thennal environment for the telescope structure and the optics and different active optics correction scenarios can be modeled. Figure I depicts the current TMT aero-thermal modeling synergy. The gray box called "Stochastic Framework" represents the TMT Monte Carlo simulation test-bed [I] and fuses together performance estimates for optical surface shapes, wind disturbances and thermal seeing. It utilizes the normalized Point Source Sensitivity (PSSn) performance metric [2]. A 3-year environmental record [3] measured on 13N, Mauna Kea, the TMT site, provides ambient condition variability and the corresponding telescope pointing record from Gemini orth provides orientation statistics. The ...

TMT telescope structure thermal model

“…After studying the latest expected inputs and estimated environment we made the following selections for the environmental conditions. This is an approximate curve-fit to the average diurnal behavior of the telescope environment [5]. The daytime temperature vertical gradient is conservative (by about a factor of 1.5 based on previous CFD analysis) and it is also assumed to be attained immediately after sunrise (also conservative).…”

Integrated modeling and systems engineering for the Thirty Meter Telescope

“…However, the gravitational load depends on the zenith angle of the telescope, which in turn is the function of the observing program carried out by observatory. The inputs to thermal deformation calculations are also random environmental and operational parameters [8]. Even the statistics of true random processes, like wind buffeting depend on operational parameters: telescope and enclosure zenith angle and azimuth position relative to wind direction, as well as the level of enclosure venting.…”

“…Addressing this coupling required data iterations between the computational fluid dynamics (CFD) and finite element analysis (FEA) models [8], [19]. We chose the "bucket brigade" method -i.e.…”

“…To understand and design the thermal management of the observatory requires modeling and following it for several days, through daytime air conditioning and nighttime environmental heat exchanges. Based on the Standard Year, a 3 days history of aero-thermal and thermal changes was developed, including (i) air conditions inside and around of the enclosure, (ii) the thermal states of solid components, like structure and optical glass, as well as (iii) their interaction [8], [13]. iii.…”

Experiments at the W.M. Keck Observatory to support the Thirty Meter Telescope design work

“…Air temperature fluctuations and motions of the air result in optical turbulence. 7 We are investigating the effect of wind on the telescope structure motions and the control system's ability to mitigate those effects. We are also seeking to validate the Computational Fluid Dynamics (CFD) models being used for the TMT design by making predictions of the conditions at the Keck telescopes, the wind and temperature fields are inputs to those models.…”