Characterization of Wind Loading
1. Introduction
Understanding the role of wind in the design of a next generation telescope will be fundamental owing to two concerns: (1) the direct effects of wind buffeting on the mechanical structure and other subsystems; and (2) the indirect effects on local seeing resulting from thermal effects and turbulence induced by airflow around structural elements. This chapter summarizes our efforts to understand these effects based on analysis and measurements of wind loading on current generation telescopes. Our studies represent first steps toward a deeper understanding of how to model the effects of wind on ELTs and how to mitigate these effects via appropriate choice of site, enclosure, structural design, and adaptive optics.
The first concern is wind buffeting – as air moves across the structure, the incident pressure causes structural deformations. These deformations affect both the primary mirror (M1) and the secondary (M2). Because of the extremely tight tolerances required for operation in Optical and Near-infrared wavelengths, such deflections must be kept to a minimum. The problem is compounded by the nature of the loading. With the advent of active optics, it is possible to correct slowly varying errors caused by gravity and changing temperature. Very high-frequency disturbances are generally small enough to be ignored. However, wind loading presents a challenge because it is a dynamic load at low-enough frequency to produce significant displacements.
Telescope designers have traditionally provided wind protection for the telescope by enclosing it in a tightly enclosed dome. However, measurements made at the MMT1,2, the AAT3 and elsewhere showed that significant improvements in seeing were possible if local temperature effects could be reduced. During the design of the current generation of large telescopes, water and wind tunnel testing and computational fluid dynamics (CFD) were used to analyze the airflow in and around telescope enclosures, with the goal of developing designs with better ventilation to flush out warm air trapped in the enclosure. As a result, modern enclosures generally provide large, adjustable vent areas or active HVAC systems to address the local seeing issue, while still mostly shrouding the telescope structure. These designs have reached a balance between providing sufficient airflow to ensure good seeing and providing sufficient blockage to protect the structure from wind buffeting under a variety of external wind loading conditions.
Many current proposals for extremely large telescopes (ELTs) are for telescopes of 30m diameter and larger. Until now, primary mirror diameters of this size have usually been associated only with radio telescopes. In contrast to O/IR designs, large radio telescopes are generally operated in the open air, but have dramatically lower surface accuracy requirements. Even so, wind buffeting is a concern for such telescopes both in the pointing accuracy and (for mm- and sub-mm-wave systems) the surface accuracy. Addressing the seeing and wind buffeting problems for ELTs is particularly formidable. They have a considerably larger cross-section to the incident wind, increasing the total loading and the quasi-static deflections. Very large structures also have lower natural frequencies, resulting in a larger dynamic response in the wind. At the same time, the demand for sharper images reduces the tolerance for wind-induced image motion.
The other concern is local seeing, which is determined by variations in temperature between the telescope, the enclosure, and the surrounding air. This effect can be dramatically reduced by allowing airflow through the enclosure and across the telescope and mirror. Such airflow removes localized warm air layers and also aids in bringing the system to equilibrium with the outside air temperature.
While it is understood that local seeing and wind buffeting cause dynamic distortion and motion of the image, there are many unknowns in designing to address the problem. To actually design a successful ELT, there are several key wind characterization issues that must be understood:
1.1. The Seeing Problem
Ventilation of a large enclosure
As the size of an enclosure increases, the size of the ventilation openings must increase as well. Indeed, if an existing enclosure design is scaled proportionally, then the ratio of vent area to the projected enclosure cross-sectional area remains constant. However, the total vent area increases by the square of the structure size, while the volume contained within the dome increases by the cube. This suggests that for the same inlet wind velocity, an enclosure for a 30m-class telescope could take three times longer to change the air than a similar enclosure for a 10m version.
While the actual requirements for either air flow rate or enclosure flushing rate are not well known, an understanding of this problem will be essential in scaling an enclosure design to accommodate an ELT. The extent to which adaptive optics systems can correct for local seeing, thus reducing ventilation requirements, must also be understood.
1.2. The Buffeting Problem
Characterization of wind buffeting
In order to design an adequate telescope structure, one must be able to predict the amplitude, distribution, and frequency content of both the incident wind load and the response of the telescope structure. For such a large leap in scale, it is advantageous to base these predictions on direct measurements of wind buffeting and structural response at large existing facilities. When combined with a finite-element analysis (FEA) of the structure, such tests allow the creation of a benchmark to show the level of prediction possible by computational fluid dynamics (CFD) and FEA.
Most of the wind characterization data for proposed sites will be measurements of wind velocity, but to predict the response of an ELT structure to wind buffeting, it is necessary to quantify the incident forces applied by the wind. This suggests that it is necessary to measure the applied forcing on the telescope both by direct measurement of pressures and by extraction from structural response data. These force measurements can then be compared with simultaneously-measured local wind velocities to determine the conversions from wind velocity to drag force that are appropriate for dynamic wind loading in this type of structural environment.
An understanding of the wind input measured at existing facilities will demonstrate the predictability obtainable from modeling, and will bound the design problem in terms of required structural stiffness, range of actuator strokes in the active and adaptive optics systems, and the required bandwidths on the control systems.
How to scale measurements to ELTs
A critical component of applying the benchmark results will be determining how to scale those results to the larger scale of an ELT, for example, the 30m GSMT point design. An understanding of such scaling must be obtained from the measurements, from simulations, and from an understanding of fluid mechanics, and will ultimately be the basis for the generation of dynamic load cases for structural analysis of an ELT.
1.3. Separating the effects of telescope wind loading
The deleterious effects of wind buffeting are primarily caused by the dynamic portion of the wind loading. In order to optimize the design of the facility to limit wind buffeting effects, it is necessary to understand the source of the dynamic content of the wind force. In general, this may be from: (1) turbulence in the incoming wind arriving at the enclosure; (2) turbulence induced by passing through openings in the enclosure; (3) turbulence generated by interaction of the wind with surfaces on the telescope structure itself. These are discussed below.
Local topography
Turbulence in the wind arriving at the observatory is a combination of the dynamic content of the wind approaching the mountain with the turbulence generated by interactions with the local topography of the summit. This is a complex situation that depends not only on local wind direction, but also on the local thermal environment. It will be necessary to simulate the wind flows at proposed sites, as was done for many of the current generation of 8m telescopes. Given the importance of the wind-buffeting problem, such modeling should be done fairly early in the site selection process (see Section 6.2).
The direct measurements of wind velocity described above should include measurements that can be used to calibrate CFD studies of the effects of local topography.
Enclosure design
In addition to the ventilation issues for a large enclosure, it is important to understand the effects of the turbulence generated by the enclosure as the wind enters the slit and the vents. CFD simulations of different types of vents and enclosure geometries, combined with the analysis of in situ measurements at existing facilities, will help guide the enclosure design.
Telescope feature shapes
Dynamic loading of telescope structures may be largely generated by turbulence induced by the interaction of the wind with the telescope structure itself. As a final mitigation against wind buffeting, fairings might be used on parts of the telescope structure to reduce vortex generation caused by the flow around the structure. An understanding of such effects would likely not alter the fundamental design of the telescope structure, but could potentially reduce the dynamic variations of wind pressures on the system and thus reduce the requirements on the active/adaptive compensation systems.
2. Summary of previous studies
Each of the large telescope projects of the current generation has had to contend with local seeing and wind buffeting effects, and a number of papers and reports have been written on wind loading of astronomical telescopes. Prior to considering what new studies are necessary to address the issues outlined in the previous section, it is worthwhile to review the results, information, and unresolved questions from previous studies of local seeing and wind loading on enclosures and telescopes. As we begin to design even larger telescopes, we need to learn everything we can from the 8-10 meter class facilities.
2.1. Local Seeing Studies
During the design of the current generation of telescopes, many of the old paradigms concerning telescope enclosure design were challenged. Traditional, relatively expensive designs, with the telescope mounted in a tightly-closed, hemispherical dome on top of a multi-purpose observatory building, were found to provide inferior seeing compared with less expensive structures such as the lower, box-like design employed by the MMT2,4. In spite of the larger opening and correspondingly greater exposure to wind, the MMT experience indicated that their best image sizes were obtained under a 10-20 mph wind5. These realizations led to a revolution in enclosure design, in which ventilation of the dome was recognized as being as essential as wind protection.
To understand how to reduce enclosure-induced seeing, many groups performed experiments via scale model testing of flushing efficiency of different dome designs. A good example of this is the Gemini Observatory enclosure study conducted at the water tunnel facility at the University of Washington6. In this test, the importance of ventilation was demonstrated dramatically (Figure 1). Comparisons of flushing times with and without vents for the various designs revealed that improvements of a factor of 2-8 were possible. Additionally, a test of a cylindrical enclosure with a large slit opening but without cross-flow ventilation from side vents showed no improvement during side winds, confirming the notion that through flow is critical to exchanging the air and improving seeing.
Figure 1. Water tunnel test of an early model of a Gemini enclosure. In this case, the side vent openings were closed and dye injected in the telescope chamber vented slowly, mostly through the observing slit. (Reprinted from reference 6.)
Since the revolution in enclosure design philosophy, existing telescopes have added active or passive ventilation systems to their enclosures, resulting in improved performance. It is clear that such an approach will also be necessary in the design of any enclosure for an ELT.
2.2. Site Wind Characterization
Previous contributions have also been made in the area of wind characterization, both in the site selection of telescopes and in the operation of existing facilities. Indeed, most observatories monitor the wind speed and direction as a standard operating procedure, and several make use of instruments capable of measuring the spectrum out to moderate frequencies. Wind power spectra have been published for several sites7,8, and vary in their behavior. While some results track the classical Davenport9 spectrum, others appear to have more energy in higher frequencies, in line with the Antoniou10 spectrum. An example of such data from reference 8, taken from San Pedro Martir, is shown in Figure 2 below. The published results of such tests have frequently been limited to a single sensor at a single location, and often do not include enough time in the sample to allow for long averaging, resulting in a noisy appearance. However, they confirm that the usual design spectra generally bound the problem.
There have been many wind tunnel tests of proposed telescope and enclosure designs11,12,13. These tests have generally been intended for calculation of survival condition loads on the enclosure or telescope, and for calculation of mean pressures on parts of the structure (e.g., the primary mirror). Such studies are helpful because they provide scale model measurements under controlled conditions and can provide information on the static component of the pressure distribution. It is worth noting that these studies cannot generally be used for investigating dynamic interactions between the structure and the flow, because it is difficult to achieve dynamic scaling and size scaling simultaneously. Flow visualization has also been performed in these tests, but is typically limited by what is visible.
To extend the wind tunnel results, it has become increasingly common to employ CFD analyses in predicting the behavior of wind flow within the environment of the enclosure and in the area around the site. This technique was employed extensively, for example, to investigate the effects of local topography at both Mauna Kea and at Cerro Pachon14. The approach has the advantage that it allows visualization and recovery of the local conditions (velocities, pressure, etc.) at a large number of points, and allows changing of flow conditions such as wind speed and direction. Figure 3 below shows the type of result available from this kind of analysis.