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 MMT 1,2, 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 submm-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:
Chapter 5: Technical Studies
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Section 5.5 Characterization of Wind Loading
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
Chapter 5: Technical Studies
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Section 5.5 Characterization of Wind Loading
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 MMT 2,4. 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.
Chapter 5: Technical Studies
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Section 5.5 Characterization of Wind Loading
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.
Chapter 5: Technical Studies
4
Section 5.5 Characterization of Wind Loading
Figure 2. Power spectral density of wind velocity
fluctuations measured on San Pedro Martir, April
17, 1999. Average horizontal wind speed was
9.2 m/sec. The ultrasonic anemometer was
located on a mast 6.8 meters above the ground.
(Reprinted from reference 8.)
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.
Figure 3. Magnitude of vorticity
in a cross section of the
atmosphere immediately above
the summit of Mauna Kea.
Vorticity increases from zero
(dark blue), or laminar flow, to
the maximum value shown as
red.
The free stream wind
speed was 20 m/s, from the
west. (Reprinted from reference
14.)
Chapter 5: Technical Studies
5
Section 5.5 Characterization of Wind Loading
In this case, the analysis was used to confirm that upwind topographic features did not have a
significant influence on performance, and that the proposed height of the telescope pier would put
the primary mirror above the turbulent boundary layer for the majority of wind conditions at the
site [Ibid].
2.3. WIND BUFFETING
The types of studies mentioned above did not produce data that are very useful for predicting
dynamic wind loading on the telescope, particularly the primary mirror. Water tunnel tests provide
visualization of flow patterns but can’t give accurate quantitative information about the wind
loading. Measurements of spectra made at observatory sites do not take into account the effects
of the enclosure and telescope in increasing wind turbulence, but several studies 4,15,16 have
reported increased wind energy at higher frequencies inside telescope enclosures, particularly
with the telescopes pointed close to the zenith rather than facing into the wind. CFD
studies13,18,19 have been limited in the amount of detail they can model, and have usually been
able to predict only average pressure variations. See Figure 4. Wind tunnel tests offer insights
into average pressures at various points on telescope models, for example at the primary mirror,
but the several orders of magnitude difference in Reynolds number makes the accurate
simulation of turbulent effects impossible, even when model features have been made
intentionally rough. In addition, the pressure taps are normally recorded individually.
The problem is that both the spatial and temporal frequency of the wind loading on the telescope,
are crucial to determining the structural response. For example, most large telescope mirror
support systems can handle uniform wind pressure on the primary mirror, since it is only a small
fraction of the mirror weight. Similarly, telescopes with active optics systems, either large-mirror
bending systems or segmented-mirror position control systems, can compensate for a quasistatic non-uniform wind pressure pattern, provided the rate of change of the pattern is slow
compared to the update cycle of the active system (slower than 0.1 Hz, for example). The
problem comes from non-uniform pressure patterns that change with time at rates faster than
about 0.1 Hz. Therefore, to understand the effect of wind buffeting on an ELT primary mirror, it is
necessary to understand both the spatial and temporal variation of wind pressure.
Several studies have attempted to measure wind pressure simultaneously at several points on a
telescope primary mirror. The Subaru Project made measurements at the Canada France
Hawai’i Telescope using differential pressure sensors to record the pressure difference between
the front and back of the CFHT mirror cover. However, only five sensors were used, so the
spatial information they obtained was limited. Similarly, Forbes and Gabor 7 measured patterns of
pressure at the MMT and at UKIRT, with differential pressure sensors recording pressure
differences between the front and back of the mirror covers, but they only used four sensors;
again, the spatial information was limited.
The most important previous measurement on wind buffeting prior to the recent tests at Gemini
South (Section 3) were those taken by ESO at La Silla16. In these tests, 13 pressure taps were
arranged on a 3.5-meter plywood dummy mirror, as shown in Figure 5, and placed in an inflatable
dome for some tests and in the NTT enclosure for others. In these tests, the 13 measurements
were fit with eight Zernike polynomials in order to gain insight into the frequency response and
pressure distribution on the mirror. In all cases, the data were normalized according to the
measured mean wind velocity for the time record. While some power spectral density (PSD) data
were published from this data set, it does not provide much information into the instantaneous
pressure distribution across the mirror or the correlation of the pressure between points. Further,
the dummy mirror was not part of a telescope structure nor located in the position it would occupy
in a dome. Even so, this was the best information available at the time of construction of the
Gemini telescopes, and the data were used as part of the basis for their analysis.
Chapter 5: Technical Studies
6
Section 5.5 Characterization of Wind Loading
Fig. 4.A. The velocity field
for a wind speed of 11 m/s
directed at 30 degrees
from the mirror surface,
evaluated at center plane
of mirror.
Fig 4.B.
Pressure field
from wind flow shown in
Fig. 4A,
evaluated at
central plane of mirror.
Red is higher pressure.
Fig 4.C.
Pressure field
from wind flow shown in
Fig. 4A, on surface of
mirror.
Figure 4. Results from computational fluid dynamics analysis of flow past a disk representing a
telescope mirror cell. (Figures reprinted from Reference 18.)
Chapter 5: Technical Studies
7
Section 5.5 Characterization of Wind Loading
Figure 5. Distribution of the pressure sensors on the ESO 3.5-meter dummy mirror. Reprinted
from reference 16.
A fairly recent measurement that is also relevant to the design of an ELT was a series of tests
performed at the Nobeyama 45m mm-wave radio telescope, described in Appendix 5.5.A. These
tests were primarily intended to investigate the pointing performance of the structure in wind, and
combined on-sky pointing measurements with direct measurement of structural motion via a large
number of accelerometers. Because the size of the structure (and thus its natural frequency) is
comparable to many ELT concepts, the data provide a useful data point on the frequency and
amplitude of structural response due to wind. For example, for a parked telescope in calm wind,
the typical motions of an accelerometer near the periphery of the primary reflector, as shown in
Figure 6, are on the order of a micron RMS, and most of this is below the 2 Hz bending mode of
the reflector.
By comparison, when the wind is blowing at about 6-8 m/s, the vibration of the structure
increases by nearly a factor of ten (Figure 7). Analysis of the corresponding deflections indicates
that the RMS motion is greater than one micron RMS unless the errors are corrected to just over
4 Hz. While some of the low frequency increase is due to rigid body tilt, the higher frequency
content results in additional modes contributing significantly to the motion of the surface. It is
worth noting that for this case the response also exhibits a group of double-peaked narrow-band
responses at low frequency, which is characteristic of vortex shedding from different parts of the
structure.
A final useful lesson from the Nobeyama test is that the tracking of the telescope introduces
almost as much vibration as the higher wind. Figure 8 shows the response of the same
accelerometer during calm wind with the telescope tracking a source. The peak at 0.9 Hz was
due to a feature within the controller. However, there is a broadband increase in the vibration
disturbance to the structure. This demonstrates clearly that tracking smoothness will be
extremely important in the design of an ELT.
The Nobeyama results also indicate that motions of the M2 support are likely to be tens of
microns.
Chapter 5: Technical Studies
8
Section 5.5 Characterization of Wind Loading
Figure 6. PSD of structural response of Nobeyama radio telescope, with telescope parked and
calm wind. Reprinted from Appendix 5.5.A.
Figure 7. PSD of structural response of Nobeyama radio telescope, with telescope parked and
wind 6-8 m/s. Reprinted from Appendix 5.5.A.
Chapter 5: Technical Studies
9
Section 5.5 Characterization of Wind Loading
Figure 8. PSD of structural response of Nobeyama radio telescope, with telescope tracking and
wind 6-8 m/s. Reprinted from Appendix 5.5.A.
3. Summary of Gemini South studies of wind buffeting
During the integration of the Gemini South telescope on Cerro Pachon in 2000, there was an
opportunity to make direct measurements of telescope wind loading. A series of measurements
were taken by Gemini staff working in collaboration with faculty and students of University of
Massachusetts Lowell and the University of Arizona, in a program designed to remedy some of
the limitations of the previous wind studies cited above.
Because of the circumstances, Gemini South was the ideal instrument for this test. It is a large
optical telescope and is located in an enclosure that has extremely large vent gates. This allows
wind-loading conditions from nearly fully protected to nearly fully exposed (Figure 9). Perhaps
even more importantly, Gemini South was an ideal choice because the telescope had reached a
point in construction at which it was capable of controlled motion but the primary mirror had not
yet been installed. The "dummy" primary mirror provided a location for direct mounting of
pressure transducers while maintaining approximately the same mass and stiffness as the final
mirror. The absence of the final optics also enabled a risk-free impact test of the structure to
provide good modal characterization, which can ultimately be used to validate or improve FE
models. Finally, the Gemini telescope design was subjected to an intensive modeling and testing
effort during its design phase, so there is a substantial set of analytical and scale-model data that
can be used to generate an analytical benchmark (as described in Section 1) to be compared
with the experimental results.
To take advantage of this testing opportunity, we conducted two sets of tests. In the first set, the
telescope was instrumented with 24 pressure sensors, 5 anemometers (each capable of
simultaneous measurement of three directions of wind speed), and 74 accelerometers. The
second set of tests included only pressure and wind speed, but employed 32 pressure sensors
and 5 anemometers (an additional anemometer was available for some tests).
Chapter 5: Technical Studies
10
Section 5.5 Characterization of Wind Loading
Figure 9. The Gemini enclosures have large ventilation gates on their sides to allow natural
ventilation with ambient air.
3.1. WIND PRESSURE TESTS: GROUP 1
In the first set of tests there were two types: modal tests and operating tests. Modal data were
taken at zenith pointing in maximally quiet conditions using an instrumented impact hammer to
put a controlled input into the structure20. The operating data were taken under a variety of
telescope and enclosure configurations using the wind as the structural disturbance. Naturally,
the response of the telescope varies widely with the wind loading conditions. Even the modal
frequencies of the telescope vary somewhat with the elevation angle. As a result, it was
impossible to cover the full testing parameter space in the time available for testing. Rather, as a
practical matter the number of tests was reduced.
The operating data tests could not be taken at a single configuration, because the goal of the test
was not only to determine the worst-case wind loading, but also to characterize the effects of the
wind under a range of conditions. As a result, the following parameters were varied for the test:
1.
2.
3.
4.
Wind azimuth angle of attack (AoA)
Telescope elevation angle (El)
Upwind vent gate position (UVG)
Downwind vent gate position (DVG)
Additionally, one test was performed with the lower windscreen raised into position.
To illustrate the difficulty in fully covering the parameter space, consider varying the azimuth AoA
from 0 (looking into the wind) to 180 (looking out of the wind) in 45-degree increments. This
results in a total of 5 values. Varying the elevation angle to 30, 45, 60, and 75 degrees provides
four distinct values. Finally, there were three vent gate positions of interest (open, half, and
closed) for each of the vent gates. Covering the full combinations of this parameter space would
result in 5*4*3*3=180 tests. Unfortunately, it would be impossible to obtain this many tests with
anything resembling constant wind conditions. Further, it is worth noting that in the entire testing
run we were able to make less than 50 operating tests, so even this coarse sampling would not
Chapter 5: Technical Studies
11
Section 5.5 Characterization of Wind Loading
have been achievable. Since it was impossible to provide full coverage of the parameter space,
we elected instead to take a statistical approach to the test using standard design of experiments
(DOE) approaches21. In this way, we obtained better resolution in each of the parameters with
many fewer tests. The main tests falling into this category were an L16 array and two L9 arrays.
In the L16, the four parameters were varied to two values apiece and all combinations were
taken. Further, since not all of the tests could be taken on the same night, the tests were
optimally 'blocked' to reject any systematic differences between the nights. Though the L16
covered a comparatively small range of parameter space, it has the advantage that it provided
information on interactions caused by varying different parameters simultaneously. The two L9
measurements were taken on different days. Each one covered a larger area of parameter space
than the L16; each parameter was varied to three different values. However, the trade-off was
that these tests could give no information on effects caused by changing two of the parameters
simultaneously.
As stated above, for this test there were a total of 74 channels of accelerometers, located on the
structure as shown in Figure 10. For most of the operating tests, however, only 62 of these were
used, to leave room in the acquisition system for the pressure transducers (the twelve channels
removed were four sets of triaxial accelerometers on the main weldment around the primary
mirror cell). There were 24 pressure transducers, and all were located on the primary mirror, with
the layout shown by the red dots in Figure 11. Finally, there were five ultrasonic anemometers,
each measuring wind speed in three directions. Three were located at the outer edge of the main
weldment (+X, -X, and -Y), one was located at M2, and the final one was installed on the top of
the dome. For each test, time data was collected and stored directly to the hard drive for later
processing. Each operating test was 5 minutes (300s) in length. The accelerometers and
pressure transducers were sampled by one system with a sample rate of 200 Hz. The pressure
transducers were also sampled (together with the anemometers) by a second acquisition system
with a sample rate of 10 Hz.
Figure 10. Locations of accelerometers on the
Gemini Telescope structure.
Chapter 5: Technical Studies
12
Figure 11.
Locations of pressure
sensors on the dummy mirror. The 24
sensors shown in red were used for
the first set of tests; the 8 sensors
shown in blue were added for the
second set.
Section 5.5 Characterization of Wind Loading
The modal tests were performed using all 74 accelerometers and an instrumented impact
hammer. Because of the large accelerometer count, the hammer was used at only a few
locations near the secondary. A modal characterization report was produced by the University of
Massachusetts Lowell Modal Analysis and Controls Laboratory22.
The operating data conditions covered a wide parameter range. A more complete description of
the tests and layout is available at:
http://www.aura-nio.noao.edu/studies/wind_tests1/index.html.
3.2. WIND PRESSURE TESTS: GROUP 2
To expand on the data taken in the first round of testing, an additional eight pressure transducers
were added, as shown by the blue dots in Figure 11. An additional anemometer was added at
the +Y location on the primary mirror cell, but was removed again to serve as a replacement
following failure of the dome sensor. As a result, most of this series of tests have measurements
at the same five locations as the Group 1 tests. In this series of tests, the opposite side of the
structure was chosen as the 'upwind' side in order to confirm symmetry of the results, and again a
large number of configurations were tested. This additional coverage includes a set of
measurements that are suitable for combination with the Group 1 tests to confirm the results.
3.3. DATA PRODUCTS
The tests produced a large volume of data. There were 116 operational test runs of five minutes
each, with up to 114 channels of sensors. This wealth of data is both a great asset and a
sizeable challenge. NIO has already invested considerable effort in data reduction, as described
in references 23 and 24 for example, but further data analysis is ongoing.
Examples of the data products currently available for each of the 116 test cases include:
Wind velocity data:

Five-minute data records, recorded at 10 readings per second, of X, Y and Z velocity
components from each anemometer

Instantaneous resultant wind speed at each anemometer

Average resultant wind speed at each anemometer

Average resultant wind speed at M1 (average of three anemometers)

Ratio of average wind speed at M1 compared to wind speed above dome

Ratio of average wind speed at M2 compared to wind speed above dome

RMS of wind speed variation relative to mean, at each anemometer

Power spectral density (PSD) of wind speed in X, Y and Z directions, at each anemometer
(first test series)

PSD of resultant wind speed at each anemometer (first test series)
Wind pressure data:

Five-minute data records, recorded at 10 readings per second, from each sensor

Maximum and minimum pressure at each sensor

Average pressure at each sensor

Time history of the average pressure over the mirror surface

Average (temporal and spatial) pressure on the mirror surface
Chapter 5: Technical Studies
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Section 5.5 Characterization of Wind Loading

Average total force normal to the mirror surface

Average moment exerted by wind pressures, about the X-axis

Average moment exerted by wind pressures, about the Y-axis

Temporal average of the spatial RMS pressure

PSD of pressure variation at each sensor (first test series)

PSD of spatial average pressure over the mirror surface (first test series)

Pressure maps over the mirror surface at each time instant, interpolated by cubic spline fits,
that can be viewed in sequence as an animation of dynamic pressure variations

Maps of the mirror surface showing the (temporal) average pressure pattern

Maps of the mirror surface showing the (temporal) RMS pressure variation as a function of
position

Structure functions for the pressure variations over the mirror surface
Figure 12 shows an example of maps of the average and RMS pressures.
Figure 12. Pressure maps for a case pointing into the wind, with both vent gates open. The map
on the left shows the average pressure pattern; the map on the right shows the RMS pressure
variation as a function of position.
Much of this data is available on the web at:
http://www.aura-nio.noao.edu/studies/.
Chapter 5: Technical Studies
14
Section 5.5 Characterization of Wind Loading
3.4. EVALUATION OF RESULTS
L16 Experiment Group
Some interesting results from the first study can be seen in an analysis of the sixteen
configurations chosen to fill out the L16 experiment. This group of tests is of particular interest
because it provides an estimate of the total random variability in the experiment. This makes it
possible to identify those effects that are significant. For example, Figure 13 shows the average
PSD of the (spatial) average pressure for the 16 tests, together with the average PSD for those
cases with the upwind vent gate open and the average PSD for the cases with the upwind vent
gate closed. Additionally, it shows the error bars per channel. From the figure it is clear that the
position of the upwind vent gate is significant, since the effect on the PSD is well outside the error
bars. It can be seen that the effect of opening the upwind vent gate was, on average, to increase
the pressure PSD by a factor of ten.
Figure 13. Comparison of the average pressure PSD for the 16 tests, with error bars, with the
average pressure PSD for those cases with the upwind vent gate open and the average PSD for
the cases with the upwind vent gate closed.
For the other parameters considered (Figures 14-16), the results are less dramatic. There is, on
average, a difference between 0 and 45 degrees of mean azimuth angle of attack, though it is not
much above the noise, and there is a similar small effect due to the position of the downwind vent
gate. The surprising result was that there was no statistically significant change in the average
PSD at the primary mirror due to changes in elevation angle of the telescope. While this is
surprising, it is worth noting that this result is consistent with a result from the La Silla testing
(reference 16), in which the average pressure was found to depend on the elevation angle, but
the pressure variations did not.
Another advantage of the L16 experiment is that it allows investigation into interactions between
factors. For this data set, the most significant interaction is between the AoA and the UVG
position. This is not surprising, as one would expect the position of the upwind gate to be more
important for a strong crosswind component when compared with a nearly direct headwind.
Chapter 5: Technical Studies
15
Section 5.5 Characterization of Wind Loading
Figure 14. Comparison of the average pressure PSD for the 16 tests, with error bars, with the
average pressure PSD for those cases with azimuth angle of attack 0 degrees, and the average
PSD for the cases with azimuth angle of attack 45 degrees.
Figure 15. Comparison of the average pressure PSD for the 16 tests, with error bars, with the
average pressure PSD for those cases with the downwind vent gate open and the average PSD
for the cases with the downwind vent gate closed.
Figure 16. Comparison of the average pressure PSD for the 16 tests, with error bars, with the
average pressure PSD for those cases with telescope elevation angle 30 degrees, and the
average PSD for the cases with telescope elevation angle 60 degrees.
Chapter 5: Technical Studies
16
Section 5.5 Characterization of Wind Loading
L9 Experiment Group
The L9 experiments were performed to provide increased resolution in the testing. Specifically,
by using the L9 experiment design (reference 21) to sample four effects at three levels each, it
has been employed here as a 'fully saturated' (i.e., maximally efficient) design. The advantage of
this type of test was that it provided information about a wide parameter space. The main
disadvantage is that it provides no estimate of the total random variability, so it is more difficult to
say which effects, if any, are statistically significant. Additionally, it provides no information on the
interaction between effects.
One of the most interesting results is evident from Figure 17 below. Even without error bars on
the experiment, it is evident that there is essentially no change in the average pressure PSD
when the upwind vent gate is changed from half to fully open. That is, for significant wind speed,
the full excitation has been reached by the time the vent gate is half open.
Figure 17. Comparison of the pressure PSDs for those cases in the L9 set with: upwind vent
gates open, half open, and closed.
3.5 PREDICTED DEFORMATION OF THE PRIMARY MIRROR
While the pressure on the Gemini primary mirror varies in a complex way with high spatial
frequency content, the deformation of the primary mirror itself is simpler. This arises because the
mass and stiffness of the primary makes it act as a 'filter' to the incoming force. As a result, most
of the deflections due to the complicated incident forcing distribution are manifest as piston, tip,
tilt, focus and astigmatism. After the removal of the rigid body modes, the remaining deflection
pattern is essentially astigmatism 23. The higher spatial frequency components of the forcing are
insignificant because the stiffness of the mirror against that mode of deformation is substantially
higher. This is well illustrated by Figure 18 below.
Based on these analyses, it was concluded that, to meet the Gemini error budget, the allowable
velocity at the primary mirror is approximately:
VM1 = 3.2 - 0.8 cos(q)
in meters per second, where q is the azimuth angle of attack of the incoming wind. Since this
value is based on deformations of the 8-meter mirror, a lower wind speed limit may be necessary
on a larger telescope.
Chapter 5: Technical Studies
17
Section 5.5 Characterization of Wind Loading
Figure 18. The surface map on the left shows the distribution of pressure on the surface of the
Gemini primary mirror, at a single moment during a wind test, with the telescope pointing into the
wind, both vent gates open, and the telescope pointing 30 degrees from the Zenith. The scale is
in Pascals. Notice that the areas with negative pressure are as large as those with positive
pressure. The surface map on the right shows the resulting mirror deformation calculated by
finite-element analysis, after removal of piston, tip, tilt and focus. The remaining aberration is
almost entirely astigmatism. The scale is in microns.
3.6. LESSONS LEARNED
While we are still in the process of developing a full understanding of the fluid mechanics involved
in telescope wind loading, a number of insights are apparent from an examination of the data.
Some of these are listed below:

Modeling of wind flow by computational fluid dynamics (CFD) is not accurate in predicting
pressure patterns if the model does not accurately reflect the shape of the telescope.
Compare Figure 4, which depicts average pressures on a disk oriented at 30 degrees to the
wind, to Figure 12, the map of pressure measurements for a similar 30-degree orientation to
the wind. In the CFD model, the leading edge of the mirror exhibits the highest pressure,
while in the measurements, the trailing edge of the mirror consistently exhibits the highest
pressures, whether the telescope is facing into the wind, or is at right angles to the wind with
side vents open. The discrepancy is apparently caused by the telescope structure around
the perimeter of the mirror, which does not look like the simple disk modeled in the CFD
analysis.

Wind velocity and pressure at the primary mirror are strongly influenced by the position of the
vent gates. At medium outside wind speeds, pressure variations at the primary mirror can be
almost two orders of magnitude higher when the vent gates are both open, than when they
are fully closed.
Chapter 5: Technical Studies
18
Section 5.5 Characterization of Wind Loading

Vent gate positions have less of an effect on the wind speed at the secondary mirror, but they
do affect the direction of airflow. In cases where only one vent gate is open, on the upwind
side, the wind enters the enclosure through the side vent and exits through the slit.

With vent gates open, the attenuation of wind velocity at the primary mirror is higher when
facing into the wind than when at 90 degrees to the wind.

Time-varying pressure patterns on the primary mirror are larger than the average pressure
pattern. This is in contrast to the wind velocity in the free air stream, where velocity variations
are substantially less than the average velocity.

Pressures on the mirror surface are negative almost as often as they are positive. Average
force normal to the mirror surface is usually very low (seldom more than 100 Newtons) even
in relatively high wind situations with vent gates open, but the instantaneous values can be
several times that (see Figure 19). Average moments on the mirror caused by wind
pressures are seldom more than 200 N-m.
Figure 19. Time history of
(spatial) average pressure
on mirror surface.
The
telescope was pointing into
the wind, 30 degrees from
the Zenith, with both vent
gates open. Note the long
period
variation
at
frequencies down to 0.01 Hz.
4. IDENTIFYING SOURCES OF PRESSURE VARIABILITY
One of the fundamental issues about wind loading is to identify the source of the pressure
variation on the telescope. It could come from several sources, including: (1) the variability of
wind velocity and direction in the free stream; (2) turbulence generated by the passage of the air
through openings in the enclosure; or (3) the induced flow and vortices caused by the wind
passing around the structure itself.
Separating these effects presents a challenge, as the
measured flow around the telescope reflects a combination of these effects. More measurements
are planned to continue investigating this question, but there are some early indications that the
pressure variation on the mirror is largely due to the flow around the telescope itself.
Many of the plots of RMS pressure variation, for example, the plot on the right side of Figure 12,
show the highest variation close to the edge of the mirror, implying that the turbulence is
generated by air passing over the structure around the mirror. Upstream vortices would be
expected to have a more uniform effect on the pressure variability across the mirror. Figure 20
illustrates the point-to-point difference in pressure variation.
Chapter 5: Technical Studies
19
Section 5.5 Characterization of Wind Loading
Sensor # 13 (c00030oo)
10
10
5
5
pressure (N/m2)
pressure (N/m2)
Sensor # 12 (c00030oo)
0
0
-5
-5
-10
-10
0
50
100
150
200
Time History: time (second)
250
0
300
50
100
150
200
Time History: time (second)
250
300
Figure 20. Pressure graphs for the test case shown in Figure 12. The graph on the left is a time
history from pressure sensor 12, located on the right side of the mirror. The graph on the right is
from sensor 13 located on the left side. Note the difference in magnitude of pressure fluctuation.
In the case shown in figure 12, the air is flowing generally in the +Y direction over the mirror (the
V components of wind velocity) and on the left side of the mirror the air is flowing generally
towards the right (the U components). An area of higher pressure variation appears above and to
the right of the central baffle, which may be caused by vortex shedding from the baffle structure.
Similar features are seen in many other plots. Clearly, this is a preliminary result, and more
detailed examination of the local flow data is planned, to look for further confirmation.
If it can be shown that interaction of the wind with the telescope structure is the primary source of
pressure fluctuations, this will put increased emphasis on understanding how this can be
ameliorated by aerodynamic design of the telescope.
5. NEXT STEPS
While the previous studies and the on-going analyses have already provided information critical
to the design of an ELT, additional work is required. NIO conducted a workshop November 2627, 2001, to consider what additional tests and analyses are needed. Participants came from
several institutions:

NIO is working in collaboration with the Tennessee State University Center for Excellence in
Information Systems, Engineering & Management, who are proposing to assist the GSMT
studies by doing CFD wind flow analyses. Dr. Yong Tao and Dr. Guanpeng Xu from TSU
participated in the workshop.

AMEC (formerly Coast Steel), a Canadian company that has built many of the world’s largest
telescope enclosures, has also been working with NIO. AMEC is developing design concepts
for ELT enclosures, and is currently doing a design study for GSMT. David Halliday and Mike
Gedig of AMEC participated in the workshop.

Dr. Oleg Likhatchev, a faculty member of the Aerospace and Mechanical Engineering
Department of the University of Arizona, is collaborating with NIO on fluid mechanics studies
related to telescope wind loading.

Dr. Mark Whorton, a senior Aerospace Engineer with NASA Marshal Space Flight Center, is
collaborating with NIO on control systems and wind modeling.

Dr. David DeYoung is a senior research physicist with NOAO who has done CFD analyses of
wind flow over several observatory summits.
Chapter 5: Technical Studies
20
Section 5.5 Characterization of Wind Loading

Several engineers from NOAO and Gemini Observatory, who have worked on wind modeling
or wind measurement, also participate in the workshop.
It was agreed that several areas deserved particular attention: (1) to model air flow over a 30meter segmented mirror; (2) to model air flow through enclosures of different geometries; (3) to
conduct additional wind measurements to investigate the question of the origin of turbulence
affecting the telescope; (4) to continue analysis of existing wind data to investigate the same
question. These initiatives are discussed briefly, below.
5.1 CFD MODELING OF 30-METER MIRROR
The consensus at the workshop was that a key source of the turbulence that produces fluctuating
pressures on the telescope is the interaction of the wind with the telescope structure itself.
Therefore, it was felt more productive to begin modeling details of proposed ELT designs rather
than perform additional analyses to more thoroughly understand the measurements made on
Gemini.
This analysis will start by modeling the 30-meter primary mirror itself, and then proceed to further
elaboration of the point-design telescope structure. It will investigate turbulent shedding effects of
air flow around a 30m primary mirror under a range of wind speeds and directions, and will
provide a first indication of likely pressure distributions and variations on the front and back
surfaces of the mirror.
TSU has offered to conduct the CFD modeling of airflow over the 30-meter mirror. Their
proposal25 states, in part:
“The research will focus on (a) understanding the flow mechanisms that affect the
dynamic wind loading on the primary mirror of a 30-meter aperture giant segmented
mirror telescope (GSMT), and (b) the development of efficient computational techniques
that model the unsteady, turbulent flow field around the primary mirror.
The research group at TSU will adopt a mathematical and numerical formulation to
analyze a flow field around a primary mirror of a 30-meter GSMT. Navier-Stokes
equations will be solved in the entire flow field. Proper turbulence models will be used for
modeling the separation. The dynamic response of the flow field to the incoming
turbulence and harmonics will be studied.
The computational effort will focus on the extension of an existing, three-dimensional (3D) Navier-Stokes solver to a GSMT primary mirror. In this approach, 3-D unsteady,
compressible Navier-Stokes equations will be solved in the entire region on a body-fitted
grid surrounding the primary mirror. The mirror leading edge under subjected airflow
inevitably results in flow separation and turbulent shedding towards the mirror surface.
Therefore, proper turbulence models for separated flow will be integrated into the solver.
The ultimate goal of this modeling work is to study the effects of turbulent shedding due
to mirror leading edge that is subject to the prescribed incoming wind with either steady
or turbulence profile (buffeting effect) on the mirror dynamic wind loading.”
5.2. CFD MODELING OF ELT ENCLOSURES
AMEC has proposed to perform CFD analyses of air flow into and around different types of
enclosures. This will allow comparison of results with the earlier CFD analyses on the Gemini
enclosures to check current scaling assumptions, and it will provide first-pass information on the
likely flushing rates and internal wind velocities for some enclosure types.
Chapter 5: Technical Studies
21
Section 5.5 Characterization of Wind Loading
There is reason to have confidence in this modeling – the CFD studies performed by DeYoung
(reference 18) modeling the patterns and velocities of air flow within the Gemini enclosure have
been verified by the wind measurements made at Gemini South.
AMEC has agreed to share the results of their studies with the other collaborators.
5.3. ADDITIONAL WIND MEASUREMENTS
Velocity measurements
Since the anemometers are still available at Gemini South, additional wind velocity
measurements can be taken as needed. One measurement that will be made in the near future
is intended to separate the effects of the variability of the free stream from the variability
introduced by the passage of the flow through the dome. Identical anemometers will be placed
on a weather tower outside the dome and on the telescope structure close to the primary mirror.
Simultaneous measurements should allow identification of turbulent vortices that are traveling in
the free air stream, and should provide a clear indication whether the level of velocity variation
seen inside the enclosure is consistently higher than in the free air stream.
While the comprehensive data measured at Gemini South is useful by itself, a separate
experiment at another facility would be an excellent complement to the data set. Particularly if
the test could be done at a facility with a markedly different enclosure, and if it could be designed
to partially match the testing done at Gemini, the resulting data could be much more useful than
the sum of the tests alone. NIO will explore collaborations with other institutions to implement
such measurements on other enclosures.
Determine forcing input
While the tests to date have measured wind velocities and structural response, the forcing
distribution on the structure is still poorly known, except at the front surface of the primary mirror
(where pressure transducers were mounted). While the forcing input can be predicted via CFD
analysis, that approach is limited because of the extremely high cost in super computer time
required to model complex detail. There are two other approaches that would help close this
issue. The first approach is to use the existing modal data together with a finite-element model of
the Gemini telescope to produce a corrected model, updated to reflect the behavior shown in the
controlled testing. This corrected model can then be reduced to the degrees of freedom
measured in the operating data testing, and the response inverted to back out the input forces.
These input force histories could then be compared with the measured wind velocities in the
vicinity of the accelerometers to determine effective drag and lift coefficients. The goal would be
to be able to predict the dynamic loading on a telescope structure based on the airflow velocities
and directions calculated by CFD analysis.
As an additional confirmation of local wind forcing, it would be worthwhile to purchase or develop
a device to locally measure force (or perhaps pressure) on existing structures. This would
provide a "ground truth" on the local forcing for comparison with either CFD predictions or
extracted force profiles.
5.4 WORKING WITH THE CURRENT DATA SET
A significant remaining task can be described as “mining” the currently available data from the
Gemini South tests. Recent analyses by Likhatchev have already revealed valuable insights into
the behavior of the flow over the primary mirror, and his work continues. It is worth noting that
much of the analysis to date has focused on the cases with significant wind speed and open vent
gates. This focus is due to the more interesting response that is obtained from higher excitation.
However, it is perhaps more important in the design of an ELT to understand the environment
under more protected conditions, since this will likely be the normal operating procedure.
Chapter 5: Technical Studies
22
Section 5.5 Characterization of Wind Loading
Therefore, in the coming months, Likhatchev plans to focus more on the protected (i.e., closedvent) configurations.
Reduction of operating data conditions and analysis of the structural results from the Group 1
data set via design of experiments techniques should be performed. Further data reduction is
also needed on the Group 2 pressure and wind velocity data set.
Finally, there are useful comparisons to be made with previous studies, including the
measurements at La Silla and some of the wind tunnel and CFD studies.
6. ACKNOWLEDGMENTS
The work summarized in this section is the result of a great deal of work by many people. The
collection of the Gemini South data was accomplished by Gemini Observatory staff members and
the testing team from Michigan Technical University Keweenaw Research Center and University
of Massachusetts Lowell Modal Lab. Data reduction has been funded by Gemini, and was
accomplished by Myung Cho of NOAO, David Smith of Merlab, P.C., and Seongho Kim of the
University of Arizona. Thanks are due to all the participants in the NIO Wind Loading Workshop.
Finally, the authors of previous and current work cited in this document provided the necessary
technical foundation for this work to be performed.
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Section 5.5 Characterization of Wind Loading
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Section 5.5 Characterization of Wind Loading
Structures, Proceedings of the 19th International Modal Analysis Conference, Kissimmee, FL,
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Chapter 5: Technical Studies
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Section 5.5 Characterization of Wind Loading
Chapter 5: Technical Studies
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Section 5.5 Characterization of Wind Loading