1. IN-VENTO 2014 XIII Conference of the Italian Association for
Wind Engineering 22-25 June 2014, Genova, Italy Performance
prediction and validation of a tuned liquid column damper with
internal baffles Stefano Cammelli1 , Yin Fai Li2 and Leejay Hsu2 1
BMT Fluid Mechanics, Teddington, UK 2 BMT Fluid Mechanics, Kuala
Lumpur, Malaysia Corresponding author: Stefano Cammelli,
[email protected] Abstract During the early design stages of a
relatively slender 42-storey high-end residential building located
in the Middle East, a series of high-frequency force balance (HFFB)
wind tunnel tests highlighted that the highest occupied floors
could experience wind-induced motion which depending on the
inherent damping of the finished structure had the potential to
exceed standard industry occupant comfort criteria. In order to
mitigate these excessive vibrations, a Tuned Liquid Column Damper
(TLCD) was proposed for this building. The performance prediction
and validation of the behaviour of such device involved: an initial
campaign of full scale measurements to validate frequencies and
inherent damping of the structure near completion; a series of
shake table tests employing a 1:20 scale physical model; and a
final full scale extrapolation study using Computational Fluid
Dynamics (CFD). 1 Introduction The location of the site of the
proposed development considered within this technical paper was
approximately 1 km from the Mediterranean coastline, with the
immediate surrounding area consisting of densely populated low to
mid-rise urban sprawl. The 50-yr return period mean-hourly basic
wind speed for the region was ~25 m/s (10 m reference height in z0
= 0.03 m) and the characteristic product of the local wind climate
was ~4. The height of the building was ~180 m, with a ~22 m ~44 m
rectangular floor plan. The lateral stability of the tower was
provided by a central reinforced concrete core. The numerically
predicted structural frequencies of the three fundamental modes of
vibration of the building were: 0.19 Hz, 0.26 Hz and 0.53 Hz, with
the first two describing pure sway of the structure along the
principal axes of the central core (exponent of these mode shapes
were ~1.5) and the third one being torsional. The damper study,
which this paper is focused on, was part of a wider range of wind
engineering consultancy services which included: wind climate
study, pedestrian and terrace / balcony level wind microclimate
study, overall wind loading study and cladding pressure study. 2
On-site full scale measurements When the construction of the
super-structure of the tower approached its completion and before
commencement of the installation of the cladding, a campaign of
on-site full scale measurements was conducted to detect some of the
key structural parameters of the building; in order to achieve
this, the 34th level of the tower was instrumented with a number of
low-range high-resolution accelerometers with the aim of acquiring
a large number of ambient data records. Before commencement of
post- processing, the different time-histories of recorded
wind-induced accelerations have been digitally low-pass filtered at
a frequency of 1 Hz to remove high-frequency noise content which
due to the nature of the site have inevitably been picked up during
the measurements.
2. Cammelli et al. Performance prediction and validation of a
TLCD with internal baffles The different time-histories have then
been analysed making use of the so-called random decrement (RD)
technique (Tamura et al., 2000 and Li et al., 1998 & 2003),
which enabled the random and chaotic part embedded in the actual
measured signals associated with the excitation from the
atmospheric turbulence to be fully removed, revealing the far more
regular signature left by the structure itself. Subsequently, modal
identification techniques (Tamura, 2005) were applied to the RD
signatures in order to identify the frequencies and damping of the
tower during construction. The first two modes of vibration of the
structure (for the specific construction stage the tower was at
during monitoring) have been found well aligned with the two
principal axes of the structural core of the building and their
frequencies in very good agreement with the prediction of the
finite element model. The level of inherent structural damping
associated with these two modes of vibration of the structure was
found to be in the region of ~1.0% of critical. It should be noted
that during the period of monitoring the strength of the wind
storms that passed through the region was lower than what expected
for a typical 1-yr return period event. 3 Concept design of the
TLCD A detailed review of the HFFB wind tunnel tests results,
performed during the early design stages of the design, revealed
that the motion along the weak direction of the building was the
key contributor to the peak combined wind-induced acceleration. It
was also estimated that, in order to achieve the desired level of
occupant comfort at the highest occupied levels of the building for
both the more frequent (1-yr return period) and the less frequent
(10-yr return period) wind events, a total damping of ~2.0% of
critical in the first mode of vibration of the structure was
required. Amongst the different types of auxiliary damping devices
which could be installed on a tall building, tuned liquid dampers
(TLDs) are the most cost-effective. The preliminary design of the
damper was conducted following the guidance provided in Vickery
(2006). It was estimated that the damping system employed would
require a total effective mass of ~80,000 kg, equating to ~0.5% of
the modal mass of the first mode of vibration of the tower, and a
natural frequency of ~99.5% of the first mode frequency of the
building. These estimates were made assuming an efficiency of the
damping device of ~75%. Deviation from zero main damping was also
duly taken into consideration during concept design. Due to the
relatively slender nature of the building here examined (the
slenderness ratio of the building in its weak axis was ~1:9), the
more compact tuned liquid column damper (TLCD) solution was adopted
from the very start of the concept design study. The TLCD damper
comprises an auxiliary vibrating system consisting of a column of
liquid moving in a tube-like container. The restoring force is
provided by gravity, whilst the energy dissipation is achieved at
the baffles installed within the horizontal duct. The above
estimates and considerations led to the selection of a pair of
identical TLCDs in the form of a U-tube water tank to be installed
just below the roof level of the building. The internal dimensions
(i.e. exclusive of the thickness of the RC wall) of one of the two
identical TLCDs are reported in Table 1 below: Table 1. Internal
dimensions of one of the two identical TLCDs. Dimensions (mm)
Length of the U-tube, Lo 7600 Internal width of each riser, W 1650
Internal breadth of the TLCD, B 4925 Internal height of the
horizontal duct, H 1100 Internal free board in each riser, R
900
3. Cammelli et al. Performance prediction and validation of a
TLCD with internal baffles The overall arrangement of one of the
TLCD is illustrated in Figure 1 below: Figure 1. Internal
arrangement of one of the two identical TLCDs. It was estimated
that the internal headroom for sloshing dR during a typical 10-yr
return period wind event was ~1000mm. It should be noted that the
number and location of the required internal baffles was at this
stage of the design only indicative. 4 Detail design of the TLCD
Physical model testing The concept design of the damper was tested
in the 6 degree-of-freedom (6DOF) shake table facility of the
Department of Civil Engineering of the University of Bristol. The
aim of the model testing was not only to verify the key resulting
parameters of the concept design but also to derive the optimal
geometry and internal arrangement of the baffles within the TLCD.
4.1 Experimental setup A 1:20 model of the damper was constructed
in Plexiglas. The construction of the damper allowed up to five
interchangeable porous screens to be inserted within its horizontal
duct. The working fluid in the model was water. The model damper
was mounted on the shake table via a piezoelectric load cell (see
Figure 2). The motion of the shake table was programmed according
to the solution of the equation of motion of the first mode of the
actual building at various levels of structural damping computed
based on wind tunnel measurements. The motion of the shake table
was then measured simultaneously with the load cell signal using
non-contact displacement transducers. Figure 2. Experimental setup
of the 1:20 TLCD model. Model of the TLCD Load cell Shake
table
4. Cammelli et al. Performance prediction and validation of a
TLCD with internal baffles A video of the model scale experiment is
presented in Figure 3. Figure 3. Video of the physical model scale
shake table testing (press on the still image to run). 4.2 Results
and discussions The energy dissipated within the model can be
derived from the simultaneous measurements of the base force
reaction and motion of the damper as follows: dtxFFdxW (1) where
xxF ,, denotes the measured force, displacement, and time
derivative of the displacement (i.e., velocity). The equivalent
damping ratio of the TLCD, i.e. the damping ratio of a non-viscous
device that would dissipate the same amount of energy per cycle of
vibration as a perfectly viscous device at the same amplitude, can
be defined as follows: 2 2 22 22 2 1 2 1 2 2 x xF nnn eq neqeq mdtx
dtxF mdtxm dtxF dtxmdtxcdtxF (2) where 22 ,,,, xxFneq m are the
damping ratio expressed as fraction of critical, mass (in kg), and
natural circular frequency (in rad/s) of the system, covariance
between measured force and velocity (in Nm/s) and variance of
velocity (m/s). It should be noted that the measured ratio of the
two covariances is not dimensionless and had therefore to be
converted to full scale in order for the equation above to apply. A
dimensional analysis revealed that the scaling of the 2 2 x xF term
would follow the geometric scale of the model raised to the power
of 2.5, i.e.: 2 25.2 2 x xF n L eq m NR (3) where LR and N are the
geometric scale and the number of TLCDs installed in the building.
Eq. (3) therefore represents a direct relationship between the
equivalent damping ratio of the full scale damper system and the
model scale 2 2 x xF term, which was measured as a function of
standard deviation of excitation displacement.
5. Cammelli et al. Performance prediction and validation of a
TLCD with internal baffles In the case of a TLCD installed on a
building with fixed structural frequencies and inherent damping,
the standard deviation of excitation amplitude during a wind event
is controlled by total system damping, which is in turn contributed
significantly by the added damping of the TLCD itself. Figure 4
plots the equivalent added damping ratio versus total system
damping. It is clear from this graph that for all configurations of
porous baffles tested, the equivalent damping generally increased
with excitation magnitude, or decreased total damping.
Configurations with a larger number of porous baffles generally
showed higher energy dissipation at low amplitude, as more energy
was dissipated when water moved across the screen. While, on the
other hand, configurations with many baffles had the potential to
prohibit the build-up of vibration amplitude of the water, hence
hindering the damping performance. The actual damping performance,
taking into account the inherent damping of the structure, is
denoted in Figure 4 by the intersection points between different
baffle configurations and different levels of inherent damping.
From this plot it is clear that both the 3 baffles and the 5
baffles configurations gave rise to an equivalent damping ratio of
~1.2% of critical which together with a ~1.0% of inherent
structural damping corresponded to ~2.0% of critical of total
system damping. Figure 4. TLCD performance curves (75% porous
baffles). The geometrical arrangement of the best performing baffle
arrangement (75% porous) is presented in Figure 5. Figure 5.
Arrangement of a 75% porous baffle (dimensions in
millimetres).
6. Cammelli et al. Performance prediction and validation of a
TLCD with internal baffles 4.3 Comparison with the solution of 2DOF
equation of motion In order to further inspect and understand the
measured results in terms of total system damping performance, the
time domain solution of equation of motion based on Clough and
Penzien (1993) for the first mode has been extended to a 2
degree-of-freedom (2DOF) system to incorporate the addition of the
TLCD. In order to solve the equation of motion in the time domain
the knowledge of the internal damping of the TLCD is required. The
internal damping of the damper system with optimal baffle
configuration was evaluated via a series of free decay model
testing. The free decay of base shear force was measured after the
damper was subjected to a step excitation. The internal damping was
calculated by applying the logarithmic decay to the measured time
histories. It was found that the damping generally reduces with
amplitude and, for the operating conditions here examined (10-yr
return period wind event), the damping is of the order of ~3.7%
3.9% of critical. A sample of a free decay time history is shown in
Figure 5 with damping estimates for different section of the time
history. Figure 5. Example of free decay force time history of the
physical model of the TLCD. The 2DOF equation of motion was solved
for the measured wind excitation, damping ratio and frequency of
the TLCD for each time step and the response with and without the
TLCD is presented in Figure 6. The peak acceleration response of
the primary mass, i.e. the building itself, has reduced from ~21.5
milli-g to ~15.5 milli-g, which is equivalent to an increase in
total system damping from ~1.0% of critical to ~1.9% of critical.
Figure 6. Solutions of the equation of motion in the time domain,
with and without TLCD.
7. Cammelli et al. Performance prediction and validation of a
TLCD with internal baffles 5 Detail design of the TLCD CFD study
The 1:20 scale physical model testing inevitably left the authors
of this technical papers with some uncertainties over the potential
for scale effects to affect the performance of the full scale
TLCDs. In order to try to quantify these, a number of CFD studies
have been undertaken. 5.1 Analysis software The multi-purpose CFD
software OpenFOAM (www.openfoam.com) was used for the study.
OpenFOAM is an open source CFD package which has gained a large
user base in commercial and academic applications which features a
wide variety of validated solvers in the area of oscillatory and
sloshing flow. 5.2 Geometry and grid The numerical work was focused
on a single TLCD, the internal volume of which was discretised with
a 3D structured mesh. Areas of particular interest were modelled
with a higher level of geometrical detail, such as the regions
around each baffle. Figure 7 shows the complete structured mesh of
a single TLCD. The green regions are open to atmospheric pressure.
The blue regions represent areas of high mesh density near the
baffles: properly capturing the flow behaviour in these regions was
a high priority. Figure 7. Perspective view of the spatial mesh.
5.3 Porous regions The baffles were modelled as anisotropic porous
regions using the Darcy-Forchheimer approach. This model is
composed of two parts: a viscous loss term known as the Darcy
permeability (first term on the right hand side of Eq. (4)) as well
as an inertial loss term known as the Forchheimer term (second term
on the right hand side of Eq. (4)): ( ) (4) Where Si is the
volumetric source term added to the momentum equations of the
baffle zones, Dij and Fij are the prescribed porous media tensors,
is fluid dynamic viscosity, is fluid density, Uj is the jth
component of the velocity vector, and is the velocity
magnitude.
8. Cammelli et al. Performance prediction and validation of a
TLCD with internal baffles 5.4 Boundary conditions Rough walls with
a no-slip condition (u, v, w = 0) were assumed for all internal
surfaces. A turbulent viscous wall function and mean roughness
height of 0.025mm (uniform sand grain roughness) were used to
simulate the surface roughness of the smooth-finish concrete walls
in the full scale simulations of the TLCD. The lateral pressure
release openings were modeled as constant atmospheric pressure
openings in the CFD grid. 5.5 Turbulence model The standard k-
turbulence model was employed in the CFD simulations when assessing
the internal flow of the water tank in order to capture
recirculation and eddy phenomena (such as the recirculation near
the inside corners as illustrated in Figure 8). This turbulence
model is one of the most widely used turbulence models for its
combination of computational speed and accuracy. Figure 8.
Recirculation near the internal corner of the TLCD. 5.6 Solver
OpenFOAMs interDyMFoam solver was used for the study. This solver
is compatible with 2-phase, isothermal, incompressible, immiscible
flows. InterDyMFoam uses a finite volume approach to represent the
Navier-Stokes equations, in which each cell in the computational
mesh is assigned a single value for each fluid property (i.e.
velocity and pressure) that represents the average of these
properties over the whole volume of the cell. 5.7 Methodology A
1:20 scale numerical model was initially set-up with the aim of
generating results which could have been directly compared with the
ones obtained from the physical model testing campaign. A number of
mesh independence studies was conducted to determine an optimal
computational mesh, as well as to locate potential areas which
would benefit from mesh refinement (e.g. regions in which vortices
and recirculation were expected, see Figure 9). Once the 1:20
numerical model was finalised, the results were compared to
experimental shake table results before performing full scale
computational analysis.
9. Cammelli et al. Performance prediction and validation of a
TLCD with internal baffles Figure 9. Streamlines under free decay
motion. The free-decay logarithmic decrement approach was used to
quantify the performance of each simulated TLCD. This method
included the excitation of the CFD model with a sinusoidal input
wave until the system reached a periodic steady state. The forced
movement of the TLCD was then stopped, and the decay of the overall
net force was measured over time. The net force measured included
the contribution from dynamic pressure acting on the walls and
baffles of the TLCD in the direction of the first mode of vibration
of the tower. Comparison of damping performance (3 baffles
configuration), as simulated in CFD and experimentally gathered in
the shake table experiments (~4.2 4.5% and ~3.7 3.9% respectively),
was satisfactory given the complex nature of unsteady multi-phase
flow (see Figure 10). Figure 10. Example of free decay force time
history of the model TLCD, CFD vs. physical testing. Numerically
computed studies on a full scale TLCD showed that the damping of
the device itself during operating conditions (10-yr return period
wind event) decreased by ~10%. This was believed to be due to the
different physics controlling the energy dissipation at the two
scales: at model scale, in fact, the contribution coming from
viscous forces is expecting to be larger than at full scale where,
on the other hand, damping at full scale will be more dominated by
inertial forces and recirculation within the TLCD. By: Yin Fai Li
Date: 18-Jun-13 Status: Final Drawing no.: 431130-FIG-18 431130
Skygate Tower
10. Cammelli et al. Performance prediction and validation of a
TLCD with internal baffles 6 Conclusions A pair of identical TLCDs
has been designed to mitigate the excessive wind-induced motion of
a 42- storey residential tower located in the Middle East. Their
concept design, based on an initial desktop study approach, has
been subsequently validated via a series of scale model tests
performed on a shake table which in turn allowed an optimal
configuration of internal porous screens to be obtained. The
internal dissipation of each TLCD, in the form of equivalent
viscous damping ratio, was extracted from the shake table
experiments using energy dissipation considerations as well as
directly solving the 2DOF equation of motion: these two analyses
led to very consistent results. Finally, the performance of the
full scale TLCDs has been evaluated using CFD in an attempt to gain
insights on the differences between model scale study and full
scale implementation. 7 Acknowledgments The authors of this paper
would like to thank Dr John Macdonald from the Department of Civil
Engineering of the University of Bristol for his support during the
course of the physical model testing campaign and Professor Michael
Graham from the Department of Aeronautics of the Imperial College
London for his input in the CFD work. References Clough, R.W. and
Penzien, J. (1993). Dynamic of Structures. 2nd Ed. McGraw Hill. Li,
Q.S., Fang, J.Q, Jeary, A.P., Wong, C.K. (1998). Full Scale
Measurements of wind effects on Tall buildings. Journal of Wind
Engineering and Industrial Aerodynamics Vol 74-76, pp 741-750. Li,
Q.S., Yang, Ke., Wong, C.K., Jeary, A.P. (2003). The effect of
amplitude-dependent damping on wind induced vibrations of a super
tall building. Journal of Wind Engineering and Industrial
Aerodynamics Vol 91, pp 1175-1198. Tamura Y. (2005). Damping in
buildings and estimation techniques. Proceedings of APCWE-VI,
Seoul, Korea. Tamura, Y., Suda, K., Sasaki, A. (2000). Damping in
Buildings for Wind Resistant Design. International Symposium on
Wind and Structures for the 21st Century, 26-28 Jan, Cheju, Korea.
Vickery, B.J. (2006). On the Preliminary Design of Passive Tuned
Mass Dampers to Reduce Wind Induced Accelerations. Australasian
Wind Engineering Society Workshop, Queenstown, New Zealand,
February.