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Static wind load measurements with a six-component high frequency balance based on piezoelectric crystals
Journal o f Wind Engineering and Industrial Aerodynamics, 24 (1986) 87--91
87
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
Short Communication
STATIC WIND LOAD MEASUREMENTS WITH A SIX-COMPONENT HIGH FREQUENCY BALANCE BASED ON PIEZOELECTRIC CRYSTALS
S.O. HANSEN and E.G. S~)RENSEN
Danish Maritime Institute*, L yngby, Copenhagen (Denmark) (Received September 5, 1985; accepted in revised form January 28, 1986) "
The outstanding qualities of piezoelectric balances compared with straingauge balances for the measuring of dynamic wind loads is well known (Cook [1] and Schewe [2] ). The natural frequency of a typical modelbalance system is in general m u c h higher for a piezoelectric balance compared with what can be obtained with a strain-gauge balance (Tschanz [3]). This implies advantages in model manufacturing, measuring and analysis procedures. The use of piezoelectric balances in wind-tunnel testing is not widespread. A frequent argument against the use of piezoelectric balances is the natural difficulties encountered when measuring static wind loads as the signals drift (Tschanz [3] ). However, Cook [1], Spescha and Volle [4], and Schewe [5] have all reported the successful measurement of static loads with piezoelectric force transducers. The purpose of the present paper is to pass on briefly the experience gained at the Danish Maritime Institute concerning the measuring of static wind loads with a Kistler balance t y p e Z 11835. The balance is of exactly the same t y p e as described by Cook in ref. 1. The balance
A detailed description of the Kistler type Z 11835 balance is given in ref. 1. A Kistler signal conditioner type 9861A5 is used for signal conditioning and interfacing to a PDP 11 computer. At present the signal conditioner supports five ranges from 1 N V -~ to 1 kN V-1. The number of ranges can easily be extended as small, easily exchangeable plugs in printed circuit boards designed for customer-specified measuring ranges can be bought for a few hundred dollars.
*The Danish Maritime Institute is a self-governing institution affiliated to the Danish Academy of Technical Sciences.
0167-6105/86/$03.50
© 1986 Elsevier Science Publishers B.V.
Operate
Reset
I
Yl ~t/~
I
Setting tunnel _ . . speed j _ Subserie averages used to determine regression line y=o~t+q
Tunnelspeed constant
Fig. 1. Principle of measuring and analysis using the piezoelectric six-component Kistler balance.
Kistler I Signal conditioner
Static load =q-offset
On-line data processing]
Subserie No.
Offset
Maximum output voltage
Output signal from balance channel
Measuring i~ero
I
I
/ //
~J!
~'x
Y=~t+q
/- ¢ ' / /
Time
O0 QO
89 Procedure for measuring and analysis The procedure for measuring the static wind load which has been successfully applied at DMI is illustrated in Fig. 1. When measuring static wind loads, the time histories of the signals are derided into subseries of constant duration (as described by Mayne and Cook [6] ). The contribution to the drift is estimated by means of a least square fit to the mean values of the subseries. A very accurate knowledge of the timing when measuring is the basis for a successful analysis. The Kistler signal conditioner, the data acquisition and the fan of the wind tunnel are controlled by a PDP 11/23 computer, thereby ensuring accurate timing. After the contribution from the static wind load has been calculated for each of the o u t p u t channels, the results can be combined to give values proportional to the three forces and the three moments. A check calibration of the accuracy of the measured static loads using the above principle of measuring and analysis was made by nine measurements using weights, scales and pulleys in the lower part (maximum 15%) of the 100 N range of the signal conditioner. This gave a regression line with a slope of 0.992 N/N, an intercept of - 0 . 0 0 8 9 N, a correlation coefficient of 0.9994 and a mean square error of 0.03 N 2. The static force was an x-force which also gave a y - m o m e n t . It should be m e n t i o n e d t h a t the procedure described above requires a measuring time larger t h a n would be necessary with a strain-gauge balance. However, this is not a serious drawback as in general investigations with six-component balances can be quickly carried out. Comparison with strain.gauge balance measurements Measurements have been carried out in order to compare the results obtained with the piezoelectric balance and the results obtained with a strain-gauge balance. The measurements were carried o u t with a model of an offshore structure situated in a natural grown b o u n d a r y layer simulating wind over rough sea. The results of the comparison are shown in Fig. 2. As can be seen the results from the two balance measurements compare well. Conclusion The results from the measurement of static wind loads with the piezoelectric balance are so good t h a t the application of this t y p e of balance in wind-tunnel testing is strongly recommended when static as well as dynamic wind loads are of importance.
90 CYP -E IZOELECTR C I 2.5 Q
i
-2.5
/, ,
,
~ N
-0,0~
,
GAUGE l
|
2.5
p.O
SLOPE
0.960
INTERCEPT CORRELATION
COEFFICIENT
=
-0.007
=
0.995
-2.5 CXP -E IZOELECTR C I 2,5
SLOPE
=
INTERCEPT
=
-0.027
1.019
=
0.995
GAUGE l
i
-
•
-2.5
l
,
l
i
2.5
CORRELATION
COEFFICIENT
-2.5 Fig. 2. Comparison between the results obtained with the. piezoelectric balance and the results obtained with a strain-gauge balance.
Acknowledgements The hospitality and interest which Dr. N.J. Cook at Building Research Establishment, Watford, U.K. has shown in this matter is greatly appreciated by the authors.
91
References 1 N.J. Cook, A sensitive 6-component high-frequency range balance for building aerodynamics, J. Phys. E., 16 (1983) 390--393. 2 G. Schewe, On the force fluctuations acting on a circular cylinder in cross flow from subcritical up to transcritical Reynolds numbers, J. Fluid Mech., in press. 3 T. Tschanz, The Base Balance Measurement Technique and Applications to Dynamic Wind Loading of Structures, BLWT-5-1983, The University of Western Ontario, Faculty of Engineering Science, London, Ontario, Canada, N6A 5B9. 4 G. Spescha and E. Voile, Piezoelectric Measuring Instruments. Unknown origin. 5 G. Schewe, A multicomponent balance consisting of piezoelectric force transducers for a high-pressur e wind tunnel, Paper given at Sensor and Systems, Los Angeles, CA, 1982. 6 J.R. Mayne and N. Cook, Acquisition analysis and application of wind loading data, Proc. 5th Int. Conf. on Wind Engineering, F o r t Collins, CO, 1979, Pergamon Press, Oxford, 1980.
87
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
Short Communication
STATIC WIND LOAD MEASUREMENTS WITH A SIX-COMPONENT HIGH FREQUENCY BALANCE BASED ON PIEZOELECTRIC CRYSTALS
S.O. HANSEN and E.G. S~)RENSEN
Danish Maritime Institute*, L yngby, Copenhagen (Denmark) (Received September 5, 1985; accepted in revised form January 28, 1986) "
The outstanding qualities of piezoelectric balances compared with straingauge balances for the measuring of dynamic wind loads is well known (Cook [1] and Schewe [2] ). The natural frequency of a typical modelbalance system is in general m u c h higher for a piezoelectric balance compared with what can be obtained with a strain-gauge balance (Tschanz [3]). This implies advantages in model manufacturing, measuring and analysis procedures. The use of piezoelectric balances in wind-tunnel testing is not widespread. A frequent argument against the use of piezoelectric balances is the natural difficulties encountered when measuring static wind loads as the signals drift (Tschanz [3] ). However, Cook [1], Spescha and Volle [4], and Schewe [5] have all reported the successful measurement of static loads with piezoelectric force transducers. The purpose of the present paper is to pass on briefly the experience gained at the Danish Maritime Institute concerning the measuring of static wind loads with a Kistler balance t y p e Z 11835. The balance is of exactly the same t y p e as described by Cook in ref. 1. The balance
A detailed description of the Kistler type Z 11835 balance is given in ref. 1. A Kistler signal conditioner type 9861A5 is used for signal conditioning and interfacing to a PDP 11 computer. At present the signal conditioner supports five ranges from 1 N V -~ to 1 kN V-1. The number of ranges can easily be extended as small, easily exchangeable plugs in printed circuit boards designed for customer-specified measuring ranges can be bought for a few hundred dollars.
*The Danish Maritime Institute is a self-governing institution affiliated to the Danish Academy of Technical Sciences.
0167-6105/86/$03.50
© 1986 Elsevier Science Publishers B.V.
Operate
Reset
I
Yl ~t/~
I
Setting tunnel _ . . speed j _ Subserie averages used to determine regression line y=o~t+q
Tunnelspeed constant
Fig. 1. Principle of measuring and analysis using the piezoelectric six-component Kistler balance.
Kistler I Signal conditioner
Static load =q-offset
On-line data processing]
Subserie No.
Offset
Maximum output voltage
Output signal from balance channel
Measuring i~ero
I
I
/ //
~J!
~'x
Y=~t+q
/- ¢ ' / /
Time
O0 QO
89 Procedure for measuring and analysis The procedure for measuring the static wind load which has been successfully applied at DMI is illustrated in Fig. 1. When measuring static wind loads, the time histories of the signals are derided into subseries of constant duration (as described by Mayne and Cook [6] ). The contribution to the drift is estimated by means of a least square fit to the mean values of the subseries. A very accurate knowledge of the timing when measuring is the basis for a successful analysis. The Kistler signal conditioner, the data acquisition and the fan of the wind tunnel are controlled by a PDP 11/23 computer, thereby ensuring accurate timing. After the contribution from the static wind load has been calculated for each of the o u t p u t channels, the results can be combined to give values proportional to the three forces and the three moments. A check calibration of the accuracy of the measured static loads using the above principle of measuring and analysis was made by nine measurements using weights, scales and pulleys in the lower part (maximum 15%) of the 100 N range of the signal conditioner. This gave a regression line with a slope of 0.992 N/N, an intercept of - 0 . 0 0 8 9 N, a correlation coefficient of 0.9994 and a mean square error of 0.03 N 2. The static force was an x-force which also gave a y - m o m e n t . It should be m e n t i o n e d t h a t the procedure described above requires a measuring time larger t h a n would be necessary with a strain-gauge balance. However, this is not a serious drawback as in general investigations with six-component balances can be quickly carried out. Comparison with strain.gauge balance measurements Measurements have been carried out in order to compare the results obtained with the piezoelectric balance and the results obtained with a strain-gauge balance. The measurements were carried o u t with a model of an offshore structure situated in a natural grown b o u n d a r y layer simulating wind over rough sea. The results of the comparison are shown in Fig. 2. As can be seen the results from the two balance measurements compare well. Conclusion The results from the measurement of static wind loads with the piezoelectric balance are so good t h a t the application of this t y p e of balance in wind-tunnel testing is strongly recommended when static as well as dynamic wind loads are of importance.
90 CYP -E IZOELECTR C I 2.5 Q
i
-2.5
/, ,
,
~ N
-0,0~
,
GAUGE l
|
2.5
p.O
SLOPE
0.960
INTERCEPT CORRELATION
COEFFICIENT
=
-0.007
=
0.995
-2.5 CXP -E IZOELECTR C I 2,5
SLOPE
=
INTERCEPT
=
-0.027
1.019
=
0.995
GAUGE l
i
-
•
-2.5
l
,
l
i
2.5
CORRELATION
COEFFICIENT
-2.5 Fig. 2. Comparison between the results obtained with the. piezoelectric balance and the results obtained with a strain-gauge balance.
Acknowledgements The hospitality and interest which Dr. N.J. Cook at Building Research Establishment, Watford, U.K. has shown in this matter is greatly appreciated by the authors.
91
References 1 N.J. Cook, A sensitive 6-component high-frequency range balance for building aerodynamics, J. Phys. E., 16 (1983) 390--393. 2 G. Schewe, On the force fluctuations acting on a circular cylinder in cross flow from subcritical up to transcritical Reynolds numbers, J. Fluid Mech., in press. 3 T. Tschanz, The Base Balance Measurement Technique and Applications to Dynamic Wind Loading of Structures, BLWT-5-1983, The University of Western Ontario, Faculty of Engineering Science, London, Ontario, Canada, N6A 5B9. 4 G. Spescha and E. Voile, Piezoelectric Measuring Instruments. Unknown origin. 5 G. Schewe, A multicomponent balance consisting of piezoelectric force transducers for a high-pressur e wind tunnel, Paper given at Sensor and Systems, Los Angeles, CA, 1982. 6 J.R. Mayne and N. Cook, Acquisition analysis and application of wind loading data, Proc. 5th Int. Conf. on Wind Engineering, F o r t Collins, CO, 1979, Pergamon Press, Oxford, 1980.