Abstract: The direct measurement of skin friction for flat plate model in shock tunnel is introduced in this paper, including the design, the manufacture, the calibration of piezoelectric skin friction balance, also including the skin friction measurement test in shock tunnel and the analyzing of the experiment results. In this research, in order to improve the precision of the skin friction balance calibration and the skin friction measurement in shock tunnel test, and to change easily between the shock tunnel test and the calibration, the skin friction balance is especially designed with a separate sensing head from the cantilever beam of the skin friction balance main body.
The sensing head of the skin friction balance could be changed in different situations for the calibration and shock tunnel test. For example, the sensing head could be changed into a calibration head while calibration. While in the shock tunnel test, it could be changed back to sensing head as measuring the skin friction of the surface of the model. The additional advantages to adopt this structure of the skin friction balance are described as follows. First, the skin friction balance can be changed easily in different shape sensing head (circle，square or triangle) and in different surface which can fit well with many kinds of measuring surface, which can enhance the test ability of the skin friction balance. Second, with a thermal insulating sensing head can improving the skin friction balance test ability in high temperature flows, which can be used as an useful tool for skin friction measurement in the flow fields such as in the scramjet inlet and combustion chamber. Third, the response frequency of the skin friction balance can be improved by using light sensing head, which would be used in high frequency test, such as the skin friction measurement in flow field changed drastically.
In the manufacture of the skin friction balance, it must paid much attention to the location and protection of the piezoelectric ceramic. The location of the piezoelectric ceramic is assured by the given tool designed specifically. After the skin friction balance signal wires is fetched out, the epoxy glue and moisture-proof adhesive are daubed on the piezoelectric ceramic to moisture-proof and seal. In order to protect the balance from the damage during the calibration and shock tunnel test, the skin friction balance is protect with a protecting house.
In calibration, the skin friction balance is fixed in a horizontal position, and then calibration load is hung vertically by thin thread which fixed to the calibration head. The calibration load rang is form -50 gram to 50 gram, with the minimal load of 0.13gram. The sensitivity of the two skin friction balances is 11.55mv/g and 5.47mv/g respectively, and the precision error is 0.36% and 0.29%. The calibration results show that the sensitivity and precision error of the skin friction balance could satisfy the skin friction measurement in shock tunnel.
In the shock tunnel test, the dimension of the sensing head surface is 20mm´20mm. The sensing head surface is mounted flush with the model surface, and there is a small gap about 0.1 to 0.2mm around the sensing head from the measuring surface. The skin friction test is conducted in shock tunnel with a flat plate model. The flow field Mach number is 5.78 , and the attack angles are 0°and -6°. The heat flux of the same location at the measured skin friction is also measured with φ2mm thin film gauges and hot film sensors. The theory calculation of the flat model skin friction is also carried out using the turbulent model with compressed modification. The shock tunnel test result shows that the skin friction coefficient of flat plate model could be test with the new developed skin friction balance. There is a strong correlation between the skin friction coefficient and the heat flux in the surface of the flat plate model. The test result is well comparable with the theory calculation. The measurement uncertainty of skin friction coefficient is also calculated, the result shows that, the skin friction measurement uncertainty is about 6.8% to 14.4%.
In the future research, the structure and manufacture of the skin friction balance will be optimized. The disturbance of the shock tunnel environment especially the pressure influence to the skin friction measurement will be considered and studied. The skin friction balance measurement technique in shock tunnel maturation will be improved. In the same time, the correlations research between skin friction and heat flux measurement will be enhanced. The purpose of skin friction measurement research is to provide high quality experiment data for the development of hypersonic vehicle in shock tunnel.
Key word: shock tunnel; skin friction balance, flat plate model, measurement
