Development of an automated calibration system for hotwire anemometers

MA-Thesis / Master von Constantin Schosser

Introduction:

In experimental fluid dynamic measurements hot-wire anemometry is used to record information about flow fields. Furthermore one can obtain the magnitude, the direction and even the time dependant behaviour of the fluid flow, if multiple-wire probes are in operation. The hot-wire measurement technique is based on the convective heat transfer from a heated element to the fluid flow, which is actually proportional to the velocity of the flow. So HWA is an indirect measurement technique. There are miscellaneous sensors which work properly in water or other liquids, air or in gas flows. As an example, Fig. 1.1 shows a cross-wire probe in a fluid flow, which can detect the velocity and its direction in two components, if the main flow direction is in one plane (2D flow).

Predominantly HWA is a research tool for turbulent flow studies, especially transient procedures. Turbulence models have to be built to represent the characteristics of the flow in numerical simulations (CFD). Therefore only detailed experimental measurements lead to reliable information about the local velocity of a turbulent flow. This can be provided by HWA on the basis of its very high spatial and temporal resolution. Although the development of HWA started at the beginning of the 19th century and new techniques like PIV or LDA (direct methods) have been established, it is still a common device in all wind tunnel labs. The analogue output signal can be optimized by filters before signal processing. It can also be deployed to arrange a spectrum analysis, due to the high temporal resolution. Moreover, unlike the digital devices the analogue signal is densely packed. The range of application is large and leads from sub- and supersonic flows, the independency of the medium to high-temperature measurements. HWA is also affordable in contrast to LDA and PIV systems. In spite of these advantages the natural contamination of the hot-wire probe increases by and by, since the particles in the fluid flow mature themselves to the probe and finally isolate it. As this effect of disturbance causes measuring errors, the hot-wire probes have to be calibrated at frequent intervals - best before and after every data acquisition series. However, HWA is an intrusive measurement technique, thus disturbing the flow. Another disadvantage is that it is not applicable in separation and backward flow regions.

The aim of this thesis is to develop an automated calibration system to implement an in situ calibration. A traverse system will move the hot-wire probe to the calibrator. Immediately after accomplishing the calibration, the traverse will adjust the probes position directly in the wind tunnel to fulfil the measuring task. Reasons for applying the in situ calibration are to minimize mistakes and to accelerate this time-consuming procedure.

The disturbance of the flow, caused by the probe and its fixation is the same during calibration and experiment. Disconnecting cables between calibration and measurement, which avoids additional errors, is not necessary anymore. Furthermore it is not needed to move the computer system and the pressure supply. This thesis consists of the mechanical design of the calibration facility, of the composition of the computer programs needed and of an experimental validation. All parts of the calibration and of the measurement program are generated in LabVIEW 8.5 - a development environment for a visual programming language by National Instruments [FD], [HD].

Table of Contents:

	Declaration	2
	Abstract	3
	Acknowledgement	4
	Abbreviations	8
1.	Introduction	9
2.	Theory of hot-wire-anemometry	11
2.1	Hot-wire probes	11
2.1.1	Measuring chain	11
2.1.2	Probe selection	12
2.1.3	Coordinate system	13
2.2	Heat transfer of HWA	15
2.2.1	Ohmic resistance of a hot-wire	15
2.2.2	Thermal balance	15
2.2.3	Influences on the sensor signal	18
2.3	Electrical circuit of HWA	18
2.3.1	CCA mode	18
2.3.2	CTA mode	20
2.3.3	Ohmic and complex resistance of a hot-wire	20
2.4	Anemometer setup	21
2.4.1	Overheat adjustment	21
2.4.2	Square wave test	21
2.4.3	Low-pass filtering	22
2.5	Reference velocity for hot-wire calibrations	22
2.5.1	Calculation of the fluid velocity	23
2.5.2	Adjusting a defined mass flow in the calibrator	24
2.6	Calibration methods	26
2.6.1	Velocity calibration	26
2.6.2	Calibration of cross-wire sensors according to the DANTEC method	27
2.6.3	Look-up matrix method	30
2.6.4	Temperature correction	32
2.7	Spatial resolution errors	33
3.	Design of the automated calibration facility	34
3.1	Initial requirements	34
3.2	Dimensioning of the mechanical components	34
3.3	Design of the calibration device	37
3.4	Important components of the measuring system	40
3.5	Positioning accuracy	40
4.	Experimental approach during calibration and measurement	42
4.1	Measurement devices	42
4.1.1	Measuring chain	42
4.1.2	Arrangement of the measuring system	44
4.1.3	Controller card ISEL IT 116	45
4.1.4	Pressure transducer SETRA 239	45
4.1.5	Hot-wire sensors	46
4.1.6	CTA unit DISA Type 55M01/55M10	47
4.1.7	Analogue-to-digital converter	48
4.2	Calibration equipment DISA Type 55D90	50
4.3	Description of the settings	52
4.3.1	Overheat setup	52
4.3.2	Dynamic bridge balancing	53
4.3.3	Precise positioning of the hot-wire probe	54
4.3.4	Calibration of the pressure transducer	54
4.4	LabView programs	57
4.4.1	Calibration program	57
4.4.2	Measurement program	64
4.4.3	step motor control	66
5.	Measurement results 67
5.1	Hot-wire calibration	67
5.1.1	Single-wire probe (steady flow)	67
5.1.2	Single-wire probe (transient flow)	68
5.1.3	Cross-wire probe (steady flow)	69
5.1.4	Cross-wire probe (transient flow)	71
5.2	Hot-wire validation	73
5.2.1	Single-wire probe (steady state flow)	73
5.2.2	Single-wire probe (transient flow)	74
5.2.3	Cross-wire probe (steady state and transient flow)	75
5.3	Summary of validation	78
5.4	Discussion and limitations	78
5.4.1	Single-wire	78
5.4.2	Cross-wire	79
6.	Perspective	82
	Bibliography	85
	List of symbols	86
	Figures and Pictures	93
A	Appendix: About hot-wire measurements	94
A.1	Spatial resolution error - turbulent velocity distribution	94
A.2	Table for quick selection of HW probes	96
B	Appendix: Datasheets	98
B.1	Hot-wire probes	98
B.1.1	Single-wire probe DISA type 55P1	98
B.1.2	Cross-wire probe DANTEC type 9055P0511	98
B.1.3	Specifications	99
B.1.4	SEL IT 116	100
B.2	NI 6036E	101
B.3	SETRA 239	103

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