CN108387483B - Wall shear stress measuring method - Google Patents
Wall shear stress measuring method Download PDFInfo
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- CN108387483B CN108387483B CN201810062589.6A CN201810062589A CN108387483B CN 108387483 B CN108387483 B CN 108387483B CN 201810062589 A CN201810062589 A CN 201810062589A CN 108387483 B CN108387483 B CN 108387483B
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- nickel foil
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N11/00—Investigating flow properties of materials, e.g. viscosity, plasticity; Analysing materials by determining flow properties
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
- G01L11/00—Measuring steady or quasi-steady pressure of a fluid or a fluent solid material by means not provided for in group G01L7/00 or G01L9/00
- G01L11/002—Measuring steady or quasi-steady pressure of a fluid or a fluent solid material by means not provided for in group G01L7/00 or G01L9/00 by thermal means, e.g. hypsometer
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
- G01M10/00—Hydrodynamic testing; Arrangements in or on ship-testing tanks or water tunnels
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N11/00—Investigating flow properties of materials, e.g. viscosity, plasticity; Analysing materials by determining flow properties
- G01N2011/006—Determining flow properties indirectly by measuring other parameters of the system
- G01N2011/0093—Determining flow properties indirectly by measuring other parameters of the system thermal properties
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- General Health & Medical Sciences (AREA)
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- Fluid Mechanics (AREA)
- Investigating Or Analyzing Materials Using Thermal Means (AREA)
- Investigating Strength Of Materials By Application Of Mechanical Stress (AREA)
Abstract
本发明提供了一种壁面切应力测量方法，属于流体力学实验技术领域。本发明的壁面切应力测量方法使用的传感器由两片镍箔和一层聚酰亚胺薄膜组成，两片镍箔分别粘贴在聚酰亚胺薄膜的两侧的对应位置。其中一片镍箔作为传感器的敏感元件与流体直接接触，通过对加热功率的测量直接换算出切应力；另一片作为敏感元件的发热衬底，与传感器加热至同一温度，保证传感器只与流体进行热量交换。该方法无需对传感器进行标定即可实现对壁面切应力的测量。
The invention provides a method for measuring wall shear stress, which belongs to the technical field of fluid mechanics experiments. The sensor used in the wall shear stress measurement method of the present invention is composed of two pieces of nickel foil and a layer of polyimide film, and the two pieces of nickel foil are respectively pasted at corresponding positions on both sides of the polyimide film. One of the nickel foils, as the sensitive element of the sensor, is in direct contact with the fluid, and the shear stress is directly converted by measuring the heating power; the other, as the heating substrate of the sensitive element, is heated to the same temperature as the sensor to ensure that the sensor only conducts heat with the fluid. exchange. The method can measure the wall shear stress without calibrating the sensor.
Description
Technical Field
The invention belongs to the technical field of fluid mechanics experiments, and relates to a wall shear stress measuring method for measuring wall shear stress in a flow field.
Background
Wall shear stress is an important parameter of the wall in the flow field that determines the distribution of wall viscous forces and velocities near the wall. The method for measuring the wall shear stress includes a direct measurement method on the wall and an indirect measurement method for testing near the wall. The direct measurement method includes oil film interference method and micro electromechanical sensor method installed on the wall. Among these measurement methods, micro-electromechanical sensors using the heat exchange principle are widely used in the fields of aviation and fluid.
A schematic diagram of a mems sensor of the heat exchange principle is shown in figure 1.
Applying a certain current to the sensor to generate heat, wherein the joule heat Q and the average wall shear stress tau are equal when the heat balance is reachedwThe following relationships exist:
in equation (1), Q is Qs + Qa, where Qa is the amount of heat transferred to the flow field, Qs is the amount of heat conducted to the wall, and a and B are related to Qs and Qa, which can be obtained by calibration. The values of a and B are typically determined in a calibrated manner. However, there are instances where calibration is not practical and changes in ambient temperature and surface conditions can cause the measurement results to drift. Therefore, it is necessary to provide a wall shear stress measurement method without calibration.
Disclosure of Invention
The technical problem to be solved by the invention is as follows: the wall shear stress measuring method can measure the shear stress of the wall in a flow field. The method does not need to calibrate the sensor, and can convert the wall shear stress by only measuring the voltage and the current of the sensor.
The technical scheme of the invention is as follows:
a wall shear stress measuring method adopts a sensor to measure; the sensor is composed of two nickel foils and a layer of polyimide film, wherein the two nickel foils are respectively adhered to the upper surface and the lower surface of the polyimide film; the nickel foil has an aspect ratio of more than 10 and a positive temperature coefficient of resistance; the two sides of the nickel foil are respectively provided with a wiring point, the wiring points are positioned at the two ends of the nickel foil in the length direction, each wiring point leads out two wires, one wire is a current wiring for conducting current passing through the nickel foil, the other wire is a voltage wiring for measuring the voltage at the two ends of the nickel foil, namely, each sensor is connected with four wires; one nickel foil is directly contacted with fluid in a flow field to serve as a sensitive element, and the other nickel foil is arranged at the corresponding position on the other side of the insulating medium; measuring the current passing through the nickel foil and the voltage at two ends of the nickel foil, calculating the resistance of the nickel foil, and calculating the temperature of the nickel foil in the state according to the resistance temperature coefficient of the metallic nickel; changing the current of two nickel foils to make the two nickel foils work under the same resistance value; at the moment, the working temperatures of the two nickel foils are the same, and the nickel foil serving as a sensitive element only exchanges heat with fluid; after the temperature is stabilized, the heat quantity of current heating of the nickel foil serving as the sensitive element is equal to the heat quantity taken away by the fluid, and at the moment, the relation between the heating power and the shear stress is calculated.
The invention has the beneficial effects that: the invention can calculate the shear stress at the position of the sensor by measuring the voltage and the current of the sensitive element and calculating the heating power of the element. The wall shear stress can be converted by measuring the voltage and the current of the sensor without calibrating the sensor, and the method is convenient and accurate.
Drawings
Figure 1 is a schematic view of a mems sensor of the heat exchange principle.
FIG. 2 is a schematic of the test of the present invention.
Detailed Description
The following further describes a specific embodiment of the present invention with reference to the drawings and technical solutions.
The sensor is composed of two nickel foils and a layer of polyimide film, wherein the two nickel foils are electrified and heated to the same temperature, and one of the nickel foils is in direct contact with fluid in a flow field and exchanges heat with the fluid to be used as a sensitive element. And the other nickel foil is arranged at the corresponding position on the other side of the polyimide film and is used as a substrate of the sensitive element. The thickness of the sensor of the whole sandwich structure is controlled within 25 microns.
A schematic of the test is shown in figure 2.
Since the two nickel foils are heated to the same temperature, the heat Qs conducted by the sensing element to the wall surface can be reduced to almost zero, so that Q is Qa, and the formula (1) becomes the following form:
wherein, the derivation is carried out according to the heat exchange theory:
wherein, TfIs nickel foil temperature, TaIs the fluid temperature, w is the nickel foil width, l is the nickel foil length, k, ρ, Cp, μ are the thermal conductivity, density, specific heat capacity and viscosity of the fluid, respectively;
according to the formula of the temperature coefficient of resistance, the temperature T of the nickel foilfThe following formula is used to calculate:
wherein, TfIs nickel foil temperature, T0Is the temperature of the nickel foil at room temperature, α is the temperature coefficient of resistance, R, of nickel0The resistance of the nickel foil at normal temperature, and R is the resistance of the nickel foil in the working state. The two nickel foils can be heated to the same temperature through the control of current and the measurement of resistance, and the heat transfer between the thermal film serving as a sensitive element and the bottom surface is negligible.
Claims (1)
1. A wall shear stress measuring method is characterized in that a sensor is adopted for measurement; the sensor is composed of two nickel foils and a layer of polyimide film, wherein the two nickel foils are respectively adhered to the upper surface and the lower surface of the polyimide film; the nickel foil has an aspect ratio of more than 10 and a positive temperature coefficient of resistance; the two sides of the nickel foil are respectively provided with a wiring point, the wiring points are positioned at the two ends of the nickel foil in the length direction, each wiring point leads out two wires, one wire is a current wiring for conducting current passing through the nickel foil, the other wire is a voltage wiring for measuring the voltage at the two ends of the nickel foil, namely, each sensor is connected with four wires; one nickel foil is directly contacted with fluid in a flow field to serve as a sensitive element, and the other nickel foil is arranged at the corresponding position on the other side of the insulating medium; measuring the current passing through the nickel foil and the voltage at two ends of the nickel foil, calculating the resistance of the nickel foil, and calculating the temperature of the nickel foil in the state according to the resistance temperature coefficient of the metallic nickel; changing the current of two nickel foils to make the two nickel foils work under the same resistance value; at the moment, the working temperatures of the two nickel foils are the same, and the nickel foil serving as a sensitive element only exchanges heat with fluid; after the temperature is stable, the heat quantity of current heating of the nickel foil serving as the sensitive element is equal to the heat quantity taken away by the fluid, and the relation between the heating power and the shear stress is calculated at the moment;
since the two nickel foils are heated to the same temperature, the heat Qs conducted by the sensing element to the wall surface is zero, so that Q is Qa, and the formula (1) becomes the following form:
wherein Q is Qs + Qa, Qa is flow direction fieldThe quantity of heat transferred in (1), Qs is the quantity of heat conducted to the wall surface, and A is related to Qs and Qa and is obtained through calibration; tau iswMean wall shear stress;
deducing according to a heat exchange theory:
wherein, TfIs nickel foil temperature, TaIs the fluid temperature, w is the nickel foil width, l is the nickel foil length, k, ρ, Cp, μ are the thermal conductivity, density, specific heat capacity and viscosity of the fluid, respectively;
according to the formula of the temperature coefficient of resistance, the temperature T of the nickel foilfThe following formula is used to calculate:
wherein, TfIs nickel foil temperature, T0Is the temperature of the nickel foil at room temperature, α is the temperature coefficient of resistance, R, of nickel0The resistance of the nickel foil at normal temperature, and R is the resistance of the nickel foil in a working state; the two nickel foils were heated to the same temperature by controlling the current and measuring the resistance, and the heat transfer between the thermal film as the sensing element and the bottom surface was ignored.
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| CN201810062589.6A CN108387483B (en) | 2018-01-23 | 2018-01-23 | Wall shear stress measuring method |
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| CN201810062589.6A CN108387483B (en) | 2018-01-23 | 2018-01-23 | Wall shear stress measuring method |
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| CN108387483B true CN108387483B (en) | 2020-04-07 |
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| CN109406097A (en) * | 2018-12-05 | 2019-03-01 | 浙江大学 | A kind of device and method for fluid wall surface shear stress in surveying laboratory sink |
| CN111351609A (en) * | 2020-03-16 | 2020-06-30 | 大连理工大学 | Method for measuring wall shear stress vector |
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| TW456041B (en) * | 2000-11-16 | 2001-09-21 | Nat Science Council | A contact type micro piezoresistive shear-stress sensor |
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Effective date of registration: 20240507 Address after: Room 1401, 14th Floor, Building B2, Zhiye Plaza, No. 135 Guangxian Road, High tech Industrial Park, Dalian City, Liaoning Province, 116023 Patentee after: Dalian Hanghua Technology Co.,Ltd. Country or region after: China Address before: 116024 No. 2 Ling Road, Ganjingzi District, Liaoning, Dalian Patentee before: DALIAN University OF TECHNOLOGY Country or region before: China |