A Method for Measuring Wall Shear Stress

Abstract

The invention belongs to the technical field of fluid measurement, and particularly relates to a parallel array hot line probe and a wall surface shearing force measurement method. According to a wallsurface shearing force measurement method, the speeds of two position points, far way from a wall surface at different heights, in a viscous bottom layer of a wall region, are simultaneously measuredin one time by the parallel array hot line probe, the fluctuating velocity is small, the additional shearing force generated by fluctuating velocity is also small, a viscous shearing force of the viscous bottom layer has a main guide effect, the speed in the viscous bottom layer and the wall surface distance are in linear distribution, and the product of slop (U2-U1) / d and a coefficient Mu of kinetic viscosity is a shearing force. The parallel array hot line probe is carried on a mechanical arm during measurement, a mechanical arm control driver is controlled to drive the mechanical arm to move and adjust by an upper computer, rapid moving of different positions of the wall surface can be achieved, and the measurement demand of the shearing force at different positions of the wall surfaceis solved very well.

Description

technical field [0001] The invention belongs to the technical field of fluid measurement, and in particular relates to a method for measuring wall shear stress. Background technique [0002] Wall shear stress is an important basic parameter for studying and judging the shape of flow field and the state of velocity boundary layer, but the measurement of wall shear stress has always been a difficult point in experimental fluid mechanics. With the continuous development of modern experimental fluid mechanics measurement technology, the new measurement technology of fluid wall shear stress has received more and more attention. [0003] Traditional wall shear stress measurement methods include direct measurement methods and indirect measurement methods. Direct measurement methods include surface oil film method, floating balance method, surface hot film method, etc. Such measurement methods generally have large calibration errors, difficult installation, and poor frequency respon...

AI Technical Summary

Problems solved by technology

[0003] Traditional wall shear stress measurement methods include direct measurement methods and indirect measurement methods. Direct measurement methods include surface oil film method, floating balance method, surface hot film method, etc. Such measurement methods generally have large calibration errors, difficult installation, and poor frequency response. Low-level problems, it is difficult to meet the requirements of accurate measurement of fluid wall shear stress, and can only roughly measure the distribution of shear stress; indirect measurement methods mainly rely on pitot tubes, hot wire anemometer (HWA), laser Doppler velocimeter (LDV) instrument measurement Velocity boundary layer, according to the distribution characteristics of the boundary layer (such as linear region, logarithmic region, etc.), the wall shear stress can be obtained indirectly, but this kind of measurement method is limited in scope, for example, Pitot tube is not applicable in supersonic flow, because There is an isolated shock wave at the head of the pitot tube, and the total pressure hole feels the total pressure after the wave, and the static pressure of the incoming flow is difficult to measure accurately; and the existing hot wire anemometer (HWA) cannot measure the velocity at different heights near the wall area at the same time , it is necessary to measure the velocity at different heights multiple times, and then calculate the wall shear stress in the near wall area; while the laser Doppler velocimeter (LDV) needs to have a certain transparency for the measured fluid, when measuring the velocity of pure water or air When the flow rate is high, it is necessary to artificially add an appropriate amount of particles as the scattering center. When the flow rate is high, it is required to increase the laser output power. Due to the high frequency of the signal, it is difficult to process the signal. The limitations are relatively large, and it is also difficult to obtain the dynamics of the wall shear stress. information

[0004] For example, Chinese patent CN108387483A discloses a method for measuring wall shear stress, which discloses the use of sensors to measure wall shear stress. The sensor is composed of two pieces of nickel foil and a layer of polyimide film. Temperature, one of the nickel foils is in direct contact with the fluid in the flow field, and exchanges heat with the fluid, as a sensitive element; the other nickel foil is installed at the corresponding position on the other side of the polyimide film, as the substrate of the sensitive element, using When the sensor measures the wall shear stress, the sensor is pasted on the wall by an adhesive, and the current passing through the nickel foil and the voltage across the nickel foil are used to calculate the heating power of the element, thereby calculating the shear stress at the sensor position , although this measurement method does not need to calibrate the sensor, there are still some problems in this measurement method. During the measurement, the sensor needs to be attached to the wall with an adhesive before measurement. If the measurement needs to be performed on the wall at another position When measuring shear stress, the sensor must be torn off from the wall first and then pasted to the position to be measured for measurement. The measurement process is more complicated, and it is easy to cause damage to the sensor during the tear-off process, resulting in the failure of the measurement process.

Images