| The measurement of boundary layer flow is one of the most important fundamental problems in the fluid mechanics research.The boundary layer transition and separation has significant impacts on the energy consumption,controllability,aerodynamic heat and safety of aircrafts.Wall shear stress is a tangential force in the direction of fluid flows over a solid body due to the viscosity,which is one of the most essential physical quantities for the study of skin friction drag and boundary layer characteristics.The measurement of shear stress can provide crucial evidence for the key engineering and basic research,promising the improved performance of air vehicles and new insights into rules and mechanisms underlying the complex boundary layer flow.Due to its small magnitude and scale,high-frequency fluctuation,the device for the shear stress measurement must possess high sensitivity and resolution.However,traditional measurement methods are unable to fulfill the shear stress measurement requirements.With the development of micro and nano fabrication technologies,diverse micro shear stress sensors have been developed for the boundary layer measurement.Among all these sensors,the micro flexible hot-film sensor has high sensitivity and resolution,short response time.Meanwhile,it is well fit on curved surfaces and for multi-point measurement.It is the most convenient and effective method to determine the detailed characteristics in the boundary layer,both simultaneously and non-intrusively.In this work,we focus on the main fundamental and technical problems in the design and fabrication of the high-performance micro flexible hot-film sensor for boundary layer flow measurement.The mathematical model of the micro flexible hot-film sensor for shear stress measurement was presented.Numerical simulation and experiments were conducted to get the non-thermal crosstalk space under various working conditions,the design criterion of sensor array space was established.According to the investigation of the influence of magnetron sputtering and electroplating process parameters on the films adhesion properties,and the annealing effect on the temperature coefficient of resistance(TCR)of thermosensitive film,the fabrication process parameters of micro flexible hot-film sensor with high sensitivity and reliability were obtained.The static and dynamic characteristics of the sensor working under constant current(CC)and constant temperature(CT)modes are tested in the shear stress calibration devices.Finally,the boundary layer transition and turbulent separation characteristics of the airfoils were measured in wind tunnels using the developed flexible hot-film sensor array.The main contents and conclusions in this dissertation are listed as follows:Firstly,based on the heat transfer energy equation of steady flow,the mathematical model between the sensor output and shear stress of the micro flexible hot-film sensor working in CC and CT modes were studied.The thermal crosstalk criterion of the sensor array under typical working condition was determined.The corresponding temperature distribution in laminar and turbulent boundary layers of the sensing element working in air and water were estimated by numerical simulation.According to the thermal crosstalk criterion,the optimal spaces between two sensing elements for various shear stress and working temperature were obtained.Flat plate wind tunnel and micro water tunnel experiments were designed and carried out to determine the thermal crosstalk to verify the above numerical simulation results.Secondly,the fabrication process flow of the high-performance micro flexible hot-film sensor was designed.The effects of sputtering power and argon pressure in magnetron sputtering process,current in electroplating process on the adhesion strength and TCR of the sensor films were studied.The optimal sputtering power of 150 W,argon pressure of 0.6 Pa,electroplating current of 1.5 A were determined.Parylene C was selected as the protective material for applications in harsh environments such as humid air or underwater.The protective film was fabricated by vapor phase deposition process.Thirdly,the effect of thermal annealing temperature and time on the TCR of flexible hot-film sensor were studied.It is found that the TCR shows a significant increase with the increasing annealing temperature below 400 ℃ as well as annealing time within 6 hours.However,when the annealing time exceeds 6 hours,the TCR almost has no change with increasing annealing time.The maximum TCR of nickel film after annealing is 5100 ppm/℃,which is the highest value of the reported flexible hot-film sensor with nickel thermistor.The sensor sensitivity after annealing was highly improved.The microstructural characteristics,surface morphological,preferred crystalline orientation,grain size,composition,and residual stress were evaluated to understand the relation between the film microscopic properties and TCR.Based on the physical models of TCR that include the effects of surface and grain boundary scattering,the improvement of TCR is determined by the grain boundary scattering.Fourthly,a detailed characterization of the static response of flexible hot-film sensors was presented in rectangular duct and flat plate shear stress calibration devices,respectively.The sensor sensitivity is decrease with increasing shear stress.In CC working mode,the sensor output voltage varies inversely as the 1/3 power of the shear stress.In contrast,the output voltage is proportional to the 1/3 power of the shear stress under CT working mode.Meanwhile,the dynamic response of flexible hot-film sensors was studied employing a plane-wave tube capable of generating known sinusoidal oscillatory wall shear stress.The dynamic sensitivity of the sensor versus frequency was observed,the sensor demonstrates a bandwidth of approximately 3 k Hz in CC mode and 7 k Hz in CT mode,respectively.In conclusion,the flexible hot-film sensor can fulfill the requirements of time average and resolve shear stress measurement in turbulent boundary layer.In the end,according to the shear stress variation during boundary layer transition and separation,the micro flexible hot-film sensor array was employed to detect the formation and separation characteristics of turbulent boundary layer over NACA0012 airfoil in the wind tunnel.The onset,50% intermittent and end of transition points at different Mach number and angle of attack(AOA)were experimental investigated.The results demonstrated that the transition region is propagated upward to the leading edge with increasing Mach number and AOA.Moreover,the static stall was caused by the leading-edge separation,the stall angles captured by the flexible hot-film sensors are same with the static pressure distribution measurement.In addition,the sensors were also used for steady and unsteady turbulent boundary layer separation measurement of ARJ21-700 aircraft model for wind tests.The static and dynamic separation angles were obtained.It is found that the dynamic separation and reattachment angle in unsteady flow has a delay compare to the steady condition. |
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