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A second generation MEMS surface fence sensor for high resolution wall shear stress measurement
Abstract
The measurement of the wall shear stress in fluid mechanics has important implications for the investigation of turbulent flow phenomena. An advanced version of the MEMS silicon surface fence sensor that measures this stress is presented herein. This sensor consists of a cantilever structure with a length of 5 mm, a height of 315 μm, and a thickness of 10 μm. Fluid flowing towards this structure causes it to bend. The resulting bending stress in the cantilever is measured with four piezoresistors, and this stress is correlated to the wall shear stress. The offset voltage has been reduced significantly. The sensor operates over a range of approximately −1 to +1 N/m2 with a resolution of 10 mN/m2.
... In the hypersonic wind tunnel experiment, obtaining the accurate friction distribution of the model points or surfaces puts forward higher requirements for the size of the sensor limited by the volume of the aircraft model. In recent years, several types of MEMS sensors have been reported to measure skin friction including capacitance type and comb differential capacitance type [1][2][3], piezoresistive type [4][5][6], and piezoelectric type [7,8]. Those MEMS sensors have similar characteristics such as small size, high sensitivity, and high resolution in a small measurement range (several Pa). ...
... For example, Jiang et al. [1] have reported a supporting cantilever and plate differential capacitive skin friction sensor for low-speed wind tunnels with high sensitivity and a minimum detection limit of 0.05 Pa in a range from 0.1-2 Pa. Von P et al. [7] have reported a wall shear stress sensor using four piezoresistors as a transducer in the cantilever, with a resolution of 0.01 Pa in the range 0-2 Pa. T Kim T et al. [9] have reported a piezoelectric floating element shear stress sensor for the wind tunnel flow measurement with the high sensitivity 56.5 pc/Pa. ...
The skin friction of a hypersonic vehicle surface can account for up to 50% of the total resistance, directly affecting the vehicle’s effective range and load. A wind tunnel experiment is an important and effective method to optimize the aerodynamic shape of aircraft, and Micro-Electromechanical System (MEMS) skin friction sensors are considered the promising sensors in hypersonic wind tunnel experiments, owing to their miniature size, high sensitivity, and stability. However, the sensitive structure including structural appearance, a gap with the package shell, and flatness of the sensor will change the measured flow field and cause the accurate measurement of friction resistance. Aiming at the influence of sensor-sensitive structure on wall-flow characteristics and friction measurement accuracy, the two-dimensional and three-dimensional numerical models of the sensor in the hypersonic flow field based on Computational Fluid Dynamics (CFD) are presented respectively in this work. The model of the sensor is verified by using the Blathius solution of two-dimensional laminar flow on a flat plate. The results show that the sensor model is in good agreement with the Blathius solution, and the error is less than 0.4%. Then, the influence rules of the sensitive structure of the sensor on friction measurement accuracy under turbulent flow and laminar flow conditions are systematically analyzed using 3D numerical models of the sensor, respectively. Finally, the sensor-sensitive unit structure’s design criterion is obtained to improve skin friction’s measurement accuracy.
... MEMS skin friction sensors are considered promising sensors in hypersonic wind tunnel experiments owing to their miniature size, high sensitivity, and stability. In recent years, several researchers have developed MEMS sensors to measure skin friction, including the capacitance-type and comb differential capacitance-type [1][2][3][4][5], piezoresistive-type [6][7][8], and piezoelectric-type [9]. For example, Mills et al. [5] reported a fully differential capacitive wall shear stress sensor for low-speed wind tunnels with the high sensitivity of 196 mV/Pa and a minimum detection limit of 12 mPa at 1000 Hz in a range from 0-10 Pa; Von, P. et al. [8] reported a wall shear stress sensor using four piezoresistors in the cantilever, and the resolution was 0.01 Pa in the range of 2 Pa; Kim, T. et al. [9] reported a piezoelectric floating element shear stress sensor for the wind tunnel flow measurement with the high sensitivity of 56.5 pc/Pa and with the frequency range of the sensor up to 800 Hz. ...
... In recent years, several researchers have developed MEMS sensors to measure skin friction, including the capacitance-type and comb differential capacitance-type [1][2][3][4][5], piezoresistive-type [6][7][8], and piezoelectric-type [9]. For example, Mills et al. [5] reported a fully differential capacitive wall shear stress sensor for low-speed wind tunnels with the high sensitivity of 196 mV/Pa and a minimum detection limit of 12 mPa at 1000 Hz in a range from 0-10 Pa; Von, P. et al. [8] reported a wall shear stress sensor using four piezoresistors in the cantilever, and the resolution was 0.01 Pa in the range of 2 Pa; Kim, T. et al. [9] reported a piezoelectric floating element shear stress sensor for the wind tunnel flow measurement with the high sensitivity of 56.5 pc/Pa and with the frequency range of the sensor up to 800 Hz. The common characteristics of those sensors are their usually high sensitivity, high resolution (10 −2 Pa), and small measurement range (several Pa). ...
Micro-electromechanical system (MEMS) skin friction sensors are considered to be promising sensors in hypersonic wind tunnel experiments owing to their miniature size, high sensitivity, and stability. Aiming at the problem of short test duration (a few milliseconds) and heavy load in a shock wind tunnel, the fast readout circuit and the sensor head structures of a MEMS skin friction sensor are presented and optimized in this work. The sensor was fabricated using various micro-mechanical processes and micro-assembly technology based on visual alignment. Meanwhile, the sensor head structure was integrated with the fast readout circuit and tested by using a centrifugal force equivalent method. The calibration results show that this sensor provides good linearity, sensitivity, and stability. The measurement ranges are 0–2000 Pa with good performance. The resolution is better than 10 Pa at 3000 Hz detection frequency of the readout circuit for the sensor in ranges from 0 to 1000 Pa. In addition, the repeatability and linearity of static calibration for sensors are better than 1%.
... A number of techniques exist for measuring surface shear stress. These include oil film interferometry [4], heated patch or heated wire measurements [5][6][7], hair-like sensors [8][9][10], surface fence measurements [11,12], and floating element techniques (see below). These techniques have been reviewed in a number of excellent papers and have various advantages and disadvantages [1,2,[13][14][15]. ...
... Using the sensitivity model given in Eqs. (8) and (9), it is postulated that the change in capacitance can be related to the shear stress and pressure gradient by (15) For each flow condition the pressure gradient and the shear stress are known, either from the flow rate according to Eqs. (12) and (13), or from direct measurement of pressure gradient. There are 24 non-zero flow conditions to evaluate and two constants to fit. ...
... Until now, most sensors have components which are exposed to the streaming fluid and are therefore intrusive. Often a variety of surface fences or hot-wires (von Papen et al. 2004; Ebefors et al. 1998; Buder et al. 2007) are used as the sensing element in the flow. As well as having the disadvantage of changing the flow, these components are very fragile and susceptible to damage from particles in the flow. ...
... The electrical connections were mostly placed downstream. In these cases, the sensor is placed flush within the wall, but the wiring not, as shown in the work of von Papen et al. (2004). In this new approach presented here, the integration of the sensors and the necessary subsystems directly into the wall is investigated. ...
In this work, the latest results of the design, fabrication and characterization of a new MEMS piezoresistive pressure sensor are presented. Significant changes in the layout as well as in the micro-fabrication process have been made, e.g. anodic bonding of a glass cover on the backside. The sensor has been developed in order to meet the special requirements of measurements in fluid mechanics, particularly with regard to the non-intrusive nature of the sensor. The sensor development, starting with the simulation of mechanical stresses within the diaphragm resulting from a pressure of up to 4 bar is described. These calculations have lead to an optimized placement of the piezoresistors in order to achieve a maximum sensitivity. Important parameters including sensitivity, resonance frequency and maximum load are described precisely. The experiments and the initial results, e.g. its linearity and its dynamic capability are demonstrated.
... An alternative device, essentially a MEMS Stanton fence whose fluctuating bending stress is sensed on a chip into which the fence is integrated, was initially described by von Papen et al. (2001). Its design has since been improved, as described by von Papen et al. (2004) and Schiffer et al. (2005). ...
... An alternative device, essentially a MEMS Stanton fence whose fluctuating bending stress is sensed on a chip into which the fence is integrated, was initially described by von Papen et al. (2001). Its design has since been improved, as described by von Papen et al. (2004) and Schiffer et al. (2005). Further work on this device is the topic of the present paper. ...
Microelectromechanical systems (MEMS) sensors are increasingly used for measurements in fluid dynamics since, because of their inherently compact size, they allow access to the information that was previously unavailable. In this paper we describe further testing of a MEMS sensor used to measure surface shear stress. It is a wall-mounted fence that bends under the influence of the pressure difference resulting from the velocity shear at the wall. The fences have been successfully calibrated in a wind tunnel and, as an example of their application, used to determine mean and fluctuating shear stress (along with spectra) in a cylindrical cavity flow.
... For avoiding these drawbacks, indirect wall shear stress measurements were developed with various methods. For example, micro-fences using a cantilever structure and piezoresistors are presented in [5]. The exploitation of optical resonances such as whispering gallery modes of dielectric microspheres is proposed in [6]. ...
In this paper, a Micro-Electro-Mechanical Systems (MEMS) calorimetric sensor with its measurement electronics is designed, fabricated, and tested. The idea is to apply a configurable voltage to the sensitive resistor and measure the current flowing through the heating resistor using a current mirror controlled by an analog feedback loop. In order to cancel the offset and errors of the amplifier, the constant temperature anemometer (CTA) circuit is periodically calibrated. This technique improves the accuracy of the measurement and allows high sensitivity and high bandwidth frequency. The CTA circuit is implemented in a CMOS FD-SOI 65 nm technology. The supply voltage is 1.2 V while the core area is 0.266 mm2. Experimental results demonstrate the feasibility of the MEMS calorimetric sensor for measuring airflow rate. The developed MEMS calorimetric sensor shows a maximum normalized sensitivity of 117 mV/(m/s)/mW with respect to the input heating power and a wide dynamic flow range of 0–26 m/s. The high sensitivity and wide dynamic range achieved by our MEMS flow sensor enable its deployment as a promising sensing node for direct wall shear stress measurement applications.
... Nevertheless, their materials' mechanical properties or sensitivities are compromised at high temperature. MEMS surface fence is a piezoresistive cantilever shear-stress sensor [7][8][9][10][11][12][13]. Deflection of the sensitive cantilever, proportional to wall shear stress, can be directly measured by stress-sensitive resistors placed in stress concentration areas. ...
A new variant of MEMS surface fence is proposed for shear-stress estimation under high-speed, high-temperature flow conditions. Investigation of high-temperature resistance including heat-resistant mechanism and process, in conjunction with high-temperature packaging design, enable the sensor to be used in environment up to 400°C. The packaged sensor is calibrated over a range of ~65 Pa and then used to examine the development of the transient flow of the scramjet ignition process (Mach 2 airflow, stagnation pressure, and a temperature of 0.8 MPa and 950 K, respectively). The results show that the sensor is able to detect the transient flow conditions of the scramjet ignition process including shock impact, flow correction, steady state, and hydrogen off.
... For avoiding these drawback indirect wall shear stress measurements were developed with various methods. For example, micro-fences using a cantilever structure and piezoresistors are presented in [10]; the exploitation of optical resonances such as whispering gallery modes of dielectric microspheres is proposed in [11] (for which the optical resonance shifts with radial deformations of the spheres due to the shear stress); the deflection of micro-pillars is presented in [12] and thermal-based sensors are presented in the next paragraphs of the present paper. The physical principle used in these latter consists in taking advantage of the convective heat transfer between an electrically heated resistor and a surrounding cooler fluid [6]. ...
The paper describes and discusses the design and testing of an efficient and high-sensitivity calorimetric thermal sensor developed for bi-directional wall shear stress measurements in aerodynamic flows. The main technical application targeted is flow separation detection. The measurement principle is based on the forced convective heat transfer from a heater element. The sensor structure is composed of three parallel substrate-free wires presenting a high aspect ratio and supported by periodic perpendicular SiO2 micro-bridges. This hybrid structure takes advantages from both conventional hot-films and hot-wires, ensuring near-wall and non-intrusive measurement, mechanical toughness and thermal insulation to the bulk substrate, and it allowed to add the calorimetric sensor functionality to detect simultaneously the wall shear stress amplitude and direction. The central wire is made of a multilayer structure composed of a heater element (Au/Ti) and a thermistor (Ni/Pt/Ni/Pt/Ni) enabling measurement of the heater temperature and a layer of SiO2 between them for electrical insulation. The upstream and downstream wires are thermistors enabling operation in the calorimetric mode. This design provides a high temperature gradient and a homogeneous temperature distribution along the wires. The sensor operates in both constant current and constant temperature modes, with a feedback on current enabled by uncoupling heating and measurement. Welded on a flexible printed circuit, the sensor was flush mounted on the wall of a turbulent boundary layer wind tunnel. The experiments, conducted in both attached and separated flow configurations, quantify the sensor response to a bi-directional wall shear stress up to 2.4 Pa and demonstrate the sensor ability to detect flow separation.
... laminar/turbulent transition, flow separation). Over the years, many types of shear-stress measurement devices have therefore been investigated, for example direct force sensors [1], indirect pressure sensors [2], or optical sensing of the deflection of micropillars [3]. Notwithstanding these developments, in practice the wall shear-stress on aerodynamic surfaces is still mainly measured with thermal sensors like near-wall hot wires [4] or more commonly hot-film sensors [5]. ...
The static calibration of two MEMS calorimetric shear-stress sensors is performed. In a first step, a calibration range of τw=±2 Pa is obtained in a two-dimensional channel-flow facility. The long-term repeatability of the sensors output, over a two-weeks period, is shown to be very good, with a standard deviation of less than 1% of the mean. In a second step, the sensors are calibrated in a large subsonic wind tunnel up to a velocity of 100 m/s, which corresponds to a range of τw=±14 Pa of wall shear-stress that is closer to realistic values in low-speed aerodynamic flows. A method to measure the frequency response of the calorimetric sensors is also proposed. At an average wall shear-stress of τw¯=1 Pa, the sensors exhibit a cut-off frequency of approximately 1 kHz. Finally, the strong influence of the inter-beam distance on the static and dynamic characteristics of calorimetric sensors is demonstrated.
... piezoelectric, piezoresistive, capacitive), applicable to various pressure ranges and with different geometric dimensions. A wall shear stress measurement may be realized using a floating element, a fence or heated wire transducing principles [5][6][7]. The measurement of pressure and wall shear stress in one miniaturized device is possible but research activities on such combinations are quite limited. ...
For simultaneous measurement of pressure and near surface flow conditions allowing indirect determination of wall shear stress in experimental water tunnel environment an integrated hybrid sensor system has been developed. In contrast to known approaches, which are limited to the use in gas atmosphere due to protruding electrical and fragile parts, our sensor system is waterproof shielded and embedded in epoxy resin. Furthermore an amplification circuit for the pressure signal based on a programmable gain amplifier is integrated in direct vicinity to the pressure sensor in order to minimize noise by electromagnetic disturbances. Also sensor systems with on-board digitalization of the pressure signal for direct digital read-out were realized. We present all aspects of system assembly and embedding to one waterproof module. Furthermore, read-out strategies as well as sensor test results in air and water are shown and watertightness is confirmed.
... The movement implies a variation of an electrical parameter: capacitive sensors 5 or cantilever based sensors 6,7 are part of this category. For indirect flow measurement, various methods have been applied as, for example, micro-fences using a cantilever structure and piezoresistors, 8 optical resonance such as whispering gallery modes of dielectric microspheres shifting with radial deformations of the spheres due to the shear stress, 9 the deflection of micro-pillars, 10 and thermal-based sensors, [11][12][13][14][15][16][17][18][19][20] which are developed below. Among all these devices, thermal sensors are widely adopted when dealing with fluid dynamics, including laminar or turbulent flows, as they do not contain mechanical moving parts and are thus less prone to wear than the others. ...
We present an efficient and high-sensitive thermal micro-sensor for near wall flow parameters measurements. By combining substrate-free wire structure and mechanical support using silicon oxide micro-bridges, the sensor achieves a high temperature gradient, with wires reaching 1 mm long for only 3 μm wide over a 20 μm deep cavity. Elaborated to reach a compromise solution between conventional hot-films and hot-wire sensors, the sensor presents a high sensitivity to the wall shear stress and to the flow direction. The sensor can be mounted flush to the wall for research studies such as turbulence and near wall shear flow analysis, and for technical applications, such as flow control and separation detection. The fabrication process is CMOS-compatible and allows on-chip integration. The present letter describes the sensor elaboration, design, and micro-fabrication, then the electrical and thermal characterizations, and finally the calibration experiments in a turbulent boundary layer wind tunnel.
... floating elements) and indirect measurement by detecting other specific physical parameters related to the wall shear stress (e.g. micro pillar, micro fence, micro optical, and thermal-based hot-wire or hot-film) (Barlian et al. 2007;Chandrasekharan et al. 2011;Ma and Ma 2016;Otugen and Sheverev 2010;Papen et al. 2004;Große and Schröder 2009). Among these techniques, the hot-wire type has been investigated extensively, due to (i) the absence of moving parts, (ii) relatively simpler fabrication and implementation process, (iii) wide flow measurement range and short response time (Kuo et al. 2012). ...
This paper reports an innovative flexible hot-wire senor microarray and its experimental studies for underwater wall shear stress measurement. 20 parallel channels of the hot-wire sensor are embedded between two polyimide layers for curved surface applications, which is fabricated based on micromachining processes, including platinum sputtering, nickel electroplating, etc. An accurate method to calculate the conversion factor of the compensation loop is proposed and verified for the temperature-compensated circuit. An average temperature coefficient of resistance (TCR) is measured 2136.7 ppm/°C with linearity better than 0.05 % for platinum thermal resistors. Output instabilities of 0.22 and 0.63 mV are obtained in static flow and moving flow, respectively. The feasibility of the hot-wire sensors for detecting underwater wall shear stress is verified with an average sensitivity of 0.0123 \({\text{V}}^{2} /{\text{Pa}}^{1/3}.\) According to the experiment results, the flexible sensor microarray is very promising for characterizing underwater wall shear stress distributions in the boundary layer.
... floating elements [12,13]) and indirect (e.g. micro fence [14], micro optical [15], micro pillar [16] and thermal-based [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31]). Among these devices, the development of thermal shear stress sensors has seen significant advancement in recent years, due to (i) the absence of moving parts, (ii) relatively simpler fabrication and packaging schemes, and (iii) possible CMOS compatible fabrication. ...
In this paper we present a novel Silicon-on-Insulator (SOI) complementary metal oxide semiconductor (CMOS) micro-electro-mechanical-system (MEMS) thermal wall shear stress sensor based on a tungsten hot-wire and a single thermopile. Devices were fabricated using a commercial 1 μm SOI-CMOS process followed by a deep reactive ion etching (DRIE) back-etch step to release a silicon dioxide membrane, which mechanically support and thermally isolates heating and sensing elements. The sensors show an electro-thermal transduction efficiency of 50 µW/°C, and very small zero flow off-set. Calibration for wall shear stress measurement in air in the range of 0 - 0.48 Pa was performed using a suction type, 2-D flow wind tunnel. The sensors were found to be extremely sensitive, up to 4 V/Pa for low wall shear stress values. Furthermore, we demonstrate the superior signal to noise ratio (up to five times higher) of a single thermopile readout configuration compared with a double thermopile readout configuration (embedded for comparison purposes within the same device). Finally, we verify that the output of the sensor is proportional to the cube root of the wall shear stress and we propose an accurate semi-empirical formula for its modelling.
... Wall shear stress (skin friction), velocity profile and Reynolds stress were measured. Wall shear stress was collected by measuring the static pressure difference across a small fence mounted within the viscous sublayer, first introduced as classical sublayer (Stanton) fence by Konstantinov and Dragnysh [32], and then further developed by von Papen et al. [33], Schiffer et al. [34] and Savelsberg et al. [35]. Arrangements of the sublayer fences and silk threads are shown in Fig. 3. Silk threads, with their middle sections stuck on the wall, visually indicated length of the recirculation zone and stability characteristic of the reattachment point. ...
... A technique described by Fernholtz et al. [28] use precision micro pressure transducers to measure the change in pressure upstream and downsteam of the fence and calculate skin friction which is a function of the pressure difference. vonPapen et al. [29,30], and Shober et al. [31] describe the development of a micro surface fence capable of measuring wall shear stress magnitude and direction. ...
Low Reynolds number boundary layer separation causes reduced aerodynamic performance in a variety of applications such as MAVs, UAVs, and turbomachinery. The inclusion of a boundary layer separation control system offers a way to improve efficiency in conditions that would otherwise result in poor performance. Many effective passive and active boundary layer control methods exist. Active methods offer the ability to turn on, off, or adjust parameters of the flow control system with either an open loop or closed loop control strategy using sensors. This research investigates the use of a unique sensor called Surface Stress Sensitive Film (S3F) in a closed loop, low Reynolds number separation control system. S3F is an elastic film that responds to flow pressure gradients and shear stress along its wetted surface, allowing optical measurement of wall pressure and skin friction. The S3F sensor was integrated into the curved airfoil surface so as not to perturb the boundary layer. In this proof of concept investigation the S3F image signal was acquired via high speed interface and analyzed using an off board control system. The S3F displacement signal was used directly in a closed loop separation control system to drive a Dielectric Barrier Discharge (DBD) plasma actuator used to control laminar boundary layer separation on an Eppler 387 airfoil over a range of low Reynolds numbers. Operation of the plasma actuator resulted in a 33% reduction in section drag coefficient and reattachment of an otherwise separated boundary layer. A simple On/Off controller and Proportional Integral (PI) controller were used to close the control loop.
... A MEMS adaptation of the conventional surface fence method of determining skin friction has also been developed [7,8]. This uses an embedded piezoresistive strain gauge to directly measure the pressure-induced deflection of a silicon fence typically 0.3 mm high by 5 mm wide and achieves sensitivities of up to about 0.6 (mV V −1 ) Pa. ...
A sensor capable of measuring small shear stresses in wind tunnel applications is presented. The sensor utilizes an in-plane cantilever concept for shear stress measurement, designed to minimize intrusiveness into the airflow and allow easy incorporation into wind tunnel test models. The sensor operates independently of input voltage, and can measure <1 Pa shear stresses with a sensitivity of 8.6 (mV V-1) Pa. Altering the geometry of the sensor has a direct effect on the sensitivity and so can be used to adapt the sensor for different applications.
... Unfortunately, details on the flow calibration are not provided other than to state the calibration was conducted in a known flow in a reference wind tunnel. [8][9][10][11] An important consideration in the use and design of any device based on an obstruction to the flow is for it not to alter the flow field. These type devices have similar issues with the validity of a calibration extending to the actual use of the sensor. ...
By surveying current research of various micro-electro mechanical systems (MEMS) shear stress sensor development efforts we illustrate the wide variety of methods used to test and characterize these sensors. The different methods of testing these sensors make comparison of results difficult in some cases, and also this comparison is further complicated by the different formats used in reporting the results of these tests. The fact that making these comparisons can be so difficult at times clearly illustrates a need for standardized testing and reporting methodologies. This need indicates that the development of a national or international standard for the calibration of MEMS shear stress sensors should be undertaken. As a first step towards the development of this standard, two types of devices are compared and contrasted. The first type device is a laminar flow channel with two different versions considered: the first built with standard manufacturing techniques and the second with advanced precision manufacturing techniques. The second type of device is a new concept for creating a known shear stress consisting of a rotating wheel with the sensor mounted tangentially to the rim and positioned in close proximity to the rim. The shear stress generated by the flow at the sensor position is simply τ = µ rω /h, where µ is the viscosity of the ambient gas, r the wheel radius, ω the angular velocity of the wheel, and h the width of the gap between the wheel rim and the sensor. Additionally, issues related to the development of a standard for shear stress calibration are identified and discussed.
... Chen et al used a flat plate with a size of 180µm×1100µm which is flapped from the base into the flow and the drag is measured with a strain gauge at the base, too. A similar sensor principle is used by von Papen et al. (2002) to measure the wall shear stress fluctuations at a single point with frequencies up to 1 kHz. The MEMS surface fence sensor consists of a 5mm long, 100-300 µm high and 7-10 µm thick silicon fence of which deflection is being transformed into an electrical signal which is proportional to the component of the wall-shear-stress perpendicular to the fence. ...
A new optical sensor technique based on a sensor film with arrays of hair-like flexible micropillars on the surface is presented to measure the temporal and spatial wall shear stress field in boundary layer flows. The sensor principle uses the pillar tip deflection in the viscous sublayer as a direct measure of the wall shear stress. The pillar images are recorded simultaneously as a grid of small bright spots by high-speed imaging of the illuminated sensor film. Two different ways of illumination were tested, one of which uses the fact that the transparent pillars act as optical microfibres, which guide the light to the pillar tips. The other method uses pillar tips which were reflective coated. The tip displacement field of the pillars is measured by image processing with subpixel accuracy. With a typical displacement resolution on the order of 0.2 μm, the minimum resolvable wall friction value is τ
w≈20 mPa. With smaller pillar structures than those used in this study, one can expect even smaller resolution limits.
... Up to now, a lot of sensors have components which rise in the streaming fluid. These sensors often have a surface fence [1] or a hot-wire [2] as a sensing element in the flow. Additional to the disadvantage of changing the flow, these components are very fragile against particles in the flow. ...
In this work, a novel silicon-based sensor for pressure and flow measurements is presented.
To meet the special requirements of the aerospace industry a new piezoresistive pressure sensor with a
flat surface has been developed, so that the flow is not affected by the sensor. To avoid bonding-wires
on top of the sensor a special through-wafer connection is presented. By making other significant
changes in the layout as well as in the micro fabrication process, a novel sensor has been created. It is
robust enough to be laminated in fibre material, which opens new possibilities for measurements. With
this sensor it is possible to characterize the condition of the flow near the separation point. This article
describes the complete process from the development to the laminated sensor.
A family of MEMS calorimetric wall shear stress sensors is experimentally and numerically investigated to determine their most significant design parameters. Fifteen sensor prototypes are first calibrated in a range of 2 Pa to generate an experimental database for validation of the subsequent numerical investigation. Then, a fully parameterized numerical setup is used to investigate the effect of three geometric design parameters, namely the cavity height, the cavity width, and the inter-beam distance, on amplitude and sensitivity of the sensor. This is done by building a surrogate model based on Gaussian Process interpolation (Kriging) in the four-dimensional space consisting of the three design parameters and the shear velocity in the flow. Thanks to this methodology, the calibration curves of all possible sensor designs in the investigated range can be estimated with an error of less than 2 %. A detailed study of this model reveals that the most significant design parameters are the inter-beam distance and the cavity width, while the cavity height is found to be of minor importance.
The principle of operation of a new MEMS calorimetric shear-stress sensor is described. The sensor is based on the detection of the thermal wake generated by a heated beam over a cavity. Numerical simulations and wind-tunnel tests are conducted to highlight the effects of the inter-beam distance and the importance of the constant-temperature mode of operation. The calibration of a sensor prototype in a range of ±1.2 Pa of wall shear-stress in air is performed and the sensor's capability to measure the mean and fluctuating wall shear-stress and the flow direction in a separation-bubble flow is demonstrated. Results of reverse-flow intermittency measurements are compared with those obtained with a classical thermal-tuft probe.
In aerodynamic structures, shear stress is the greatest contributor to a body's total parasitic skin friction drag. This drag is proportional to the local wall shear stress on a surface. Measurement difficulties, high errors and the cost of fabrication have motivated innumerable efforts to develop precise and inexpensive methods for measuring the local shear stress in fluid structures. This is especially important in the supersonic aerodynamic environment, where the interaction between the sensor and air flow induces even higher errors. In order to further improve the efficiency of aircraft and other aerodynamic bodies, sensitive measurements on small scales are required. The present study introduces a novel electrochemical microfluidic shear stress sensor enabling the measurement of the wall shear stress in wind tunnel models. Our company proposes a paradigm shift in shear stress measurements which will take advantage of the complete sensing package offered in micro electro-mechanical systems (MEMS) without the need for moving mechanical parts or expensive manufacturing. The sensor contains a cavity, capped by a thin membrane. The air flow above the membrane deflects the membrane and induces fluid motion within the cavity, which accordingly changes the conductance of the electrolyte solution inside the cavity. This allows the direct measurement of the shear stress by measuring the electrical current under a fixed voltage applied. The proposed sensor is tested inside a subsonic wind tunnel at different air flow rates, using optical experiments and image processing techniques. These measurements enable the comparison of the shear stress measured by the sensor to that obtained by boundary layer measurements and cavity convection measurements. These results imply that the electrochemical shear stress sensor offers a precise and robust measurement system capable of quantifying wall shear stress in air flows.
The latest AeroMEMS surface fence design is presented herein. This sensor is used for high resolution wall shear stress measurements in turbulent flows as well as in flow reversals. The deflection of the fence - proportional to wall shear stress - is directly measured by a Wheatstone bridge with four implanted resistors. This unique sensor design enables the detection of the flow direction directly by means of the output signal polarity. The fence is 5 mm wide, max. 700 μm high with a thickness of either 7 μm or 13 μm. (The sensor height is infinitely adjustable to a maximum viscious sublayer thickness of 700 μm.) The sensor designs feature a sensitivity up to 6 mV/(V·N/m 2) and a resolution up to 10-4 N/m2 in a measurement range of ±0.3 N/m2 that is well suited for measurement of wall shear stresses below ±1 N/m2. Additionally, two pn-diodes (with a sensitivity of-2 mV/K @ 100 μA) were implemented for temperature monitoring and temperature drift compensation.
There is a need for a sensor to measure global, spatially and temporally resolved wall shear stress on wall bounded flows in various engineering fields. A wall shear stress sensor using a micro pillar array made out of silicone rubber is presented. This sensor is based on the principle that, if such a pillar is inside the viscous sub layer the deflection of the pillar is proportional to the drag forced experienced by the pillar, which in turn is proportional to the wall shear stress. The displacements of individual pillars in the array are tracked to obtain the wall shear stress field in a turbulent boundary layer flow. Design and manufacturing considerations are discussed along with typical sensor calibrations in a fully developed turbulent channel flow. Based on the resolution needed the sensor can be tuned for various applications. To demonstrate the feasibility of these types of sensors, the turbulent statistics in a fully developed channel flow is studied. The instantaneous wall shear stress distribution around a cylinder in cross flow was also mapped. Copyright © 2008 by the American Institute of Aeronautics and Astronautics, Inc.
In this work, the latest results of the design, fabrication and characterization of a new MEMS piezoresistive pressure sensor are presented. It is made of silicon using a boron diffusion process to create piezoresistors. Significant changes in the layout as well as in the micro-fabrication process have been made, e.g. anodic bonding of a Pyrex cover on the backside. These lead to a very precise pressure sensor, which is tailor made for high dynamic measurements in fluids with a total pressure up to 4 bar. This new piezoresistive pressure sensor has been developed in order to meet the special requirements of measurements in fluid mechanics, particularly with regard to the non-intrusive nature of the sensor. The sensor development, starting with the simulation of mechanical stresses within the diaphragm is described. These calculations have lead to an optimized placement of the piezoresistors in order to achieve a maximum sensitivity. The result of this work is a sensor which has well known properties. Important parameters including sensitivity, resonance frequency and maximum load are described precisely. These are necessary to enable new measurements in the boundary layer of fluids. The experiments and the initial results, e.g. its linearity and its dynamic capability are demonstrated in several figures.
We present a cantilever fabricated from the polymer SU-8 for the measurement of wall shear stress in fluid flows. The pressureinduced deflection of the cantilever, measured using an calibrated and integrated nichrome strain gauge, can be related to the wall shear stress on the surface. The initial degree of curvature of the cantilever can be controlled via the exposure dose, which allows a small positive deflection to be achieved, and so minimises the intrusion into the flow. Wind tunnel testing results show a sensitivity greater than 2.5 mV/Pa, with a shear stress of 0.38 Pa and excitation of 1V.
A sophisticated sensor design of the aeroMEMS surface fence probe for wall shear stress measurements in turbulent flow is presented. The fence deflection (proportional to the wall shear stress) is directly measured by a piezoresistive Wheatstone bridge. The fence is 5 mm wide, 700 μm (max.) high and 7 μm and 13 μm thick, respectively. The cantilever height is 1700 μm or 2200 μm. The sensing element rests on a silicon body of 5200×7500 μm<sup>2</sup>. A sensitivity up to 6 mV/(V·N/m<sup>2</sup>), a resolution of up to 10<sup>-4</sup> N/m<sup>2</sup> and a symmetrical behavior for bidirectional measurements was obtained in a measurement range of ±0.3 N/m<sup>2</sup>. Two pn-diodes were implemented for temperature monitoring (sensitivity: -2 mV/K @ 100 μA).
The first three dimensional (3D) flow sensor probe based on
polyimide joints has been fabricated and successfully tested. The new 3D
sensor, which is specially designed for turbulent gas flow measurements,
is based on the anemometer principle, i.e. gas cooling of electrically
heated hot-wires. The sensor probe consists of three perpendicular 500
μm×5 μm×2 μm polysilicon hot-wires giving a
measuring volume sufficiently small to resolve the small eddies in a
turbulent flow. A bulk micromachining process in combination with
sacrificial etching is used to form the hot-wire probes. The hot-wires
are connected to 30 μm thick bulk silicon beams which are defined by
double sided KOH etching. Two wires (x and y) are located in the wafer
plane and the third z-wire is rotated out of the plane using a new
robust micro-joint. The self-assembly micro-joint with small bending
radius is based on thermal shrinkage of polyimide in V-grooves. High
flow sensitivity for the anemometric hot-wires has been measured. Time
constants of 120 and 330 μs were measured for the cooling and heating
of the hot-wires, respectively
We present in this paper the design and test results of
micromachined individual sensors and sensor arrays for shear stress
measurements in air flows. The authors previously (1996) reported on the
sensing principle and fabrication process. The floating-element sensors
have since been extensively tested in laminar flows. Static calibrations
have shown this sensor to have a maximum nonlinearity of 1% over four
orders of wall shear stress (0.0014-10 Pa). In addition, for the first
time, the dynamic response of this sensor has also been experimentally
verified to 4 kHz. An improved sensing scheme has been developed and has
been successfully implemented in a second generation of sensor arrays
A new MEMS shear stress sensor imager has been developed and its
capability of imaging surface shear stress distribution has been
demonstrated. The imager consists of multi-rows of vacuum-insulated
shear stress sensors with a 300 μm pitch. This small spacing allows
it to detect surface flow patterns that could not be directly measured
before. The high frequency response (30 kHz) of the sensor under
constant temperature bias mode also allows it to be used in high
Reynolds number turbulent flow studies. The measurement results in a
fully developed turbulent flow agree well with the numerical and
experimental results previously published
A new sensor for aerodynamic research providing high dynamics as well as flow reversal detection is presented. The MEMS surface fence sensor consists of a 5mm long, 100–300 µm high and 7–10 µm thick silicon fence as well as a “body” for contacting and handling. In a flow, a pressure difference between both sides of the fence occurs and deflects it. Four piezoresistors, connected to a Wheatstone bridge, measure the deflection and transforms it into an electrical signal. Three different designs, the most sensitive of them allowing the detection of a wall-shear-stress below 20 mN/m2, are discussed. The device is fabricated using microfabrication processes and double side polished Si-wafers as base material.
The latest AeroMEMS surface fence design is presented herein. This sensor is used for high resolution wall shear stress measurements in turbulent flows as well as in flow reversals. The deflection of the fence - proportional to wall shear stress - is directly measured by a Wheatstone bridge with four implanted resistors. This unique sensor design enables the detection of the flow direction directly by means of the output signal polarity. The fence is 5 mm wide, max. 700 μm high with a thickness of either 7 μm or 13 μm. (The sensor height is infinitely adjustable to a maximum viscious sublayer thickness of 700 μm.) The sensor designs feature a sensitivity up to 6 mV/(V·N/m 2) and a resolution up to 10-4 N/m2 in a measurement range of ±0.3 N/m2 that is well suited for measurement of wall shear stresses below ±1 N/m2. Additionally, two pn-diodes (with a sensitivity of-2 mV/K @ 100 μA) were implemented for temperature monitoring and temperature drift compensation.
The wall-shear-stress is the force parallel to a surface caused by a flow, which propagates parallel to the surface. Although there are several methods in use, there is a lack of sensors able to provide a high time resolution as well as the detection of flow reversals. This paper presents a micromechanical surface fence sensor, which consists of a 100–300μm high and 7–10μm thick silicon fence and a “body” for contacting and packaging. Four piezoresistors, connected to a Wheatstone bridge, measure the deflection of the fence and transform it into an electrical signal. Different fence geometries have been designed, simulated by FEA, fabricated and tested in a probe system as well as in a wind tunnel. With the most sensitive design, a resolution of 20mN/m2 has been achieved. With this performance, the device is suitable for a number of applications in aerodynamic research.
The use of single and multiple hot-wire/hot-film probes for subsonic flows is discussed with emphasis on the digital methods and procedures. An account is given of the major factors which influence the reliability of measurements and the application of many of the correction procedures required. The factors considered are: probe calibration, nonlinearity of the sensor-response equation, temperature effects, temporal and spatial resolution and probe design.
A new experimental technique for the investigation of near-wall turbulence using laser Doppler anemometry is presented, which
allows an accurate measurement of the flow field very close to the wall, with good resolution and a high data rate. Such a
technique is tested in a fully developed turbulent flow (with Reynolds numbers between 4,300 and 67,000) by carrying out a
careful statistical analysis of the streamwise and wall-normal velocity components within the near-wall region, at distances
from the wall ranging from approximately y
+ = 1 to y
+ = 100. The velocity profiles, Reynolds stresses and higher-order moments of the two-dimensional boundary layer are presented.
The results, which are in agreement with the most recent data in the literature, testify the validity of the proposed experimental
solution. Moreover, the accuracy of the results allows the friction velocity to be calculated as the intercept at the wall
of the best linear fit of the total stress profile; in this way, an unambiguous examination of the normalized statistics is
possible.
This experimental investigation compares two configurations of pulsed-wire probe for measurements in the very near wall region,
as introduced by Castro and Dianat (1990) and Devenport et al. (1990). Diffusion of the thermal wake causes significant errors
in regions where the velocity is low and the velocity gradient high. These errors, which are about the same for either configuration,
are large in the viscous sub-layer. An analysis of diffusional effects is made, and a method of correction is given that applies
equally in laminar and turbulent flow. It is necessary to calibrate a probe for the effects of shear in a known flow.
Pulsed Hot-Wire Anemometry (PHWA) measurements are performed in well defined two- and three-dimensional turbulent wall jets.
For the two-dimensional wall jet the objective is to study reported differences between conventional Hot-Wire Anemometry (HWA)
and Laser Doppler Anemometry (LDA) results. In the three dimensional wall jet, new improved data are provided, employing a
measuring technique suitable for highly turbulent flows. This, since only hot-wire results previously have been published
for this flow. The pulsed wire results show good agreement with existing Laser Doppler anemometer data in the two-dimensional
wall jet, both reporting significantly higher turbulence levels in the outer region of the flow than hot-wires do. The hot-wire
anemometer errors generally increase with increasing local turbulence intensity and since the three-dimensional wall jet has
a higher turbulence level than its two-dimensional equivalent, the new pulsed hot-wire results improve the information available
for the turbulence field in this flow significantly.