Range Of Wall Shear Stress Research Articles

Scale and shear investigation of the viability and consequences of controlling viscous heating for Argon flows in nano-channels

Molecular dynamics (MD) is used to investigate the combined effects of channel size and wall shear stress on force-driven Argon flows in nanochannels. The adaptive wall thermostat (AWT) is used to automatically adjust wall temperature refraining the viscous heating upsurge with the increased body force from increasing mean fluid temperature. AWT is functional up to “wall shear stress limit” (WSSL), classifying wall shear stress range into controllable and uncontrollable viscous heating ranges (CVH and UVH) before and beyond WSSL, respectively. With increasing wall shear stress in CVH, liquid Argon at ∼119.9 K exhibits more non-uniform temperature profile in larger channels. Consequently, further shear dependence of density profile and fluid molecular layering which leads to controversial size-dependent slip length. Within UVH, supercritical Argon emerges from the significant increase of temperature and pressure exceeding critical state; consequently, density profile and molecular layering decrease noticeably drifting toward walls which results in minor and proximate values of slip length in all channels approaching no-slip condition. The apparent viscosity is higher in larger channels with decreasing and increasing trends within CVH and UVH, respectively, associated with changing fluid state and phase. Finally, the continuum solution suggests the possibility of estimating the mass flow rate with reasonable accuracy relative to MD if implemented with appropriate properties and slip boundary condition.

Research on process modeling and simulation of spent lead paste desulfurization enhanced reactor

In the reaction process of carbonate desulfurization lead paste, the produced PbCO3 is easily wrapped in the outer periphery of PbSO4 to form a product layer, hindering the mass transfer process. Therefore, it is necessary to break the PbCO3 product layer. In this work, the rotor stator-reinforced reactor was selected as the enhanced desulfurization reactor for the purpose of breaking the PbCO3 product layer and promoting mass transfer. The breakage process of the PbCO3 product layer generated during the PbSO4 desulfurization was modeled. Computational fluid dynamics simulation to the rotation conditions was carried out to theoretically analyze the fluid flow characteristics of PbSO4 slurry and the wall shear stress affecting the breakage of PbCO3 product layer. By optimizing the rotation conditions, the distribution ratio of effective rotor wall shear stress range achieved 96.1%, and the stator wall shear stress range reached 99.15% under a rotation of 2000 r·min−1. The research work provides a reference for analysis of the mechanism of product layer breakage in the PbSO4 desulfurization process, and gives a clear and intuitive systematic study on the fluid flow characteristics and wall shear stress of the desulfurization reactor.

Numerical simulation of flow resistance characteristics in a rod bundle channel under rolling conditions

Flow resistance characteristics in a 3 × 3 rod bundle channel under rolling motion are investigated by numerical simulation to overcome the limitation of experimental conditions and measurement methods. Severe rolling conditions are selected to make up for the lack of test range. The local parameters, including flow field, temperature field and wall shear stress are given to analyze the influence mechanism of the rolling motion on friction resistance of the rod bundle channel. Under rolling conditions, the absolute value of flow rate does not influence the fluctuation amplitude of frictional pressure drop when inlet flow rate does not fluctuate. Additional pressure drop is the main factor affecting the fluctuation amplitude of frictional pressure drop under rolling conditions. When the rolling motion becomes severe, the velocity deviation increases, while it has no obvious influence on velocity peak value within cross section. Increasing the flow rate weakens the effect of rolling motion on axial velocity distribution. The variation of flow rate does not affect the magnitude and distribution of radial velocity, and the radial secondary flow intensity is mainly determined by the intensity of rolling motion. Different from velocity distribution, the wall shear stress distribution is obviously affected by rolling motion and is unconnected with flow rate, while the flow rate affects the relative value of wall shear stress. Under the same rolling parameters, the variation range of boundary layer thickness increases with decreasing flow rate, which leads to an increase in the relative variation range of wall shear stress with decreasing flow rate.

Modelling bacterial twitching in fluid flows: a CFD-DEM approach

Bacterial habitats are often associated with fluid flow environments. Bacterial twitching is important for initial bacterial colonization and biofilm formation. The existing research about bacteria twitching is largely experimental orientated. There is a lack of models of twitching motility of bacteria in shear flows, which could provide fundamental understanding about how bacterial twitching would be affected by bacteria associated properties such as number of pili and their distribution on the cell body and environmental factors such as flow and surface patterns. In this work, a three-dimensional modelling approach of Computational Fluid Dynamics (CFD) coupled with the Discrete Element Method (DEM) proposed to study bacterial twitching on flat and groove surfaces under shear flow conditions. Rod-shaped bacteria are modelled as groups of spherical particles and Type IV pili attached to bacteria are modelled as dynamic springs which can elongate, retract, attach and detach. The CFD-DEM model of rod-shape bacteria is validated against orbiting of immotile bacteria in shear flows. The effects of fluid flow rate and surface topography on twitching motility are studied. The model can successfully predict upstream twitching motility of rod-shaped bacteria in shear flows. Our model can predict that there would be an optimal range of wall shear stress in which bacterial upstream twitching is most efficient. The results also indicate that when bacteria twitch on groove surfaces, they are likely to accumulate around the downstream side of the groove walls.

Characterising wall-slip behaviour of carbopol gels in a fully-developed poiseuille flow

In this paper, we examine the wall-slip behaviour of Carbopol gel, as a simple yield-stress fluid, in a Poiseuille flow by analysing its velocity distribution across a glass capillary tube and near the wall. Optical coherence tomography (OCT) is used to image the flow of the gel seeded by buoyant tracing particles, and particle tracking velocimetry (PTV) is applied to the flow images to obtain the velocity profiles near the wall. In addition, the dynamics of the flow is measured through a viscometric unit integrated with the flow cell. Our velocimetric and viscometric measurements reveal apparent slip of Carbopol gels on the capillary wall, in a wide range of wall shear stresses below and above the yield stress of the material. Our results confirm the presence of a fully plugged flow within the precision of our velocimetric measurement, where the wall stress lies below the yielding point. Above the yielding point, the presence of wall slip is confirmed over a wide range of shear rates. Our results indicate that the slip velocity increases with the wall shear stress in both regimes. We relate the slip velocity to the excess shear stress on the wall, i.e. the wall shear stress minus the shear stress at the onset of slip, using a power-law scaling, with an exponent independent of the microstructure of the fluid. The effect of microstructure of the gel on the wall slip is studied by either varying the concentration of the gel or the stirring rate during preparation. We find that the higher the concentration of the gel or the higher the stirring rate is, the lower the slip velocity is. Moreover, our experiments indicate that increasing the viscosity of the solvent leads to a decrease in the slip velocity.

Numerical Simulation of Myocardial Bridging in Patients with Hypertrophic Cardiomyopathy

A myocardial bridge (MB), a congenital anomaly of the coronary artery, occurs when a segment of the epicardial coronary artery goes underneath the myocardium. MBs are often observed in the middle part of the left anterior descending (LAD) artery. MB squeezes the vessel wall periodically and induces hemodynamic abnormalities which are correlated with angina and myocardial ischemia. The level of hemodynamics disturbances induced by MB depends on the myocardial bridge length, the degree of myocardial contractility, thickness, and location [1]. Hypertrophic cardiomyopathy (HCM), characterized by abnormal thickening of the heart wall, is a leading cause of death in patients of all ages [2]. The prevalence and severity of myocardial bridging are higher among patients with HCM, up to 30% [3]. Therefore, understanding the effects of MB in HCM patients is of clinical significance. The purpose of this study was to determine the role of MB on coronary hemodynamics in HCM patients with various level of MB using computational fluid dynamics (CFD) modeling. We analyzed three HCM patients with MB in the middle LAD who underwent coronary angiography at Fuwai hospital (Tab. 1). All patients had a significant MB (diameter stenosis ≥ 50%) as diagnosed by arteriography. <br/> Biplane angiographic images were recorded at 15 frames/s using Artis zee III floor (Siemens Ltd. China) with a pixel spacing of 154μm. 2D vessel centerlines of two different oblique views were extracted from angiographic data and then combined to create a 3D vessel centerline in SolidWorks^®. Lumen diameter of LAD at both end-diastole and end-systole were measured to reconstruct the LAD model (Fig. 1). Linear interpolation was used to predict vessel wall movements during the cardiac cycle using a user-defined function (UDF). The arbitrary Lagrangian-Eulerian (ALE) method was employed for the moving boundaries. Blood was assumed to be homogenous Newtonian fluid with a density of 1050 kgm^-3 and dynamic viscosity of 0.00316 Pa.s. To ensure fully developed flow, extended lengths of 10-diameter and 15-diameter were added to the inlet and outlet sections, respectively [4]. Intracoronary pressure waveforms were applied as the inlet and outlet boundary conditions [5]. The model was meshed with hexahedral elements with a finer mesh near the wall. The shear stress transport (SST) turbulence model was used for simulating flow in ANSYS Fluent (v 17.1). The Navier-Stokes equations were solved using semi-implicit method (Simple) with the maximum residual of less than 10^-6. To achieve a periodic solution, three cardiac cycles were simulated and the last cycle results were used for the interpretation. <br/> To better understand the effect of MB, simulations were repeated for the same patient without MB (vessel compression was artificially removed). CFD results demonstrated that the presence of MB reduced the time-averaged wall shear stress (TAWSS) in the proximal (0.83 ± 0.29 Pa with MB versus 1.25 ± 0.59 without MB) and distal (2.76 ± 0.85 Pa with MB versus 3.82 ± 0.85 without MB) segments while increased TAWSS in the bridge segment (4.04 ± 1.29 Pa with MB versus 2.72 ± 1.23 without MB). The normal range of wall shear stress at arterial wall is from 1 to 7 Pa [6]. MB increased the maximum residence time of blood particles (Tab. 2), which may trigger thrombosis in the proximal segment. Increasing the degree of stenosis showed a significant decrease in the volume-averaged velocity and volumetric flow rate. Our analysis illustrated that the MB had significant effects on the distribution of wall shear stress and blood flow in HCM patients.

Channelization in porous media driven by erosion and deposition.

We develop and validate a new model to study simultaneous erosion and deposition in three-dimensional porous media. We study the changes of the porous structure induced by the deposition and erosion of matter on the solid surface and find that when both processes are active, channelization in the porous structure always occurs. The channels can be stable or only temporary depending mainly on the driving mechanism. Whereas a fluid driven by a constant pressure drop in general does not form steady channels, imposing a constant flux always produces stable channels within the porous structure. Furthermore we investigate how changes of the local deposition and erosion properties affect the final state of the porous structure, finding that the larger the range of wall shear stress for which there is neither erosion nor deposition, the more steady channels are formed in the structure.

Force and torque on spherical particles in micro-channel flows using computational fluid dynamics

To delineate the influence of hemodynamic force on cell adhesion processes, model in vitro fluidic assays that mimic physiological conditions are commonly employed. Herein, we offer a framework for solution of the three-dimensional Navier–Stokes equations using computational fluid dynamics (CFD) to estimate the forces resulting from fluid flow near a plane acting on a sphere that is either stationary or in free flow, and we compare these results to a widely used theoretical model that assumes Stokes flow with a constant shear rate. We find that while the full three-dimensional solutions using a parabolic velocity profile in CFD simulations yield similar translational velocities to those predicted by the theoretical method, the CFD approach results in approximately 50% larger rotational velocities over the wall shear stress range of 0.1–5.0 dynes cm−2. This leads to an approximately 25% difference in force and torque calculations between the two methods. When compared with experimental measurements of translational and rotational velocities of microspheres or cells perfused in microfluidic channels, the CFD simulations yield significantly less error. We propose that CFD modelling can provide better estimations of hemodynamic force levels acting on perfused microspheres and cells in flow fields through microfluidic devices used for cell adhesion dynamics analysis.

SAPHENOUS VEIN GRAFT PATENCY AFTER GEOMETRY REMODELING

Tortuous saphenous vein graft (SVG) hemodynamics was investigated using computational fluid dynamics (CFD) techniques. Computed tomography (CT) technology is used for noninvasive bypass graft assessment seven days after surgery. CT investigation shows two regions with severe shape remodeling, one is an angle type contortion and the other one is a sharp curvature with tortuous area reduction. The numerical analysis carefully examines the effect of an SVG geometry remodeling through flow separation, particle deposition, and wall shear stress (WSS). During the cardiac cycle, overall pressure drop increases from 2.6[Formula: see text]mmHg to 4.4[Formula: see text]mmHg. In the accelerating part of the systolic phase, particles released in the inlet section move downstream toward the first narrowed part (elbow type contortion) with a helical motion. WSS range along the cardiac cycle varies from 2[Formula: see text]Pa to 42[Formula: see text]Pa, enough to damage the endothelial cells. Vessel torsion induced helical flow can reduce the flow disturbance and separation. Additionally, in the distal end of the graft, the high particle concentrations can promote the inflammatory processes in the vessels.

A numerical study on the effect of hematocrit on hemodynamic characteristics in arteriovenous graft

Stenosis at an arteriovenous graft is related with the critical ranges of hemodynamic characteristics. Hematocrit has a significant effect on the blood viscosity. During hemodialysis, hematocrit is changed by the dialysis machine. The effect of hematocrit on hemodynamic characteristics is investigated by numerical study. A multiphase non-Newtonian blood model was used to analyze the changes of hematocrit. The hematocrit of blood flows at injection needle changed 40%, 50%, and 60%. As a result, the blood viscosity increased by about 6% point. Also, the high wall shear stress region (over 3 Pa) increased about 6% point when the hematocrit at the vein anastomosis increased by about 2% point. When the hematocrit increased by 4% at the vein anastomosis, an extremely high wall shear stress region (over 7.5 Pa) increased by 3 times. Thus, the variation of hematocrit should be predicted using a multiphase blood model to avoid the critical range of wall shear stress when hematocrit changes regionally.

Microfluidic shear stress-regulated surfactant secretion in alveolar epithelial type II cells in vitro

We investigated the role of flow-induced shear stress on the mechanisms regulating surfactant secretion in type II alveolar epithelial cells (ATII) using microfluidic models. Following flow stimulation spanning a range of wall shear stress (WSS) magnitudes, monolayers of ATII (MLE-12 and A549) cells were examined for surfactant secretion by evaluating essential steps of the process, including relative changes in the number of fusion events of lamellar bodies (LBs) with the plasma membrane (PM) and intracellular redistribution of LBs. F-actin cytoskeleton and calcium levels were analyzed in A549 cells subjected to WSS spanning 4-20 dyn/cm(2). Results reveal an enhancement in LB fusion events with the PM in MLE-12 cells upon flow stimulation, whereas A549 cells exhibit no foreseeable changes in the monitored number of fusion events for WSS levels ranging up to a threshold of ∼8 dyn/cm(2); above this threshold, we witness instead a decrease in LB fusion events in A549 cells. However, patterns of LB redistribution suggest that WSS can potentially serve as a stimulus for A549 cells to trigger the intracellular transport of LBs toward the cell periphery. This observation is accompanied by a fragmentation of F-actin, indicating that disorganization of the F-actin cytoskeleton might act as a limiting factor for LB fusion events. Moreover, we note a rise in cytosolic calcium ([Ca(2+)]c) levels following stimulation of A549 cells with WSS magnitudes ranging near or above the experimental threshold. Overall, WSS stimulation can influence key components of molecular machinery for regulated surfactant secretion in ATII cells in vitro.

Assessment of the accuracy of MRI wall shear stress estimation using numerical simulations

To investigate the accuracy of wall shear stress (WSS) estimation using MRI. Specifically, to investigate the impact of different parameters and if MRI WSS estimates are monotonically related to actual WSS. The accuracy of WSS estimation using methods based on phase-contrast (PC) MRI velocity mapping, Fourier velocity encoding (FVE) and intravoxel velocity standard deviation mapping were studied using numerical simulations. The influence of spatial resolution, velocity encoding, wall segmentation, and voxel location were investigated over a range of WSS values. WSS estimates were found to be sensitive to parameter settings in general and spatial resolution in particular. All methods underestimated WSS, except for the FVE-based method, which instead was extremely sensitive to voxel position relative to the wall. Methods using PC-based WSS estimation with wall segmentation showed to be accurate for low WSS, but were sensitive to segmentation errors. Even in the absence of noise and for relatively simple velocity profiles, MRI WSS estimates cannot always be assumed to be linearly or even monotonically related to actual WSS. High WSS values cannot be resolved and the estimates depend on parameter setting. Nevertheless, distinguishing areas of low and moderate WSS may be feasible.

Sensitivity of hemodynamics in a patient specific cerebral aneurysm to vascular geometry and blood rheology

Newtonian and generalized Newtonian mathematical models for blood flow are compared in two different reconstructions of an anatomically realistic geometry of a saccular aneurysm, obtained from rotational CTA and differing to within image resolution. The sensitivity of the flow field is sought with respect to geometry reconstruction procedure and mathematical model choice in numerical simulations. Taking as example a patient specific intracranial aneurysm located on an outer bend under steady state simulations, it is found that the sensitivity to geometry variability is greater, but comparable, to the one of the rheological model. These sensitivities are not quantifiable a priori. The flow field exhibits a wide range of shear stresses and slow recirculation regions that emphasize the need for careful choice of constitutive models for the blood. On the other hand, the complex geometrical shape of the vessels is found to be sensitive to small scale perturbations within medical imaging resolution. The sensitivity to mathematical modeling and geometry definition are important when performing numerical simulations from in vivo data, and should be taken into account when discussing patient specific studies since differences in wall shear stress range from 3% to 18%.

A Comparative Study of Shear Stresses in Collagen-Glycosaminoglycan and Calcium Phosphate Scaffolds in Bone Tissue-Engineering Bioreactors

The increasing demand for bone grafts, combined with their limited availability and potential risks, has led to much new research in bone tissue engineering. Current strategies of bone tissue engineering commonly use cell-seeded scaffolds and flow perfusion bioreactors to stimulate the cells to produce bone tissue suitable for implantation into the patient's body. The aim of this study was to quantify and compare the wall shear stresses in two bone tissue engineering scaffold types (collagen-glycosaminoglycan (CG) and calcium phosphate) exposed to fluid flow in a perfusion bioreactor. Based on micro-computed tomography images, three-dimensional numerical computational fluid dynamics (CFD) models of the two scaffold types were developed to calculate the wall shear stresses within the scaffolds. For a given flow rate (normalized according to the cross-sectional area of the scaffolds), shear stress was 2.8 times as high in the CG as in the calcium-phosphate scaffold. This is due to the differences in scaffold geometry, particularly the pore size (CG pore size approximately 96 microm, calcium phosphate pore size approximately 350 microm). The numerically obtained results were compared with those from an analytical method that researchers use widely experimentalists to determine perfusion flow rates in bioreactors. Our CFD simulations revealed that the cells in both scaffold types were exposed to a wide range of wall shear stresses throughout the scaffolds and that the analytical method predicted shear stresses 12% to 21% greater than those predicted using the CFD method. This study demonstrated that the wall shear stresses in calcium phosphate scaffolds (745.2 mPa) are approximately 40 times as high as in CG scaffolds (19.4 mPa) when flow rates are applied that have been experimentally used to stimulate the release of prostaglandin E(2). These findings indicate the importance of using accurate computational models to estimate shear stress and determine experimental conditions in perfusion bioreactors for tissue engineering.

Microfluidic device functionalized with P‐selectin reveals discontinuous rolling of leukocytes in mouse whole blood

Intravital studies of mouse cremaster have shown evidence that leukocytes roll discontinuously on endothelial P‐selectin. We use a microfluidic device to study the rolling of leukocytes in heparinized mouse blood over a coverglass with adhered P‐selectin at known site density. The microfluidic device has an array of 300×30 μm channels, and is reversibly sealed against the coverglass using a magnetic clamp. Leukocytes are observed to undergo periods of stable rolling separated by jumps indicating brief periods of detachment from the substrate. During the jumps, cells are observed to move at velocities comparable with the theoretical estimates for a cell freely flowing in a shear flow. In the entire range of wall shear stress (2‐8 dyes/cm2), the average rolling velocities of leukocytes linearly increase with the shear stress. This increase is due to faster translocation of leukocytes during the jumps rather than increased velocity of rolling. The velocity of leukocytes during the periods of stable rolling remains nearly unchanged in the shear stress range studied. The rolling is completely inhibited by a mAb against mouse P‐selectin, demonstrating molecular specificity. We conclude that the apparent variation of rolling velocity with wall shear stress is mainly caused by increased distances traveled by leukocytes during periods of detachment. This study was supported by NIH EB 02185 (K.L) and NSF NIRT 0608863 (A.G).

Human erythrocyte filterability at low driving pressure

In this study, the human RBC capillary flow has been modeled by passing 11 microl of RBC suspension (Hematocrit = 6%) in phosphate buffer solution (PBS) of a viscosity of 1 and 2.6 cP (in the presence of 2% Dextran) through 5 microm pore diameter polycarbonate Nuclepore filters. We have developed a digitally controlled experimental system for measuring the RBC filterability at a constant driving pressure, in the range of 10-400 Pa, producing a wall shear stress range of 1-50 Pa. The RBC filterability was evaluated by measuring the cell suspension flow rate normalized by the PBS flow rate. The RBC filterability has been found to be a nonlinear function of the driving pressure, having a single minimum locus at 25 Pa. Lowering the driving pressure below 25 Pa revealed an unexpected increase of the RBC filterability.The maximal RBC filterability (near unity) was detected at the lowest driving pressure (10 Pa) and the corresponding estimated RBC linear velocity while traveling through the capillary pore was as high as 800 microm/s. Increasing the driving pressure above 25 Pa confirmed previous results, where RBC filterability is monotonically and asymptotically increasing. Increasing the PBS medium viscosity from 1 to 2.26 cP significantly attenuated the RBC filterability and led to the anomalous increase of RBC deformability at the 10 Pa pressure range. We propose that the anomalous increase in RBC deformability was caused by RBCs undergoing spontaneous mechanical fluctuations.

Laminar flow in radial flow cell with small aspect ratios: Numerical and experimental study

Studies on the effect of wall shear stress on soil and biofilm attachment and removal from a surface are one of the many applications of radial axisymmetrical flow. The particular nature of this flow allows taking advantage of a wide range of wall shear stress applied at the analyzed surface in a single experiment. This type of experiments provides a critical radius up to which soil removal occurs. Good models are, however, still needed to convert the experimental data into critical wall shear stress. Analytical models are already available for creeping flow but Computational Fluid Dynamics must be applied for experiments performed at higher Reynolds numbers. The present study is a numerical analysis of the radial axisymmetrical flow for aspect ratios of 0.125, 0.25, 0.5 and 1 with inlet pipe Reynolds numbers varying from 0 to 2000, aiming at computing the wall shear stress distribution at any distance from the center. The simulations provided a thorough description of the complex flow pattern encountered close to the inlet section, which were validated for the laminar regime by dye injection. A total of up to four recirculation zones were identified in both numerical and experimental investigations. The experimental positions of these recirculation zones corresponded well to the numerical predictions. Based on this work, a map of the flow for the different aspect ratios was developed, which can be particularly interesting for the design of experimental devices involving axisymmetrical flow.

Preliminary study of hemodynamics in human carotid bifurcation by computational fluid dynamics combined with magnetic resonance angiography

A longstanding hypothesis that correlates fluid dynamic forces and atherosclerotic disease has led to numerous analytical, numerical, and experimental studies over the years because it is very difficult to measure the hemodynamic variables of blood in vivo. To investigate the technique of visualization and quantitation of hemodynamic variables at carotid artery bifurcation in vivo by combining computational fluid dynamics (CFD) and vascular imaging. Twenty-six healthy volunteers underwent magnetic resonance (MR) angiography of the bilateral carotid artery by a 3.0T whole-body scanner. Hemodynamic variables at these carotid bifurcations were calculated and visualized by combining vascular imaging post-processing and CFD. The average velocity of the carotid bifurcation in the systolic phase and the diastolic phase was 0.46+/-0.24 m/s and 0.23+/-0.05 m/s, respectively. Eddy current and back flows were observed at bifurcation and the lateral part of the proximal internal carotid arteries (ICA) and external carotid arteries (ECA), and the shapes of them changed with phases of the cardiac cycle, which were significant at the middle of the systolic phase and faded out quickly downstream of the ICA and ECA. The average range of wall shear stress (WSS) at the bifurcation was 4.36+/-1.32 Pa, and the maximum WSS was 18.02+/-4.11 Pa. The WSS map revealed a large region of low WSS at the carotid bulb and extended to the outer wall in the proximal end of the ICA (the lowest value was below 0.5 Pa), and there was also a small region of low WSS at the outer wall in the proximal end of the ECA. CFD combined with vascular imaging can calculate and visualize hemodynamic variables at carotid bifurcation in vivo individually.

Family of Micromachined Wall Hot-Wire Sensors on Polyimide Foil

Micromachined wall hot-wire sensors composed of a highly sensitive, nickel, thin-film resistor spanning an air-filled cavity in a mechanically flexible substrate are presented. Cavity design and sensor materials are optimized to reduce thermal losses, thus enabling measurement of high-frequency fluctuations in fluid flows. Successfully realized sensors featuring wire widths of 2 and 5 μm, wire lengths from 400 to 2000 μm, and various cavity dimensions were characterized in wind-tunnel experiments. Static sensor calibration, cutoff frequency determination using a sine sweep, and recording of angular characteristics were conducted at wall shear stresses of up to 1 N/m 2 , obtained at 20 m/s freestream velocity in an open wind tunnel on a flat plate with fully developed turbulent flow. An overheat ratio of 1.8 was used for hours without thermal failure of the sensors, and a maximum cutoff frequency in still air of 73 kHz was obtained. The highest average sensitivity of 0.196 V/(N/m 2 ) was recorded for a sensor of 5-μm wire width and a length-to-width ratio of 400 in a wall shear stress range from 0 to 1 N/m 2 with a power consumption of less than 30 mW.

AeroMEMS Wall Hot-Wire Anemometer on Polyimide Substrate Featuring Top Side or Bottom Side Bondpads

Design, manufacturing, calibration, and basic characterization of a microelectromechanical systems (MEMS) wall hot wire sensor on a flexible polyimide substrate are presented. A configuration exhibiting bond pads on the top side of the foil, as well as an improved setup featuring a through-foil metallization and bottom side bond pads were established. Both sensor designs make use of a highly sensitive nickel thin-film resistor spanning a reactive ion etched cavity in a polyimide substrate. The polyimide base material enables the sensor to be adapted to curved aerodynamic surfaces, e.g., airfoils and turbine blades. A mismatch of curvature of aerodynamic surface and silicon sensor surface, as observed with previously presented MEMS hot-wire anemometers is avoided. The combination of polyimide's low thermal conductivity and a cavity featuring FEM-optimized dimensions accounts for a very low-power consumption (<25 mW). Fluctuations in wall shear stress up to 85 kHz can be resolved in constant-temperature mode. An average sensitivity of 0.166 V/(N/m2) is achieved in a wall shear stress range from 0 to 0.72 N/m2. The specifically designed through-foil metallization process allows for electrical contacts to be positioned on the backside of the substrate, thus effectively minimizing aerodynamic disturbances.