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E-skins consisting of soft pressure sensors are enabling technology for soft robots, bio-integrated devices, and deformable touch panels. A well-known bottleneck of capacitive pressure sensors (CPS) is the drastic decay in sensitivity with increasing pressure. To overcome this challenge, we have invented a hybrid-response pressure sensor (HRPS) that exhibits both the piezoresistive and piezocapacitive effects intrinsic to a highly porous nanocomposite (PNC) with carbon nanotube (CNT) dopants. The HRPS is constructed with two conductive electrodes sandwiching a laminated PNC and a stiff dielectric layer. We have simplified the hybrid response into a parallel resistor–capacitor circuit, whose output depends on the AC (alternating current) frequency used for the capacitance measurement. Herein, through theoretical analysis, we discover a dimensionless parameter that governs the frequency responses of the HRPS. The master curve is validated through experiments on the HRPS with various doping ratios, subject to different compressive strains, under diverse AC frequencies. In addition, the relative contribution of piezoresistive and piezocapacitive mechanisms are also found to vary with the three parameters. Based on this experimentally validated theory, we establish a very practical guideline for selecting the optimal AC frequency for the capacitance measurement of HRPSs.Free, publicly-accessible full text available November 16, 2023
This work investigates surface pressure unsteadiness on a compliant panel under a shockwave/boundary-layer interaction (SBLI) induced by a 2D compression ramp with an angle of 20o in a Mach 2 wind tunnel. High-speed digital image correlation (DIC) and fast-response pressure-sensitive paint (PSP) measurements are used to measure the panel displacement and panel and ramp-face surface pressure fluctuations at 5kHz and 20kHz, respectively. The data reduction technique of POD (proper orthogonal decomposition) was employed both for pressure and displacement fields. POD mode distribution for the pressure fields reveals that the first six modes have 60% of the total energy and exhibit low-frequency content for both rigid and compliant panels. The vibration of the compliant panel was seen to alter the energy distribution of the high energy modes as compared to the rigid panel case. The cross-correlations between the displacement and pressure modes were made using the time coefficients. This analysis shows significant correlations were present among the lower modes. The highest correlation was between displacement mode 1 and the pressure mode 4, which stemmed from the upstream of the intermittent region. The analysis was also made for the surrogate shock foot and reattachment lines. The correlation shows that panel vibration lowersmore »
A thin compliant panel was tested in a Mach 2 wind tunnel. The panel was mounted flush with the tunnel floor and was of dimensions L=121.9 mm (chord), W=63.5 mm (span) and h=0.254 mm (thickness). A 20 degree compression ramp was placed 5 mm downstream of the model, which induced a shock/boundary layer interaction with fully separated flow over parts of the panel. Full-field deformation was measured using Digital Image Correlation and the surface pressure field was obtained from fast-response pressure-sensitive paint. Analysis of the shock foot motion was performed using a curve-fitting method. Comparison of the shock motion between a rigid and compliant panel case showed no difference in the size of the intermittent region but found that the shock motion over the compliant panel is affected by the panel vibration. Proper Orthogonal Decomposition revealed that the surface pressure is dominated by low-frequency unsteady shock motion, in both cases (rigid and compliant panel). The sixth POD mode clearly shows the streamwise shock foot motion oscillates at the first panel vibration frequency. The surface pressure field upstream of the shock foot is dominated by piston-theory aerodynamics and thus correlated to the slope of the compliant panel. The Sparse Identification ofmore »
Experimental Investigation of Flow-Structure Interaction for a Compliant Panel under a Mach 2 Compression-RampThis experimental study focuses on fluid-structure interaction (FSI) for a thin compliant panel under a shock/boundary layer interaction (SBLI) generated by a 2D compression ramp in a Mach 2 wind tunnel. In previous work, we have studied the FSI for this configuration using simultaneous fast-response pressure-sensitive paint (PSP) and digital image correlation (DIC). Simultaneous PSP/DIC allows for examination of the relationship between the dynamic panel displacement and surface pressure loading, respectively. Spectral analysis showed that pressure fluctuations within the interaction region and shock-foot unsteadiness tend to lock to the first mode resonant frequency of the compliant panel. The current study aims to utilize synchronous high-speed stereoscopic PIV (25 kHz) and DIC (5 kHz) techniques to better understand the coupling between the flow field and the panel displacement field. The PIV is obtained in a streamwise-spanwise plane located at 15% of the boundary layer height. Thin compliant polycarbonate panel with thicknesses of 1 mm is utilized, which has a first-mode vibrational frequency of 407 Hz. The 1 mm panel out-of-plane displacement amplitude was up to 15% of the boundary layer thickness. The analysis includes low-pass and band-pass filtering of the velocity data, including the surrogate separation line, and cross-correlation analysis betweenmore »
Effect of Structural Modifications on Vibratory Response of Panel under Ramp-Induced Shock Wave Boundary Layer InteractionThe vibration of a compliant panel under a shock / boundary layer interaction (SBLI) induced by a compression ramp in a Mach 2 flow, is investigated experimentally. The panel is made from brass shim stock of length (streamwise), width (spanwise) and thickness of 122 mm by 63.5 mm by 0.25 mm, respectively. The 20° compression ramp is placed near the downstream edge of the compliant panel, and it creates a shock-induced turbulent separated flow that extends over the downstream 20% of the panel. Large pressure fluctuations occur in the region of the separation shock foot unsteadiness. The pressure fluctuations increase vibration amplitudes of the higher panel modes, especially the second mode, which has an antinode near the shock foot region. In this work, the authors use structural modifications of the baseline compliant panel to mitigate vibrations induced by the large pressure fluctuations of the shock foot unsteadiness. A thin rib is attached in the spanwise direction to the lee side of the panel at the location of SBLI. In one configuration, the rib is attached to the panel using epoxy adhesive, which creates a stiff connection. In another configuration, the rib is attached to the panel via double-sided viscoelastic tape,more »
Investigation of flow-structure coupling for a compliant panel under a shock/boundary-layer interaction using fast-response PSPThis experimental study aims to investigate and compare unsteady surface pressure fluctuations on rigid and compliant panels under a shock-wave/boundary-layer interaction (SBLI) generated by a 20o compression ramp in a Mach 2 wind tunnel. The compliant panel was made of 1mm thick polycarbonate and had a first mode resonant frequency of 407 Hz. High-speed simultaneous pressure-sensitive paint (PSP) and digital image correlation (DIC) techniques allow for examination of the panel surface pressure and panel displacement, respectively, with acquisition frequencies of 20 kHz and 5 kHz respectively. The PSP measurements were also made on the face of the rigid compression ramp and so the effect of fluid-structure interaction on the reattachment dynamics could be explored. The rigid panel pressure measurements demonstrated the spectral content inherent to the SBLI. Spectral analysis of the surface pressure fields revealed that the SBLI shock foot and separation shear layer behave as low- and high-frequency filters, respectively, and the compliant panel showed the similar behavior with the peak frequency at the first mode frequency of the panel (~407Hz). The spectral comparison of pressure fields also depicted that the other modes of the structural panel affected the flow ( at frequency 624 Hz) near the compliant panel’smore »
A Chest‐Laminated Ultrathin and Stretchable E‐Tattoo for the Measurement of Electrocardiogram, Seismocardiogram, and Cardiac Time Intervals
Seismocardiography (SCG) is a measure of chest vibration associated with heartbeats. While skin soft electronic tattoos (e‐tattoos) have been widely reported for electrocardiogram (ECG) sensing, wearable SCG sensors are still based on either rigid accelerometers or non‐stretchable piezoelectric membranes. This work reports an ultrathin and stretchable SCG sensing e‐tattoo based on the filamentary serpentine mesh of 28‐µm‐thick piezoelectric polymer, polyvinylidene fluoride (PVDF). 3D digital image correlation (DIC) is used to map chest vibration to identify the best location to mount the e‐tattoo and to investigate the effects of substrate stiffness. As piezoelectric sensors easily suffer from motion artifacts, motion artifacts are effectively reduced by performing subtraction between a pair of identical SCG tattoos placed adjacent to each other. Integrating the soft SCG sensor with a pair of soft gold electrodes on a single e‐tattoo platform forms a soft electro‐mechano‐acoustic cardiovascular (EMAC) sensing tattoo, which can perform synchronous ECG and SCG measurements and extract various cardiac time intervals including systolic time interval (STI). Using the EMAC tattoo, strong correlations between STI and the systolic/diastolic blood pressures, are found, which may provide a simple way to estimate blood pressure continuously and noninvasively using one chest‐mounted e‐tattoo.