Multi-Resolution, Time-Resolved PIV Measurements of a Decelerating Turbulent Boundary Layer near Separation

We report on measurements of the time-evolving velocity profile of a turbulent boundary layer subjected to a strong adverse pressure gradient (APG) at Reynolds numbers up to Re_theta ~55000 with an upstream friction Reynolds number exceeding Re_tau ~10000. Near the point of flow separation high-resolution imaging at high camera frame rates captured the time evolving velocity profile using the so-called "profile-PIV" technique in a nested imaging configuration of two cameras operating at different image magnifications. One camera used an image magnification better than unity to resolve the viscous scales directly at the wall while the remainder of the roughly 200 mm thick boundary layer is simultaneous captured by the second camera. In the APG the variance of the stream-wise velocity exhibits no "inner peak" commonly found in turbulent boundary layers without pressure gradient influence. Spectral analysis further shows that the peak energy within the boundary layer shifts away from the wall toward lower frequencies. The overlap between the simultaneously imaged areas allows to assess and, to first order, correct for the effect of spatial smoothing on statistical quantities, spectra and related quantities. A multi-frame cross-correlation algorithm was used to process the extensive database. In addition, a newly developed 2D-2C "Shake-The-Box" algorithm(STB) provided highly resolved particle tracking data beyond the reach of conventional PIV processing

A fully-printed electrochemical platform for assisted colorimetric detection of phosphate in saliva: Greenness and whiteness quantification by the AGREE and RGB tools

Herein, we report the environmental impact quantification of a newly developed fully printed electrochemical device to assist a colorimetric detection of phosphate in saliva. The evaluation of the analytical procedure was per formed according to the principles of Green Analytical Chemistry and White Analytical Chemistry. The standard method for phosphate detection relies on a reaction between phosphate and molybdate in presence of antimony potassium tartrate and ascorbic acid, using strong acid conditions and high volumes of reagents (100–500 mL). To deliver an eco-friendly method, we have combined a screen-printed electrode with a liquid electrolyte battery and inkjet-printed conductive paths to develop a fully printed device on a flexible polymer substrate avoiding the use of ascorbic acid and using a small amount of reagents. The printed sensor was first developed and optimized for phosphate detection in saliva, allowing for a detection limit equal to 26 μM and satisfactory repeatability (relative standard deviation value of 7.5%). Finally, the AGREE and the RGB assessment tools were applied for a quantitative evaluation of the proposed sensor and reference method, in agreement with the Green Analytical and White Analytical principles. The results demonstrated the lower environmental impact of the proposed sensor, as well as the suitability of this novel approach for phosphate detection in saliv

Exciton tuning in monolayer WSe2_2 via substrate induced electron doping

We report on large exciton tuning in WSe$_2$ monolayers via substrate induced non-degenerate doping. We observe a redshift of $\sim$62 meV for the $A$ exciton together with a 1-2 orders of magnitude photoluminescence (PL) quenching when the monolayer WSe$_2$ is brought in contact with highly oriented pyrolytic graphite (HOPG) compared to the dielectric substrates such as hBN and SiO$_2$. As the evidence of doping from HOPG to WSe$_2$, a drastic increase of the trion emission intensity was observed. Using a systematic PL and Kelvin probe force microscopy (KPFM) investigation on WSe$_2$/HOPG, WSe$_2$/hBN, and WSe$_2$/graphene, we conclude that this unique excitonic behavior is induced by electron doping from the substrate. Our results propose a simple yet efficient way for exciton tuning in monolayer WSe$_2$, which plays a central role in the fundamental understanding and further device development.Comment: 14 pages, 10 figure

Modification of turbulence models for pressure-induced separation on smooth surfaces using the DLR VicToria experiment

A new experiment of a turbulent boundary layer flow at a large adverse pressure gradient at a high Reynolds number is presented. The strong pressure gradient leads to pressure-induced separation on the smooth surface of the geometry model with a thin separation bubble. The experiment was performed within the DLR internal project VicToria. First, the design of the test case, the set-up in the wind tunnel, and the measurement technique using both large-scale and high-magnification particle imaging and Lagrangian particle tracking are described. Then the experimental results for the mean velocity are described as the flow evolves downstream from the zero-pressure gradient region into the adverse pressure gradient region. From the measurement data a wall law for the mean velocity with a thin log-law region and a half-power law region above the log-law is observed in the adverse pressure gradient region. Then the differential Reynolds stress transport model SSG/LRR-omega is considered. Based on the observation that the length-scale equation is not consistent with the assumed wall laws at adverse-pressure gradient, a modification of the equation for the dissipation rate omega in the model is proposed, so that the modified model can predict the observed wall law at adverse-pressure gradient. Finally, the numerical results using the modified SSG/LRR-omega model are shown. The modification causes a reduction of the mean velocity in the inner part of the boundary layer at adverse-pressure gradients, making the modified model more susceptible for flow separation. The numerical predictions of the modified model are found to be in good agreement with the experimental data

Multi-resolution, time-resolved particle image velocimetry of a turbulent boundary layer approaching separation

The evolution and characteristics of turbulent boundary layers subjected to a positive pressure gradient are common in many flows of industrial relevance and appear, for instance, on the suction side of high-lift wing sections or turbomachinery blading. The pressure gradient is associated with a deceleration of the external flow which results in a thickening of the boundary layer and reduction of the wall friction. Under strong adverse pressure gradient conditions, the wall friction reduces to zero as the flow begins to separate. From a numerical perspective these flows are difficult to model with methods of engineering relevance, such as Reynolds averaged Navier-Stokes simulations (RANS) and motivate experiments such as the one presented here to provide relevant validation data

Biomedical Diagnostics Enabled by Integrated Organic and Printed Electronics

© 2017 American Chemical Society. Organic and printed electronics integration has the potential to revolutionize many technologies, including biomedical diagnostics. This work demonstrates the successful integration of multiple printed electronic functionalities into a single device capable of the measurement of hydrogen peroxide and total cholesterol. The single-use device employed printed electrochemical sensors for hydrogen peroxide electroreduction integrated with printed electrochromic display and battery. The system was driven by a conventional electronic circuit designed to illustrate the complete integration of silicon integrated circuits via pick and place or using organic electronic circuits. The device was capable of measuring 8 μL samples of both hydrogen peroxide (0-5 mM, 2.72 × 10 -6 A·mM -1 ) and total cholesterol in serum from 0 to 9 mM (1.34 × 10 -8 A·mM -1 , r 2 = 0.99, RSD < 10%, n = 3), and the result was output on a semiquantitative linear bar display. The device could operate for 10 min via a printed battery, and display the result for many hours or days. A mobile phone "app" was also capable of reading the test result and transmitting this to a remote health care provider. Such a technology could allow improved management of conditions such as hypercholesterolemia

Experimental evidence of near-wall reverse flow events in a zero pressure gradient turbulent boundary layer

This study reports on experimentally observed rare near-wall reverse flow events in a fully developed turbulent flat plate boundary layer at zero pressure gradient with Reynolds numbers between Re_\theta \approx 2500 and Re_\theta \approx 8000 (Re_\tau = 800-2400). The reverse flow events are captured using high magnification particle image velocimetry sequences with record lengths varying from 50 000 to 126 000 samples. Time resolved particle image sequences allow singular reverse flow events to be followed over several time steps whereas long records of nearly statistically independent samples provide a variety of single snapshots at a higher spatial resolution. The probability of occurrence lies in the order of 0.012-0.018% which matches predictions from direct numerical simulations (DNS). The typical size of the reverse flow bubble is about 30 wall units in length and 5 wall units in height which agrees well with similar observations made in existing DNS data

Modification of the SSG/LRR-omega RSM for adverse pressure gradients using turbulent boundary layer experiments at high Re

A modification of the SSG/LRR-omega model for turbulent boundary layers in adverse pressure gradient is presented. The modification is based on a new wall law for the mean velocity at adverse pressure gradient. The wall law is found from two new joint DLR/UniBw experiments and from the analysis of a data base from the literature. The mean velocity profile in the inner layer is found to consist of a log-law region, which is thinner than its zero pressure gradient counterpart, and a half-power law region above the log law. An empirical correlation for the wall-distance of the transition from the log-law to the half-power law is presented. Then a modification of the omega-equation to account for a half-power law behaviour of the mean velocity is described. The modified SSG/LRR-omega model is then applied to the two joint DLR/UniBw experiments. The modification leads to a reduction of the mean velocity in the inner part of the boundary layer and makes the model more susceptible for flow separation, which is in good agreement with the experimental data

Multi-PIV Measurements of an Adverse Pressure Gradient Turbulent Boundary Layer

We report on a multi-national measurement campaign aimed at providing highly resolved flow field data of a turbulent boundary layer subjected to an adverse pressure gradient (APG). In the case of APGs the structure and dynamics of large scale turbulent flow structures along with their significance on the statistical properties of the flow is not well understood. Hence the fundamental aim was to resolve and characterise the large-scale coherent structures in an APG boundary layer flow. In addition to large-field-of-view PIV measurements using 16 sCMOS cameras along a 3.5m length, stereoscopic PIV measurements were performed at specific locations in order to also resolve the span-wise velocity statistics. Long-distance, high-speed micro-PIV measurements provided near wall statistics at selected locations including the time-resolved wall shear stress. The measurements were performed in the boundary layer wind tunnel of the Laboratoire de Mécanique de Lille (LML) and funded by EuHIT (www.euhit.org)