Experiments and simulations of MEMS thermal sensors for wall shear-stress measurements in aerodynamic control applications

MEMS thermal shear-stress sensors exploit heat-transfer effects to measure the shear stress exerted by an air flow on its solid boundary, and have promising applications in aerodynamic control. Classical theory for conventional, macroscale thermal shear-stress sensors states that the rate of heat removed by the flow from the sensor is proportional to the 1/3-power of the shear stress. However, we have observed that this theory is inconsistent with experimental data from MEMS sensors. This paper seeks to develop an understanding of MEMS thermal shear-stress sensors through a study including both experimental and theoretical investigations. We first obtain experimental data that confirm the inadequacy of the classical theory by wind-tunnel testing of prototype MEMS shear-stress sensors with different dimensions and materials. A theoretical analysis is performed to identify that this inadequacy is due to the lack of a thin thermal boundary layer in the fluid flow at the sensor surface, and then a two-dimensional MEMS shear-stress sensor theory is presented. This theory incorporates important heat-transfer effects that are ignored by the classical theory, and consistently explains the experimental data obtained from prototype MEMS sensors. Moreover, the prototype MEMS sensors are studied with three-dimensional simulations, yielding results that quantitatively agree with experimental data. This work demonstrates that classical assumptions made for conventional thermal devices should be carefully examined for miniature MEMS devices

A parametrized three-dimensional model for MEMS thermal shear-stress sensors

This paper presents an accurate and efficient model of MEMS thermal shear-stress sensors featuring a thin-film hotwire on a vacuum-isolated dielectric diaphragm. We consider three-dimensional (3-D) heat transfer in sensors operating in constant-temperature mode, and describe sensor response with a functional relationship between dimensionless forms of hotwire power and shear stress. This relationship is parametrized by the diaphragm aspect ratio and two additional dimensionless parameters that represent heat conduction in the hotwire and diaphragm. Closed-form correlations are obtained to represent this relationship, yielding a MEMS sensor model that is highly efficient while retaining the accuracy of three-dimensional heat transfer analysis. The model is compared with experimental data, and the agreement in the total and net hotwire power, the latter being a small second-order quantity induced by the applied shear stress, is respectively within 0.5% and 11% when uncertainties in sensor geometry and material properties are taken into account. The model is then used to elucidate thermal boundary layer characteristics for MEMS sensors, and in particular, quantitatively show that the relatively thick thermal boundary layer renders classical shear-stress sensor theory invalid for MEMS sensors operating in air. The model is also used to systematically study the effects of geometry and material properties on MEMS sensor behavior, yielding insights useful as practical design guidelines

The effect of internal pipe wall roughness on the accuracy of clamp-on ultrasonic flow meters

Clamp-on transit-time ultrasonic flowmeters (UFMs) suffer from poor accuracy compared with spool-piece UFMs due to uncertainties that result from the in-field installation process. One of the important sources of uncertainties is internal pipe wall roughness which affects the flow profile and also causes significant scattering of ultrasound. This paper purely focuses on the parametric study to quantify the uncertainties (related to internal pipe wall roughness) induced by scattering of ultrasound and it shows that these effects are large even without taking into account the associated flow disturbances. The flowmeter signals for a reference clamp-on flowmeter setup were simulated using 2-D finite element analysis including simplifying assumptions (to simulate the effect of flow) that were deemed appropriate. The validity of the simulations was indirectly verified by carrying out experiments with different separation distances between ultrasonic probes. The error predicted by the simulations and the experimentally observed errors were in good agreement. Then, this simulation method was applied on pipe walls with rough internal surfaces. For ultrasonic waves at 1 MHz, it was found that compared with smooth pipes, pipes with only a moderately rough internal surface (with 0.2-mm rms and 5-mm correlation length) can exhibit systematic errors of 2 in the flow velocity measurement. This demonstrates that pipe internal surface roughness is a very important factor that limits the accuracy of clamp on UFMs

Printing of wirelessly rechargeable solid-state supercapacitors for soft, smart contact lenses with continuous operations

Recent advances in smart contact lenses are essential to the realization of medical applications and vision imaging for augmented reality through wireless communication systems. However, previous research on smart contact lenses has been driven by a wired system or wireless power transfer with temporal and spatial restrictions, which can limit their continuous use and require energy storage devices. Also, the rigidity, heat, and large sizes of conventional batteries are not suitable for the soft, smart contact lens. Here, we describe a human pilot trial of a soft, smart contact lens with a wirelessly rechargeable, solid-state supercapacitor for continuous operation. After printing the supercapacitor, all device components (antenna, rectifier, and light-emitting diode) are fully integrated with stretchable structures for this soft lens without obstructing vision. The good reliability against thermal and electromagnetic radiations and the results of the in vivo tests provide the substantial promise of future smart contact lenses

Overview of sensors suitable for active flow control methods

Hlavným cieľom tejto bakalárskej práce bolo vytvorenie prehľadu vyvíjaných a už aplikovaných senzorov pre účely aktívneho riadenia prúdov. Senzory musia splňovať niektoré podmienky, preto výber senzorov bol naviazaný na reálnych výsledkoch testovacích programov, popis ktorých tvorí prvú časť tejto bakalárskej práce. Opis technológie a princíp fungovania senzorov je popísaný v druhej časti tejto práce.The main purpose of this bachelor thesis was to create the overview of the sensors developed for the future active flow control applications and overview the sensors already used in the active flow control applications. The sensors have to fulfil several requirements, so selection for the overview was based on the real flight test programs results, which were described in the first part of the thesis. The sensors technology description and operation principles were included in the second part of the thesis

A versatile platform for three-dimensional dynamic suspension culture applications

In the last decades, the rapid upgrading in cell biological knowledge has bumped the interest in using cell-based therapeutic approaches as well as cell-based model systems for the treatment of diseases. Given the rapid translation towards cell-based clinical treatments and the consequent increasing demand of cell sources, three-dimensional (3D) suspension cultures have demonstrated to be an advantageous alternative to monolayer techniques for large scale expansion of cells and for the generation of three-dimensional model systems in a scale-up perspective. In this scenario, a versatile bioreactor platform suitable for 3D dynamic suspension cell culture under tuneable shear stress conditions is developed and preliminarily tested in two different biotechnological applications. By adopting simple technological solutions and avoiding rotating components, the bioreactor exploits a laminar hydrodynamics, enabling dynamic cell suspension in an environment favourable to mass transport. Technically, the bioreactor is conceived to produce dynamic suspension cell culture under tuneable shear stress conditions without the use of moving components (from ultralow to moderate shear stress). A multiphysics computational modelling strategy is applied for the development and optimization of the suspension bioreactor platform. The in silico modelling is used to support the design and optimization phase of the bioreactor platform, providing a comprehensive analysis of its operating principles, also supporting the development/optimization of culture protocols directly in silico, and thus minimizing preliminary laboratory tests. After the technical assessment of the functionality of the device and a massive number of in silico simulations for its characterization, the bioreactor platform has been employed for two preliminary experimental applications, in order to determine the suitability of the device for culturing human cells under dynamic suspension. In detail, the bioreactor platform has been used to culture lung cancer cells for spheroid formation (Calu-3 cell line) under ultralow shear stress conditions, and for human induced pluripotent stem cell (hiPSC) dynamic suspension culture. The use of the bioreactor platform for the formation of cancer cell spheroids under low shear stress conditions confirms the suitability of the device for its use as dynamic suspension bioreactor. In fact, compared to static cell suspension, after 5 days of dynamic suspension culture the bioreactor platform preserves morphological features, promotes intercellular connection, increases the number of cycling cells, and reduces double strand DNA damage. Calu-3 cells form functional 3D spheroids characterized by more functional adherence junctions between cells. Moreover, the computational model has been used as a tool for assisting the setup of the experimental framework with the extraction of the fluid dynamic features establishing inside the bioreactor culture chamber. As second proof of concept application, the bioreactor platform has been tested for the dynamic suspension of hiPSCs. Starting from the ‘a priori’ knowledge gained by the development of the in silico culture protocol, the agglomeration of human induced pluripotent stem cells has been modulated by means of the combination of moderate intermittent shear stress and free-fall transport within the bioreactor culture chamber. The inoculation of single cells suspensions inside the bioreactor chamber promotes cell-cell interaction and consequently the formation of human induced pluripotent stem cell aggregates. In conclusion, the impeller-free functioning principle characterizing the proposed bioreactor platform demonstrates to be promising for human cell dynamic suspension culture. In the future, this bioreactor platform will be further optimized for the realization of impeller-free dynamic suspension bioreactors dedicated and optimized to specific applications in stem cell and cancer cell culture

Mechanical BioMEMS Technologies for Advanced Label-free Sensing of Biomolecular Species in Microfluidic Channels

The aim of this PhD project is to investigate alternative sensing methodologies that can possibly improve the sensing performances of lab–on–chip (LOC) designed for biochemical applications. Suspended microchannel resonator (SMR) for bio–mechanical sensing applications have become very popular as detection of weigths of chemicals integrated in LOC. They exploit laser doppler vibrometry (LDV) for dynamic mode detection. In this thesis two different SMRs designs have been investigated, involving either technological challenges – the use of polymers as material and processing techniques based on laser micromachining – and different sensing phenomena – the use of the parametric resonance rather than the standard harmonic resonance response. The flexibility of two–photon direct laser writing is exploited to optimize a highly– versatile fabrication strategy based on a shell–writing procedure with the aim to reduce fabrication time of big inlet/outlet sections compatible with most microfluidic systems for LOCs. With respect to standard microfabrication techniques, requiring several technological steps to obtain suspended hollow structures, this method allows to fabricate complex SMR sensors in only one fabrication step, by virtue of its intrinsically three– dimensional nature. A SMR fixed-fixed beam has been fabricated and characterized by LDV. A different sensing mechanism based on the parametric resonance instead of the harmonic resonance has been investigated to develop a novel platform for the characterization of biomolecules in free–flow with unique specificity, sensitivity, and speed: to this purpose a PDMS based device was realized by laser machining, a rapid prototype fabrication technique; beside to it, a commercial fused silica capillary tubing was also employed in the realization of a prototype for this sensing mechanism, and both solutions were tested through LDV

Boundary layer simulator improvement

Boundary Layer Integral Matrix Procedure (BLIMPJ) has been identified by the propulsion community as the rigorous boundary layer program in connection with the existing JANNAF reference programs. The improvements made to BLIMPJ and described herein have potential applications in the design of the future Orbit Transfer Vehicle engines. The turbulence model is validated to include the effects of wall roughness and a way is devised to treat multiple smooth-rough surfaces. A prediction of relaminarization regions is examined as is the combined effects of wall cooling and surface roughness on relaminarization. A turbulence model to represent the effects of constant condensed phase loading is given. A procedure is described for thrust decrement calculation in thick boundary layers by coupling the T-D Kinetics Program and BLIMPJ and a way is provided for thrust loss optimization. Potential experimental studies in rocket nozzles are identified along with the required instrumentation to provide accurate measurements in support of the presented new analytical models