The Experts below are selected from a list of 234 Experts worldwide ranked by ideXlab platform
Alexander Smits - One of the best experts on this subject based on the ideXlab platform.
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modelling and operation of sub miniature constant temperature hot wire Anemometry
Measurement Science and Technology, 2016Co-Authors: Milad Samie, J H Watmuff, T Van Buren, N Hutchins, Ivan Marusic, Marcus Hultmark, Alexander SmitsAbstract:High-Reynolds number flows are very common in technological applications and in nature, and Hot-Wire Anemometry is the preferred method for measuring the time-series of fluctuating velocity in such flows. However, measurement of very high-Reynolds number flows requires Hot-Wires with higher temporal and spatial resolution than is available with conventional probes. Much effort has therefore been devoted to decreasing the size of the Hot-Wire probes and this has led to associated challenges with operation. It is this latter operation problem which is the focus of this paper. To this end, an existing theoretical model of constant-temperature Hot-Wire anemometers (Perry 1982 Hot-Wire Anemometry (New York: Oxford University Press), Watmuff 1995 Exp. Therm. Fluid Sci. 11 117-34) is applied, and its accuracy is tested for the first time by comparison to measurements using an in-house constant temperature anemometer (CTA) for both conventional 5μm-diameter wires and sub-miniature Hot-Wires. With the aid of this model, we propose modifications to the CTA design and demonstrate successful operation of the CTA with the Princeton nano-scale thermal Anemometry probe (NSTAP) (Bailey et al 2010 J. Fluid Mech. 663 160-79). It is also shown that the transfer function obtained from the model can be utilized to estimate the true frequency response and cut-off frequency of a Hot-Wire-CTA system to the velocity fluctuations, which is essential in accurate measurements of energy spectrum and higher order statistics of turbulent flows.
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a new criterion for end conduction effects in hot wire Anemometry
Measurement Science and Technology, 2011Co-Authors: Marcus Hultmark, Anand Ashok, Alexander SmitsAbstract:The effect of end conduction on constant temperature Hot-Wire Anemometry was studied. A new parameter, , is proposed to describe the significance of end conduction more comprehensively than the commonly used length-to-diameter ratio l/d, in that it allows for material property variations, resistance ratio and Reynolds number effects. Numerical and experimental data are used to show that Γ improves the correlation of the attenuation of measured turbulence fluctuations, and it is found that Γ > 14 is required to avoid such effects.
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Further support for Townsend’s Reynolds number similarity hypothesis in high Reynolds number rough-wall pipe flow
Physics of Fluids, 2007Co-Authors: Gary J. Kunkel, James J. Allen, Alexander SmitsAbstract:Measurements of turbulence behavior in high Reynolds number fully developed rough-wall pipe flow are presented. The data are acquired with single-component conventional Hot-Wire Anemometry in the Princeton/ONR Superpipe fitted with a honed rough pipe. Streamwise turbulence intensities, higher-order moments, and spectra are compared with the corresponding results from a previous smooth-wall pipe flow study in the same facility. Accepting the experimental challenges of conducting Hot-Wire Anemometry studies in such a facility, the rough-wall data agree well with the smooth-wall data in the outer region of the flow, strongly supporting Townsend’s Reynolds number similarity hypothesis.
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Turbulence characteristics in high-Reynolds-number rough-wall pipe flow
36th AIAA Fluid Dynamics Conference and Exhibit, 2006Co-Authors: Gary J. Kunkel, Alexander SmitsAbstract:Measurements of turbulence behavior in high-Reynolds-number fully-developed roughwall pipe ∞ow are presented. The data are acquired with conventional Hot-Wire Anemometry in the Princeton/ONR Superpipe, which has recently been fltted with a honed rough pipe. Turbulence intensities, higher order moments, and spectra are compared with the corresponding results from an earlier smooth-wall pipe ∞ow study in the same facility. Accepting the experimental challenges of conducting Hot-Wire Anemometry studies in such a facility, the rough-wall data agrees with the smooth-wall data in all statistics analyzed here.
Snezhana I Abarzhi - One of the best experts on this subject based on the ideXlab platform.
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whittle maximum likelihood estimate of spectral properties of rayleigh taylor interfacial mixing using hot wire Anemometry experimental data
Physical Review E, 2020Co-Authors: D Pfefferle, Snezhana I AbarzhiAbstract:Investigating the power density spectrum of fluctuations in Rayleigh-Taylor (RT) interfacial mixing is a means of studying characteristic length, timescales, anisotropies, and anomalous processes. Guided by group theory, analyzing the invariance-based properties of the fluctuations, our paper examines raw time series from Hot-Wire Anemometry measurements in the experiment by Akula et al. [J. Fluid Mech. 816, 619 (2017)JFLSA70022-112010.1017/jfm.2017.95]. The results suggest that the power density spectrum can be modeled as a compound function presented as the product of a power law and an exponential. The data analysis is based on Whittle's approximation of the power density spectrum for independent zero-mean near-Gaussian signals to construct a maximum likelihood estimator of the parameters. Those that maximize the log-likelihood are computed numerically through Newton-Raphson iteration. The Hessian of the log-likelihood is used to evaluate the Fisher information matrix and provide an estimate of the statistical error on the obtained parameters. The Kolmogorov-Smirnov test is applied to analyze the goodness of fit, by verifying the hypothesis that the ratio between the observed periodogram and the estimated power density spectrum follows a χ^{2} probability distribution. The dependence of the parameters of the compound function is investigated on the range of mode numbers over which the fit is performed. In the domain where the relative errors of the power-law exponent and the exponential decay rate are small and the goodness of fit is excellent, the parameters of the compound function are clearly defined, in agreement with the theory developed in the paper. The study of the power-law spectra in RT mixing data suggests that rigorous physics-based statistical methods can help researchers to see beyond visual inspection.
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whittle maximum likelihood estimate of spectral properties of rayleigh taylor interfacial mixing using hot wire Anemometry experimental data
arXiv: Fluid Dynamics, 2019Co-Authors: D Pfefferle, Devesh Ranjan, Snezhana I AbarzhiAbstract:The Rayleigh-Taylor instability (RTI) occurs in a broad range of processes in nature and technology. Analysing the power density spectrum of fluctuations in Rayleigh-Taylor (RT) flow is a means of highlighting characteristic length- and time-scales, anisotropies and anomalous processes. Raw time series from Hot-Wire Anemometry measurements of Rayleigh-Taylor interfacial mixing experiment by Akula et al., JFM 816, 619-660 (2017) are considered as a sample case to adjust the parameters of a model power density spectrum. The results suggest that the power density spectrum of one of the flow components can be confidently modelled as the product of a power law and an exponential. The data analysis is based on Whittle's approximation of the power density spectrum for independent zero-mean near-Gaussian signals to construct a Maximum likelihood Estimator (MLE) of the parameters. Those that maximise the log-likelihood are computed numerically through Newton-Raphson iteration. The Hessian of the log-likelihood is used to evaluate the Fisher information matrix and provide an estimate of the statistical error on the obtained parameters. The Kolmogorov-Smirnov test is used to verify the hypothesis that the ratio between the observed periodogram and the estimated power density spectrum follows a chi-squared probability distribution. This step is performed to show goodness-of-fit. We also study the dependence of the model parameters on the range of mode numbers over which the fit is performed.
D Pfefferle - One of the best experts on this subject based on the ideXlab platform.
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whittle maximum likelihood estimate of spectral properties of rayleigh taylor interfacial mixing using hot wire Anemometry experimental data
Physical Review E, 2020Co-Authors: D Pfefferle, Snezhana I AbarzhiAbstract:Investigating the power density spectrum of fluctuations in Rayleigh-Taylor (RT) interfacial mixing is a means of studying characteristic length, timescales, anisotropies, and anomalous processes. Guided by group theory, analyzing the invariance-based properties of the fluctuations, our paper examines raw time series from Hot-Wire Anemometry measurements in the experiment by Akula et al. [J. Fluid Mech. 816, 619 (2017)JFLSA70022-112010.1017/jfm.2017.95]. The results suggest that the power density spectrum can be modeled as a compound function presented as the product of a power law and an exponential. The data analysis is based on Whittle's approximation of the power density spectrum for independent zero-mean near-Gaussian signals to construct a maximum likelihood estimator of the parameters. Those that maximize the log-likelihood are computed numerically through Newton-Raphson iteration. The Hessian of the log-likelihood is used to evaluate the Fisher information matrix and provide an estimate of the statistical error on the obtained parameters. The Kolmogorov-Smirnov test is applied to analyze the goodness of fit, by verifying the hypothesis that the ratio between the observed periodogram and the estimated power density spectrum follows a χ^{2} probability distribution. The dependence of the parameters of the compound function is investigated on the range of mode numbers over which the fit is performed. In the domain where the relative errors of the power-law exponent and the exponential decay rate are small and the goodness of fit is excellent, the parameters of the compound function are clearly defined, in agreement with the theory developed in the paper. The study of the power-law spectra in RT mixing data suggests that rigorous physics-based statistical methods can help researchers to see beyond visual inspection.
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whittle maximum likelihood estimate of spectral properties of rayleigh taylor interfacial mixing using hot wire Anemometry experimental data
arXiv: Fluid Dynamics, 2019Co-Authors: D Pfefferle, Devesh Ranjan, Snezhana I AbarzhiAbstract:The Rayleigh-Taylor instability (RTI) occurs in a broad range of processes in nature and technology. Analysing the power density spectrum of fluctuations in Rayleigh-Taylor (RT) flow is a means of highlighting characteristic length- and time-scales, anisotropies and anomalous processes. Raw time series from Hot-Wire Anemometry measurements of Rayleigh-Taylor interfacial mixing experiment by Akula et al., JFM 816, 619-660 (2017) are considered as a sample case to adjust the parameters of a model power density spectrum. The results suggest that the power density spectrum of one of the flow components can be confidently modelled as the product of a power law and an exponential. The data analysis is based on Whittle's approximation of the power density spectrum for independent zero-mean near-Gaussian signals to construct a Maximum likelihood Estimator (MLE) of the parameters. Those that maximise the log-likelihood are computed numerically through Newton-Raphson iteration. The Hessian of the log-likelihood is used to evaluate the Fisher information matrix and provide an estimate of the statistical error on the obtained parameters. The Kolmogorov-Smirnov test is used to verify the hypothesis that the ratio between the observed periodogram and the estimated power density spectrum follows a chi-squared probability distribution. This step is performed to show goodness-of-fit. We also study the dependence of the model parameters on the range of mode numbers over which the fit is performed.
Saher Al Shakhshir - One of the best experts on this subject based on the ideXlab platform.
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Applying Hot-Wire Anemometry to directly measure the water balance in a proton exchange membrane fuel cell – Part 2: Experimental
International Journal of Hydrogen Energy, 2016Co-Authors: Saher Al Shakhshir, Soren Juhl Andreasen, Torsten BerningAbstract:In order to better understand and more accurately measure the water balance in a proton exchange membrane fuel cell our group has recently proposed to apply hot wire Anemometry in the fuel cell's anode outlet. It was theoretically shown that the electrical signal obtained from the hot wire sensor can be directly converted into the fuel cell water balance. In this work an ex-situ experimental investigation is performed to examine the effect of the wire diameter and the outlet pipe diameter on the voltage signal. For a laboratory fuel cell where the mass flow rate the anode outlet is small, it is found important to use a small output pipe diameter to obtain a sufficiently strong convection effect and hence clear voltage readings. Depending on the hot wire diameter and the inner pipe diameter, the resulting values for the exponent of the Reynolds number Re in the determination of the Nusselt number Nu range between m = 0.267 and m = 0.329. In general, it is shown that applying hot wire Anemometry yields in fact very clear voltage readings with high frequency, and it can be used as a diagnosis tool in various fuel cell applications.
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Applying Hot-Wire Anemometry to directly measure the water balance in a proton exchange membrane fuel cell for a pre-humidified hydrogen stream
International Journal of Hydrogen Energy, 2016Co-Authors: Torsten Berning, Saher Al ShakhshirAbstract:Abstract In a recent publication it has been shown how the water balance in a proton exchange membrane fuel cell can be determined employing hot wire Anemometry. The hot wire sensor has to be placed into the anode outlet pipe of the operating fuel cell, and the voltage signal E that is read from the sensor has to be divided by a pre-determined voltage signal E0 that has been obtained for a stream of dry hydrogen where the molar flow rate corresponds to a total current I of the fuel cell stack and a stoichiometric flow ratio, ξ. Because the last two properties are usually continuously known in fuel cell experiments, E0 is also continuously known. There is a one-to-one correlation between the relative voltage signal E/E0 and the fuel cell water balance, and therefore the fuel cell water balance can be a continuous output signal similar to the fuel cell voltage and the high frequency resistance. This method was originally believed to be limited to the fuel cell anode operating on dry hydrogen. In the current work, it is expanded for the case of a pre-humidified hydrogen stream. In addition, useful correlations are derived that link the fuel cell water balance with the anode side inlet and outlet thermodynamic state. Finally, it will be shown how previously developed dew point diagrams for the anode side in a fuel cell can be corrected for a humidified hydrogen inlet stream.
Torsten Berning - One of the best experts on this subject based on the ideXlab platform.
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Applying Hot-Wire Anemometry to directly measure the water balance in a proton exchange membrane fuel cell – Part 2: Experimental
International Journal of Hydrogen Energy, 2016Co-Authors: Saher Al Shakhshir, Soren Juhl Andreasen, Torsten BerningAbstract:In order to better understand and more accurately measure the water balance in a proton exchange membrane fuel cell our group has recently proposed to apply hot wire Anemometry in the fuel cell's anode outlet. It was theoretically shown that the electrical signal obtained from the hot wire sensor can be directly converted into the fuel cell water balance. In this work an ex-situ experimental investigation is performed to examine the effect of the wire diameter and the outlet pipe diameter on the voltage signal. For a laboratory fuel cell where the mass flow rate the anode outlet is small, it is found important to use a small output pipe diameter to obtain a sufficiently strong convection effect and hence clear voltage readings. Depending on the hot wire diameter and the inner pipe diameter, the resulting values for the exponent of the Reynolds number Re in the determination of the Nusselt number Nu range between m = 0.267 and m = 0.329. In general, it is shown that applying hot wire Anemometry yields in fact very clear voltage readings with high frequency, and it can be used as a diagnosis tool in various fuel cell applications.
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Applying Hot-Wire Anemometry to directly measure the water balance in a proton exchange membrane fuel cell for a pre-humidified hydrogen stream
International Journal of Hydrogen Energy, 2016Co-Authors: Torsten Berning, Saher Al ShakhshirAbstract:Abstract In a recent publication it has been shown how the water balance in a proton exchange membrane fuel cell can be determined employing hot wire Anemometry. The hot wire sensor has to be placed into the anode outlet pipe of the operating fuel cell, and the voltage signal E that is read from the sensor has to be divided by a pre-determined voltage signal E0 that has been obtained for a stream of dry hydrogen where the molar flow rate corresponds to a total current I of the fuel cell stack and a stoichiometric flow ratio, ξ. Because the last two properties are usually continuously known in fuel cell experiments, E0 is also continuously known. There is a one-to-one correlation between the relative voltage signal E/E0 and the fuel cell water balance, and therefore the fuel cell water balance can be a continuous output signal similar to the fuel cell voltage and the high frequency resistance. This method was originally believed to be limited to the fuel cell anode operating on dry hydrogen. In the current work, it is expanded for the case of a pre-humidified hydrogen stream. In addition, useful correlations are derived that link the fuel cell water balance with the anode side inlet and outlet thermodynamic state. Finally, it will be shown how previously developed dew point diagrams for the anode side in a fuel cell can be corrected for a humidified hydrogen inlet stream.