Quasi-laminar stability and sensitivity analyses for turbulent flows: Prediction of low-frequency unsteadiness and passive control

2014 ◽  
Vol 26 (4) ◽  
pp. 045112 ◽  
Author(s):  
Clément Mettot ◽  
Denis Sipp ◽  
Hervé Bézard
Keyword(s):  
Turbulent Flows ◽  
Passive Control ◽  
Low Frequency ◽  
Sensitivity Analyses

Cardiovascular risk in 159 934 frequent blood donors while addressing the healthy donor effect

Heart ◽  
2019 ◽  
Vol 105 (16) ◽  
pp. 1260-1265 ◽  
Author(s):  
Karlijn Peffer ◽  
Martin den Heijer ◽  
Wim L A M de Kort ◽  
André L M Verbeek ◽  
Femke Atsma
Keyword(s):  
Cardiovascular Risk ◽  
High Frequency ◽  
Healthy Donor ◽  
Blood Donors ◽  
Cox Regression ◽  
Blood Donation ◽  
Low Frequency ◽  
Sensitivity Analyses ◽  
Hospital Data ◽  
Dutch Hospital

ObjectiveTo investigate whether regular blood donation decreases cardiovascular risk.MethodsAll 159 934 Dutch whole-blood donors with an active donation career of at least 10 years were categorised into sex-specific donation tertiles based on the number of donations during this 10-year qualification period. Cardiovascular endpoints were based on hospital discharge diagnoses and death certificates from Dutch Hospital Data and Statistics Netherlands and occurring after the 10-year qualification period. Cox regression was used to estimate the age-adjusted and starting year-adjusted hazard rate ratio (HRR).ResultsFemale high-frequency blood donors had a reduced cardiovascular morbidity (HRR=0.91, 95% CI 0.85 to 0.98) compared with low-frequency blood donors. No effect was observed in men (HRR=1.00, 95% CI 0.95 to 1.05). To rule out a residual healthy donor effect (HDE), additional sensitivity analyses using a 5-year qualification period were conducted. The results supported the absence of a residual HDE.ConclusionsThis study showed a protective effect of long-term, high-frequency blood donation against cardiovascular disease. This effect was only observed in women and not in men.

Estimating Zonal Flow Contributions in Deep Water Assets From Pressure and Temperature Data

2017 ◽  
Author(s):  
A. Rashid Hasan ◽  
Rayhana N. Sohel ◽  
Xiaowei Wang
Keyword(s):  
Turbulent Flows ◽  
Deep Water ◽  
Fluid Pressure ◽  
Zonal Flow ◽  
Temperature Data ◽  
Sensitivity Analyses ◽  
Momentum Balance ◽  
Flow Profile ◽  
Flowing Fluid ◽  
Energy And Momentum

Producing hydrocarbon from deep water assets is extremely challenging and expensive. A good estimate of rates from multiple pay zones is essential for well monitoring, surveillance, and workover decisions. Such information can be gleaned from flowing fluid pressure and temperature; deep-water wells are often well instrumented that offers such data on a continuous basis. In this study a model is presented that estimates zonal flow contributions based on energy and momentum balances. Kinetic and heat energy coming from the reservoir fluid to the production tubing is accounted for in the model. The momentum balance for wellbore takes into account differing flow profile in laminar and turbulent flows. In addition, when sandface temperature data are not available, a recently developed analytical model to estimate the effect of Joule-Thompson expansion on sandface temperature was used to estimate sandface temperature from reservoir temperature. The model developed can be applied to any reservoir with multiple pay zones and is especially useful for deep-water assets where production logging is practically impossible. Available field data for multiphase flow was used to validate the model. Sensitivity analyses were performed that showed accurate temperature data is essential for the model to estimate zonal contribution accurately.

A Three-Equation Eddy-Viscosity Model for Reynolds-Averaged Navier–Stokes Simulations of Transitional Flow

2008 ◽  
Vol 130 (12) ◽  
Author(s):  
D. Keith Walters ◽  
Davor Cokljat
Keyword(s):  
Boundary Layer ◽  
Turbulent Flows ◽  
Eddy Viscosity ◽  
Flow Behavior ◽  
Applied Pressure ◽  
Plate Boundary ◽  
Low Frequency ◽  
Transitional Flow ◽  
Test Cases ◽  
Eddy Viscosity Model

An eddy-viscosity turbulence model employing three additional transport equations is presented and applied to a number of transitional flow test cases. The model is based on the k-ω framework and represents a substantial refinement to a transition-sensitive model that has been previously documented in the open literature. The third transport equation is included to predict the magnitude of low-frequency velocity fluctuations in the pretransitional boundary layer that have been identified as the precursors to transition. The closure of model terms is based on a phenomenological (i.e., physics-based) rather than a purely empirical approach and the rationale for the forms of these terms is discussed. The model has been implemented into a commercial computational fluid dynamics code and applied to a number of relevant test cases, including flat plate boundary layers with and without applied pressure gradients, as well as a variety of airfoil test cases with different geometries, Reynolds numbers, freestream turbulence conditions, and angles of attack. The test cases demonstrate the ability of the model to successfully reproduce transitional flow behavior with a reasonable degree of accuracy, particularly in comparison with commonly used models that exhibit no capability of predicting laminar-to-turbulent boundary layer development. While it is impossible to resolve all of the complex features of transitional and turbulent flows with a relatively simple Reynolds-averaged modeling approach, the results shown here demonstrate that the new model can provide a useful and practical tool for engineers addressing the simulation and prediction of transitional flow behavior in fluid systems.

Local and global fluid-flow effects on dipole flexural waves

Geophysics ◽  
2020 ◽  
Vol 85 (1) ◽  
pp. D1-D11
Author(s):  
Elliot J. H. Dahl ◽  
Kyle T. Spikes
Keyword(s):  
Fluid Flow ◽  
Low Frequency ◽  
Summation Method ◽  
Sensitivity Analyses ◽  
Flexural Wave ◽  
Biot Theory ◽  
S Wave ◽  
Local Flow ◽  
Flow Effects ◽  
Wave Induced

Wave-induced fluid flow (WIFF) can significantly alter the effective formation velocities and cause increasing waveform dispersion and attenuation. We have used modified frame moduli from the theory of Chapman together with the classic Biot theory to improve the understanding of local- and global-flow effects on dipole flexural wave modes in boreholes. We investigate slow and fast formations with and without compliant pores, which induce local flow. The discrete wavenumber summation method generates the waveforms, which are then processed with the weighted spectral semblance method to compare with the solution of the period equation. We find compliant pores to decrease the resulting effective formation P- and S-wave velocities, that in turn decrease the low-frequency velocity limit of the flexural wave. Furthermore, depending on the frequency at which the local-flow dispersion occurs, different S-wave velocity predictions from the flexural wave become possible. This issue is investigated through changing the local-flow critical frequency. Sensitivity analyses of the flexural-wave phase velocity to small changes in WIFF parameters indicate the modeling to be mostly sensitive to compliant pores in slow and fast formations.

Development of an Analog Controller for Tuning an Adaptive-Passive Control Device

2005 ◽  
Author(s):  
Marty Johnson ◽  
Edward C. Diggs
Keyword(s):  
Passive Control ◽  
Low Frequency ◽  
Helmholtz Resonator ◽  
Dc Motor ◽  
Control Device ◽  
Inner Product ◽  
Variable Cross ◽  
Error Signal ◽  
Cross Sectional ◽  
Drive Frequency

Adaptive-passive devices such as adaptive Helmholtz Resonators (HR) and tunable vibration absorbers have been shown to be suitable for controlling both narrowband disturbances and lightly damped structural/acoustic modes driven by broadband disturbances. In order to track changes in the disturbance or changes in the modes, the natural frequency of the absorber, ωn, is tuned to match the observed signals. This is achieved by altering some physical parameter of the control device such as the stiffness of a vibration absorber or the neck cross-sectional area of a Helmholtz resonator. In order to automatically adjust these devices, control systems and tuning algorithms have been developed, most of which involve a digital controller. However, this paper looks specifically at the development of a simple analog controller used to drive a DC motor in order to tune a mechanical device. A two sensor dot product method is employed where one sensor is placed inside of the control device, such as a Helmholtz Resonator, and the other on/in the system under control, such as in a room. The outputs from the two sensors are multiplied together and subsequently low passed in order to extract a low frequency “DC” voltage which acts as an error signal. The error signal is related to the relative phase of the two sensor signals and determines the direction in which the device should be tuned. When the two signals are 90° apart, the system is tuned (i.e. the inner product produces zero DC level). If the drive frequency ω is different than the tuned frequency, then the system is mis-tuned. The relationship between the mis-tuning, ωn-ω, and the error is not linear, but for small perturbations a linear approximation can be used to investigate the stability and performance of the system. The gradient of the function is shown to be largest when the mis-tuning error is zero and is inversely proportional to the damping level in the control device. Once stability of the system has been ensured the ability of the system to track changes in drive frequency is investigated experimentally. The control system is demonstrated using an adaptive Helmholtz resonator which has a variable cross-sectional neck via an iris diaphragm. The iris is controlled using a small DC motor; two microphones (one mounted internally and one externally) are used to supply the driving signal to the circuit.

Analysis of Unsteady Compressible Viscous Layers

1991 ◽  
Vol 113 (4) ◽  
pp. 644-653 ◽  
Author(s):  
G. D. Power ◽  
J. M. Verdon ◽  
K. A. Kousen
Keyword(s):  
Flat Plate ◽  
Turbulent Flows ◽  
Compressible Flows ◽  
Numerical Technique ◽  
Low Frequency ◽  
Nonlinear Effects ◽  
Asymptotic Results ◽  
Minimal Impact ◽  
Solution Proceeds ◽  
Unsteady Effects

The development of an analysis to predict the unsteady compressible flows in blade boundary layers and wakes is presented. The equations that govern the flows in these regions are transformed using an unsteady turbulent generalization of the Levy–Lees transformation. The transformed equations are solved using a finite difference technique in which the solution proceeds by marching in time and in the streamwise direction. Both laminar and turbulent flows are studied, the latter using algebraic turbulence and transition models. Laminar solutions for a flat plate are shown to approach classical asymptotic results for both high and low-frequency unsteady motions. Turbulent flat-plate results are in qualitative agreement with previous predictions and measurements. Finally, the numerical technique is also applied to the stator and rotor of a low-speed turbine stage to determine unsteady effects on surface heating. The results compare reasonably well with measured heat transfer data and indicate that nonlinear effects have minimal impact on the mean and unsteady components of the flow.

The use of a fibre anemometer in turbulent flows

1963 ◽  
Vol 16 (2) ◽  
pp. 269-281 ◽  
Author(s):  
D. J. Tritton
Keyword(s):  
Velocity Field ◽  
Turbulent Flows ◽  
Theoretical Study ◽  
Low Frequency ◽  
Frequency Theory ◽  
Background Information ◽  
Approximate Representation ◽  
Practical Situation ◽  
Quartz Fibre ◽  
The Mean

Quartz fibre anemometers have been used (as described in subsequent papers) to survey the velocity field of turbulent free convective air flows. This paper discusses the reasons for the choice of this instrument and provides the background information for its use in this way. Some practical points concerning fibre anemometers are mentioned. The rest of the paper is a theoretical study of the response of a fibre to a turbulent flow. An approximate representation of the force on the fibre due to the velocity field and the equation for a bending beam, representing the response to this force, form the basis of a consideration of the mean and fluctuating displacement of the fibre. Emphasis is placed on the behaviour when the spectrum of the turbulence is largely in frequencies low enough for the fibre to respond effectively instantaneously (as this corresponds to the practical situation). Incomplete correlation of the turbulence along the length of the fibre is taken into account. Brief mention is made to the theory of the higher-frequency (resonant) response in the context of an experimental check on the applicability of the low-frequency theory.

The generation of combustion noise by chemical inhomogeneities in steady, low-Mach-number duct flows

1980 ◽  
Vol 99 (2) ◽  
pp. 383-397 ◽  
Author(s):  
Y. L. Sinai
Keyword(s):  
Heat Transfer ◽  
Mach Number ◽  
Turbulent Flows ◽  
Low Frequency ◽  
Combustion Noise ◽  
Low Mach Number ◽  
First Order ◽  
Specific Heats ◽  
Chemically Reacting ◽  
Frequency Character

The low-frequency character of two model problems is exploited in order to illustrate the acoustic consequences of the interactions between chemically reacting (or relaxing) inhomogeneities and flames or constrictions in ducts. The monopole of the former is associated with heat transfer in a fluid which exhibits variations in its specific heats, while in the latter there is an extension of the classical phenomenon associated with the pulsations of an inhomogeneity of the fluid compressibility. This second mechanism is found to be insignificant, but the heat-conduction source is considered to be very powerful at sufficiently low Mach numbers; in fact, to first order it is independent of the flow Mach number for laminar, as well as a certain class of turbulent, flows.

scholarly journals Development of a High-Sensitivity Optical Accelerometer for Low-Frequency Vibration Measurement

Sensors ◽  
2018 ◽  
Vol 18 (9) ◽  
pp. 2910 ◽  
Author(s):  
Rui-Jun Li ◽  
Ying-Jun Lei ◽  
Zhen-Xin Chang ◽  
Lian-Sheng Zhang ◽  
Kuang-Chao Fan
Keyword(s):  
Passive Control ◽  
Low Cost ◽  
Low Frequency ◽  
High Sensitivity ◽  
Dynamic Responses ◽  
Vibration Measurement ◽  
Leaf Spring ◽  
Seismic Mass ◽  
Low Frequency Vibration ◽  
Low Amplitude

Low-frequency vibration is a harmful factor that affects the accuracy of micro/nano-measuring machines. Low-frequency vibration cannot be completely eliminated by passive control methods, such as the use of air-floating platforms. Therefore, low-frequency vibrations must be measured before being actively suppressed. In this study, the design of a low-cost high-sensitivity optical accelerometer is proposed. This optical accelerometer mainly comprises three components: a seismic mass, a leaf spring, and a sensing component based on a four-quadrant photodetector (QPD). When a vibration is detected, the seismic mass moves up and down due to the effect of inertia, and the leaf spring exhibits a corresponding elastic deformation, which is amplified by using an optical lever and measured by the QPD. Then, the acceleration can be calculated. The resonant frequencies and elastic coefficients of various seismic structures are simulated to attain the optimal detection of low-frequency, low-amplitude vibration. The accelerometer is calibrated using a homemade vibration calibration system, and the calibration experimental results demonstrate that the sensitivity of the optical accelerometer is 1.74 V (m·s−2)−1, the measurement range of the accelerometer is 0.003–7.29 m·s−2, and the operating frequencies range of 0.4–12 Hz. The standard deviation from ten measurements is under 7.9 × 10−4 m·s−2. The efficacy of the optical accelerometer in measuring low-frequency, low-amplitude dynamic responses is verified.

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