Momentum Flux Measurements Research Articles

Shadowing effects on multi-step Langmuir probe array on HL-2A tokamak

Multi-step Langmuir probe arrays have been designed and installed on the HL-2A tokamak [1]–[2] to study the turbulent transport in the edge plasma, especially for the measurement of poloidal momentum flux, Reynolds stress Rs. However, except the probe tips on the top step, all other tips on lower steps are shadowed by graphite skeleton. It is necessary to estimate the shadowing effects on equilibrium and fluctuation measurement. In this paper, comparison of shadowed tips to unshadowed ones is presented. The results show that shadowing can strongly reduce the ion and electron effective collection area. However, its effect is negligible for the turbulence intensity and coherence measurement, confirming that the multi-step LP array is proper for the turbulent transport measurement.

A comparison of computational fluid dynamics predicted initial liquid penetration using rate of injection profiles generated using two different measurement techniques

The rate of injection profile is a key parameter describing the fuel injection process for diesel injection. It is also an essential input parameter for computational fluid dynamics simulations of spray flows. In the present work, rate of injection profiles of a multi-hole diesel injector were measured using the Zeuch method and the momentum flux method. The rate of injection profiles measured by the momentum flux method had a faster rise in rate of injection during the initial ramp-up phase than with the Zeuch method. The measured rate of injection profiles were applied in three-dimensional computational fluid dynamics simulations of diesel sprays under non-vaporizing and vaporizing conditions with sweeps in injection pressure, bulk charge gas density, and bulk charge gas temperature. Analytical results were compared against experimental data for liquid penetration generated under those conditions. Computational fluid dynamics results with the rate of injection profile measured by the Zeuch method under-predict liquid penetration during the initial ramp-up phase, while computational fluid dynamics results with the rate of injection profiles measured by the momentum flux method showed much better agreement with the experimental data of liquid length and penetration. This suggests that current computational fluid dynamics spray models may be able to more accurately model transient liquid penetration when using the velocity profile developed from momentum flux measurements. Further study is needed to evaluate how computational fluid dynamics predictions of combustion and emissions of affected when using these two rate of injection profiles.

Numerical and experimental analysis of the spray momentum flux measuring on a GDI injector

In direct injection combustion systems, the spray momentum flux measurement can provide significant insight in the fuel jet development and the in-cylinder fuel-air mixing potential. The spray momentum flux can be determined by means of the impact force method, which was proved to be completely consistent when the basic measuring hypotheses are fulfilled, i.e. during the steady phase of the injection process. Conversely, in the transients the measurement technique details can significantly affect the results. An appropriate analysis of the possible effects exerted by the experimental setup is thus mandatory. In order to deepen the knowledge about this measurement technique applied to GDI (Gasoline Direct Injection) systems and to support the design of the experiment, in this paper a CFD 3-D model of a single-hole, research injector was developed and assessed with experimental data in terms of spray penetration curve, overall shape and droplet sizing and velocity.The validated numerical tool was then used to simulate the spray momentum measuring procedure in order to investigate the possible effects of the experimental set-up details on the momentum flux results. To this end, numerical results were compared to experimental data for different values of the main measurement parameters: target size, nozzle/target distance, discharge ambient pressure. This analysis confirmed both the experimental procedure sensitivity to some setup details and the model ability to capture the spray-target interaction phenomenon, supporting the need of a combined use of numerical and experimental approaches to obtain an adequate insight in the spray evolution.

Numerical simulation of injection rate of each nozzle hole of multi-hole diesel injector

The relative differences in injection rates within nozzle holes of multi-hole diesel injectors significantly influences the combustion and emission characteristics of diesel engines. To study systematically the injection rate of each nozzle hole of a multi-hole diesel injector, a three-dimensional gas-liquid two-phase model of cavitation flow was developed, taking the influence of injection conditions on bubble number density into consideration. To verify validity of the model, the injection rate of each nozzle hole of the injector was experimentally measured on a fuel injection system based on the transient measurement of spray momentum flux. The compared results of the measured and simulated injection rates of each nozzle hole shows that the developed model has relatively high precision and can be used to simulate the injection rate of each nozzle hole accurately.

Solids flux measurements via alternate techniques in a gas-fluidized bed

A transportable fluidization column, operating under identical conditions at three different locations, was employed to compare three experimental solids flux measurement techniques for hydrodynamic characterization of gas-fluidized beds. This paper compares measurements of solids mass and momentum flux obtained by radioactive particle tracking at the Ecole Polytechnique, positron emission particle tracking at University of Birmingham, and borescopic high speed particle image velocimetry at PSRI, carried out with FCC particles of mean diameter 107μm. These techniques provided broadly similar time-average solids flux profiles, but there were significant quantitative differences. Analysis of the results, focusing on the fundamentals of each measurement technique, provides valuable insights into the reasons for the discrepancies. The results also add to a unique hydrodynamic database for validation of CFD and other models.

The effect of nozzle geometry over internal flow and spray formation for three different fuels

The influence of internal nozzle flow characteristics over macroscopic spray development is studied experimentally for two different nozzle geometries and three fuels. The measurements include a complete hydraulic characterization consisting of instantaneous injection rate and spray momentum flux measurements, followed by a high-speed visualization of isothermal liquid spray in combination with cylindrical and conical nozzle configurations. Two of the fuels are pure components—n-heptane and n-dodecane—while the third fuel consists of a three-component surrogate to better represent the physical and chemical properties of diesel fuel. The cylindrical nozzle with 8.6% larger diameter, in spite of higher mass flow rate and momentum flux, shows slower spray tip penetration when compared to the conical nozzle. The spreading angle is found to be inversely proportional to the spray tip penetration. The spreading angle is largely influenced by the nozzle geometry and the ambient density. Rail pressure was found to have weak influence on the near-field spreading angle and no influence on the standard deviation of the spreading angle. n-Heptane spray shows slowest penetration rates while n-dodecane and the surrogate fuel mixture show very similar spray behavior for variations in injection pressure and back pressure. However, the surrogate fuel mixture shows higher penetration than n-dodecane when using the conical nozzle and lower penetration than n-dodecane when using cylindrical nozzle.

Measurements and analyses on the transient discharge coefficient of each nozzle hole of multi-hole diesel injector

The objective of this paper is to propose a measuring method based on the spray momentum flux measurement of each nozzle hole that could be used to determine the transient discharge coefficient of each nozzle hole of a multi-hole diesel injector. For this purpose, a measurement system for the transient discharge coefficient of each nozzle hole was established utilizing a conventional injection system of pump-line-nozzle and a dedicated constructed experimental rig. By measuring the spray momentum flux of each nozzle hole of the multi-hole fuel injector and injection pressure of the pump-pipe-injector fuel delivery system, the discharge coefficient of each nozzle hole was obtained, and analyzed throughout the injection duration and with different injection pump speeds and cycle fuel injection quantities. The results show that the transient discharge coefficient of each nozzle hole changes constantly, meanwhile, the variations of the transient discharge coefficient of the nozzle holes were similar. However, the discharge coefficient of the nozzle holes were not uniform at the same operating condition and that of the No.5 was apparently lower than that of the others. With increasing cam speed, the fluctuations of the discharge coefficients were variably stable at the maximum needle position.The fluctuations of the discharge coefficients of the nozzle holes were higher for the smaller cycle fuel injection quantity, but as the fluctuation gradually became smaller, the mean discharge coefficient of each nozzle hole increased slightly with increasing cycle fuel injection quantity.

Wind Stress Dynamics in Chesapeake Bay: Spatiotemporal Variability and Wave Dependence in a Fetch-Limited Environment

AbstractThe spatiotemporal variability of wind stress dynamics in Chesapeake Bay has been investigated using a combination of observations and numerical modeling. Direct measurements of momentum and surface heat fluxes were collected using an ultrasonic anemometer deployed on a fixed tower in the middle reaches of Chesapeake Bay in the spring of 2012 along with collocated wave measurements. These measurements were compared to bulk estimates of wind stress using wave-dependent formulations of the Charnock parameter (alpha). Results indicate that a constant alpha value of 0.018 reasonably represents observed stress values, but estimates can be improved by the inclusion of surface wave information in the parameterization of alpha. Using a wave age formulation of alpha in combination with an optimally interpolated 10-m neutral wind field, a third-generation numerical wave model, Simulating Waves Nearshore (SWAN), was employed to investigate the spatiotemporal variability of wind stress across the estuary. Alpha values were found to be wind speed dependent and displayed spatial distributions that ranged between open-ocean values and strongly fetch-limited values. Model results suggest that variable wind stress dynamics stemming from a combination of variable surface winds and fetch-limited wave growth may result in the 10-m neutral drag coefficient varying by a factor of 2 across the estuary. Up to 20% of these changes can be directly attributed to the effects of variable waves.

Motion-correlated flow distortion and wave-induced biases in air–sea flux measurements from ships

Abstract. Direct measurements of the turbulent air–sea fluxes of momentum, heat, moisture and gases are often made using sensors mounted on ships. Ship-based turbulent wind measurements are corrected for platform motion using well established techniques, but biases at scales associated with wave and platform motion are often still apparent in the flux measurements. It has been uncertain whether this signal is due to time-varying distortion of the air flow over the platform or to wind–wave interactions impacting the turbulence. Methods for removing such motion-scale biases from scalar measurements have previously been published but their application to momentum flux measurements remains controversial. Here we show that the measured motion-scale bias has a dependence on the horizontal ship velocity and that a correction for it reduces the dependence of the measured momentum flux on the orientation of the ship to the wind. We conclude that the bias is due to experimental error and that time-varying motion-dependent flow distortion is the likely source.

Hydraulic characterization of Diesel and water emulsions using momentum flux

Diesel and water emulsions have the potential to be used in compression ignition engines to control the emissions of NOx and PM. Very little is known about the influence emulsification will have on the fuel sprays formed during injection. This paper outlines the measurement of the momentum flux of injection sprays of Diesel fuel and Diesel fuel emulsions containing 10% and 20% water, with the goal of hydraulically characterizing the sprays and identifying the influence emulsification may have on them. The momentum flux, mass flow, instantaneous mass flow, discharge coefficient, injection velocity, momentum coefficient and momentum efficiency have been examined. The injections were carried out in a high pressure chamber filled with nitrogen. The measured momentum flux is observed to increase with increasing injection pressure in a linear form. Increasing the ambient density in the chamber resulted in a decrease in the measured momentum flux. The emulsified fuel sprays had a very similar momentum flux as the neat Diesel fuel sprays. The total mass of emulsified fuel injected was less than for neat Diesel at corresponding condition. The instantaneous mass flow rate was determined using a normalized form of the momentum flux measurement and the independently measured total mass injected. The emulsions tended to have a lower discharge coefficient and there is no evidence that the nozzle is cavitating at these conditions. The emulsified fuels have tended to have a higher injection velocity than the neat Diesel fuel sprays. The momentum efficiency is introduced, which uses the instantaneous mass measurement and the theoretical velocity of the spray. The emulsified fuels have a larger momentum efficiency, a result of their high injection velocity compared with the neat Diesel fuel.

Direct Flux Measurements from Mobile Platforms at Sea: Motion and Airflow Distortion Corrections Revisited

AbstractShip-based measurements of wind speed and direct fluxes are affected by airflow distortion that can lead to a tilt of the wind vector as well as acceleration or deceleration of the wind speed. Direct flux measurements are additionally affected by the fluctuating velocity of the platform. The classic approach is to first correct the wind speed for angular and translational platform velocities and thereafter rotate the wind vector into the mean flow. This study finds that for ships under way, this leads to an overestimation of the vector tilt and biased flux estimates. This may explain the common observation that flux estimates from ships in transit have lower quality than measurements taken on station. Here an alternative approach is presented, where the flow-distortion-induced tilt of the wind vector is estimated from the 3D wind speed measurements and applied to the apparent wind vector. The tilt correction is carried out after correction for the fluctuating part of the platform velocity but before removing the ship’s mean translational velocity. This new method significantly improved the agreement of direct momentum flux measurements made from a ship under way with the parameterization of the COARE3.5 bulk model. The sensitivity of the eddy covariance measurements of momentum and scalar fluxes to the choice of the tilt-motion correction method is analyzed, and this study proposes that a reanalysis of previous direct flux measurements with the new method discussed here can improve researchers’ understanding of air–sea interaction.

In situ Measurements of Momentum Fluxes in Typhoons

Abstract One of the scientific objectives of the U.S. Office of Naval Research–sponsored Impact of Typhoons on the Ocean in the Pacific (ITOP) campaign was improved understanding of air–sea fluxes at high wind speeds. Here the authors present the first-ever direct measurements of momentum fluxes recorded in typhoons near the surface. Data were collected from a moored buoy over 3 months during the 2010 Pacific typhoon season. During this period, three typhoons and a tropical storm were encountered. Maximum 30-min sustained wind speeds above 26 m s−1 were recorded. Data are presented for 1245 h of direct flux measurements. The drag coefficient shows evidence of a rolloff at wind speeds greater than 22 m s−1, which occurred during the passage of a single typhoon. This result is in agreement with other studies but occurs at a lower wind speed than previously measured. The authors conclude that this rolloff was caused by a reduction in the turbulent momentum flux at the frequency of the peak waves during strongly forced conditions.

Development and Testing of Instrumentation for UAV-Based Flux Measurements within Terrestrial and Marine Atmospheric Boundary Layers

Abstract Instrumentation packages have been developed for small (18–28 kg) unmanned aerial vehicles (UAVs) to measure momentum fluxes as well as latent, sensible, and radiative heat fluxes in the atmospheric boundary layer (ABL) and the topography below. Fast-response turbulence, hygrometer, and temperature probes permit turbulent momentum and heat flux measurements, and shortwave and longwave radiometers allow the determination of net radiation, surface temperature, and albedo. UAVs flying in vertical formation allow the direct measurement of fluxes within the ABL and, with onboard high-resolution visible and infrared video and laser altimetry, simultaneous observation of surface topography or ocean surface waves. The low altitude required for accurate flux measurements (typically assumed to be 30 m) is below the typical safety limit of manned research aircraft; however, with advances in laser altimeters, small-aircraft flight control, and real-time kinematic differential GPS, low-altitude flight is now within the capability of small UAV platforms. Flight tests of instrumented BAE Systems Manta C1 UAVs over land were conducted in January 2011 at McMillan Airfield (Camp Roberts, California). Flight tests of similarly instrumented Boeing Insitu ScanEagle UAVs were conducted in April 2012 at the Naval Surface Warfare Center, Dahlgren Division (Dahlgren, Virginia), where the first known measurements of water vapor, heat, and momentum fluxes were made from low-altitude (down to 30 m) UAV flights over water (Potomac River). This study presents a description of the instrumentation, summarizes results from flight tests, and discusses potential applications of these UAVs for (marine) atmospheric boundary layer studies.

Improved analysis of all-sky meteor radar measurements of gravity wave variances and momentum fluxes

Abstract. The advantages of using a composite day analysis for all-sky interferometric meteor radars when measuring mean winds and tides are widely known. On the other hand, problems arise if this technique is applied to Hocking's (2005) gravity wave analysis for all-sky meteor radars. In this paper we describe how a simple change in the procedure makes it possible to use a composite day in Hocking's analysis. Also, we explain how a modified composite day can be constructed to test its ability to measure gravity wave momentum fluxes. Test results for specified mean, tidal, and gravity wave fields, including tidal amplitudes and gravity wave momentum fluxes varying strongly with altitude and/or time, suggest that the modified composite day allows characterization of monthly mean profiles of the gravity wave momentum fluxes, with good accuracy at least at the altitudes where the meteor counts are large (from 89 to 92.5 km). In the present work we also show that the variances measured with Hocking's method are often contaminated by the tidal fields and suggest a method of empirical correction derived from a simple simulation model. The results presented here greatly increase our confidence because they show that our technique is able to remove the tide-induced false variances from Hocking's analysis.

Flow and turbulence in an industrial/suburban roughness canopy

A field study conducted to investigate the flow and turbulence structure of the urban boundary layer (UBL) over an industrial/suburban area is described. The emphasis was on morning and evening transition periods, but some measurements covered the entire diurnal cycle. The data analysis incorporated the dependence of wind direction on morphometric parameters of the urban canopy. The measurements of heat and momentum fluxes showed the possibility of a constant flux layer above the height $$z\approx 2{H}$$ , wherein the Monin-Obukhov Similarity Theory (MOST) is valid; here $$H$$ is the averaged building height. For the nocturnal boundary layer, the mean velocity and temperature profiles obeyed classical MOST scaling up to $$\sim 0.5\Lambda \left( {\sim 6{H}}\right) $$ , where $$\Lambda $$ is the Obukhov length scale, beyond which stronger stratification may disrupt the occurrence of constant fluxes. For unstable and neutral cases, MOST scaling described the mean data well up to the maximum measured height $$(\sim 6{H})$$ . Available MOST functions, however, could not describe the measured turbulence structure, indicating the influence of additional governing parameters. Alternative turbulence parameterizations were tested, and some were found to perform well. Calculation of integral length scales for convective and neutral cases allowed a phenomenological description of eddy characteristics within and above the urban canopy layer. The development of a significant nocturnal surface inversion occurred only on certain days, for which a criterion was proposed. The nocturnal UBL exhibited length scale relationships consistent with the evening collapse of the convective boundary layer and maintenance of buoyancy-affected turbulence overnight. The length and velocity scales so identified are useful in parameterizing turbulent dispersion coefficients in different diurnal phases of the UBL.

Assessment of gravity wave momentum flux measurement capabilities by meteor radars having different transmitter power and antenna configurations

Measurement capabilities of five meteor radars are assessed and compared to determine how well radars having different transmitted power and antenna configurations perform in defining mean winds, tidal amplitudes, and gravity wave (GW) momentum fluxes. The five radars include two new‐generation meteor radars on Tierra del Fuego, Argentina (53.8°S) and on King George Island in the Antarctic (62.1°S) and conventional meteor radars at Socorro, New Mexico (34.1°N, 106.9°W), Bear Lake Observatory, Utah (∼41.9°N, 111.4°W), and Yellowknife, Canada (62.5°N, 114.3°W). Our assessment employs observed meteor distributions for June of 2009, 2010, or 2011 for each radar and a set of seven test motion fields including various superpositions of mean winds, constant diurnal tides, constant and variable semidiurnal tides, and superposed GWs having various amplitudes, scales, periods, directions of propagation, momentum fluxes, and intermittencies. Radars having higher power and/or antenna patterns yielding higher meteor counts at small zenith angles perform well in defining monthly and daily mean winds, tidal amplitudes, and GW momentum fluxes, though with expected larger uncertainties in the daily estimates. Conventional radars having lower power and a single transmitting antenna are able to describe monthly mean winds and tidal amplitudes reasonably well, especially at altitudes having the highest meteor counts. They also provide reasonable estimates of GW momentum fluxes at the altitudes having the highest meteor counts; however, these estimates are subject to uncertainties of ∼20 to 50% and uncertainties rapidly become excessive at higher and lower altitudes. Estimates of all quantities degrade somewhat for more complex motion fields.

Experimental and numerical momentum flux evaluation of high pressure Diesel spray

In the present paper, a detailed numerical and experimental investigation of high pressure Diesel spray evolution is reported. The analysis is mainly focused on the spray momentum flux measurement, which is increasingly used to characterise both the in-nozzle flow in terms of cavitation intensity and the flow uniformity among the different jets emerging from the same nozzle. In the present study, the potentially critical aspects of the spray momentum flux measurement methodology based on the impact force on a flat target have been investigated comparing CFD-3D results and experimental data pertaining to a wide range of operating conditions in terms of injection pressure, energizing time and back-pressure. The available experimental data, pertaining to a commercial common-rail injector, include the spray momentum flux time-histories and steady values, the injection rate and the spray imaging during momentum flux tests and free-jet evolution. Globally, a good agreement was found among experimental and numerical momentum flux histories; furthermore, CFD allowed to evaluate the significant contribution of the spray gaseous phase to the measured momentum flux, which rapidly tends to become predominant as the measurement station distance from the nozzle exit increases. The obtained results also suggest that some details of the measurement system can significantly affect the spray momentum detection accuracy. Hence, a detailed numerical analysis of the spray impact force was developed to evaluate the target intrusiveness, pointing out the irrelevance in terms of momentum flux time-history of the target presence into the simulated domain with respect to the free jet evolution. A practical strategy to define the optimal combination of target-diameter and nozzle–target distance, for each set of operating conditions, as a compromise between the extreme configurations of the involved parameters, is discussed.

A contribution to the understanding of cavitation effects in Diesel injector nozzles through a combined experimental and computational investigation

In this paper a combined experimental and computational study was carried out in order to assess the ability of a homogeneous equilibrium model in predicting the experimental behaviour observed from the hydraulical characterization of a nozzle. The nozzle used was a six-orifice microsac nozzle, with cylindrical holes, and therefore inclined to cavitate. The experimental results available for the validation purpose comprised measurements of mass flow rate and spray momentum flux, which correctly combined provide also fundamental information such as discharge coefficient, nozzle exit effective velocity and area contraction. The model was proved to be able of reproducing the experimental results with high degree of confidence and, through the exploration of the internal flow, allowed the explanation of widely reported experimental findings related to cavitation phenomena: the mass flow choking induced by cavitation and the increment of effective injection velocity.

Drake Antarctic Agile Meteor Radar first results: Configuration and comparison of mean and tidal wind and gravity wave momentum flux measurements with Southern Argentina Agile Meteor Radar

A new generation meteor radar was installed at the Brazilian Antarctic Comandante Ferraz Base (62.1°S) in March 2010. This paper describes the motivations for the radar location, its measurement capabilities, and comparisons of measured mean winds, tides, and gravity wave momentum fluxes from April to June of 2010 and 2011 with those by a similar radar on Tierra del Fuego (53.8°S). Motivations for the radars include the “hotspot” of small‐scale gravity wave activity extending from the troposphere into the mesosphere and lower thermosphere (MLT) centered over the Drake Passage, the maximum of the semidiurnal tide at these latitudes, and the lack of other MLT wind measurements in this latitude band. Mean winds are seen to be strongly modulated at planetary wave and longer periods and to exhibit strong coherence over the two radars at shorter time scales as well as systematic seasonal variations. The semidiurnal tide contributes most to the large‐scale winds over both radars, with maximum tidal amplitudes during May and maxima at the highest altitudes varying from ∼20 to >70 ms−1. In contrast, the diurnal tide and various planetary waves achieve maximum winds of ∼10 to 20 ms−1. Monthly mean gravity wave momentum fluxes appear to reflect the occurrence of significant sources at lower altitudes, with relatively small zonal fluxes over both radars, but with significant, and opposite, meridional momentum fluxes below ∼85 km. These suggest gravity waves propagating away from the Drake Passage at both sites, and may indicate an important source region accounting in part for this “hotspot.”

On the 11 year solar cycle signature in global total ozone dynamics

AbstractThe main aim of the present study is to investigate further the association between total ozone (TOZ) and the 11 year solar cycle (SC) during the period 1979–2010, by employing satellite observations of TOZ made by Nimbus‐7, Meteor‐3, Earth Probe Total Ozone Mapping Spectrometer (TOMS) and Ozone Monitoring Instrument (OMI) instrumentation. A statistically significant correlation between the annual mean TOZ over both hemispheres and sunspot number (SN) is found. On the contrary, focusing on the January and February mean monthly TOZ fluctuations from the equator to the high latitudes of the Northern Hemisphere, no association between TOZ and SN is derived. This is attributed to the existence of the quasi‐biennial‐oscillation (QBO) and the El Niño‐Southern oscillation (ENSO) in the TOZ time series. The latter oscillation is herewith expressed by the recently introduced Ozone ENSO Index (OEI). However, when considering the TOZ zonal means centred at 17.5 and 27.5°N during the east phase years of the QBO in the equatorial zonal wind at 50 hPa, a statistically significant correlation between TOZ and SN is revealed. It is an indication that the quasi‐periodic fluctuations (i.e. QBO, ENSO) strongly contaminate the relationship between TOZ and solar activity. Plausible mechanisms are discussed, exploring the momentum flux (MF) measurements between 45 and 75°N, in the periods of increased dynamical variability. The findings obtained point to the conclusion that the 11 year solar cycle response in TOZ is caused by dynamical changes which are caused by solar activity. These are of crucial importance because solar radiation is a major driving force of the climate system. Copyright © 2012 Royal Meteorological Society