Effects Of Flow Distortion Research Articles

EFFECT OF FLOW DISTORTIONS ON PIN FINS IN THE ENTRY REGION OF CIRCULAR CHANNELS

Abstract Utilizing heat transfer augmentation features at the inlet of aircraft engines as an additional heat sink requires further understanding of entry region heat transfer and flow field interactions. The effect of asymmetric flow fields or distortions on entry region heat transfer has not been studied. Because the addition of augmentation features will create a near-wall distortion that can impact downstream aircraft components, comparing entry region features to standard distortion screens can provide insight on the performance impact to aircraft engines. This study investigates the effect of inlet distortion screens on pin fin entry region heat transfer, in order to understand if flow field disturbances disrupt the benefits of augmented entry region heat transfer. The superposition of the distortion caused by pin fin features onto a screen distorted smooth entry region is also tested. Three distortion screens are investigated with pin fin arrays, each based on SAE standard designs. Reynolds numbers ranging from 1.5*105 to 3.50*105 are investigated. From this study, inlet heat transfer augmentation due to pin fins is not significantly impacted by inlet distortion flowfields. This is due to the near wall boundary layer impact of augmentation features overshadowing the effect of distortion. Due to the insensitivity of the near wall boundary layer for pin fin arrays with distortions, superimposing the smooth entry region distorted flow with the non-distorted pin fin array flow provides an adequate estimate of the flow field of the pin fin array with a distorted inlet.

Field validation of a yaw misalignment observer for wind farm control

In this work, a load-based yaw misalignment observer was tested and validated with turbine and mast data collected during a wake steering and characterization campaign. A shallow feed-forward neural network was used to map the relation between the yaw misalignment and the in-and out-of-plane blade load harmonics for a 3.5 MW machine, and its performance was analysed over about 108 full days of useful data. Confirming previous findings, this simple neural network was able to accurately estimate the yaw misalignment, with an average 10-minute absolute error of at most 4°.The performance of the yaw misalignment observer was compared to the one of the standard onboard wind vane during the wake steering campaign. Results indicate that the wind vane may significantly overestimate the misalignment for large angles, possibly on account of the wake rotation and flow distortion effects caused by the nacelle. The observer on the other hand, sampling the flow at the rotor disk and not behind it, is not affected by such phenomena and could therefore provide a more accurate measurement of the misalignment angle, possibly improving the performance of wake steering. When the turbine is already equipped with load sensors, this is obtained without the need to install and maintain extra hardware, which instead is the case with spinner-mounted anemometers or lidars.

Measurements of Ice Crystal Fluxes from the Surface at a Mountain Top Site

New observations of anomalously high cloud ice crystal concentrations at the Jungfraujoch research station (Switzerland, 3.5 km a.s.l.) are presented. High-resolution measurements of these ice crystals using a high-speed 2D imaging cloud particle spectrometer confirm that the concentrations far exceed those expected from any known primary ice production mechanisms and are at temperatures well below those for known secondary ice production processes to contribute. The most likely explanation is due to a strong surface source generated by the interaction of turbulent deposition of supercooled droplets to fragile ice-covered snow surfaces. This process enhances the detachment of crystal fragments wherein the smaller size mode is turbulently re-suspended even at low wind speeds below expected blowing snow thresholds. These then continue to grow, adding significantly to the ice crystal number concentrations whose size and habit is determined by the transport time between the ice crystal source and measurement location and liquid water profile within the cloud. We confirm, using eddy covariance measurements of ice particle number fluxes, that the likely source is significantly far upwind to preclude flow distortion effects such that the source plume has homogenised by the time they are measured at the mountain top summit.

Effects of Inlet S-duct on Inducer Performance and Cavitation Instability

To investigate the effect of inlet flow distortion on two-bladed inducer performance and cavitation instability, the s-shaped duct was installed at the inlet of the inducer to induce secondary flow. Inducer cavitation performance with and without s-duct were compared while varying the cavitation number. Also, the effect of s-duct on the occurrence of alternate blade cavitation was investigated using unsteady casing pressure measurement and high-speed camera visualization. It was shown that the s-duct decreases the strength of alternate blade cavitation and lowers the head breakdown cavitation number without considerable head loss by reducing the tip leakage vortex cavitation.

Numerical study of the effect of Inlet paired Swirl on Performance and Stability of Axial compressor

In order to study the effect of inlet paired swirl distortion on the performance and stability of compressor, a full-annulus unsteady numerical simulation of the effects of swirl distortion on performance and stability of transonic rotor Rotor 37 is carried out in this paper. The result shows that: the rotor performance deteriorates and the stable working range decreases under the effect of vortex flow distortion; the circumferential side flow caused by vortex swirl distortion leads to the uneven circumferential distribution of flow parameters at the inlet and the outlet of the compressor; the rotor blade has a very significant attenuation effect on the swirl distortion, and the swirl distortion intensity decreases along the axial direction; the circumferentially inhomogeneous distribution of rotor angle causes excessive local aerodynamic load on the rotor, and high intensity leakage vortices and excitation vortices are generated at the blade tip. The circumferentially unevenly distributed swirl angle causes excessive local aerodynamic load of the rotor, resulting in high-intensity leakage vortex and shock wave interference in the blade tip area, which severely blocks the flow path and eventually leads to unstable flow in the Rotor37.

Flow Distortion Effects of a Three-Dimensional Ultrasonic Anemometer and Its Impact on Measurements

The sonic anemometer is widely recognized as a precise and accurate instrument for measuring and studying atmospheric wind speed and turbulence that works on the principle of measuring the difference in the transit time of acoustic pulses along a known path length. Desirable characteristics of the sonic anemometer include lack of moving parts, linear dynamic response, and good directional response. However, the sensor probes and support structures inevitably lead to a deformation of the flow field being examined resulting in transducer shadowing and flow distortion errors. An empirical method of determining the effects of flow distortion errors on measurements is utilized. While deviations in the horizontal wind are negligible, investigations clearly indicate the need for correction of raw data on vertical wind measurements. Simulations have been conducted using a synthetic time series to determine the impact of observed errors on turbulence, with indications that measurements have a dependence on the angle of attack. Using the series at angles of elevation varying between -12° and 12°, a deviation in turbulence by over 30% is observed for certain wind directions. Matrices derived from the errors at different angles of attack have been used to correct standard ten-minute time series of field data resulting in a decrease in the measured turbulence strength by between 7 and 10%.

A member-based porous method for predicting flow distortions around a lattice tower

In the wind field measurement, the readings of an anemometer mounted on a lattice tower will be influenced by the tower. Therefore, evaluation of the flow distortions generated by a lattice tower is essential for correcting the wind speed data or guiding the arrangement of anemometers. Wall-refined CFD is a powerful tool to simulate the flow field but is quite time consuming for lattice structures. As an alternative approach for improving the computational efficiency of CFD, a member-based porous method was proposed by this article. It employs porous zones to substitute the real members of a lattice tower, so that the computational resources can be considerably saved. Wind tunnel test of two lattice tower segments were conducted to validate the member-based porous method. Models of wall-refined CFD were also established. Results of the member-based porous method are in acceptable agreement with those of test and wall-refined CFD. The method is proved to be an effective way to predict flow distortions of a lattice tower. Both the test and numerical simulations (wall-refined CFD and member-based porous) reveal that disparities exist between the flow field of different measurement planes. The results highlight the need to consider the impacts of member arrangement in predictions of the flow distortion. At last, some crucial issues on the flow distortion effects of a lattice tower were also discussed, results of the classic theoretical methods were compared with those of the member-based porous method.

Evaluation of Profiles of Standard Deviation of Vertical Wind in the Urban Area of Rome: Performances of Monin–Obukhov Similarity Theory Using Different Scaling Variables

The parametrizations of meteorological variables provided by the Monin–Obukhov similarity theory (MOST) is of major importance for pollutant dispersion assessment. However, the complex flow pattern that characterizes the urban areas limits the applicability of the MOST. In this work, the performance of different existing parametrizations of the standard deviation of vertical wind velocity were tested in the city of Rome. Results were compared with experimental data acquired by a sonic detection and ranging (SODAR) and a sonic anemometer. Different scaling variables estimated from the anemometer data by considering two coordinate systems—one aligned with the geodetic reference frame and the other following the flow streamlines—were used to evaluate the effects of flow distortion due to the presence of buildings. Results suggest that the MOST parametrizations perform better if the scaling variables obtained using the coordinate system following the flow streamlines are used. This estimation of the scaling variables would make it possible to overcome the difficulties in conducting measurements of turbulent fluxes, either at different altitudes or even in the constant flux layer.

Flow distortion effect on electromagnetic flowmeter and mitigation using magnetic flux manipulation

Abstract The effect of velocity profile distortion on the performance of flow sensors like the electromagnetic flowmeter has been a popular topic in flow measurement research. In many of the investigations computational modeling has been useful as a tool to understand the impact of flow distortion on flowmeter measurement accuracy. However, a more realistic investigation can be conducted by a model with ability to account for three-dimensional variation in fluidic and electromagnetic characteristics. In this research paper, a multiphysics model of the electromagnetic flowmeter accounting for three-dimensional flow distortion effects and independent of experimental measurements is developed and validated. The model integrates the flow and magnetic fields to yield the induced electrical signal, and is a departure from the popular weight function calculation approach. The model predicts flow distortion effects realistically and is employed in evaluating an idea to mitigate flow distortion problems: manipulating the magnetic field to minimize distortion effects. It is seen that while flow distortion induces a 3 to 4 % error in a flowmeter designed arbitrarily, proper design of the coils can reduce this error to an insignificant level.

Accuracy of Wind Observations from Open-Ocean Buoys: Correction for Flow Distortion

AbstractThe comparison of equivalent neutral winds obtained from (i) four WHOI buoys in the subtropics and (ii) scatterometer estimates at those locations reveals a root-mean-square (RMS) difference of 0.56–0.76 m s−1. To investigate this RMS difference, different buoy wind error sources were examined. These buoys are particularly well suited to examine two important sources of buoy wind errors because 1) redundant anemometers and a comparison with numerical flow simulations allow us to quantitatively assess flow distortion errors, and 2) 1-min sampling at the buoys allows us to examine the sensitivity of buoy temporal sampling/averaging in the buoy–scatterometer comparisons. The interanemometer difference varies as a function of wind direction relative to the buoy wind vane and is consistent with the effects of flow distortion expected based on numerical flow simulations. Comparison between the anemometers and scatterometer winds supports the interpretation that the interanemometer disagreement, which can be up to 5% of the wind speed, is due to flow distortion. These insights motivate an empirical correction to the individual anemometer records and subsequent comparison with scatterometer estimates show good agreement.

Comparison of turbulence measurements by a CSAT3B sonic anemometer and a high-resolution bistatic Doppler lidar

Abstract. Accurate measurements of turbulence statistics in the atmosphere are important for eddy-covariance measurements, wind energy research, and the validation of atmospheric numerical models. Sonic anemometers are widely used for these applications. However, these instruments are prone to probe-induced flow distortion effects, and the magnitude of the resulting errors has been debated due to the lack of an absolute reference instrument under field conditions. Here, we present the results of an intercomparison experiment between a CSAT3B sonic anemometer and a high-resolution bistatic Doppler lidar, which is inherently free of any flow distortion. This novel remote sensing instrument has otherwise very similar spatial and temporal sampling characteristics to the sonic anemometer and hence served as a reference for this comparison. The presented measurements were carried out over flat homogeneous terrain at a measurement height of 30 m. We provide a comparative statistical analysis of the resulting mean wind velocities, the standard deviations of the vertical wind speed and the friction velocity and investigate the reasons for the observed deviations based on the turbulence spectra and co-spectra. Our results show an agreement of the mean wind velocity measurements and the standard deviations of the vertical wind speed with a comparability of 0.082 and 0.020 m s−1, respectively. Biases for these two quantities were 0.003 and 0.012 m s−1, respectively. Slightly larger differences were observed for friction velocity. Analysis of the corresponding co-spectra showed that the CSAT3B underestimates this quantity systematically by about 3 % on average as a result of co-spectral losses in the frequency range between 0.1 and 5 s−1. We also found that an angle-of-attack-dependent transducer-shadowing correction does not improve the agreement between the CSAT3B and the Physikalisch-Technische Bundesanstalt (PTB) lidar effectively.

Toward Future Installations: Mutual Interactions of Short Intakes With Modern High Bypass Fans

In this paper, we investigate the coupled interaction between a new short intake design with a modern fan in a high-bypass ratio civil engine, specifically under the off-design condition of high incidence. The interaction is expected to be much more significant than that on a conventional intake. The performance of both the intake-alone and rotor-alone configurations are examined under isolation. Subsequently, a comprehensive understanding on the two-way interaction between intake and fan is presented. This includes the effect of fan on intake angles of attack (AoA) tolerance (FoI) and the effect of circumferential and radial flow distortion induced by the intake on the fan performance (IoF). In the FoI scenario, the rotor effectively redistributes the mass flow at the fan-face. The AoA tolerance of the short-intake design has increased by ≈4 deg when compared with the intake-alone configuration. Dynamic nature of distortion due to shock unsteadiness has been quantified. ST plots and power spectral density (PSD) of pressure fluctuations show the existence of a spectral gap between the shock unsteadiness and blade passing, with almost an order of magnitude difference in the corresponding frequencies. In the IoF scenario, both the “large” (O(360 deg)) and “small” scale distortion (O(10–60 deg)) induced by the intake results in a non-uniform inflow to the rotor. Sector analysis reveals a substantial variation in the local operating condition of the fan as opposed to its steady characteristic. Streamline curvature, upwash, and wake thickening are identified to be the three key factors affecting the fan performance. These underlying mechanisms are discussed in detail to provide further insights into the physical understanding of the fan-intake interaction. In addition to the shock-induced separation on the intake lip, the current study shows that shorter intakes are much more prone to the upwash effect at higher AoA. Insufficient flow straightening along the engine axis is reconfirmed to be one of the limiting factors for the short-intake design.

A method to assess the accuracy of sonic anemometer measurements

Abstract. We propose a method to assess the accuracy of atmospheric turbulence measurements performed by sonic anemometers and test it by analysis of measurements from two commonly used sonic anemometers, a Metek USA-1 and a Campbell CSAT3, at two locations in Denmark. The method relies on the estimation of the ratio of the vertical to the along-wind velocity power spectrum within the inertial subrange and does not require the use of another measurement as reference. When we correct the USA-1 to account for three-dimensional flow-distortion effects, as recommended by Metek GmbH, the ratio is very close to 4∕3 as expected from Kolmogorov's hypothesis, whereas non-corrected data show a ratio close to 1. For the CSAT3, non-corrected data show a ratio close to 1.1 for the two sites and for wind directions where the instrument is not directly affected by the mast. After applying a previously suggested flow-distortion correction, the ratio increases up to ≈1.2, implying that the effect of flow distortion in this instrument is still not properly accounted for.

Investigation of inlet distortion effects on axial compressor performance based on streamline curvature method

In this paper, the effects of inlet flow distortion on performance maps and details of the flow field are investigated using the Streamline Curvature Method. The results are presented for both design and off-design conditions and compared with experimental data, which shows good agreement. The effects of inlet flow distortion are investigated by inlet total pressure variation in three different cases in the way that the average total pressure remains constant and equal to the design value. The results show that a relative increase in the total pressure at tip causes an increase in the pressure ratio and efficiency as well as a better performance in the choking region. Alternatively, a relative increase in the total pressure in hub causes opposite behavior and a better performance at the surging region.

Calculations of low-frequency wind noise along a low two-dimensional hill surface

Measurement of outdoor sound propagation is often limited by wind noise, i.e. pressure fluctuations from atmospheric turbulence, especially for infrasound and low audible frequencies. Over a flat ground surface, the spectral density of wind noise can be predicted by the shear-turbulence mechanism for static pressure fluctuations using a mirror flow atmospheric turbulence model for the inhomogeneous surface-blocking effect. This study moves beyond a flat ground model to consider the effects of flow distortion by weak topography on surface wind noise, specifically, flow over a low two-dimensional hill, free from separation, at large Froude number. The integral solution for the pressure Poisson equation is used as a starting point, with turbulence modeled by the mirror flow in surface-following coordinates. The perturbation analysis of Hunt et al. [Q. J. R. Meteorol. Soc. 114(484), 1435–1470 (1988)] is used to model the mean shear rate as a function of elevation and distance over the hill. For upwind, downwi...

Determination and Implication of Ultimate Water Cut in Well-Spacing Design for Developed Reservoirs With Water Coning

Theoretically, ultimate water-cut (WCult) defines stabilized well's oil and water production rates for uncontained oil pay underlain with water. However, in a real multiwell reservoir, the well's drainage area is contained by a no-flow boundary (NFB) that would control water coning, so the WCult concept should be qualified and related to the well-spacing size. Also, the presently used WCult formula derives from several simplifying assumptions, so its validity needs to be verified. The study shows that in multiwell bottom-water reservoirs, the production water-cut would never stabilize (after initial rapid increase) but would continue increasing at slow rate dependent on the production rate and well-spacing size. At each production rate, there is a minimum well-spacing size above which water-cut becomes practically constant at the value defined here as pseudoWCult. A new formula—developed in this study—correlates the minimum well-spacing with reservoir properties. Further, formula for pseudoWCult is derived by considering radial flow distortion effects in the oil and water zones. It is found that for well-spacing larger than the minimum well-spacing, the two effects-when combined-do not change the water-cut value. Thus, in practical applications, for sufficiently large well-spacing, the pseudoWCult values can be computed from the presently used WCult formula. The pseudoWCult concept has potential practical use in well-spacing design for ultimate recovery determined by the water cut economic limit, WCec. When the water-cut economic margin (WCec–WCult) is large, well-spacing has little effect on the ultimate recovery, so large well-spacing could be designed. However, when the water-cut economic margin is small, reservoir development decision should consider increase of final recovery by reducing well-spacing below the minimum well-spacing.

Experimental/Numerical Study of Ducted-Fan Noise: Effect of Duct Inlet Shape

The paper presents an investigation of flow and noise produced by the generic fan of an environmental control system installed in a circular duct, with a focus on the evaluation of the effect of mean flow distortions and elevated turbulence levels upstream of the fan, which are here created by replacing a baseline smooth bell-mouth duct inlet by inlets with a rectangular-to-circular transition of the duct cross section and with a T junction formed by two circular pipes. The study includes both experiments, which are aimed primarily at supplying data for a validation of simulation approaches and numerical simulations based on a hybrid Reynolds-averaged Navier–Stokes/large-eddy simulation approach. An acoustic modal decomposition of the measured and computed unsteady pressure fields is then carried out, which allows extracting “pure” (with filtered out hydrodynamic/turbulent fluctuations) acoustic modes propagating upstream and downstream under reflection-free conditions. A comparison with the experiment is performed for both the “raw” pressure spectra on the duct walls and the extracted individual acoustic modes. It is shown that, in terms of acoustics, a replacement of the clean inlet by the rectangular/circular one is almost neutral, whereas the T-junction inlet causes a strong noise penalty.

Improvement in the performance of the vortex flowmeter using contraction cone

The present experimental study investigates a new configuration of vortex flowmeter to improve the turndown ratio and flow distortion sensitivity. An optimized contraction cone is introduced upstream to the vortex shedder to enhance the strength of the shed vortices and to suppress upstream flow distortion effects. A trapezoidal bluff body with two different blockage ratios (19% and 28%) is studied as a vortex shedder. The performance of this new configuration is studied under fully developed flow condition and also evaluated with various flow disturbances. The flow distortion is introduced with 90° degree in-plane and out-of-plane elbows. Transient wall pressure measurement technique is used to capture the vortex shedding frequency. It is found that 28% blockage ratio with optimized contraction cone is the best combination to improve the turn down ratio of vortex flowmeter. The installation of contraction cone helps in reducing the non-uniformity in the velocity profiles which improves the strength and stability of the shed vortices. Minimum measurable Reynolds number obtained with this technique is 5000 which is significantly lower than conventional vortex flowmeters. The turn down ratio achieved with this technique is 1:80 as opposed to 1:20 for conventional vortex flowmeters.

Stability assessment of an airflow distorted military engine’s FAN

Military aircraft are often subjected to severe flight maneuvers with high angles of attack and angles of sideslip. These flight attitudes induce non-uniformity in flow conditions to their gas turbine engines, which may include distortion of inlet total pressure and total temperature at the aerodynamic interface plane. Operation of the downstream engine’s compression system may suffer reduced aerodynamic performance and stall margin, and increased blade stress levels. The present study presents a methodology of evaluating the effect of inlet flow distortion on the engine’s fan stability. The flow distortion examined was induced to the aerodynamic interface plane by means of changing the aircraft’s flight attitude. The study is based on the steady-state flow results from 27 different flight scenarios that have been simulated in computational fluid dynamics. As a baseline model geometry, an airframe inspired by the General Dynamics/LMAERO F-16 aircraft was chosen, which has been exposed to subsonic incoming airflow with varying direction resembling thus different aircraft flight attitudes. The results are focused on the total pressure distribution on the engine’s (aerodynamic interface plane) face and how this is manifested at the operation of the fan. Based on the results, it was concluded that the distorted conditions cause a shift of the surge line on the fan map, with the amount of shift to be directly related to the severity of these distorted conditions. The most severe flight attitude in terms of total pressure distortion, among the tested ones, caused about 7% surge margin depletion comparing to the undistorted value.

Effect of flow distortion on fuel/air mixing and combustion in an upstream-fueled cavity flameholder for a supersonic combustor

This paper describes an experimental study of the effects of an incident shockwave on the flow field, fuel distribution and combustion within a cavity flameholder with upstream fuel injection. Two impingement locations are employed: (1) near the fuel injector (the so-called shock-on-jet case) and (2) on the cavity shear layer (the shock-on-cavity case). Shadowgraph is used to characterize the flow field. Air seeded with nitric oxide (NO) is used as the simulated fuel and the resulting planar laser-induced fluorescence (NO-PLIF) from NO molecules is used to characterize fuel/air mixing while planar laser-induced fluorescence of OH molecules to characterize the actual combustion process. The shadowgraph and NO-PLIF images are compared with a CFD (Computational Fluid Dynamics) solution of the Reynolds-averaged-Navier Stokes (RANS) for assessment and explanation of experimental results of non-reacting tests. The effect of the shock on the cavity shear layer is to control the fuel distribution within the cavity. The effect of the shock on the jet is to force the shear layer deep within the cavity, which results in higher fuel concentrations near the cavity centerline. The shock-on-cavity case causes the shear layer to separate upstream of the cavity. Mixing uniformity is enhanced by the increased breakup of the fuel plume. Combustion is stronger and more uniform with the shock impinging on the cavity, while it is limited to the edges of the cavity with shock impingement on the jet. The greater mixing afforded in the shock-on-cavity case reduces the fuel concentration near the centerline and allows stronger burning in the center of the cavity. Doubling the fuel injection momentum flux ratio does not strongly affect the pattern of fuel distribution in either case, but combustion in the shock-on-cavity case is reduced, because the fuel concentration at the centerline is high.