Upstream Distance Research Articles

Investigation of the incoming wind vector for improved wind turbine yaw-adjustment under different atmospheric and wind farm conditions

Regardless of the evolution of wind energy harvesting, the way in which turbines obtain in-situ meteorological information remains the same - i.e. using traditional wind vanes and cup anemometers installed at the turbine's nacelle, right behind the blades. As a result, misalignment with the mean wind vector is common and energy losses up to 4.6% can be experienced as well as increases in loading and structural fatigue. A solution for the near-blade monitoring is to install wind LIDAR devices on the turbines' nacelle. This technique is currently under development as an alternative to traditional in-situ wind anemometry because it can measure the wind vector at substantial distances upwind. But at what upwind distance should they interrogate the atmosphere? and, what is the optimal average time in which to learn about the incoming flow conditions? This work simulates wind fields approaching isolated wind turbines and wind turbine arrays within large wind farms using Large Eddy Simulations. The goal is to investigate the existence of an optimal upstream scanning distance and average time for wind turbines to measure the incoming wind conditions under different ambient atmospheric conditions. Results reveal no significant differences when measuring the incoming wind vector at different upstream distances, regardless of the atmospheric stratification. Within this framework a 30 min readjustment period is observed to perform the best.

Fan–Intake Interaction Under High Incidence

In this paper, we present an extensive numerical study on the interaction between the downstream fan and the flow separating over an intake under high incidence. The objectives of this investigation are twofold: (a) to gain qualitative insight into the mechanism of fan–intake interaction and (b) to quantitatively examine the effect of the proximity of the fan on the inlet distortion. The fan proximity is altered using the key design parameter, L/D, where D is the diameter of the intake, and L is the distance of the fan from the intake lip. Both steady and unsteady Reynolds-averaged numerical simulations (RANS) were carried out. For the steady calculations, a low-order fan model has been used, while a full 3D geometry has been used for the unsteady RANS. The numerical methodology is also thoroughly validated against the measurements for the intake-only and fan-only configurations on a high bypass ratio turbofan intake and fan, respectively. To systematically study the effect of fan on the intake separation and explore the design criteria, a simplified intake–fan configuration has been considered. In this fan–intake model, the proximity of the fan to the intake separation (L/D) can be conveniently altered without affecting other parameters. The key results indicate that, depending on L/D, the fan has either suppressed the level of the postseparation distortion or increased the separation-free operating range. At the lowest L/D (∼0.17), around a 5 deg increase in the separation-free angle of incidence was achieved. This delay in the separation-free angle of incidence decreased with increasing L/D. At the largest L/D (∼0.44), the fan was effective in suppressing the postseparation distortion rather than entirely eliminating the separation. Isentropic Mach number distribution over the intake lip for different L/D's revealed that the fan accelerates the flow near the casing upstream of the fan face, thereby decreasing the distortion level in the immediate vicinity. However, this acceleration effect decayed rapidly with increasing upstream distance from the fan-face.

On the Upstream Track Deflection of Tropical Cyclones Past a Mountain Range: Idealized Experiments

Abstract Upstream track deflection of a propagating cyclonic vortex past an isolated mountain range is investigated by using idealized simulations with both boundary layer turbulent mixing and cloud effects. The westbound vortex past a shorter mountain range may experience an earlier northward deflection prior to landfall. The vortex then takes a sudden southward turn as it gets closer to the mountain range, in response to the effects of the stronger northerly wind over the mountain due to the effects of channeling flow. The vortex may deflect southward when approaching a longer mountain range and then rebound northward upstream of the mountain ridge. The southward deflection is primarily induced by the convergence (stretching) effect due to the combination of the speedy core at the southwestern flank of the vortex and a northerly jet between the vortex and the mountain. The vortex then rebounds northward to pass over the mountain as the speedy core rotates counterclockwise to the eastern flank of the vortex. The track deflection near the mountain is also affected as either of both physics is deactivated. Sensitivity experiments show that for a given steering flow and mountain height, a linear relationship exists between the maximum upstream deflection distance and the nondimensional parameter Rmw/Ly, where Rmw is the vortex size (represented by the radius of the maximum wind) and Ly is the north–south length scale of the mountain. The southward deflection distance increases with smaller Rmw/Ly and higher mountains for both weaker and stronger steering flow. When the steering-flow intensity is doubled, the southward deflection is roughly reduced by 50%.

Screening the Suitability of Levee Protected Areas for Strategic Floodplain Reconnection Along the LaGrange Segment of the Illinois River, USA

AbstractLevee‐protected floodplains along the 125‐km LaGrange Segment (LGS) of the Illinois River were screened for their abiotic suitability for alternative ecosystem services (ESs), including wetland creation, habitat, floodwater denitrification and flood‐tolerant agriculture. The suitability framework developed for this study builds upon the Land Capability Potential Index and is informed by the current understanding of the linkages between river hydrology, hydraulics, floodplain soils, floodplain vegetation and floodplain nutrient cycling. In addition to screening these floodplain areas for alternative ESs, we demonstrate how this framework can be combined with economic assessments of current floodplain services to inform how strategic floodplain reconnection (i.e. restoration of hydrologic linkages between river and floodplain for the purpose of flood mitigation and provisioning of alternative ESs) could be used to work towards sustainable floodplain management. Results show that ESs increase with upstream distance from the LaGrange Lock and Dam. This is attributed to the operation of the lock and dam generating a water level that would result in the inundation of a substantial portion of floodplain (>70 km2) up to ˜60 km upstream if the levees were to be removed or set back. Comparison of the profits from current floodplain agriculture with the potential profits of flood‐tolerant agriculture suggests that overcoming the economic costs of floodplain reconnection within the LGS will likely require both conversion of reconnected floodplain lands to flood‐tolerant agriculture and payments for ESs. Copyright © 2016 John Wiley & Sons, Ltd.

TH‐AB‐BRA‐03: Backscatter Dose Factors Re‐Evaluated for Inhomogeneities in the Presence of a 1.5 T Magnetic Field Using the GPUMCD Monte Carlo Algorithm

Purpose:To quantify the backscatter dose factors near the interfaces for clinically relevant high atomic number materials using GPUMCD for the Elekta MRI Linac.Methods:Backscatter dose factors (BSDF) were calculated as the ratio of the dose with and without the presence of the heterogeneity. The BSDF's were calculated either in the absence or presence of an orthogonal 1.5 T magnetic field. Doses were scored in small voxels of side 1 mm in a water phantom with dimensions of 20×20×20 cm using GPUMCD (Elekta). The minimum uncertainty in dose calculations was kept to 0.5%. A slab of thickness 2 cm, representing the inhomogeneity, was placed inside the phantom with variable position from the surface of the phantom. The slab was filled with either bone, aluminum, titanium, stainless steel, or dental amalgam. The phantom was irradiated using particles sampled from a histogram which represented the true MRI Linac spectrum.Results:With the application of the 1.5 T magnetic field (B‐On), all of the BSDF's were reduced by at least 8% compared to the no magnetic field (B‐Off) cases. For the B‐Off case, the BSDF decreases exponentially with the upstream distance away from the interface. With B‐On, the BSDF decreases exponentially for titanium, SS, and amalgam. However, it remains constant for Aluminum. In the case of bone, the BSDF increases up to a distance of 4 mm away from the interface in the presence of the magnetic field.Conclusion:The BSDF does not depend upon the thickness of the homogeneous material above the inhomogeneity for either the B‐Off or B‐On cases. For all the materials investigated, the BSDF is lower at the interface for the B‐On case. The exponential fall‐off of the BSDF away from the interface is not valid for all the materials when the magnetic field is turned ON.Funding support for this research was provided by Elekta

Large Eddy Simulation of the fuel transport and mixing process in a scramjet combustor with rearwall-expansion cavity

Large Eddy Simulation (LES) was employed to investigate the fuel/oxidizer mixing process in an ethylene fueled scramjet combustor with a rearwall-expansion cavity. The numerical solver was first validated for an experimental flow, the DLR strut-based scramjet combustor case. Shock wave structures and wall-pressure distribution from the numerical simulations were compared with experimental data and the numerical results were shown in good agreement with the available experimental data. Effects of the injection location on the flow and mixing process were then studied. It was found that with a long injection distance upstream the cavity, the fuel is transported much further into the main flow and a smaller subsonic zone is formed inside the cavity. Conversely, with a short injection distance, the fuel is entrained more into the cavity and a larger subsonic zone is formed inside the cavity, which is favorable for ignition in the cavity. For the rearwall-expansion cavity, it is suggested that the optimized ignition location with a long upstream injection distance should be in the bottom wall in the middle part of the cavity, while the optimized ignition location with a short upstream injection distance should be in the bottom wall in the front side of the cavity. By employing a cavity direct injection on the rear wall, the fuel mass fraction inside the cavity and the local turbulent intensity will both be increased due to this fueling, and it will also enhance the mixing process which will also lead to increased mixing efficiency. For the rearwall-expansion cavity, the combined injection scheme is expected to be an optimized injection scheme.

Pluto's interaction with the solar wind

AbstractThis study provides the first observations of Plutogenic ions and their unique interaction with the solar wind. We find ~20% solar wind slowing that maps to a point only ~4.5 RP upstream of Pluto and a bow shock most likely produced by comet‐like mass loading. The Pluto obstacle is a region of dense heavy ions bounded by a “Plutopause” where the solar wind is largely excluded and which extends back >100 RP into a heavy ion tail. The upstream standoff distance is at only ~2.5 RP. The heavy ion tail contains considerable structure, may still be partially threaded by the interplanetary magnetic field (IMF), and is surrounded by a light ion sheath. The heavy ions (presumably CH4+) have average speed, density, and temperature of ~90 km s−1, ~0.009 cm−3, and ~7 × 105 K, with significant variability, slightly increasing speed/temperature with distance, and are N‐S asymmetric. Density and temperature are roughly anticorrelated yielding a pressure ~2 × 10−2 pPa, roughly in balance with the interstellar pickup ions at ~33 AU. We set an upper bound of <30 nT surface field at Pluto and argue that the obstacle is largely produced by atmospheric thermal pressure like Venus and Mars; we also show that the loss rate down the tail (~5 × 1023 s−1) is only ~1% of the expected total CH4 loss rate from Pluto. Finally, we observe a burst of heavy ions upstream from the bow shock as they are becoming picked up and tentatively identify an IMF outward sector at the time of the NH flyby.

Weak-Shock Interactions with Transonic Laminar Mixing Layers of Fuels for High-Speed Propulsion

This paper extends to transonic mixing layers an analysis of Lighthill (“Reflection at a Laminar Boundary Layer of a Weak Steady Disturbance to a Supersonic Stream, Neglecting Viscosity and Heat Conduction,” Quarterly Journal of Mechanics and Applied Mathematics, Vol. 54, No. 3, 1950, pp. 303–325.) on the interaction between weak shocks and laminar boundary layers. As in that work, the analysis is carried out under linear-inviscid assumptions for the perturbation field, with streamwise changes of the base flow neglected, as is appropriate given the slenderness of the mixing-layer flow. The steady-disturbance profile is determined by taking a Fourier transform along the longitudinal coordinate. Closed-form analytical functions for the pressure field are derived in the small- and large-wave-number limits, and vorticity disturbances are obtained as functions of the pressure perturbations. The analysis is particularized to ethylene–air and hydrogen–air mixing layers, for which the dynamics are of current interest for hypersonic propulsion. The results provide, in particular, the effective distance of upstream influence of the pressure perturbation in the subsonic stream. The resulting value, which scales with the thickness of the subsonic layer, is much smaller than the upstream influence distances encountered in boundary layers. This study may serve as a basis to understand shock-induced autoignition and flameholding phenomena in simplified versions of non-premixed supersonic-combustion problems.

Performance Benefits of Connected Vehicles for Implementing Speed Harmonization

This paper reports on the combined microsimulation and small scale demonstration evaluation of the freeway performance effects of a specific connected vehicle implementation of speed harmonization (The Prototype). The microsimulation testing was performed on a 10 mile stretch of freeway in San Mateo County, California. The small scale field demonstration was performed with 10 equipped vehicles on 17 miles of the I-5 freeway in Seattle, Washington.The microsimulation analysis evaluated the effects of the Prototype speed harmonization algorithm on shockwaves, queues, throughput, speed variation, travel time, and reliability, for different response rates for vehicles on the facility. The “response rate” is defined as the combination of the percent of the vehicle fleet with connected vehicle capabilities, the percent of connected vehicles that successfully receive the speed recommendation in a timely manner, and the percent of drivers who having received the recommendation also choose to comply with the recommended speed.Results from the simulation analysis found that the Prototype is likely to significantly reduce the magnitude of the speed differences (shockwaves) between vehicles, even at the 10-percent response rate. This is considered to benefit safety by reducing the probability of collisions where free-flowing traffic meets the back of a queue. The trade-off for the improved safety is that the Prototype increases the geographic impact of existing bottlenecks on freeway speeds by expanding the upstream distance that is affected by congestion.The small scale field demonstration evaluated the performance of a cell phone based application for communicating vehicle status and receiving recommended speeds and advanced queue warnings using a combination of the cellular network and roadside dedicated short range communication devices via a DSRC antenna installed in the vehicle. The small scale demonstration also evaluated the ability to detect queues via a combination of installed loop detectors in the road and basic safety messages transmitted by the connected vehicles.The small scale field demonstration showed the feasibility of connected vehicle data capture and dissemination functionality using both cellular communications and Dedicated Short Range (DSRC) communications. It demonstrated that the Prototype has the communication speeds, communication reliability, and computed processing speed to support speed harmonization application functionality in an operational traffic environment.This paper is based upon work supported by the Federal Highway Administration under contract number DTFH61-12-D-00044. Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the Federal Highway Administration.

Assessing spatial and temporal effects due to a crash on a freeway through traffic simulation

The focus of this paper is to simulate, evaluate and assess spatial and temporal effects in travel time variation and upstream distance or length of upstream links affected due to a crash. Traffic simulations were conducted for different conditions in VISSIM 5.30™ to obtain travel times at various points upstream of the crash location, over time, along a freeway corridor. Travel time variation between fatal crash condition and no crash condition (baseline condition) and between injury crash condition and no crash condition (baseline condition) were computed and compared to evaluate the effect of a crash on a freeway. Results obtained showed that fatal crash on freeways has an effect under low, moderate, and high traffic volume conditions, whereas injury crash has an effect only under moderate and high traffic volume conditions. The travel time variation and upstream distance affected due to a fatal crash on the right-most lane was generally higher than fatal crash on the left-most lane. The trends remained fairly consistent irrespective of the lane on which an injury crash occurred. The upstream distance affected due to a fatal crash varied from 1.5 miles to ∼7.5 miles based on traffic volume and lane on which the fatal crash occurred. It varied from ∼0.5 miles to 7 miles due to an injury crash and traffic condition. Queue may start dissipating at least 15min after blocked lanes are re-opened for normal traffic flow depending on the type of crash, traffic volume and lane in which the crash occurred. The results and findings from this research can be applied to emulate dynamic message signs over time and space so as to alert the motorists about the length and duration of congestion depending on the severity of crash and lane on which the crash occurred.

Plant ecological traits highlight the effects of landscape on riparian plant communities along an urban–rural gradient

Urban landscapes are characterized by an urban matrix often unfavorable for biodiversity, interspersed with remnant corridors such as riparian areas. Those are increasingly threatened by urban expansion and land use change worldwide. We investigated the effect of the two components, matrix versus corridor, by comparing the riparian plant diversity and the community-level ecological traits along an urbanization gradient. Species distribution was surveyed at a local-scale along an urban riparian corridor in Strasbourg, eastern France. Ninety plots were sampled along an urbanization gradient. Several plant metrics were measured using both plant richness (R) and mean ecological trait values of species weighted by their abundance (CWM). The surroundings of each plot were first described by selecting representative variables of matrix and corridor. Secondly, the distribution of plant species according to a given i ecological trait was analyzed in relation with different levels of urbanization. Using mixed effects models, we verified whether matrix or corridor variables best explain the distribution of traits. Three levels of urbanization were detected, termed urban, suburban and peri-urban, based on landscape composition. Neither the peri-urban nor the suburban level affected plant metrics. At the urban level, and whatever indicator value was considered, the CWMi metrics clearly decreased, whereas species richness Ri increased. The upstream distance to the nearest natural area and tree cover were the most influential variables on CWMi metrics, whereas Ri metrics were mainly driven by built component and landscape heterogeneity. Matrix variables were more important in explaining Ri metrics while corridor features affected CWMi metrics. These results highlight the preponderance of the corridor effect on plant ecological types and the importance of the matrix on the selection and/or the implantation of novel species. Thus, the urbanization gradient may operate simultaneously on the abundance of local species and impose the recruitment of new co-existing species. This study suggests that ecological type responds to the urbanization gradient and may be an alternative tool to understand plant distribution rather than plant diversity.

How Big of an Effect Do Small Dams Have? Using Geomorphological Footprints to Quantify Spatial Impact of Low-Head Dams and Identify Patterns of Across-Dam Variation.

Longitudinal connectivity is a fundamental characteristic of rivers that can be disrupted by natural and anthropogenic processes. Dams are significant disruptions to streams. Over 2,000,000 low-head dams (<7.6 m high) fragment United States rivers. Despite potential adverse impacts of these ubiquitous disturbances, the spatial impacts of low-head dams on geomorphology and ecology are largely untested. Progress for research and conservation is impaired by not knowing the magnitude of low-head dam impacts. Based on the geomorphic literature, we refined a methodology that allowed us to quantify the spatial extent of low-head dam impacts (herein dam footprint), assessed variation in dam footprints across low-head dams within a river network, and identified select aspects of the context of this variation. Wetted width, depth, and substrate size distributions upstream and downstream of six low-head dams within the Upper Neosho River, Kansas, United States of America were measured. Total dam footprints averaged 7.9 km (3.0–15.3 km) or 287 wetted widths (136–437 wetted widths). Estimates included both upstream (mean: 6.7 km or 243 wetted widths) and downstream footprints (mean: 1.2 km or 44 wetted widths). Altogether the six low-head dams impacted 47.3 km (about 17%) of the mainstem in the river network. Despite differences in age, size, location, and primary function, the sizes of geomorphic footprints of individual low-head dams in the Upper Neosho river network were relatively similar. The number of upstream dams and distance to upstream dams, but not dam height, affected the spatial extent of dam footprints. In summary, ubiquitous low-head dams individually and cumulatively altered lotic ecosystems. Both characteristics of individual dams and the context of neighboring dams affected low-head dam impacts within the river network. For these reasons, low-head dams require a different, more integrative, approach for research and management than the individualistic approach that has been applied to larger dams.

Freely vibrating circular cylinder in the vicinity of a stationary wall

We present a numerical study on vortex-induced vibration (VIV) of a freely vibrating two degree-of-freedom circular cylinder in close proximity to a stationary plane wall. Fully implicit combined field scheme based on Petrov–Galerkin formulation has been employed to analyze the nonlinear effects of wall proximity on the vibrational amplitudes and hydrodynamic forces. Two-dimensional simulations are performed as a function of decreasing gap to cylinder diameter ratio e/D∈[0.5,10] for reduced velocities U⁎∈[2,10] at ReD=100 and ReL=2900, where ReD and ReL denote the Reynolds numbers based on the cylinder diameter and the upstream distance, respectively. We investigate the origin of enhanced streamwise oscillation of freely vibrating near-wall cylinder as compared to the isolated cylinder counterpart. For that purpose, detailed analysis of the amplitudes, frequency characteristics and the phase relations has been performed for the isolated and near-wall configurations. Initial and lower branches in the amplitude response are found from the gap ratios of 0.75 to 10, similar in nature to the isolated cylinder laminar VIV. A third response branch has been found between the initial and the lower branch at the gap ratio of e/D≤0.60. For near-wall cases, phase relation between drag force and streamwise displacement varies from close to 0° to 180°. Between e/D∈[5,7.5], the effect of wall proximity on the frequency response tends to disappear. The effect of mass-ratio is further investigated. Finally, we introduce new correlations for characterizing peak amplitudes and forces as a function of the gap ratio for a cylinder vibrating in the vicinity of a stationary plane wall.

Proximity effect on low-rise building surrounded by similar-sized buildings

Wind pressures on a low-rise building in a large group were measured to investigate the proximity effects of surrounding buildings. Five area densities and a long upstream distance were considered under a suburban flow. Results show that variations of pressure coefficients within one area density are relatively small, and that local pressure coefficients increase significantly because of a rapid decrease in local velocity pressures at the model position. Focusing on the peak pressure coefficients, sensitivity study of area density and building distance were conducted for two area densities. The effect of an upstream building is the largest, and the effect of modified arrangement of all buildings is similar to that of the upstream building. The effects of side and downstream buildings are smaller than that of area density.

Prediction of flash flood hazard impact from Himalayan river profiles

AbstractTo what extent can we treat topographic metrics such as river long profiles as a long‐term record of multiple extreme geomorphic events and hence use them for hazard prediction? We demonstrate that in an area of rapid mountain erosion where the landscape is highly reactive to extreme events, channel steepness measured by integrating area over upstream distance (chi analysis) can be used as an indicator of geomorphic change during flash floods. We compare normalized channel steepness to the impact of devastating floods in the upper Ganga Basin in Uttarakhand, northern India, in June 2013. The pattern of sediment accumulation and erosion is broadly predictable from the distribution of normalized channel steepness; in reaches of high steepness, channel lowering up to 5 m undercut buildings causing collapse; in low steepness reaches, channels aggraded up to 30 m and widened causing flooding and burial by sediment. Normalized channel steepness provides a first‐order prediction of the signal of geomorphic change during extreme flood events. Sediment aggradation in lower gradient reaches is a predictable characteristic of floods with a proportion of discharge fed by point sources such as glacial lakes.

A statistical study of the dawn‐dusk asymmetry of ion temperature anisotropy and mirror mode occurrence in the terrestrial dayside magnetosheath using THEMIS data

AbstractWe present a statistical study of ion temperature anisotropy and mirror mode activity in the Earth's dayside magnetosheath using 6 years of Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations focusing on the quantification of dawn‐dusk asymmetry as a function of upstream conditions and distance from the magnetopause. Our statistical data show a pronounced dusk favored asymmetry of T⊥/T∥ which drives a similar asymmetry of mirror mode activity. T⊥/T∥ decreases with increasing solar wind Alfvén Mach number, whereas mirror mode occurrence increases. In both cases, the relative asymmetry between the dawn and dusk flanks decrease with increasing Alfvén Mach number. In addition, during the transition from low/moderate MA, there was a shift in our data set from dips to peaks, suggesting that the magnetosheath strongly favors peaks during intervals of higher Mach number. We also observed more mirror modes and larger asymmetry during atypical inward ortho‐Parker spiral interplanetary magnetic field compared to the more statistically relevant Parker spiral configuration. Our results are consistent with previous experimental studies of mirror modes and to some extent, numerical models.

Reduction of Local Scour Around A Bridge Pier Using Triple Rectangular Plates

The performance of vertical triple plates as a new countermeasure in control of local scour around a cylindrical model pier is studied. Two ones of the plates are attached to the side wall one of the pier at one pier diameter distance, extending toward upstream distance and the third one is located in the middle, attached to the pier nose. All the three plates are parallel to flow direction. Experiments are conducted for five different depths of flow and two different lengths of the lateral plates, namely 8 and 14 cm. all the runs are performed under the clear water flow over uniform sediment. The results showed a maximum efficiency of 76% in scour reduction for 8 cm long side plates and 85% for 14 cm long ones.it is also found that the proposed setup act simultaneously as both the bed armoring and flow altering countermeasure.

Large-eddy Simulation of flow and dispersion around an isolated building: Analysis of influencing factors

CFD simulations are known to be very sensitive to many physical and numerical parameters. Many previous studies have analyzed the factors that influence RANS modeling, but few have considered how these factors affect LES modeling. This study investigates the factors that influence the LES modeling of flow and dispersion around an isolated building. The LES model is validated against two wind tunnel experimental datasets. The factors analyzed are upstream distance of computational domain, mesh resolution, length of sampling period, inflow fluctuating algorithm, subgrid-scale (SGS) Schmidt number, SGS model, and Smagorinsky constant. Based on the experimental data, the influences of these factors on the numerical results are discussed and appropriate selections are recommended. Specifically, this study finds that the fluctuating intensity of the velocity components provided by a Spectral synthesizer (an inflow fluctuating algorithm) is less than half of that provided by the Vortex method (an inflow fluctuating algorithm). In addition, the velocity field is less sensitive to some of the influencing factors than the concentration field. Therefore, different optimum values for specific influencing factors are suggested to predict velocity and concentration fields. The findings of this study are expected to improve the quality of LES modeling of flow and dispersion in built environments.

Passive control of flow-excited acoustic resonance in rectangular cavities using upstream mounted blocks

A passive method for controlling the flow-excited acoustic resonance resulting from subsonic flows over rectangular cavities in channels is investigated. A cavity with length to depth ratio of \(L/D=1\) is tested in air flow of Mach number up to 0.45. When the acoustic resonance is excited, the sound pressure level in the cavity reaches 162 dB. Square blocks are attached to the surface of the channel and centred upstream of the cavity leading edge to suppress the flow-excited acoustic resonance in the cavity. Six blocks of different widths are tested at three different upstream distances. The results show that significant attenuation of up to 30 dB of the excited sound pressure level is achieved using a block with a width to height ratio of 3, while blocks that fill the whole width of the channel amplify the pressure of the excited acoustic resonance. Moreover, it is found that placing the block upstream of the cavity causes the onset of the acoustic resonance to occur at higher flow velocities. In order to investigate the nature of the interactions that lead to suppression of the acoustic resonance and to identify the changes in flow patterns due to the placement of the block, 2D measurements of turbulence intensity in the shear layer and the block wake region are performed. The location of the flow reattachment point downstream of the block relative to the shear layer separation point has a major influence on the suppression level of the excited acoustic resonance. Furthermore, higher attenuation of noise is related to lower span-wise correlation of the shear-layer perturbation.

Spatial-temporal distribution of salinity and temperature in the Oued Loukkos estuary, Morocco: using vertical salinity gradient for estuary classification

BackgroundSeveral different classifications to characterize estuarine systems have been proposed. In this present paper, one of the most important estuaries in North Africa, the Oued Loukkos (Morocco), forms a case-study for proposing a systematic classification of this particular tidal estuary according to the vertical salinity gradient.FindingsThis study, conducted using a CTD, shows that the spatial-temporal distribution of salinity depends on the stage of the tide and the upstream distance from the mouth of the river. In this case, it is also evident that the morphology of the bottom was capable of impacting the distribution of salinity by locally changing the water circulation.ConclusionsBased on the vertical salinity gradient measurement, the Oued Loukkos represents an estuarine environment with one section near its mouth that can be characterized as a mixed mesotidal estuary and another section upstream which can be characterized as a stratified mesotidal estuary. Between, there is an intermediate zone with a low vertical gradient of salinity, classified as a partially mixed mesotidal estuary. When the effect of terrestrial inputs is low compared to marine inputs, the river bed topography plays a role in the stratification of salinity by either disrupting the vertical stratification of the water or by changing the lateral distribution of salinity. The proposed classification deepens our hydrological knowledge and provides descriptive labels to the Oued Loukkos estuary. It provides a valid starting point for predicting the environmental impact of future recreational, agricultural and commercial activities on the estuary.