Turbulent Reduction Research Articles

On the noise reduction mechanisms of porous aerofoil leading edges

This paper is predominantly an experimental study into the reduction of turbulence - aerofoil interaction noise by the introduction of aerofoil porosity. In this paper we study three scenarios applied to flat plates: (a) when the flat plate is fully porous, (b) when the flat plate is partially porous from the leading edge and (c) when porosity is introduced downstream of the leading edge. This paper shows that the noise reduction spectra collapse when plotted against non-dimensional frequency fl/U, where l is the length of porous section and U is the flow velocity. Narrow band measurements on a partially porous aerofoil have shown that its noise reduction spectra is characterised by a number of narrow peaks. This paper proposes two main mechanisms for explaining this behaviour. The noise reduction mechanisms are validated against noise reductions measured on a realistic aerofoil at relatively low angles of attack. One of the key findings of this paper is that, by using only a single row of holes downstream of the aerofoil leading edge one can obtain significant levels of noise reduction. This use of downstream porosity is specifically shown to be capable of providing low-frequency noise reductions without increasing the radiated noise at higher frequencies.

Numerical investigation of spatial-developing turbulent heat transfer in forced convections at different supercritical pressures

Direct numerical simulations have been adopted to study the turbulent heat transfer in forced convections of supercritical water at two different supercritical pressures P=23 MPa and P=25 MPa in a heated pipe with constant wall heat flux and a bulk Reynolds number of Re0=5400. The present study aims to reveal the mechanisms of turbulent heat transfer of superciritical fluids at different pressures in a spatial-developing flow. The results show that at the smaller pressure ratio Pr=P/Pc, where Pc is the critical pressure, the property variations become more drastic, and both the skin friction coefficient and Nusselt number become smaller. The decompositions of skin friction and Nusselt number show that it is mainly due to the large turbulence reduction along the streamwise direction. The analyses of turbulent kinetic energy (TKE), the turbulent shear stress, and the production of TKE confirm this point. Moreover, it was found that the thermophysical property fluctuations are very large and significantly influence the turbulent statistics in the supercritical fluid flows. Due to the large property fluctuations, it was found that the density-fluctuation-related terms are significant and their values are actually comparable to the mean-density-related terms. Due to their negative contributions to turbulent shear stress and turbulent heat flux, the turbulence and heat transfer are severely attenuated by the large thermophysical property fluctuations. For near-wall scaling in spatial-developing flows at supercritical pressures, the semi-local velocity transformation with a semi-local coordinate shows a better agreement in the logarithmic region. However, a clear deviation still exists, especially for mean temperature because all the transformations only incorporate the local mean property variations and cannot consider their fluctuations.

3D modelling of MHD mixed convection flow in a vertical duct with transverse magnetic field and volumetric or surface heating

Understanding phenomena associated with the multiple effects/interactions of the fusion nuclear environment on liquid metal flow is required to correctly design liquid metal (LM) blankets for fusion facilities. These effects are investigated in the present work by numerically simulating 3D LM MHD flow. The simulated geometry consists of a straight, vertical duct which runs perpendicular to a strong, fringing applied magnetic field. There is also a region of applied heating as the primary goal is to explore buoyancy effects in MHD duct flows. Results are presented for both buoyancy assisted (upwards) and buoyancy opposed (downwards) flows in conducting and insulating ducts for a range of Hartmann numbers (Ha) up to 100, Reynolds numbers (Re) from 103 to 104 and Grashof (Gr) numbers from 107 to 108. While increasing Gr or decreasing Re increases buoyancy effects, increasing Ha was shown to increase maximum temperature through turbulence reduction. The extent to which the MHD mixed convection flows are quasi-2D is analyzed and buoyant effects, in competition with electromagnetic forces, are shown to bring about 3D flow features not seen in purely MHD flows. Volumetric nuclear heating with steep gradients is applied to the vertical MHD flows for comparison to flows with surface heating only. Surface heating generates stronger buoyancy effects than volumetric heating of the same total power; however, many of the same phenomena occur. Therefore, surface heating, the only option for lab experiments, can provide indication of the effects of volumetric heating in MHD flows.

Computational Fluid Dynamics (CFD) Transient Pressure and Temperature Simulation of a Natural Gas– Hydrogen Gas Blend Transportation Pipeline

The need for a CFD exploratory investigation into the possible effect of hydrogen injection into existing natural gas pipeline network, was the driver for this study. Recent technical feasibility reports on transporting a blend of both gases (JIR, 2019), did not mention if a study was done to rule out the possible effect of natural gas-hydrogen blend on hydrate formation in a hydrate-prone gas pipeline. A CFD model of 0.0508m and 10m length, with a mesh size of 20,000 elements was developed and simulated using ANSYS Fluent version R19.3. Hydrogen concentration by volume of 5%, 10%, 15% and 20% were simulated at velocities of 5m/s and 10m/s. The results indicated that increasing the volume concentration of hydrogen gas in the blend led to a reduction of downstream temperature. Again, the force on the inner wall decreased as the volume of hydrogen injected increased. The inference was that the reduction in temperature could lead to hydrates formation, whereas the reduction in exerted force on the pipe wall by the fluid could suggest a reduction in turbulence. However, this study was exploratory in nature and it is recommended that a detailed experimental study be carried out to clearly define the hydraulic and thermal behavior of a pipeline transporting a natural gas-hydrogen blend under various subcooling temperatures.

Role of sheared E × B flow in self-organized, improved confinement states in magnetized plasmas

A major scientific success story of magnetic fusion research in the past several decades has been the theoretical development and experimental testing of the process of turbulence decorrelation and stabilization by sheared E × B flow, which shows that E × B shear effects are ubiquitous in magnetized plasmas. This concept of turbulence decorrelation and stabilization has the universality needed to explain the H-mode edge transport barriers seen in limiter and divertor tokamaks, stellarators, and mirror machines; the broader edge transport barrier seen in VH-mode plasmas; and the core transport barriers formed in tokamaks. Similar effects are seen in linear devices. These examples of confinement improvement are of considerable physical interest; it is not often that a system self-organizes to reduce transport when an additional source of free energy is applied to it. The transport decrease associated with E × B velocity shear is also of great practical benefit to fusion research, since it contributed to substantially increased fusion yield in all DT magnetic fusion experiments conducted to date. The fundamental physics involved in transport reduction is the effect of E × B shear on the growth, radial extent, and phase correlation of turbulent eddies in the plasma. The same basic transport reduction process can be operational in various portions of the plasma because there are a number of ways to change the radial electric field Er. An important secondary theme in this area is the synergistic effect of E × B velocity shear and magnetic shear. Although the E × B velocity shear appears to have an effect on broader classes of microturbulence, magnetic shear can mitigate some potentially harmful effects of E × B velocity shear and facilitate turbulence stabilization. Our present understanding in this area is the result of a multi-decade, intertwined effort in theory, modeling, and diagnostic development combined with continuing experimental investigations. These experiments have clearly demonstrated that increased E × B shear causes reductions in turbulence and transport. The experimental results are generally consistent with the basic theoretical models although considerable work remains to be done before we have a fully predictive theory of transport in magnetized plasmas including E × B shear effects.

Predictive multi-channel flux-driven modelling to optimise ICRH tungsten control and fusion performance in JET

The evolution of the JET high performance hybrid scenario, including central accumulation of the tungsten (W) impurity, is reproduced with predictive multi-channel integrated modelling over multiple confinement times using first-principle based core transport models. Eight transport channels () are modelled predictively, with self-consistent sources, radiation and magnetic equilibrium, yielding a system with multiple non-linearities: This system can reproduce the observed radiative temperature collapse after several confinement times. W is transported inward by neoclassical convection driven by the main ion density gradients and enhanced by poloidal asymmetries due to centrifugal acceleration. The slow evolution of the bulk density profile sets the timescale for W accumulation. Modelling this phenomenon requires a turbulent transport model capable of accurately predicting particle and momentum transport (QuaLiKiz) and a neoclassical transport model including the effects of poloidal asymmetries (NEO) coupled to an integrated plasma simulator (JINTRAC). The modelling capability is applied to optimise the available actuators to prevent W accumulation, and to extrapolate in power and pulse length. Central NBI heating is preferred for high performance, but gives central deposition of particles and torque which increase the risk of W accumulation by increasing density peaking and poloidal asymmetry. The primary mechanism for ICRH to control W in JET is via its impact through turbulence in reducing main ion density peaking (which drives inward neoclassical convection), increased temperature screening and turbulent W diffusion. The anisotropy from ICRH also reduces poloidal asymmetry, but this effect is negligible in high rotation JET discharges. High power ICRH near the axis can sensitively mitigate against W accumulation, and dominant ion heating (e.g. He-3 minority) is predicted to provide more resilience to W accumulation than dominant electron heating (e.g. H minority) in the JET hybrid scenario. Extrapolation to DT plasmas finds 17.5 MW of fusion power and improved confinement compared to DD, due to reduced ion-electron energy exchange, and increased Ti/Te stabilisation of ITG instabilities. The turbulence reduction in DT increases density peaking and accelerates the arrival of W on axis; this may be mitigated by reducing the penetration of the beam particle source with an increased pedestal density.

Turbulent drag reduction in magnetohydrodynamic and quasi-static magnetohydrodynamic turbulence

In hydrodynamic turbulence, the kinetic energy injected at large scales cascades to the inertial range, leading to a constant kinetic energy flux. In contrast, in magnetohydrodynamic (MHD) turbulence, a fraction of kinetic energy is transferred to the magnetic energy. Consequently, for the same kinetic energy injection rate, the kinetic energy flux in MHD turbulence is reduced compared to its hydrodynamic counterpart. This leads to relative weakening of the nonlinear term ⟨|(u·∇)u|⟩, (where u is the velocity field) and turbulent drag, but strengthening of the velocity field in MHD turbulence. We verify the above using shell model simulations of hydrodynamic and MHD turbulence. Quasi-static MHD turbulence too exhibits turbulent drag reduction similar to MHD turbulence.

Investigation of turbulence rotation in the SOL and plasma edge of W7-X for different magnetic configurations

The W7-X stellarator is optimized with respect to neoclassical transport. Therefore turbulent transport plays an important role. It is equipped with an inertial cooled graphite divertor which intersects the island chain at the plasma edge depending on the magnetic configuration. Additional control coils and the plasma current modify the iota profile at the plasma edge and shift the position of the island chain. To monitor the effects on the poloidal propagation velocity in the scrape-off layer (SOL) and the plasma edge, an O-mode Poloidal Correlation Reflectometer (PCR) is used which simultaneously monitors the propagation of low-k turbulence. Operating in the density range of 0.6 × 1019 m−3 to 2 × 1019 m−3 it covers a large part of the SOL and the plasma edge and allows for the experimental determination in the last closed flux surface (LCFS) and the associated shear layer in low to mid-density discharges. In this paper it is shown that the propagation in the shear layer and its vicinity is determined best, when based on an elliptical model. Different magnetic configurations with magnetic edge topology of five independent islands for ι = 1 and six linked islands for ι = 0.81 are investigated. Also the effects of the plasma current and additional control coils on the edge magnetic topology are studied. The coherence spectra of antenna pairs for different poloidal separations is investigated. Using a decomposition method for the measured coherence spectra the characterization of turbulence spectra is possible with respect to e.g. broad band turbulence and quasi coherent modes. A strong reduction of the broad band turbulence is observed in the vicinity of the LCFS which is evidence for the suppression of low-k turbulence at the shear layer.

Experimental Investigation of Jet Flow Fields with Chevron Nozzles

With the rapid development of the aircraft industry, more regulations on aircraft noise are established. The turbulent jet is one of the major sources of noise in the aircraft, extensive researches on noise reduction has been investigated, especially for chevron. In this project, the near-nozzle flow fields of three chevron nozzles and one circular nozzle from a subsonic jet are investigated experimentally. This paper presents the results the comparison between four nozzles (CN0, CN1, CN2 and CN3) in terms of radial mean velocity, turbulent intensity and the selected nozzles are compared to baseline and each other with respect to turbulent kinetic energy spectra. CN0 is the baseline circular nozzles, CN1 is the standard chevrons nozzle, CN2 is a chevron with alternating angles and CN3 is chevron with more number of chevrons. Fluctuated velocity profiles were measured at x/D = 2 at different radial locations along characteristic direction using hot-wire technique. The purpose of these measurements and comparison is to study the variations in the flow field with the different configurations of chevron, which might be related to the acoustic field. CN1 nozzle has a 40dB turbulent kinetic energy reduction compared to the baseline nozzle CN0, and CN2 has the highest turbulent flow field, which is a mixture of low, medium and high velocities corresponds to three directions. CN3 has the most similar mean velocity and turbulence intensity profiles, this may be caused by the under developing jet flow field.

Entanglement of near-surface optical turbulence to atmospheric boundary layer dynamics and particulate concentration: implications for optical wireless communication systems.

Localized reduction in optical turbulence due to enhanced atmospheric heating caused by the solar absorption of aerosol black carbon (BC) is reported. Immediate response of atmospheric turbulence to BC-induced atmospheric warming strongly depends on the available solar radiation (time of the day), BC concentration, and atmospheric boundary layer dynamics. Besides the significant climate implications of a reduction in turbulence kinetic energy, a large reduction in the refractive index structure parameter (Cn2) resulting from BC-induced warming would affect the atmospheric propagation of laser beams. Interestingly, aerosols contribute significantly (up to 25%) to the signal deterioration in optical wireless communication systems during convectively stable atmospheric conditions when higher signal-to-noise ratios are expected otherwise due to the reduced thermal convection. Competing effects of the fractional contributions of aerosol extinction and scintillations on beam attenuation are reported; daytime being largely dominated by scintillation effects while the nighttime being dependent on the ambient aerosol concentration as well. We put forward the entanglement of optical turbulence to aerosol concentration, atmospheric boundary layer dynamics, and surface-reaching solar radiation, and discuss the possible implications for optical propagation.

Effects of favorable pressure gradient on turbulence structures and statistics of a flat-plate supersonic turbulent boundary layer

A Mach 2.9 flat-plate supersonic turbulent boundary layer subject to a moderate favorable pressure gradient (FPG) induced by external expansion waves is investigated through direct numerical simulation and compared with a zero pressure gradient (ZPG) boundary layer. It is found that under FPG, the logarithmic region in the van Driest transformed velocity profile is lifted above the log law, while the wake region deviates below its ZPG counterpart. The near-wall streaks are elongated in the streamwise direction with wider spanwise spacing, which leads to an attenuated meandering effect compared to the ZPG case. Although small-scale motions in the outer layer are evidently suppressed, they survive mostly in the inner layer. On the other hand, large-scale motions tend to correlate further with the lifted fluid from upstream due to bulk dilatation. However, their relative locations within the boundary layer remain unchanged. Different responses of turbulence structures in the inner and the outer layer to FPG show that this two-layer feature within the boundary layer is mainly associated with the bulk dilatation rather than the wall curvatures. The profiles of turbulent kinetic energy (TKE) and turbulent Mach number also show a two-layer behavior, where the reduction in turbulence in the outer layer is more prominent than in the inner. Positive convection occurs from the buffer to the outer layer according to the TKE budget analysis, which compensates the production and resists the decrease in the turbulence level.

Cooperative lane control application for fully connected and automated vehicles at multilane freeways

Advances in communications technology have made the development of connected and automated vehicle (CAV) applications that may potentially replace traditional, gantry operated lane control signals (LCSs) possible. This paper develops the concept of a prototype CAV-enabled LCS application and provides a preliminary assessment of the potential improvement it offers over traditional LCS. Real-world data obtained from an LCS on I-66 in Northern Virginia was used to calibrate and validate a baseline simulation model. Further, the current lane control scenarios utilized in the Northern Virginia LCS were identified and relevant data resulting from implementation of those scenarios were collected to model the LCS in a simulation environment. The performance of real-world LCS was then compared to a prototype CAV-enabled LCS application developed in this research. The CAV-enabled LCS application consistently outperformed the traditional LCS with increased throughput and speeds. On average, an increase of 18.4%, 9.6% and 12.8% in throughput was observed for three selected scenarios under the best case of a 1 sec headway. Furthermore, the CAV-enabled LCS application was also found to reduce volatility, represented by variation in acceleration and deceleration regimes, by an average of 25.6% and 49.6%. The reduction in turbulence in the traffic stream may indicate potential improved safety with the CAV system.

Interaction of magnetic islands with turbulent electron temperature fluctuations in DIII-D and in GENE nonlinear gyrokinetic simulations

We present localized measurements of interactions between neoclassical tearing modes (NTMs) and turbulent electron temperature fluctuations () in the expected kθρs = 0–0.5 wave-number range of the ion temperature gradient (ITG) instability measured via correlation electron cyclotron emission (kθ and ρs are the poloidal wave-number and ion-sound Larmor radius, respectively). Comparison to GENE gyrokinetic simulations, shows qualitative agreement with being reduced (increased) inside (outside) of the islands due to modified local gradients, and being higher (lower) in the region outside the island where lower (higher) flow shear is expected due to radially asymmetric island shape. These results are consistent with previously reported modifications of long and intermediate wavelength turbulent density fluctuations and cross-field electron thermal diffusivity reduction at the O-point of magnetic islands. Interestingly, a 30% increase of is detected at the NTM rational surface when the island width is a few times ρs, which is replicated by GENE through zonal flow damping due to the destroyed magnetic field topology of the NTM rational surface.

Numerical study on the ventilation performance of a livestock house built in porous panels in Cold Regions

This paper presents a numerical prediction on the assessment of natural ventilation on a livestock house built in porous panels, aided by Computational Fluid Dynamics techniques. A typical climatic environment in Cold Regions was taken into consideration. The ventilation performances have been assessed by the distribution of indoor air velocity, turbulence intensity and local mean age of air through the cases: the house without porous panels, with wind erosion porous panels, and with snow erosion porous panels. The study has found that the porous panels improve the indoor environment in term of the reduction of air velocity and turbulence, and the wind erosion panel performs better than the snow erosion panel. However, applying porous panels may have negative effects on air exchange effectiveness. The presented numerical model is easy to be constructed, flexible to investigate different case scenarios, and quick and cheap to get solutions.

Effect of fluid injection on turbulence and noise reduction of a supersonic jet.

Supersonic jets, such as the ones used in high-performance military aircraft, have both downstream and upstream noise components due to the large-scale turbulent structures and the presence of shock cells in the jet plume. The fluid insert technology is a noise reduction method that has been shown to effectively reduce both these noise components. This paper analyses the unsteady flow changes associated with different fluid insert configurations with a goal of helping to understand the detailed noise reduction mechanisms. Using direct cross-correlations of the near-field data with the far-field microphone signals, it is found that even the use of a single injector as a fluid insert helps break up the large-scale structures of the flow. However, a more azimuthally distributed blowing is required to reduce the upstream broadband shock-associated noise (BBSAN). Addition of upstream injectors at each azimuthal location further enhances the BBSAN reduction. Decomposition of the jet flow-field into hydrodynamic and acoustic modes shows that fluid insert nozzles reduce the amplitude and convection speed of the coherent acoustic mode in the plane of highest noise reduction. This article is part of the theme issue 'Frontiers of aeroacoustics research: theory, computation and experiment'.

Electromagnetic turbulence suppression by energetic particle driven modes

In recent years, a strong reduction of plasma turbulence in the presence of energetic particles has been reported in a number of magnetic confinement experiments and corresponding gyrokinetic simulations. While highly relevant to performance predictions for burning plasmas, an explanation for this primarily nonlinear effect has so far remained elusive. A thorough analysis finds that linearly marginally stable energetic particle driven modes are excited nonlinearly, depleting the energy content of the turbulence and acting as an additional catalyst for energy transfer to zonal modes (the dominant turbulence saturation channel). Respective signatures are found in a number of simulations for different JET and ASDEX Upgrade discharges with reduced transport levels attributed to energetic ion effects.

Effect of turbine nacelle and tower on the near wake of a utility-scale wind turbine

Super-large-scale particle image velocimetry using natural snowfall is used to investigate the influence of nacelle and tower generated flow structures on the near-wake of an operational 2.5 MW wind turbine. The measurement provides the velocity field over the entire rotor span in a plane centered behind the support tower, revealing a region of accelerated flow around the nacelle and a reduction in velocity behind the tower, causing asymmetry in the velocity deficit profile. The in-plane turbulent kinetic energy field shows increased turbulence in the regions of large shear behind the blade tips and nacelle, and a reduction in turbulence behind the tower. The nacelle wake meandering frequency is found to scale with the nacelle dimension rather than the rotor dimension, corresponding to the vortex shedding frequency of an Ahmed body. Persistent nacelle wake deflection is observed and shown to be connected with the turbine yaw error. Strong interaction between the tower- and blade-generated structures, quantified by the co-presence of two dominant frequencies, demonstrates the influence of the tower on blade tip vortex breakdown. This study highlights the influence of the tower and nacelle on the behavior of the near-wake, informing model development and elucidating the mechanisms that influence wake evolution.

Large-eddy simulation of compressible turbulent flow in convergent-divergent nozzles with isothermal wall

We present results from high-order accurate large-eddy simulations of compressible flow through convergent-divergent nozzles with and without bell-shaped divergent portions. The nozzles have isothermal walls and circular cross-sections. The incoming flow is a turbulent, subsonic fully-developed pipe flow at M=0.37 and friction Reynolds number 214. A smooth transition from subsonic to supersonic flow occurs in the nozzles when the back pressure is kept small, accompanied by a reduction of turbulence intensities in the diverging portion of the nozzles. Strong reductions in production and pressure-strain correlations are observed as the flow undergoes acceleration and expansion. When the back pressure is high, the flow in the nozzle remains subsonic, accompanied by a reduction in turbulence in the convergent portion and an amplification in the divergent portion.

Reduction of edge plasma turbulence via cross-phase decrease by zonal fields

Transport of the near-adiabatic plasma is simulated numerically. The zonal flow (ZF) V along the poloidal direction is found to emerge self-consistently from the non-zonal turbulence. Zonal fields are found to feed back to turbulence by reducing the cross phase. As a result the turbulence is localized with depressed peaks. Turbulence is very low and the cross phase δ is negative near V = 0, where the gradient (vorticity) of ZF along the radial direction is large. The fluctuations are almost localized where the zonal-density gradient is a positive peak and V is locally large. Positive or positive is shown to reduce the cross phase between the electric potential and the density fluctuation.

Effect of local and upwind stratification on flow and dispersion inside and above a bi-dimensional street canyon

The effects of a stably-stratified boundary layer on flow and dispersion in a bi-dimensional street canyon with unity aspect ratio have been investigated experimentally in a wind tunnel in combination with differential wall heating. Laser-Doppler anemometry together with a fast flame ionisation detector and cold-wire anemometry were employed to sample velocities, concentration, temperatures and fluxes.A single-vortex pattern was observed in the isothermal case, preserved also when leeward wall was heated, but with a considerable increment of the vortex speed. Heating the windward wall, instead, was found to generate a counter-rotating vortex, resulting in the reduction of velocity within the canopy. The stable stratification also contributes reducing the speed, but only in the lower half of the canyon. The largest values of turbulent kinetic energy were observed above the canopy, while inside they were concentrated close to the windward wall, even when the leeward one was heated. An incoming stable stratification produced a significant and generalised turbulence reduction in all the cases. Windward heating was found to produce larger temperature increments within the canopy, while in the leeward case heat was immediately vacated above the canopy. A stable approaching flow reduced both the temperature and the heat fluxes.A passive tracer was released from a point source located at ground level at the centre of the street canyon. The resulting plume cross-section pattern was mostly affected by the windward wall heating, which produced an increment of the pollutant concentration on the windward side by breaking the main vortex circulation. The application of an incoming stable stratification created a generalised increment of pollutant within the canopy, with concentrations twice as large. Turbulent pollutant fluxes were found significant only at roof level and close to the source. On the other hand, in the windward wall-heated case the reduction of the mean flux renders the turbulent component relevant in other locations as well.The present work highlights the importance of boundary layer stratification and local heating, both capable of creating significant modifications in the flow and pollutant fields at microscale range.