Reduction In Turbulent Kinetic Energy Research Articles

Simulation research on aerodynamic noise characteristics of a compressor under different working conditions.

The shear stress transport turbulence model is employed to conduct a detailed study of flow characteristics at the highest efficiency point and near-stall point in a full-channel 1.5-stage compressor in this paper. The simulation results for the compressor's total pressure ratio and efficiency exhibit good agreement with experimental data. Emphasis is placed on examining the internal flow structure in the tip area of the compressor rotor under near-stall conditions. The results reveal that significant differences in flow structure primarily occur in the tip area as the compressor approaches stall. Specifically, a reduction in turbulent kinetic energy is observed in a region spanning approximately 20%-60% of the chord length on the rotor suction face near-stall conditions. Two additional peak frequencies, at 0.8 and 1.6 times the blade passage frequency, are observed, and the intricate flow phenomena are elaborated at the near-stall point. The near-stall point exhibits greater noise levels than the highest efficiency point, where the intensity of the surface source increases by more than 10 dB, peaking at 20 dB. This additional peak serves as a significant supplementary noise source near the stall point, leading to a maximum increase of 33.3 dB in the free radiated sound power. The acoustic response within the duct indicates that the compressor operating at the near-stall point continues to produce substantial noise on the actual test bench, showing an average increase of 6 dB in noise levels, and the distribution of the additional peak single-tone noise at the entrance significantly differs from that observed at the highest efficiency point.

Exploring implications of wave characteristics on the functionality of mimetic tubular sponges

Marine sponges influence the flow hydrodynamics by suction/pumping mechanism. They provide a recirculation flow and remove pollutants from their residence. Previous studies have validated the efficacy of mimetic sponges in improving momentum transfer in marine environments. This study endeavors to explore the impact of wave characteristics (i.e., wave height and period) on the momentum transfer near the mimetic sponges. Both physical and numerical simulations were undertaken to explore this phenomenon. Experimental results unveiled that the efficiency of the mimetic sponge is contingent upon some circumstances. When waves have a short period and height, the suction effects prevail. In such cases, orbital velocity tends to be in a negative direction, leading to suppressed fluctuating velocities and a reduction in turbulent kinetic energy throughout the water column. Thus, the penetration of the shear layer to the water column and occurrence of wake-street diminish. Conversely, an increase in wave height enhances the momentum exchange through the water column. Surface Line Integral Convolution and Three-Dimensional stream trace visualizations using OpenFOAM provide insights into the generation of spirals and vortices through the water column induced by mimetic sponges.

Characterization of very-large-scale motions in supersonic and hypersonic turbulent boundary layers

Very-large-scale motions are commonly observed in moderate- and high-Reynolds-number wall turbulence, constituting a considerable portion of the Reynolds stress and skin friction. This study aims to investigate the behaviour of these motions in high-speed and high-Reynolds-number turbulent boundary layers at varying Mach numbers. With the aid of high-precision numerical simulations, numerical experiments and theoretical analysis, it is demonstrated that the very-large-scale motions are weakened in high-Mach-number turbulence at the same friction Reynolds numbers, leading to the reduction in turbulent kinetic energy in the outer region. Conversely, the lower wall temperature enhances the very-large-scale motions but shortens the scale separation between the structures in the near-wall and outer regions.

Numerical studies of shock–vortex interaction over a wedge during shock-wave diffraction—A new approach

In this study, shock wave diffraction has been investigated through a numerical simulation of a moving normal shock incident on a sharp-edged wedge. Schardin's problem is revisited using some existing and new mathematical tools. Two-dimensional compressible Navier–Stokes equation is solved using a higher-order version of the rhoCentralFoam solver in the OpenFOAM platform. Overall flow structures are captured with high efficacy. The divergence of the Lamb vector is used to probe the interaction between vorticity bearing and fluid straining motion, which increases dramatically inside the primary vortex after collision with the reflected Mach stem and increases the turbulent kinetic energy (TKE). In the separated shear layer that emerges from the wedge tip, there is a reduction of TKE after the collision between the lambda shock and accelerated shock. The vorticity pumping into the mean flow by the baroclinic torque is dominant in the separated shear layer before the above collision, whereas after collision it is only dominant inside the primary vortex. A new vector M is introduced here, which is the cross product of the vorticity vector and the pressure gradient vector. The divergence of M shows the interaction between the pressure gradient and the vorticity gradient. This interaction affects the separation bubble formed over the base wall of the wedge due to the shock-wave/boundary layer interaction induced by the lambda shock. Dynamic mode decomposition shows a dominant mode at a frequency of 125.7 Hz, which is due to low-frequency shock oscillation.

Improvement of turbulent flow in a bend using rotational guide vane: A shape optimization

The present study aims to minimize velocity disruption using rotational guide vane (RGV) before the 90° pipe bend where the curvature ratio is r/D = 1.0 at Re = 3 × 104. The combination of computational fluid dynamics and genetic algorithm is used as a tool for the geometric optimization of RGV. The SST k–ω turbulence model was preferred in flow analysis. The number of blade, the blade angle, and the location of RGV are the geometric parameters to be optimized. A new evaluation method of the velocity distribution after the bend is proposed. The objective function is defined as velocity distribution mean squared error that is based on the comparison of the velocity distribution on the cross section after the bend with the fully developed flow in a straight pipe under the same conditions. As a result of a series of optimization processes, velocity distribution at the 90° bend exit is compared between optimized RGV and without guide vane. The results show that the optimized RGV improves velocity distribution at the bend exit. Compared to the without guide vane case, the optimized RGV has delayed flow separation from α = 37° to 56°, and the flow reattachment point moves from x/D = 0.5 to 0 at the bend exit. Thanks to optimized RGV, the counter-rotating Dean vortices merged to form a single vortex at the center of the pipe. In addition, significant reduction in turbulent kinetic energy was observed, approximately 50% when using the optimized RGV compared to the without guide vane case.

Effect of a canopy patch made of streamwise-oriented plates on turbulence in an open-channel flow

AbstractThe paper examines the flow through a highly porous canopy patch made of streamwise-oriented thin plates arranged in a staggered configuration and placed in a rough-bed open channel. This patch geometry contrasts with the patches made of bluff bodies, which are nearly exclusively used in the literature. Particle Image Velocimetry was used to measure the flow upstream, within and downstream of the patch. The canopy patch has the effect of drastically reducing the turbulence level of the incoming flow, especially the turbulence shear stress, which is reduced by 85%. Spectral analysis of the velocity shows that the reduction in turbulent kinetic energy occurs at all length scales. Yet, at the entrance of the patch, the energy from the smallest scales up to the scale of the water surface increases. This suggests a spectral shortcut mechanism by which the large-scale structures of the incoming flow are disintegrated by the group of plates instead of decaying through the energy cascade. The increased small-scale turbulent energy then dissipates through the patch.

Pore-scale large eddy simulation of turbulent flow and heat transfer over porous media

• Fluid flow and heat transfer in a composite porous-fluid system is explored using LES. • Portion of the fluid entering to the porous region leaks to the non-porous region. • Counter-rotating vortex pairs is observed within and above the porous region. • Contour of temperature shows Kelvin-Helmholtz instabilities above the interface. • In contrast to the solid block, flow laminarisation occurs on the porous block. This paper investigates turbulent fluid flow and heat transfer over a porous medium in a channel using pore-scale large eddy simulation. Special attention is placed on the exchange of heat and flow between the porous and non-porous regions through the interface between the two regions. For this purpose, two different porous systems made of a packed bed of spheres and rectangular rods are analysed and the results are compared against a solid block case of the same size. Flow visualization shows that a significant portion of the fluid entering the porous blocks leaks from the porous region to the non-porous region through the porous-fluid interface. To discuss the effects of this flow leakage on the flow features and heat transfer, discussions are made regarding velocity, pressure, and temperature fields, as well as coherent structures, and turbulence production. The flow pattern inside the porous region indicates that the flow leakage clogs the pore channels inside the porous medium which induces a significant reduction in the streamwise momentum of the pore flow. In addition, coherent structures show that flow leakage leads to the creation of counter-rotating vortex pairs of fluid flow within and above the porous block that results in the formation of organized hairpin structures. Finally, the comparison of turbulence production for the porous and solid cases together with the onset growth of the Kelvin-Helmholtz instability on the porous-fluid interface show a reduction in turbulent kinetic energy above the leading edge of porous blocks. This observation implies that for the porous cases the transition to turbulence is postponed to the downstream of the porous block and it is not achieved as fast as the solid block.

Investigation on ultralight particle dispersions in dense two-phase flow using a kinetic friction stress model

The graphite particle dispersion characteristics with irregular shapes of dense gas-solid turbulent flow in downer reactor is numerically investigated using an improved kinetic friction stress model and a adopted drag coefficient describing momentum transfer between gas and non-spherical particles. Hydrodynamic predictions are in better line with experimental results. Redistributions of Reynolds normal and shear stresses with strongly anisotropic dispersions are captured. Comparisons to glass bead, ultralight particles elongate the second acceleration length, gather the concentration peaks at near wall region, and weak the maximum dense ring. Maximum kinetic energy at fully development region is approximately 1.2 times larger than that of second acceleration region. Lower particle sphericities are contribution to the enhancement of particle velocity, particle concentration, particle shear stress, particle collision frequency, and granular temperature, except for the reduction of turbulent kinetic energy. The negative amplitudes of shear stresses at fully development regions are approximately 4.0 times larger than those of second acceleration regions.

A Lagrangian probability-density-function model for turbulent particle-laden channel flow in the dense regime

Modeling particle-laden turbulent flows at high volume fractions requires accounting for the coupling between phases. The latter is often a sensitive point, and proper closure of the exchange and production terms due to the presence of particles is not straightforward. In the present work, a Lagrangian probability-density-function model developed for homogeneous cluster-induced turbulence is extended to a channel flow. The key features are consistent two-way coupling and the decomposition of the particle velocity into spatially correlated and uncorrelated components, which is crucial for dense flows and which allows dealing with collisions from a statistical point of view. A numerical scheme for the coupled solution of the stochastic differential equations for the particles and a Reynolds-stress model for the fluid is developed. Tests with tracer particles without two-way coupling are done to assess the validity and the consistency of the numerical scheme. Finally, two sets of numerical simulations with particles with different diameters in a turbulent channel flow at a shear Reynolds of Reτ=300 are reported. The effect of two-way coupling by varying the mass loading of the dispersed phase in the mass-loading range φ= 0–2 is analyzed, and the results are compared to previous Eulerian–Lagrangian and Eulerian–Eulerian direct-numerical simulation (DNS) studies. Mean velocities and turbulent kinetic energy show good agreement with DNS, especially regarding the trend with respect to mass loading. Consistent with prior work, increased mass loading causes a drastic reduction of turbulent kinetic energy in the range φ= 0–2.

The effect of Langmuir turbulence under complex real oceanic and meteorological forcing

In this study, we expand previous large eddy simulation (LES) modeling investigations of Langmuir turbulence (LT) to real ocean conditions using field observations collected under the multi-platform field campaign “Coupled Air–Sea Processes and Electromagnetic (EM) ducting Research (CASPER-East)”. The measurement site has strong local variabilities of temperature and salinity and experienced large variations in wind forcing and several cooling events.Although LT enhances the turbulence in the water column and deepens the mixed layer during most of the simulation period, being consistent with previous studies, strong reduction of turbulent kinetic energy (TKE) in the mixed layer is observed in the simulation with Stokes drift compared to that without Stokes drift during a short period. Analysis of the meteorological forcing and the TKE budget have revealed that in the circumstance of swell dominated wave fields with young wind seas, the presence of Stokes drift reduces shear production more than the Stokes production it generates, and a reduction of total TKE in the mixed layer may be expected whether or not the Stokes drift is aligned with the wind.Weak reduction of TKE due to the inclusion of Stokes drift is also observed beneath the mixed layer during a cooling event possibly due to the fact that the upwelling associated with Langmuir circulation at the base of the mixed layer counteracts on the downwelling associated with the deep convection and reduces the total turbulence level in the water column.While both resolved Reynold stresses and the bulk eddy viscosity decrease with the increase of wind-wave misalignment angle θww and become smaller than that in the case without Stokes drift when θww exceed 60°, the subgrid scale (SGS) part of the momentum flux increases with the increase of θww, suggesting that the LES solutions in cases with large wind-wave misalignment become more sensitive to the SGS models used and need to be dealt with caution.

Large eddy simulations of a turbulent channel flow with a deforming wall undergoing high steepness traveling waves

Backward traveling waves over a wall of a fully developed turbulent channel are known to reduce the drag coefficient, flow separation, and turbulence intensity. Based on previous studies of traveling waves with a small steepness (s = a/λ < 0.0625, a: nondimensional amplitude, λ: nondimensional wavelength), it is thought that the nondimensional wave-speed (C = C*/U*, C*: dimensional wave-speed, U*: mean channel velocity) is required to be more than one to have a zero net drag and a high reduction in the turbulent kinetic energy (TKE). This idea is tested here for waves with higher wave steepness (0.05 < s < 0.15) at various wave-speeds using large eddy simulations of a fully developed turbulent channel in which one wall is undergoing a traveling wave. It is found that the increase in wave steepness decreases the wave-speed at which a net zero drag is obtained, e.g., for waves with steepness of s = 0.05, 0.075, and 0.15, the wave-speed is, approximately, C = 1.6 ± 0.1, 0.9 ± 0.1, and 0.7 ± 0.1, respectively. Similarly, the increase in wave steepness decreases the wave-speed at which TKE of the flow in the vicinity of the wave is highly reduced, e.g., qualitatively minimized. In fact, the wave-speeds at which the high reduction in TKE is observed in this study are C = 1.2, 1.2, and 0.6 for waves with wave steepness of 0.05, 0.075, and 0.15, respectively.

Four-dimensional flow magnetic resonance imaging reveals the reduction in turbulent kinetic energy after percutaneous transluminal septal myocardial ablation in hypertrophic obstructive cardiomyopathy.

Four-dimensional flow magnetic resonance imaging reveals the reduction in turbulent kinetic energy after percutaneous transluminal septal myocardial ablation in hypertrophic obstructive cardiomyopathy.

Study on two wall temperature peaks of supercritical fluid mixed convective heat transfer in circular tubes

Heat transfer deterioration is numerically studied for supercritical fluids flowing upward in circular tubes at high heat fluxes and low mass fluxes. The simulations are conducted with Shear Stress Transport (SST) k-ω turbulent model in commercial software Fluent 15.0. Both water and CO2 are simulated and the results are consistent well with the experiments. It is found that there are two peaks of wall temperature when the heat transfer deterioration occurs, the first peak is narrow and sharp while the second peak is lower and broader. The mechanism of two wall temperature peaks is analyzed in details based on radial distributions of velocity and turbulent kinetic energy at different axial positions. It is found that the mechanism of the first peak is quite different from the second peak. The first peak is caused by buoyancy effect, which flattens the velocity distribution in the near wall region and leads to the reduction of turbulent kinetic energy and the impairment of heat transfer in the near wall region. For the second peak, it is the shear stress that flattens the velocity distribution in the main flow region and leads to the reduction of turbulent kinetic energy, which again impairs the heat transfer and causes the second peak.

Black carbon solar absorption suppresses turbulence in the atmospheric boundary layer.

The introduction of cloud condensation nuclei and radiative heating by sunlight-absorbing aerosols can modify the thickness and coverage of low clouds, yielding significant radiative forcing of climate. The magnitude and sign of changes in cloud coverage and depth in response to changing aerosols are impacted by turbulent dynamics of the cloudy atmosphere, but integrated measurements of aerosol solar absorption and turbulent fluxes have not been reported thus far. Here we report such integrated measurements made from unmanned aerial vehicles (UAVs) during the CARDEX (Cloud Aerosol Radiative Forcing and Dynamics Experiment) investigation conducted over the northern Indian Ocean. The UAV and surface data reveal a reduction in turbulent kinetic energy in the surface mixed layer at the base of the atmosphere concurrent with an increase in absorbing black carbon aerosols. Polluted conditions coincide with a warmer and shallower surface mixed layer because of aerosol radiative heating and reduced turbulence. The polluted surface mixed layer was also observed to be more humid with higher relative humidity. Greater humidity enhances cloud development, as evidenced by polluted clouds that penetrate higher above the top of the surface mixed layer. Reduced entrainment of dry air into the surface layer from above the inversion capping the surface mixed layer, due to weaker turbulence, may contribute to higher relative humidity in the surface layer during polluted conditions. Measurements of turbulence are important for studies of aerosol effects on clouds. Moreover, reduced turbulence can exacerbate both the human health impacts of high concentrations of fine particles and conditions favorable for low-visibility fog events.

Drag-reduction in buoyant and neutrally-buoyant turbulent flows over super-hydrophobic surfaces in transverse orientation

The present study involves Direct Numerical Simulations (DNS) of a turbulent channel flow subject to passive-control of super-hydrophobic surfaces (SHS) employed in form of ridges/posts at the bottom-wall of the channel, oriented in transverse direction to the flow. The simulations have been carried out for a fixed friction Reynolds number Reτ=180 to investigate the effect of thermal forcing in tandem with SHS at a fixed friction Richardson number Riτ=15. It is observed that on decreasing the width to depth (w/d) ratio of the ridge topology, the drag-reduction increases and this reduction is found to be maximum for the topology of SHS posts. A key effect of this control is reduction in cross-flow fluctuations in the buffer-layer (i.e. 10<z+⩽50) which lead to generation of weaker and more stable low-speed streaks which results in reduction in bursting of these near-wall streaks. This eventually causes considerable reduction in turbulent kinetic energy (TKE) which leads to turbulent skin-friction drag reduction at the controlled wall generally of the order of 10–22%. Unstable-stratification of the SHS flow results in the wall-normal convection of turbulent-vorticity which enhances cross-flow fluctuations leading to an increase in Reynolds shear-stresses near to the bottom-wall. In total the effect of heating is found to mitigate the net-reduction produced in turbulent drag by 6–7% for different cases employing ridge/post topology.

Cloud Shading Effects on Characteristic Boundary-Layer Length Scales

We studied the effects of shading by shallow cumulus (shallow Cu) and the subsequent effect of inducing heterogeneous conditions at the surface on boundary-layer characteristics. We placed special emphasis on quantifying the changes in the characteristic length and time scales associated with thermals, shallow Cu and induced thermal circulation structures. A series of systematic numerical experiments, inspired by Amazonian thermodynamic conditions, was performed using a large-eddy simulation model coupled to a land-surface model. We used four different experiments to disentangle the effects of shallow Cu on the surface and the response of clouds to these surface changes. The experiments include a ‘clear case’, ‘transparent clouds’, ‘shading clouds’ and a case with a prescribed uniform domain and reduced surface heat flux. We also performed a sensitivity study on the effect of introducing a weak background flow. Length and time scales were calculated using autocorrelation and two-dimensional spectral analysis, and we found that shading controlled by shallow Cu locally lowers surface temperatures and consequently reduces the sensible and latent heat fluxes, thus inducing spatial and temporal variability in these fluxes. The length scale of this surface heterogeneity is not sufficiently large to generate circulations that are superimposed on the boundary-layer scale, but the heterogeneity does disturb boundary-layer dynamics and generates a flow opposite to the normal thermal circulation. Besides this effect, shallow Cu shading reduces turbulent kinetic energy and lowers the convective velocity scale, thus reducing the mass flux. This hampers the thermal lifetime, resulting in a decrease in the shallow Cu residence time (from 11 to 7 min). This reduction in lifetime, combined with a decrease in mass flux, leads to smaller clouds. This is partially compensated for by a decrease in thermal cell size due to a reduction in turbulent kinetic energy. As a result, inter-cloud distance is reduced, leading to a larger population of smaller clouds, while maintaining cloud cover similar to the non-shading clouds experiment. Introducing a $$1\,\hbox {m}\, \hbox {s}^{-1}$$ background wind speed increases the thermal size in the sub-cloud layer, but the diagnosed surface–cloud coupling, quantified by characteristic time and length scales, remains.

Numerical investigation of turbulent-drag reduction induced by active control of streamwise travelling waves of wall-normal velocity

Direct Numerical Simulations studies in a fully-developed turbulent channel flow were performed to analyse the efficacy of the active control of streamwise travelling waves of wall-normal velocity on turbulent drag-reduction. A key result of application of control was reduction in cross-flow velocity fluctuations in the buffer layer zone (10<z+≤40). This leads to the generation of more stable near-wall streaks of weaker magnitude which results in suppression of bursting events in the buffer layer. This eventually caused significant reduction in turbulent kinetic energy which further lead to reduction of turbulent skin-friction drag at the controlled wall (generally around 10%–11%). Upstream travelling waves of wall-normal velocity were most effective in bringing about considerable drag-reduction. For the case of downstream travelling waves, low phase-speed waves produced drag-reduction while waves with phase-speeds approaching c+≃10 showed a significant increase in turbulent drag. The considerations of power-budget show the practical efficacy of upstream travelling waves and low phase-speed downstream travelling waves in producing net-power savings.

Low level jet intensification by mineral dust aerosols

Abstract. Modification of the intensity of a low level jet (LLJ) and near-surface wind speed by mineral dust is important as it has implications for dust emission and its long-range transport. Using the Weather Research and Forecasting with Chemistry (WRF/Chem) regional model, it is shown that direct radiative forcing by mineral dust reduces temperature in the lower atmosphere, but increases it in the layers aloft. The surface cooling is shown to be associated with a reduction of turbulent kinetic energy (TKE) and hence vertical mixing of horizontal momentum. Changes in the vertical profile of temperature over the regions that are under the influence of a LLJ are shown to result in an intensification of the LLJ and near-surface wind speed, but a decrease of winds aloft. These changes in the wind speed profile differ from results of previous research which suggested a decrease of wind speed in the lower atmosphere and its increase in the upper boundary layer.

Atmospheric Surface-Layer Response to the Annular Solar Eclipse of 15 January 2010 over Thiruvananthapuram, India

On 15 January 2010, Thiruvananthapuram in India (8.5°N, 76.9°E) witnessed one of the longest possible noontime annular solar eclipses spanning a period of about 7 min centred at 1314 local time. Here, we present a case study on the behaviour of the atmospheric surface layer by comparing the eclipse-induced observations with similar measurements recorded on cloud-free/clear-sky days. During the peak period of the eclipse, the incoming solar irradiance was reduced by 87% of its normal values, resulting in an air-temperature decrease near the surface of 1.2°C in association with a significant reduction in turbulent kinetic energy, momentum flux and sensible heat flux. The rate of instantaneous decay in solar radiation and sensible heat flux from the first contact of the eclipse to its annularity was greater than that seen during normal evening hours.

How does vegetation affect sedimentation on tidal marshes? Investigating particle capture and hydrodynamic controls on biologically mediated sedimentation

Plants are known to enhance sedimentation on intertidal marshes. It is unclear, however, if the dominant mechanism of enhanced sedimentation is direct organic sedimentation, particle capture by plant stems, or enhanced settling due to a reduction in turbulent kinetic energy within flows through the plant canopy. Here we combine several previously reported laboratory studies with an 18 year record of salt marsh macrophyte characteristics to quantify these mechanisms. In dense stands of Spartina alterniflora (with projected plant areas per unit volume of &gt;10 m−1) and rapid flows (&gt;0.4 m s−1), we find that the fraction of sedimentation from particle capture can instantaneously exceed 70%. In most marshes dominated by Spartina alterniflora, however, we find particle settling, rather than capture, will account for the majority of inorganic sedimentation. We examine a previously reported 2 mm yr−1 increase in accretion rate following a fertilization experiment in South Carolina. Prior studies at the site have ruled out organic sedimentation as the cause of this increased accretion. We apply our newly developed models of particle capture and effective settling velocity to the fertilized and control sites and find that virtually all (&gt;99%) of the increase in accretion rates can be attributed to enhanced settling brought about by reduced turbulent kinetic energy in the fertilized canopy. Our newly developed models of biologically mediated sedimentation are broadly applicable and can be applied to marshes where data relating biomass to stem diameter and projected plant area are available.