Influence of the upper mixed layer depth on Langmuir turbulence characteristics

The upper mixed layer depth(h)has a significant seasonal variation in the real ocean and the low-order statistics of Langmuir turbulence are dramatically influenced by the upper mixed layer depth.To explore the influence of the upper mixed layer depth on Langmuir turbulence under the condition of the wind and wave equilibrium,the changes of Langmuir turbulence characteristics with the idealized variation of the upper mixed layer depth from very shallow(h=5 m)to deep enough(h=40 m)are studied using a non-hydrostatic large eddy simulation model.The simulation results show that there is a direct entrainment depth induced by Langmuir turbulence(h_(LT))within the thermocline.The normalized depthaveraged vertical velocity variance is smaller and larger than the downwind velocity variance for the ratio of the upper mixed layer to a direct entrainment depth induced by Langmuir turbulence h/h_(LT)<1 and h/h_(LT)>1,respectively,indicating that turbulence characteristics have the essential change(i.e.,depth-averaged vertical velocity variance(DAVV)DADV for Langmuir turbulence)between h/h_(LT)<1 and h/h_(LT)>1.The rate of change of the normalized depth-averaged low-order statistics for h/h_(LT)<1 is much larger than that for h/h_(LT)>1.The reason is that the downward pressure perturbation induced by Langmuir cells is strongly inhibited by the upward reactive force of the strong stratified thermocline for h/h_(LT)<1 and the eff ect of upward reactive force on the downward pressure perturbation becomes weak for h/h_(LT)>1.Hence,the upper mixed layer depth has significant influences on Langmuir turbulence characteristics.

Aero-Hydrodynamic Coupled Dynamic Characteristics of Semi-Submersible Floating Offshore Wind Turbines Under Inflow Turbulence

In this study,the frequency characteristics of the turbulent wind and the effects of wind-wave coupling on the low-and high-frequency responses of semi-submersible floating offshore wind turbines(FOWT)are investigated.Various wave load components,such as first-order wave loads,combined first-and second-order difference-frequency wave loads,combined first-and second-order sum-frequency wave loads,and first-and complete second-order wave loads are taken into consideration,while different turbulent environments are considered in aerodynamic loads.The com-parison is based on time histories and frequency spectra of platform motions and structural load responses and statistical values.The findings indicate that the second-order difference-frequency wave loads will significantly increase the natural frequency of low-frequency motion in the responses of the platform motion and structure load of the semi-submersible platform,which will cause structural fatigue damage.Under the action of turbulent wind,the influences of second-order wave loads on the platform motion and structural load response cannot be ignored,especially under extreme sea conditions.Therefore,in order to evaluate the dynamic responses of semi-submersible FOWT more accurately,the actual environment should be simulated more realistically.

EXPERIMENTAL STUDY ON TURBULENT FEATURES IN THE NEGATIVE TRANSPORT REGION OF ASYMMETRIC PLANE CHANNEL FLOW

Turbulent features of streamwise and vertical components of velocity in the negative transport region of asymmetric plane channel flow have been studied experimentally in details. Experiments show that turbulent fluctuations in negative transport region are suppressed, and their probability distributions are far from Gaussian. Besides, the skewness factors attain their negative maxima at the position of the maximum mean velocity, whereas the flatness factors attain their positive maxima at the same position.

Eddy Covariance Tilt Corrections over a Coastal Mountain Area in South-east China:Significance for Near-Surface Turbulence Characteristics

Turbulence characteristics of an atmospheric surface layer over a coastal mountain area were investigated under different coordinate frames. Performances of three methods of coordinate rotation： double rotation （DR）, triple rotation （TR）, and classic planar-fit rotation （PF） were examined in terms of correction of eddy covariance flux. Using the commonly used DR and TR methods, unreasonable rotation angles are encountered at low wind speeds and cause significant run-to-run errors of some turbulence characteristics. The PF method rotates the coordinate system to an ensemble-averaged plane, and shows large tilt error due to an inaccurate fit plane over variable terrain slopes. In this paper, we propose another coordinate rotation scheme. The observational data were separated into two groups according to wind direction. The PF method was adapted to find an ensemble-averaged streamline plane for each group of hourly runs with wind speed exceeding 1.0 m s-1. Then, the coordinate systems were rotated to their respective best- fit planes for all available hourly observations. We call this the PF10 method. The implications of tilt corrections for the turbulence characteristics are discussed with a focus on integral turbulence characteristics, the spectra of wind-velocity components, and sensible heat and momentum fluxes under various atmospheric stabilities. Our results show that the adapted application of PF provides greatly improved estimates of integral turbulence characteristics in complex terrain and maintains data quality. The comparisons of the sensible heat fluxes for four coordinate rotation methods to fluxes before correction indicate that the PF10 scheme is the best to preserve consistency between fluxes.

Impact of depth ratio on flow structure and turbulence characteristics of compound open channel flows

Compound open channel flows appear in most natural rivers are of great importance in river management and flood control.In this study,large eddy simulations were carried out to simulate the compound open channel flows with four different depth ratios(hr=0.10,0.25,0.50,and 0.75).The main flow velocity,secondary flow,Reynolds stress,and bed shear stress were obtained from numerical simulations.The depth-averaged stream wise momentum equation was used to quantify the lateral momentum exchange between the main channel and floodplain.The instantaneous coherent structures were presented by the Q criterion method.The impact of hr on flow structure and turbulence charac-teristics was analyzed.The results showed that with the increase of hr,the high velocity area in the main channel shifted to the floodplain,and the dip phenomenon became more obvious;the Reynolds stress largely contributed to the lateral momentum exchange within the flows near the side walls of floodplain;and the vortex structures were found to significantly increase in the floodplain region.

Turbulence Characteristics of Swirling Reacting Flow in a Combustor with Staged Air Injection

This paper presents an experimental investigation of the turbulent reacting flow in a swirl combustor with staged air injection. The air injected into the combustor is composed of the primary swirling jet and the secon-dary non-swirling jet. A three dimension-laser particle dynamic analyzer (PDA) was employed to measure the in-stantaneous gas velocity. The probability density functions (PDF) for the instantaneous gas axial and tangential ve-locities at each measuring location, as well as the radial profiles of the root mean square of fluctuating gas axial and tangential velocities and the second-order moment for the fluctuating gas axial and tangential velocities are ob-tained. The measured results delineate the turbulence properties of the swirling reacting flow under the conditions of staged combustion.

Turbulent forced convection in a heat exchanger square channel with wavy-ribs vortex generator

Turbulent forced convective heat transfer and flow con figurations in a square channel with wavy-ribs inserted diagonally are examined numerically. The in fluences of the 30° and 45° flow attack angles for wavy-ribs, blockage ratio, R B= b/H = 0.05–0.25 with single pitch ratio, R P= P/H = 1 are investigated for the Reynolds number based on the hydraulic diameter of the square channel, Re = 3000–20000. The use of the wavy-ribs, which inserted diagonal in the square channel, is aimed to help to improve the thermal performance in heat exchange systems.The finite volume method and SIMPLE algorithm are applied to the present numerical simulation. The results are presented on the periodic flow and heat transfer pro files, flow con figurations, heat transfer characteristics and the performance evaluations. The mathematical results reveal that the use of wavy-ribs leads to a higher heat transfer rate and friction loss over the smooth channel. The heat transfer enhancements are around 1.97–5.14 and 2.04–5.27 times over the smooth channel for 30° and 45° attack angles, respectively. However, the corresponding friction loss values for 30° and 45° are around 4.26–86.55 and 5.03–97.98 times higher than the smooth square channel, respectively. The optimum thermal enhancement factor on both cases is found at R B= 0.10 and the lowest Reynolds number, Re = 3000, to be about 1.47 and 1.52, respectively, for 30° and 45° wavy-ribs.

Three-Dimensional Bursting Phenomena in Meander Channel

Experiments were conducted in a U-shaped open-channel flume with the intention of investigating the bursting phenomena in the meander channel. The experimental results of the secondary flow fields and the Reynolds shear stress distributions show that the velocity and velocity fluctuation in the transverse direction are not negligible. Moreover, the bursting process is investigated using the three-dimensional quadrant analysis, which is more accurate than using the traditional two-dimensional quadrant analysis for the meandering channel. It is obtained from the experimental results that the internal group of events occurs more frequently than the external group, particularly the internal ejection and internal sweep events. In addition, the transition probabilities of the movements, which are defined as the changes of events from the current situation to the next situation in a time series, show that the stable organizations of events are the most possible movements, whereas the cross organizations of events have the least possible movements.

Numerical Study of Two-Dimensional Viscous Flow over Dams

In this paper, the characteristics of two-dimensional viscous flow over two dams were numerically investigated. The results show that the behavior of the vortices is closely related to the space between two dams, water depth, Fr number and Reynolds number. In addition, the flow properties behind each dam are different, and the changes over two dams are more complex than over one dam. Finally, the relevant turbulent characteristics were analyzed.

Analysis of atmospheric turbulence in the upper layers of sea fog

Atmospheric turbulence plays a vital role in the formation and dissipation of fog. However,studies of such turbulence are typically limited to observations with ultrasonic anemometers less than 100 m above ground. Thus,the turbulence characteristics of upper fog layers are poorly known. In this paper,we present 4-layers of data,measured by ultrasonic anemometers on a wind tower about 400 m above the sea surface; we use these data to characterize atmospheric turbulence atop a heavy sea fog. Large differences in turbulence during the sea fog episode were recorded. Results showed that the kinetic energy,momentum flux,and sensible heat flux of turbulence increased rapidly during the onset of fog. After onset,high turbulence was observed within the uppermost fog layer. As long as this turbulence did not exceed a critical threshold,it was crucial to enhancing the cooling rate,and maintaining the fog. Vertical momentum flux and sensible heat flux generated by this turbulence weakened wind speed and decreased air temperature during the fog. Towards the end of the fog episode,the vertical distribution of sensible heat flux reversed,contributing to a downward momentum flux in all upper layers. Spatial and temporal scales of the turbulence eddy were greater before and after the fog,than during the fog episode. Turbulence energy was greatest in upper levels,around 430 m and 450 m above mean sea level(AMSL),than in lower levels of the fog(390 m and 410 m AMSL); turbulence energy peaked along the mean wind direction. Our results show that the status of turbulence was complicated within the fog; turbulence caused fluxes of momentum and sensible heat atop the fog layer,affecting the underlying fog by decreasing or increasing average wind speed,as well as promoting or demoting air temperature stratification.

EXPERIMENTAL STUDY ON TURBULENT BOUNDARY LAYER CHARACTERISTICS OVER STREAMWISE RIBLETS

Measurements of characteristics by means of a two-component Laser DopplerVelocimeter (LDV) were carried out in turbulent boundary layers over both a symmetric V-shapedribbed plate and a smooth one in a low speed wind tunnel. The present results clearly indicate thatthe logarithmic velocity profile over the riblets surface is shifted upward with a 30. 9% increasein the thickness of the viscous sublayer. Also a change in the log-law region is found. And themaximum value of streamwise velocity fluctuations is reduced by approximately 17%. The skewness andflatness factors do not show any change besides those in the region of y^+ 【 0. 6 . It is evidentthat the Reynolds shear stress over the riblets is reduced. Further more, in log-law region, theReynolds shear stress has a larger reduction of up to 18%.

PIV analysis and high-speed photographic observation of cavitating flow field behind circular multi-orifice plates

Based on a self-developed hydrodynamic cavitation device with different geometric parameters for circular multi-orifice plates,turbulence characteristics of cavitating flow behind multi-orifice plates,including the effects of orifice number and orifice layout on longitudinal velocity,turbulence intensity,and Reynolds stress,were measured with the particle image velocimetry(PIV)technique.Flow regimes of the cavitating flow were also observed with high-speed photography.The experimental results showed the following:(1)high-velocity multiple cavitating jets occurred behind the multi-orifice plates,and the cavitating flow fields were characterized by topological structures;(2)the longitudinal velocity at each cross-section exhibited a sawtooth-like distribution close to the multi-orifice plate,and each sawtooth indicated one jet issuing from one orifice;(3)there were similar magnitudes and forms for the longitudinal and vertical turbulence intensities at the same cross-section;(4)the variation in amplitude of Reynolds stress increased with an increase in orifice number;and(5)the cavitation clouds in the flow fields became denser with the increase in orifice number,and the clouds generated by the staggered layout of orifices were greater in number than those generated by the checkerboard-type one for the same orifice number.The experimental results can be used to analyze the mechanism of killing pathogenic microorganisms through hydrodynamic cavitation.

An Experimental Study of Three-Dimensional Characteristics of Turbulent Wakes of Axial Compressor Rotors

This paper presents an experimental investigation of the characteristics of three-dimensional turbulent wakes of an isolated axial compressor rotor and a single-stage axial compressor rotor. The wakes were measured from hub to tip using a single-slant hot-wire and a four-hole conical high frequency pressure probe. The experiments were made at both design and near stall conditions. Variations of mean velocities, total pressure, static pressure and turbulence stresses in the wakes are shown and interpreted. The experimental data from the isolated compressor rotor wake are compared with that from the single-stage compressor rotor.

Turbulence,aeration and bubble features of air-water flows in macro-and intermediate roughness conditions

Free surface flows aeration potential causing the in macro- and intermediate flow characteristics to vary roughness conditions have a high with slopes and discharges. The underlying mechanism of two-phase flow characteristics in macro- and intermediate roughness conditions were analyzed in an experimental setup assembled at the Laboratory of Hydraulic Protection of the Territory （PITLAB） of the University of Pisa, Italy. Crushed angular rocks and hemispherical boulders were used to intensify the roughness of the bed. Flow rates per unit width ranging between 0.03 m^2/s and 0.09 m^2/s and slopes between 0.26 and 0.46 were tested over different arrangements of a rough bed. Analyses were mainly carried out in the inner flow region, which consists of both bubbly and intermediate flow regions. The findings revealed that the two-phase flow properties over the rough bed were much affected by rough bed arrangements. Turbulence features of two-phase flows over the rough bed were compared with those of the stepped chute data under similar flow conditions. Overall, the results highlight the flow features in the inner layers of the two-phase flow, showing that the maximum turbulence intensity decreases with the relative submergence, while the bubble frequency distribution is affected by the rough bed elements.

Observation of Stable Marine Boundary Layer by Shipborne Coherent Doppler Lidar and Radiosonde over Yellow Sea

Shipborne observations obtained with the coherent Doppler lidar(CDL)and radiosonde during 2014 campaign were used to study the structure of marine boundary layer in the Yellow Sea.Vertical wind profiles corrected for ship motion was used to derive higher-order statistics,showing that motion correction is required and significant for turbulence analysis.During a day with weak mesoscale activity,a complexed three-layer structure system was observed.The lowest layer showed a typical stable boundary layer structure feature.An aerosol layer with abrupt variation in wind speed and relative humidity always appeared at the middle layer,the formation of which may be due to Kelvin-Helmholz instability.The top layer encountered a dramatic change in wind direction,which may result from the warm advection from the Eurasian continent on the basis of backward trajectory analysis.Furthermore,the MABL height in stable regime was derived from potential temperature,CDL signal-to-noise ratio(SNR)and CDL vertical velocity variance,respectively.The stable boundary layer(SBL)height in SBL can be derived from the inversion layer of potential temperature profile,and the mixing height in SBL can be retrieved from the vertical velocity variance gradient method.Neither the SBL height nor the mixing height is in agreement with the height retrieved from CDL SNR gradient method because of different definition and criterion.One of the limitations of SNR gradient method for MABL retrieval is that it is easier to be affected by the lofted decoupled aerosol layer,where the retrieved result is less suitable.Finally,the higher-order vertical velocity statistics within the marine stable boundary layer were investigated and compared with the previous studies,and different turbulence mechanisms have an important effect on the statistics deviation.

Turbulent structure and bursting process in multi-bend meander channel

The flow characteristics in a meander channel are fully three-dimensional. With the primary flow in the streamwise direction, the secondary flow in the transverse and vertical directions induced by the channel bends are significant in the analyses of the turbulent structure. Some of the analyses in the straight channel, for instance, the quadrant analysis for the bursting phenomena, are inadequate in investigating the meander channel, since the flow in the transverse direction is not taken into account. In order to reveal the flow characteristics in a multi-bend meander channel, especially, the bursting process, experiments are conducted in the present study. With the three-dimensional quadrant analysis, the influence of the transverse flow velocity during the bursting process could be correctly addressed. The analyses and discussions are presented in this paper.

Particle effects on the hydrodynamics in slurry bubble column reactors:A review from multiscale mechanisms Author links open overlay panel

Particles can appear as catalyst,reactant or product in various gas-liquid-solid three-phase production processes.Slurry bubble column reactors(SBCRs),as a kind of three-phase reactors,are preferred for phase contacting and mixing.However,literature studies concerning the effects of particles on the hydrodynamics of SBCRs are manifold and inconsistent in conclusions.Essentially,the multiscale interactions between particles,turbulent eddies and bubbles determine the reactor performance.This review focuses on revealing the particle effects in SBCRs from the perspective of multiscale mechanisms.Macroscopic hydrodynamic changes due to particle effects in literature are summarized.Dimensionless parameters,including the Stokes number,the solid-to-liquid density ratio,the ratio of particle and liquid characteristic lengths,the contact angle and the particle volume fraction are adopted to evaluate the characteristics of gas-liquid-solid flows.The relationships between particle influencing mechanisms and these parameters are analyzed and determined.Inconsistent experimental results are explained by different ranges of these dimensionless parameters.Moreover,particle effects at the mesoscale and microscale,such as the influence on the bubble dynamics and the pivoting effect on the turbulence energy spectrum,are elaborated.Finally,progress in modeling the SBCRs with consideration of particles effects using the Euler method are introduced.This review aims to improve the overall understanding of the complex hydrodynamics in the SBCRs.

NUMERICAL STUDY ON TURBULENT COUNTER-GRADIENT-TRANSPORT PHENOMENA IN ASYMMETRIC TURBULENT CHANNEL FLOW

In this paper, the turbulence characteristics were numerically investigatedin an asymmetric turbulent channel flow and the computational results were compared with therelevant experimental data. It shows that the results are consistent with the experiments and thereexist Counter-Gradient Momentum Transport (CGMT) phenomena in the central region near the smoothwall, and this region is as large as 6 percent of the channel width. In addition, a region, in whichCounter-Gradient-Transport (CGT) phenomena occur more evidently, is found close to the rough wall.These results can help to gain a deeper insight into the mechanism of CGT phenomena.

Assessment of the Turbulence Characteristics of Shaped Film Cooling Hole with Scale Resolving Simulation

The turbulence characteristics of the shaped hole film cooling are very complex.In this study,Large Eddy Simulation(LES)and Reynolds-averaged Navier-Stokes(RANS)are used to study the film cooling of the shaped hole.The time-averaged results are compared with the experimental data in the literature.Because of the eddy-viscosity model,the RANS method roughly deals with the simulation of boundary layer,which leads to a large deviation.The RANS results are compared with the LES results to identify the weaknesses of the Realizable k-e model in predicting the turbulence characteristics of the shaped hole film cooling.The eddy viscosity hypothesis and the temperature gradient diffusion hypothesis are evaluated using LES data.Furthermore,the turbulence characteristics of the in-hole flow are analysed with the help of the incremental Proper Orthogonal Decomposition(iPOD).The turbulence presents strong anisotropy and some convection structures are induced from the shear zone.

Alluvial channel hydrodynamics around tandem piers with downward seepage

In this paper,we report the turbulent flow structures and the scour geometry around two piers with different diameters.An experiment was conducted on a non-uniform sand bed with two types of tandem arrangements,namely,pier(T1)with a 75 mm front and 90 mm rear,and pier(T2)with a 90 mm front and 75 mm rear,with and without-seepage flows,respectively.A strong wake region was observed behind the piers,but the vortex strength diminished with downward seepage.Streamwise velocity was found to be maximum near the bed downstream of the piers and at the edge of the scour hole upstream of the piers.Quadrant analysis was used to recognize the susceptible region for sediment entrainment and deposition.Upstream of the piers near the bed,the moments,turbulent kinetic energy(TKE),and TKE fluxes were found to decrease with downward seepage,in contrast to those in a plane mobile bed without piers.The reduction percentages of scour depth at the rear pier compared with the front one were approximately 40%for T1 and 60%for T2.Downward seepage also resulted in restrained growth of scouring with time.