Turbulence Levels Research Articles

Impact of Low Freestream Turbulence and Surface Roughness on the Flat-Plate Boundary Layer Transition Under a High-Lift Airfoil Pressure Gradient

Abstract Effects of low incoming freestream turbulence level (Tu < 1%) and surface roughness on the transition of flat-plate boundary layers under a high-lift airfoil pressure gradient have been investigated. Time-resolved streamwise and wall-normal velocity fields with surface roughness values of Ra/C = 0.065 × 10−5, 4.417 × 10−5, and 7.428 × 10−5 have been measured at a fixed Reynolds number of 5.2 × 105 and freestream turbulence intensity of 0.2%. For the reference smooth surface of Ra/C = 0.065 × 10−5, a laminar separation bubble forms from about 64% to 83% of the chord length. Increasing surface roughness has little impact on the laminar boundary layer separation onset but reduces the height and length of the separation bubble and induces earlier transition. For Ra/C = 4.417 × 10−5, displacement thickness during transition is slightly thinner and the overall momentum deficit is slightly lower than those for Ra/C = 0.065 × 10−5. For Ra/C = 7.428 × 10−5, the separation bubble becomes hardly visible as the transition mode approaches the attached mode, and turbulent mixing by the wall-bounded turbulence becomes dominant. In addition, the portion of turbulent wetted area increases due to earlier transition, and momentum deficit increases more rapidly in the turbulent wetted area. Thus, the overall momentum deficit for Ra/C = 7.428 × 10−5 is larger than that for Ra/C = 0.065 × 10−5.

Rapid Restratification Processes Control Mixed Layer Turbulence and Phytoplankton Growth in a Deep Convection Region

AbstractThe Gulf of Lion, Northwestern Mediterranean Sea, is one of few oceanic regions where deep convection occurs. We investigate the restratification following a convection event using measurements from an ocean glider equipped with turbulence microstructure sensors. This unique combination of instruments provides a high‐resolution description of the mixed layer with regard to turbulence, stratification and chlorophyll. We observe a rapid restratification process that proceeds over a timescale of days to one week. We find that restratification exerts a leading order control on surface mixed layer turbulence variability, as abrupt changes in turbulence dissipation rates are associated with the formation of near‐surface stratification. The near‐surface formation of stratification occurs through both the diurnal variability in surface buoyancy fluxes and through lateral advective processes. We conclude that daily near‐surface processes that influence stratification control mixed layer turbulence levels, and thus the phytoplankton response in the critical transition period to spring bloom.

Comparative analysis of C n2 estimation methods for sonic anemometer data

Wind speed and sonic temperature measured with ultrasonic anemometers are often utilized to estimate the refractive index structure parameter C n 2, a vital parameter for optical propagation. In this work, we compare four methods to estimate C n 2 from C T 2, using the same temporal sonic temperature data streams for two separated sonic anemometers on a homogenous path. Values of C n 2 obtained with these four methods using field trial data are compared to those from a commercial scintillometer and from the differential image motion method using a grid of light sources positioned at the end of a common path. In addition to the comparison between the methods, we also consider appropriate error bars for C n 2 based on sonic temperature considering only the errors from having a finite number of turbulent samples. The Bayesian and power spectral methods were found to give adequate estimates for strong turbulence levels but consistently overestimated the C n 2 for weak turbulence. The nearest neighbors and structure function methods performed well under all turbulence strengths tested.

Features of Adaptive Phase Correction of Optical Wave Distortions under Conditions of Intensity Fluctuations

An analysis of the features of measurements and correction of phase distortions in optical waves propagating in the atmosphere at various levels of turbulence was performed. It is shown that with increasing intensity fluctuations, the limiting capabilities of phase correction decrease, and the phase of an optical wave that has passed through a turbulence layer consists of two components: potential and vortex. It was found that even in the region of weak fluctuations there is an overlap of spectral filtering functions for intensity and phase fluctuations. Areas of turbulence inhomogeneities have been identified that will have mutual influence and negatively affect the operation of the phase meter. It is noted that correlation functions, both phase and intensity, are less susceptible to this compared to structural functions. The results of experimental studies on the reconstruction of the wavefront of laser radiation distorted by atmospheric turbulence using a Shack–Hartmann wavefront sensor during vignetting and central screening of the entrance pupil in the optical system are presented. Studies have been carried out on the propagation of laser radiation along a horizontal atmospheric path for various levels of turbulence. The results are analyzed in terms of Zernike polynomials.

Reduction of electrostatic turbulence in a quasi-helically symmetric stellarator via critical gradient optimization

We present a stellarator configuration optimized for a large threshold (‘critical gradient’) for the onset of the ion temperature gradient (ITG) driven mode, which achieves the largest critical gradient we have seen in any stellarator. Above this threshold, gyrokinetic simulations show that the configuration has low turbulence levels over an experimentally relevant range of the drive strength. The applied optimization seeks to maximize the drift curvature, leading to enhanced local-shear stabilization of toroidal ITG modes, and the associated turbulence. These benefits are combined with excellent quasi-symmetry, yielding low neoclassical transport and vanishingly small alpha particle losses. Analysis of the resulting configuration suggests a trade-off between magnetohydrodynamic (MHD) and ITG stability, as the new configuration possesses a vacuum magnetic hill.

Experimental study on diesel spray flame and wall heat transfer of two-dimensional combustion chamber: effect of common rail pressure

Reducing the heat transfer occurring during the spray combustion of diesel engines remains a challenging task. This study investigated the spray flame behaviour, wall heat flux, and heat transfer phenomena of diesel engines at varying common rail pressures. A rapid compression and expansion machine (RCEM) was employed to simulate a single cycle of diesel spray combustion using split injection sprays. The two-dimensional (2D) stepped piston cavity is a uniquely shaped model used to describe the spray flame behaviour in the combustion chamber. To investigate the heat transfer phenomena, we installed wall heat flux sensors in the cylinder and cavity side. The results indicated that higher common rail pressures increased the peak value of the wall heat flux and reduced the combustion duration and the luminous flame. The higher common rail pressure led to an increase in spray velocity, thereby enhancing the turbulence level. This increase in turbulence to higher peak value of the wall heat flux. The highest peak value of the wall heat flux was reached by the cylinder head owing to the intense combustion region, followed by the cavity bottom owing to the highest flame temperature occurring around this location. Additionally, the heat transfer phenomena in diesel engines were successfully represented by local Nu-Re correlations. Our results contribute to the discussion on heat transfer phenomena that influence the thermal efficiency of diesel combustion engines.

Core electron temperature turbulence and transport during sawtooth oscillations in the DIII-D tokamak

Sawteeth are one of the concerning instabilities in ITER and future burning plasma experiments. Sawtooth dynamics and its interaction with broadband plasma turbulence has been a challenge for predictive simulations of core transport in future fusion devices. This study provides new observations of core turbulence behavior during sawtooth oscillations in DIII-D hydrogen L-mode neutral beam injection heated plasmas in an inner wall limited configuration. A strong correlation of electron temperature and density turbulence levels with the sawtooth oscillation phase has been observed at locations inside the T e inversion radius and/or safety factor q = 1 magnetic surface. The T e turbulence amplitude in the core during the sawtooth ramp exhibits a critical T e gradient behavior inside but not near the T e inversion radius/q = 1 magnetic surface. The most unstable mode calculated from the trapped gyro-landau fluid turbulence simulations reveal a change from low-k ion-type to low-k electron-type modes from pre- to post- sawtooth crash time periods.

Prospects for the use of Simulation Modeling for Managing Student Entrepreneurship at the University

The relevance of the article is in the choice and application of research methods for the decision-making process in a university. The level of complexity of management objects is growing due to the increasing level of business turbulence. Universities are no exception, based on their ongoing transformation and the process of becoming a player in the market economy. The use of simulation modeling is a promising topic and deserves more attention. The purpose of the study is to present and demonstrate the possibilities of using a simulation tool to manage university processes. Research hypothesis: despite the fact that simulation modeling is becoming an increasingly popular method of forecasting in social systems, it is not used in the field of university management. The paper presents a model of student entrepreneurship at IRNRTU (Irkutsk), built with the help of AnyLogic. It demonstrates its work from the point of view of the behavior of the model under normal conditions and conditions of stimulating a separate competition (Program “Umnik” (Foundation for Assistance to Small Innovative Enterprises), Program “START” (Foundation for Assistance to Small Innovative Enterprises) and the “Startup” program (Foundation for Assistance to Small Innovative Enterprises with the Federal project “University Technological Entre-preneurship Platform”). As the software, the AnyLogic program (Russia) was selected and applied. The group of researchers selected a discrete-event model for ease of collection statistics and selection of the winner. As a result, it was concluded that the model is able to predict the most likely outcome of the underlying actions or interests in a shorter period of time compared to other forecasting tools. The theoretical significance of the work is in expanding the list of issues under study for the use of simulation modeling in higher education. The practical significance is in the application of the results to assess the effectiveness of student entrepreneurship and it might be of interest to both university administrations and higher education researchers.

Performance evaluation of modified pintle-type fuel injector for LRE (liquid rocket engine)

This research focuses on the fluid dynamics analysis and empirical validation of a modified pintle-type fuel injector through computational modeling. The primary aim is to assess the injector's performance metrics, including spray dispersion and orientation, flow velocity, turbulence levels, and vortex formation, by adjusting the pintle's positioning, which inherently modulates the flow rate. The observed effects draw parallels with the interactions seen in conical spray and hollow cone swirl atomizers. A pintle-type fuel injector model was developed using geometric design software, and computational fluid dynamics simulations were conducted to ensure effective fuel and oxidizer blending. This study evaluates key performance indicators, such as thrust output and combustion efficiency, under specific operational pressures and velocities at the injector's orifice.

The second data release from the European Pulsar Timing Array

The European Pulsar Timing Array (EPTA) and Indian Pulsar Timing Array (InPTA) collaborations have measured a low-frequency common signal in the combination of their second and first data releases, respectively, with the correlation properties of a gravitational wave background (GWB). Such a signal may have its origin in a number of physical processes including a cosmic population of inspiralling supermassive black hole binaries (SMBHBs); inflation, phase transitions, cosmic strings, and tensor mode generation by the non-linear evolution of scalar perturbations in the early Universe; and oscillations of the Galactic potential in the presence of ultra-light dark matter (ULDM). At the current stage of emerging evidence, it is impossible to discriminate among the different origins. Therefore, for this paper, we consider each process separately, and investigated the implications of the signal under the hypothesis that it is generated by that specific process. We find that the signal is consistent with a cosmic population of inspiralling SMBHBs, and its relatively high amplitude can be used to place constraints on binary merger timescales and the SMBH-host galaxy scaling relations. If this origin is confirmed, this would be the first direct evidence that SMBHBs merge in nature, adding an important observational piece to the puzzle of structure formation and galaxy evolution. As for early Universe processes, the measurement would place tight constraints on the cosmic string tension and on the level of turbulence developed by first-order phase transitions. Other processes would require non-standard scenarios, such as a blue-tilted inflationary spectrum or an excess in the primordial spectrum of scalar perturbations at large wavenumbers. Finally, a ULDM origin of the detected signal is disfavoured, which leads to direct constraints on the abundance of ULDM in our Galaxy.

Experimental investigations on an FSO-fiber converged communication system under fog and turbulence

This paper presents experimental results for a free-space optical-fiber converged (FSO-FC) communication system under varying turbulence and foggy conditions. Based on the experimental measurements, we statistically characterize the FSO channel under different levels of turbulence and fog in terms of their respective probability density functions (PDFs). Our experimental PDFs fit well with the theoretical PDFs proposed in the literature. We experimentally evaluate the average bit error rate (ABER) of the considered FSO-FC communication system under different levels of turbulence and fog. Further, to compensate for the effects of turbulence and fog, we use multimode fiber as the receiver and compare the results with single-mode fiber as the receiver. Finally, we show the improvement in ABER under varying turbulence and fog by adding cyclic redundancy check bits to data bits.

Experimental study of a separated shear layer transition under acoustic excitation

The paper discusses experimental results on the effect of broadband acoustic excitation on laminar-to-turbulent transition in a separated shear layer developing on a flat plate subjected to an adverse pressure gradient (APG) and freestream turbulence level equal to Tu ≅ 1 %. The study encompasses the influence of Reynolds number (Rex = 185 000 and 370 000) and sound pressure level (SPL). The inherent complexity of the problem is simplified by providing an acoustic excitation from a controlled source (loudspeaker), acting on the boundary layer developing on the flat plate with a given streamwise pressure gradient. Two types of instabilities were identified in the pre-transitional boundary layer in unexcited flows. One was related to the inviscid Kelvin-Helmholtz (K-H) instability, while the second one was associated with formation of streamwise-oriented Klebanoff streaks (so-called Klebanoff mode).In the low Reynolds number case (Rex = 185 000), the K-H was responsible for transition onset, while in the high Reynolds number flow (Rex = 370 000), the Klebanoff distortions dominated the turbulent breakdown with the minor effect of the K-H instability. In addition to the naturally developing boundary layer, the flow was exposed to a pink noise characterized by SPL = 125 dB and 135 dB. In the low Reynolds number case, the acoustic excitation enhanced the K-H instability. It resulted in an earlier laminar-to-turbulent transition in case with higher sound pressure level (135 dB). In the high Reynolds number flow, the acoustic excitation enhanced the mixed-type transition mechanism with dominant role of the Klebanoff streaks. Shrinking or complete suppression of the separation bubbles was observed, depending on the applied sound pressure level (125 and 135 dB).

Dynamic modelling of subgrid scalar dissipation rate in premixed and partially premixed flames with differential filter

Large eddy simulation (LES) paradigms are used in the present work to predict premixed and partially premixed turbulent flames with flamelets based thermochemistry and presumed filtered density function approach for turbulence-chemistry interaction modelling. The combustion model requires a closure for the scalar dissipation rate of a progress variable, in which a modelling constant must be chosen. The present work focuses on the computation of the model constant through dynamic procedures based on the scale-similarity assumption, which requires the application of test-filters. In particular, two test-filtering approaches for LES, based respectively on an algebraic formulation and a newly proposed differential equation, are tested for flame configurations at different levels of turbulence, and using block-structured and unstructured meshes. The analysis shows that the differential filter, unlike the algebraic one, is handled well in situations of weak turbulence at comparable computational costs. At higher turbulence conditions the outcome looks less dependent on the test-filter and mesh topology used, although quantitative differences in the behaviour of the dynamically-computed model constant are still observable and discussed. Further analyses to understand the behaviour of the two filters are presented in the paper.

Fundamental insights on turbulence characterization, vortex motion and ignition mechanism of sub/supersonic turbulent jet flames

This paper presents turbulence characterization and vortex behavior measurements of sub/supersonic jet flames subjected to extreme levels of turbulence. Specifically, sub/supersonic CH4/O2/N2 premixed jet flames were studied under calculated Reynolds (Re) number and measured jet tip velocity (Vtip) of around 20,000 and 100 m/s or greater, respectively. The vortex motion on the flame surface was measured by Schlieren image velocimetry (SIV), and ignition mechanisms of sub/supersonic jets are discussed by analyzing vortex motion and free radical reactions, respectively. Results show that the maximum vortex motion is maintained in the jet top edge region. In contrast, the velocity of vortex motion in the middle of the jet, i.e., near the side of the jet orifice, is reduced by a factor of 4–5, which visually shows a “mild” velocity gradient on the jet surface. The direction turning points of X/D = 7.25, X/D = 6.06, X/D = 6.06 and X/D = 3.40 were found in the flow direction (Y-direction) for four typical cases of supersonic jets with top primary swirling motion, supersonic jets with top secondary swirling motion, supersonic jets with weak top swirling motion and subsonic jets without top flame vortex motion. It is speculated that the ignition location is strongly correlated with the direction turning point of the vortex movement along the Y-direction. The vortex propagating along the flow direction disperses many small-scale vortices moving in the X-direction, the X-direction being perpendicular to the flow direction. Moreover, the ignition mechanisms of sub/supersonic jet were explored separately.

Plasma beta dependence of turbulent transport suggesting an advantage of weak magnetic shear from local and global gyrokinetic simulations

A higher plasma β is desirable for realizing high performance fusion reactor, in fact, one of the three goals of JT-60SA project is to achieve a high-β regime. We investigate key physical processes that regulate the β dependence of turbulent transport in L-mode plasmas by means of both local and global gyrokinetic simulations. From local simulations, we found that the turbulent transport does not decrease as β increases, because the electromagnetic stabilizing effect is canceled out by the increase of the Shafranov shift. This influence of the Shafranov shift is suppressed when the magnetic shear is weak, and thus the electromagnetic stabilization is prominent in weak shear plasmas, suggesting an advantage of weak magnetic shear plasmas for achieving a high-β regime. In high β regime, local gyrokinetic simulations are suffered from the non-saturation of turbulence level. In global simulations, by contrast, the electromagnetic turbulence gets saturated by the entropy advection in the radial direction to avoid the zonal flow erosion due to magnetic fluctuations. This breakthrough enables us to explore turbulent transport at a higher β regime by gyrokinetic simulations.

Analysis of wind characteristics and wind energy resource assessment for Tonga using eleven methods of estimating Weibull parameters

Analysisof wind characteristics and assessment of wind energy resource is carried out at a location in Tonga with the help of twelve months of measurements carried out at 34 m and 20 m heights above ground level. The daily, monthly and annual averages are computed. The wind shear analysis and its diurnal variation were studied and compared with the temperature variation. The turbulence levels at the two heights were estimated for the entire measurement period as well as for some typical days. For estimating the Weibull parameters, eleven methods were employed along with goodness of fit and error estimates to find the best method. The overall averaged wind speed for the entire period of study is estimated to be 4.41 m/s at 34 m above ground level. The predominant wind direction was south-east for Tonga. ‘Moments’ is seen to be the best method to determine accurate Weibull parameters. The average net annual energy production from one Vergnet 275 kW wind turbine is 198.57 MWh. A payback period of 8.95 years by installing five turbines near the measurement location was estimated, which is very encouraging in terms of investment. Installing wind turbines will lower the heavy reliance on the imported fossil fuels in the country and also help in achieving Sustainable Development Goal 7.

Transonic buffet simulation using a partially-averaged Navier-Stokes approach

The present article assesses the capability of the partially averaged Navier-Stokes (PANS) method to reproduce accurately the self-sustained shock oscillations, also known as transonic buffet, occurring on airfoils and wings at transonic regime under certain conditions of Mach number and angle of attack. The test case under analysis is an OAT15A unswept wing at Mach number M∞=0.73 and Reynolds number Rec=3×106. The three-dimensional flow is studied by accounting for the wind tunnel walls adopted in the experiments of Jacquin et al. [1] in the simulations. The computations on a large-span, confined configuration reveal a strong three-dimensionality of the flow both before and after the buffet onset. Attention is paid to the comparison with unsteady Reynolds-averaged Navier Stokes (URANS) results, to show the benefits of PANS in resolving flow unsteadiness at different flow resolutions, especially on affordable CFD grids, at limited additional cost. In this context, the role of the mesh metrics and the local turbulence level in the formulation of the model is described, as well as the relation of this latter with the spatiotemporal discretization used for the numerical simulations. The aim is to extend the use of PANS and obtain accurate predictions of flow cases involving shock-wave boundary layer interactions without expensive approaches.

Unraveling formal and informal business ties in shaping foreign subsidiary capabilities

PurposeDrawing upon the resource-based view this study aims to examine the connections between formal and informal business relationships and resource-bridging and adaptive capabilities within the context of foreign subsidiaries of multinational enterprises (MNEs) operating in Thailand. Based on prior literature emphasizing business network ties as sources of competitive advantage in emerging markets, this study extends the discourse by investigating the moderating effects of technological turbulence, power distance and assertiveness.Design/methodology/approachThis study uses a quantitative research approach, using data obtained from a self-administered survey conducted among 168 foreign subsidiaries spanning diverse industries in Thailand. The data were analyzed by using multiple-group structural equation modeling to test the hypothesized relationships.FindingsCultivating different types of business ties enables foreign subsidiaries to improve different types of capabilities. While interpersonal relationships (i.e. informal businessties) enable them to develop their abilities to combine various resources (i.e. resource-bridging capability), rigid contractual-based relationships (i.e. formal businessties) help them to be more adaptive (i.e. adaptive capability). These relationships are also contingent upon the levels of technological turbulence, host-country power distance and host-country assertiveness.Originality/valueThis research builds upon prior research on network ties and capability building by delineating the specific nature of capabilities. Contradicting to the previous findings, demonstrating a negative relationship between formal business ties and capabilities, this study found that each type of business tie enables foreign subsidiaries to enhance different types of capabilities under different circumstances. Moreover, this study adopts a lens of host-country national culture rather than home-country culture in investigating the moderating effects of power distance and assertiveness.

Estimation of modified Zernike coefficients from turbulence-degraded multispectral imagery using deep learning

We investigate how wavelength diversity affects the performance of a deep-learning model that predicts the modified Zernike coefficients of turbulence-induced wavefront error from multispectral images. The ability to perform accurate predictions of the coefficients from images collected in turbulent conditions has potential applications in image restoration. The source images for this work were a point object and extended objects taken from a character-based dataset, and a wavelength-dependent simulation was developed that applies the effects of isoplanatic atmospheric turbulence to the images. The simulation utilizes a phase screen resampling technique to emulate the simultaneous collection of each band of a multispectral image through the same turbulence realization. Simulated image data were generated for the point and extended objects at various turbulence levels, and a deep neural network architecture based on AlexNet was used to predict the modified Zernike coefficients. Mean squared error results demonstrate a significant improvement in predicting modified Zernike coefficients for both the point object and extended objects as the number of spectral bands is increased. However, the improvement with the number of bands was limited when using extended objects with additive noise.

Laminar separation bubble analysis by means of single–shot lifetime temperature sensitive paint in a water towing tank

A laminar separation bubble is studied on an SD7003 foil in a water towing tank at a Reynolds number of 6⋅104 and an angle of attack of 6∘ by means of the temperature sensitive paint single-shot lifetime method in order to resolve the footprints and dynamics of vortical structures at low inflow turbulence levels. A heat flux is created by applying a carbon based heating layer on the suction side of the foil. The influence of the surface heating on the transition behaviour is analyzed using 2D2C-PIV and found to be negligible. The results demonstrate the capability of the single-shot lifetime method to quantify salient time-averaged flow characteristics, as well as to resolve and characterize the footprints of the dominant coherent structures.