Influence Of Turbulence Intensity Research Articles

Study on influence of turbulence intensity on blade airfoil icing mass & aerodynamic performance

Wind is an emerging renewable energy resource, but more useful in cold regions. With the increasing threat of climate change and global warming, the unpredictability of wind energy patterns has been affected. With continual threats from extremes and uncertainties, icing on wind turbines has been noted to grow affecting aerodynamic performance. The effect of turbulence intensity at its impact on aerodynamic performance was numerically done using ANSYS Fluent and FENSAP ICE software. Conditions considered for the study included turbulence intensities, median volume diameter (MVD), liquid water content (LWC), angle of attack, and ambient temperature for 180 min. The study's conditions aimed at providing a wide range of effects covering the in-cloud icing and freezing drizzle. The mass of ice increased with an increase in LWC when it increased from 0.05 g/m3 to 0.3 g/m3, and MVD with 1000 μm compared to 40 μm, but when temperature decreased led to an increase from −1 °C to −15 °C. Increasing the angle of attack led to reduced aerodynamic performance with stall angle occurring at α = 0–18°. An increase in the turbulence intensity from 0.01 % to 50 % resulted in decreased CL/CD.

Numerical study of free stream turbulence effects on dynamic stall of pitching tubercled wings

The feasibility of a tubercled leading-edge in dynamic stall control has been validated, and the impacts of an upstream cylinder wake have already been discussed in the previous research. However, the influences of wake coherent characteristics on the dynamic stall process are inevitable. To this end, the synthetic turbulence method was adopted in the present study to generate incoming turbulence. The suppression of dynamic stall could be observed according to the shrink of hysteresis loop of aerodynamic lift. Then, the dynamic stall mechanisms have been analyzed with the help of mode decomposition method. The emergence and development of the leading-edge vortex indicate the increase in aerodynamic lift, while he detachment of dynamic stall vortex leads to the lift degradation during the onset of dynamic stall. Eventually, the influence of turbulence intensity (TI) has also been discussed through the comparison of TI = 5% and TI = 10% cases. It turns out that flow separation is more remarkable at both peak and trough sections in the case of TI = 5%, which is intrinsically caused by the suppression of resistance of adverse pressure gradient.

An improved method combined modified drag model and modified erosion model based on LES for solid-liquid multiphase flow and erosion with application in a 90° elbow

In various industrial processes, solid-liquid multiphase flows involving continuous carrier and discrete particle phases have erosion challenges. In this work, a modified drag model that considers the influence of turbulence intensity and a modified erosion model that considers the effect of low-angle impact are developed. An improved method combined modified drag model and modified erosion model based on LES is proposed for numerical simulation of solid-liquid multiphase flow and erosion. The commercial software ICEM for grid generation and ANSYS FLUENT for LES are used in the present work. In validation test of velocity distribution of solid-liquid multiphase flow in a vertical pipe, the mean relative error of the liquid phase pulsation velocity is 10.01 % for modified drag model, whereas the mean relative error is 16.11 % for original drag model. In validation test of the maximum erosion depth from a submerged impact test at a tilt angle of 45°, the relative error is 0.23 % for modified erosion model, while the relative error is 13.39 % for original erosion model. Then, the improved method is employed to compute the characteristics of solid-liquid flow and erosion in a 90° elbow. Particles have a significant influence on liquid flow in the shear layer area where the inner arch separation zone interacts with the main flow, and this influence is most pronounced at the position of r* = 0.3 at yd = 0.4D. At this position, particles result in 15.27 % reduction in liquid velocity and 8.75 % increase in turbulence intensity. The mean relative error of the elbow erosion rate by using the improved method is 16.95 %, whereas it is 31.34 % of the reference work. The erosion is affected by impact velocity, angle, and particle number, and the most significant parameter is the particle number in the severe erosion area.

Aerodynamic characteristics of a streamlined box girder under shear flow considering oncoming turbulence

Non-synoptic winds, such as typhoons and downbursts, are frequently characterized by shear flow associated with turbulence, which affects the aerodynamic performance of long-span bridges. To reveal the aerodynamic characteristics of streamlined box girders under non-synoptic winds, multi-fan wind tunnel (MFWT) tests were used to investigate the aerodynamic effect around a streamlined box girder considering the action of shear flow with different velocity gradients, turbulence intensities, and integral scales. In the MFWT tests, the high shear rate and large turbulence intensity were observed to magnify the mean wind pressure coefficient, whereas the variation in the turbulence integral scale had a slight effect on the mean wind pressure coefficient distribution. An increase in the shear rate was observed to be beneficial in reducing the drag and moment coefficients, as well as in increasing the lift coefficient. The empirical aerodynamic prediction relationships revealed that the influence of turbulence intensity on the aerodynamic coefficient is non-linear, whereas that of the turbulence integral scale and shear rate on the aerodynamic coefficient is linear. Additionally, the large eddy simulation (LES) method was used to study the vortex-shedding behaviors and aerodynamic spectrum characteristics of the streamlined box girder under shear flow. The LES results showed that larger shear parameters amplify the amplitudes of high-frequency aerodynamic forces. The vortex frequently begins from the low-velocity side of the streamlined box girder, which induces a suction effect on the low-velocity side that is greater than that on the high-velocity side.

Influence of Inlet Boundary Conditions on the Prediction of Flow Field and Hemolysis in Blood Pumps Using Large-Eddy Simulation

Inlet boundary conditions (BC) are one of the uncertainties which may influence the prediction of flow field and hemolysis in blood pumps. This study investigated the influence of inlet BC, including the length of inlet pipe, type of inlet BC (mass flow rate or experimental velocity profile) and turbulent intensity (no perturbation, 5%, 10%, 20%) on the prediction of flow field and hemolysis of a benchmark centrifugal blood pump (the FDA blood pump) and a commercial axial blood pump (Heartmate II), using large-eddy simulation. The results show that the influence of boundary conditions on integral pump performance metrics, including pressure head and hemolysis, is negligible. The influence on local flow structures, such as velocity distributions, mainly existed in the inlet. For the centrifugal FDA blood pump, the influence of type of inlet BC and inlet position on velocity distributions can also be observed at the diffuser. Overall, the effects of position of inlet and type of inlet BC need to be considered if local flow structures are the focus, while the influence of turbulent intensity is negligible and need not be accounted for during numerical simulations of blood pumps.

Wind tunnel test on the aerodynamic admittance of a rail vehicle in crosswinds

The aerodynamic admittance of a rail vehicle was investigated by wind tunnel test. Aerodynamic force was measured in the cases of three typical railway structures, including on the flat ground, above an embankment, and on a bridge, under two turbulent flow fields. First of all, three-component aerodynamic coefficients of the vehicle on each structure were obtained under uniform flow with respect to three wind attack angles and four wind direction angles. Secondly, the aerodynamic forces on the vehicle and the corresponding wind speed were evaluated to establish an aerodynamic admittance function of the vehicle. The aerodynamic admittance of the rail vehicle approximated a constant value in the low-frequency domain, but decreased with the reduced frequency increasing. The effects of the different reduced frequencies on the drag are greater than the lift admittance of the vehicle, while moment admittance stays steady-state. Finally, in order to reflect the unsteady characteristics of the buffeting force on the vehicle, the aerodynamic admittance functions of the vehicle were fitted to the expression of the frequency response function of a mass-spring-damping system, which was then verified. Furthermore, the effects of flat terrain and mountainous terrain were investigated, revealing that the influence of turbulence intensity on aerodynamic admittance is significant.

Effect of initial turbulence on explosion behavior of stoichiometric methane-ethylene-air mixtures in confined space

The turbulent explosion process of stoichiometric CH4/C2H4/air mixtures was experimentally studied by using a standard 20 L sphere explosion system and it’s powder storage tank as the turbulence generator. Taking the initial turbulent environment and ethylene volume fraction as variables, the effects of turbulence intensity and fuel composition on explosion characteristics were studied. The results show that both turbulence intensity and ethylene content promote the explosion of the mixtures and increase the severity of the explosion, but the influence of turbulence intensity on combustion evolution mainly focuses on heat loss, while the influence of ethylene content mainly focuses on adiabatic explosion. When the ethylene ratios increase to 0.5 or more, the increase of explosion severity caused by turbulence is not obvious. The analysis of experimental results and numerical simulation shows that the influence of initial turbulence generated by powder storage tank on explosion is the result of the combined action of turbulent kinetic energy and combustible gas redistribution.

Measurement of the convective heat transfer coefficient of the human body in the lift-up design

So far, the research on the convective heat transfer coefficient (hc) in outdoor thermal comfort has mainly employed CFD simulation and wind tunnel experiments, which are difficult to fully restore the complex microclimate wind environment. In the traditional thermal comfort model, the influence of turbulence intensity (TI) on the hc might be underestimated. This study aims to measure the hc of the human body surface in the outdoor environment. A thermal manikin was placed in a lift-up building. The ambient wind speed ranged from 0.5m/s to 4m/s, with the TI ranging from 4% to 55%. The experimental results show that under the same wind speed, the difference in hc between high and low TI can be up to 15%. Based on that, the regression formula for predicting hc related to wind speed and TI was proposed. This experimental study supplements the lack of field measurement of hc in outdoor thermal comfort research, which is helpful to improve the accuracy of the outdoor thermal comfort model.

Performance analysis of amplify and forward parallel relaying free-space optical system over M distribution

We propose a parallel relaying coherent orthogonal frequency division multiplexing free-space optical (FSO) communication system model with amplify-and-forward channel-state-information-assisted relaying in an M distribution. Considering the combined effects of path loss, pointing error, and atmospheric turbulence, the closed-form expression of moment generating function, probability density function, and cumulative distribution function of the upper limit of equivalent signal-to-noise ratio are derived. Furthermore, the Meijer G closed-form expressions for outage probability and symbol error rate (SER) over M aggregated channel are obtained. The influences of turbulence intensity, the number of parallel paths, normalized beam bandwidth, normalized jitter error, and pointing error on the outage probability and SER of the proposed FSO system are analyzed by simulation. Simulation results indicate that controlling the normalized beam width to a moderate range and reducing the normalized jitter error can reduce the outage probability and improve system reliability. After the pointing error exceeds 7, the SER decline tends to be flat, which provides a theoretical reference for the precision of the pointing technology, the selection of the optical antenna transmission aperture and the receiver sensitivity index in engineering design.

Aerodynamic Characteristics of Different Airfoils under Varied Turbulence Intensities at Low Reynolds Numbers

A numerical study was conducted on the influence of turbulence intensity and Reynolds number on the mean topology and transition characteristics of flow separation to provide better understanding of the unsteady jet flow of turboelectric distributed propulsion (TeDP) aircraft. By solving unsteady Reynolds averaged Navier-Stokes (URANS) equation based on C-type structural mesh and γ - Re ˜ θ t transition model, the aerodynamic characteristics of the NACA0012 airfoil at different turbulence intensities was calculated and compared with the experimental results, which verifies the reliability of the numerical method. Then, the effects of varied low Reynolds numbers and turbulence intensities on the aerodynamic performance of NACA0012 and SD7037 were investigated. The results show that higher turbulence intensity or Reynolds number leads to more stable airfoil aerodynamic performance, larger stalling angle, and earlier transition with a different mechanism. The generation and evolution of the laminar separation bubble (LSB) are closely related to Reynolds number, and it would change the effective shape of the airfoil, having a big influence on the airfoil’s aerodynamic characteristics. Compared with the symmetrical airfoil, the low-Reynolds-number airfoil can delay the occurrence of flow separation and produce more lift in the same conditions, which provides guidance for further airfoil design under TeDP jet flow.

Investigation and validation of 3D wake model for horizontal-axis wind turbines based on filed measurements

To further investigate the three-dimensional wake profiles for horizontal-axis wind turbines, a three-dimensional Jensen-Gaussian (3DJG) wake model is proposed and validated using field measurement data. The proposed 3DJG model takes the wind shear effect into account and considers the wind speed difference between wind shear inflow and uniform inflow. In addition, the influence of turbulence intensity in wake region on anisotropic wake expansion rate k is also modified. Different from previous ones, this model does not need multiple trial calculations to determine some empirical parameters, such as the wake expansion rate k when predicting the wake profile. The value of k is assigned based on the incoming wind conditions firstly and followed by the wake profile predictions. Performance of the newly proposed 3DJG model is compared and validated with previous models as well as the field measurement data. The corresponding experiment using two ground-based scanning LiDARs was conducted to detect the 3D wake characteristics of horizontal-axis wind turbines in a wind farm located in northern Hebei Province, China. Results show that the 3DJG model has higher accuracies in both horizontal plane and vertical height direction in the prediction of wake profiles. Guidelines for the quantification of turbines’ wake characteristics and model modifications can be provided by this study.

Effect of turbulent inlet conditions on the prediction of flow field and hemolysis in the FDA ideal medical device

Hemolysis is one of the most important issues of blood contacting artificial organs. Computational fluid dynamics, in conjunction with hemolysis models, has been widely used to predict the flow field and hemolysis during the development phase of these devices. It is widely accepted that hemolysis is related to flow field parameters, such as stress and energy dissipation. It is known that inlet boundary conditions such as turbulent intensity have important effects on flow fields. Nonetheless, the influence of inlet turbulent intensity on hemolysis is not yet clear. This study investigates the influence of turbulent intensity on the prediction of flow field and hemolysis in the FDA benchmark nozzle model. Three configurations are investigated: first is the original nozzle model (with a gradual contraction, throat, and sudden expansion); the second is composed only of the throat and sudden expansion; the third one is similar with the second, but with a shorter throat. Four turbulent intensities are considered, ranging from 1% to 20%. For the first configuration, the influence of turbulent intensity on both flow field and hemolysis is very small and limited prior to the gradual contraction, while for the other two configurations, the influence is considerable. The jet breaks down sooner with increased turbulent intensity. Both turbulence dissipation rate and Reynolds stress increase in the throat with increased turbulent intensity, but decrease after the jet breaks down. The influence of turbulent intensity is more considerable for the configuration with a shorter inlet. The influence of turbulent intensity on the prediction of hemolysis is up to 38.5%. This study shows that turbulent intensity can have considerable influence on the prediction of flow field and hemolysis in blood-contacting devices, thus should be taken in account when conducting blood compatibility design.

Influence of Inlet Turbulence Intensity on Transport Phenomenon of Modified Diamond Cylinder: A Numerical Study

Flows around bluff cylinder have received the attention of many researchers over the years. Therefore, the purpose of this paper was to study the effect of turbulence intensity on the transport phenomena over modified diamond cylinders which is investigated in this work. The bluff cylinders considered are of diamond shape and extruded diamond shape. The hydraulic diameter of bluff bodies is taken as the non-dimensional length scale. The simulation is done to cover cross-flow covering the laminar and turbulent regime with the Reynolds number reaching up to 10,000, while the inlet turbulent intensity is varied between 5 and 20%. The influence of turbulent intensity on enhancing heat transfer from the body has been emphasized in this work. The transition SST models along with governing equations (continuity, momentum, and energy equations) are solved numerically with ANSYS Fluent 19.2. The simulation results are validated with established correlations, and excellent agreement is found. This work demonstrates that the transition SST model can effortlessly bridge all flow regimes for predicting the heat transfer. The study computes the influence of inlet turbulence intensity on augmenting heat transfer from the bluff cylinders. The pressure and drag coefficients are found to be unaffected by the inlet turbulent intensity.

Span-wise coherence of fluctuating forces on twin bridge decks and the turbulence effect

Unlike the adequate researches on a single-box girder, the understanding of span-wise force correlation on twin decks is limited. A sectional model pressure testing of a widely slotted twin-box girder has been carried out in the background of a 5000-m-long bridge. Three grid configurations separated the effect of turbulence intensity and integral scale. Seven test sections were set on the model with totally 532 pressure taps. After the correction of pressure distortion, the correlation coefficients and integral scales of forces were compared on the leeward and the windward decks. Root coherence of lift was analyzed in detail at several separation distances. Pressure signals were used to explain why force coherence is stronger than flow and why “low frequency curve” phenomenon occurs. Considering the shortage of existing expressions, a simple but efficient form was proposed which can fit the coherence on the leeward, the windward, and the whole decks. The parameters in different flows can be reconstructed by linear functions which benefits new model’s application. In the final, the influences of turbulence intensity and integral scale on force coherence were illustrated.

Numerical investigations on the effects of turbulence intensity on knocking combustion in a downsized gasoline engine

In this work, the influence of turbulence intensity on knocking characteristics was studied. Different levels of initial swirl ratio inside cylinder were firstly performed to investigate the effect of turbulence intensity on combustion process. The results show that the enhanced turbulence intensity with increasing initial swirl ratio accelerates the spark-ignited flame (main flame) propagation, resulting in a shortage of combustion phasing (or an advance combustion phasing) and a faster flame speed. Under low turbulence intensity, the faster flame propagation can facilitate knocking combustion because of the enhanced compression of SI flame on the improvement of end-gas thermodynamic conditions. However, further increases in turbulence intensity and flame speed suppress the knocking combustion due to the insufficient time for end-gas autoignition occurrence. Further analysis shows that knock intensity mainly depended on the Unburned Mass Fraction (UBMF). Under lower levels of initial swirl ratios, the enhanced compression of SI flame with the increase of turbulence intensity induces an advanced knock onset, which leads to a larger UBMF and heavier knock intensity. However, under higher swirl ratio scenarios, UBMF and knock intensity exhibited an opposite trend because the consumption of end-gas by the fast main flame played a dominating role.

Study on the influence of turbulence intensity on vortex signal

Numerical simulation software FLUENT was used to compare the turbulence intensity of the fluid at the outlet of the linear shrinkage tube and curvilinear shrinkage tube. Through the detection points behind the bluff body in the measurement segment, the static pressure distribution is obtained, and the frequency and intensity of the vortex street signal are analysed. The results show that the fluid turbulence intensity in the curvilinear contraction tube is small, and the vortex signal intensity of the measurement segment is higher and the noise signal is less, which is favourable to the measurement of the vortex signal.

Flame stability and propagation characteristics for combustion in air for an equimolar mixture of hydrogen and carbon monoxide in turbulent conditions

In this study, the influence of turbulence intensity on flame local structural and local propagation characteristics was studied. In order to achieve this, 50% H2/50% CO with equivalence ratio of 0.8 were conducted under different turbulence intensities ranging from 0 to 1.31 m/s at atmospheric pressure and temperature. The distribution of the flame local radius and flame local propagation speed and the correlation between the two were investigated. The results show that the turbulence can increase the disturbance in the flame front, and enhance the fluctuation of the flame local radius, resulting in an increase in the fluctuation of the flame local propagation speed. With the development of flame, the distribution of the flame local propagation speed tended to become uniform, while the intersection zone of the flame local propagation speed and the flame local radius distribution gradually decreased. When the turbulence intensity ranged between 0.49 m/s to 0.74 m/s and between 1.08 m/s to 1.31 m/s, the sensitivity of the flame local propagation speed to its local characteristics reduced with the increase of the turbulence intensity.

Analysis on the effects of turbulent inflow conditions on spray primary atomization in the near-field by direct numerical simulation

It is widely acknowledged that the development of sprays in the near-field is of primary importance for the spray formation downstream, as it affects both the spray angle, as well as the intact core length. In this frame, the present work aims to study the effects of turbulence inlet boundary condition on the spray formation by means of Direct Numerical Simulations on a real condition at low Reynolds number. To this extent, the code Paris-Simulator has been used, while a digital filter-based algorithm was used in order to generate synthetic turbulence at the inlet boundary condition. The influence of turbulence intensity and lengthscale on the atomization process has been studied and analyzed through 3 simulation for which these parameters have been varied. The results clearly highlight how the atomization is heavily affected by the inlet turbulence configuration. An analysis of the different atomizing conditions has been conducted, aiming to understand how the variation introduced by the inlet boundary condition on the velocity field is affecting the local atomization dynamics.

Numerical simulation of boundary-layer transition flow of a model propeller and the full-scale propeller for studying scale effects

Propeller scale effects were widely studied from boundary-layer flow, friction and pressure distributions, propeller performance and tip vortices. The γ-Reθ transition model and two turbulence models (STAR-CCM+) were applied for the model propeller simulation, and the influence of turbulence intensity on the boundary-layer flow was studied. The transition model was also applied for the full-scale propeller simulation, and numerical results were compared with that predicted by a commonly used turbulence model. Data from open water tests and velocity measurements of the model propeller were used for validation. Results show that scale effects of propeller performance are mainly caused by different boundary-layer flows: the boundary-layer flow of the full-scale propeller is basically turbulent, while the flows over the blade face and blade back of the model propeller are complex, different and sensitive to the Reynolds number. By studying the effect of varying Reynolds numbers on propeller performance, boundary-layer flow and tip vortices, why the Reynolds number of the model propeller in experiments needs to be greater than the critical Reynolds number was studied. Last, scale effects and propeller performance of the full-scale propeller predicted by numerical methods and the ITTC recommended scaling method are close.

Influence of turbulence intensity on wind turbine power curves

In this study the effect of turbulence on wind turbine performance measurements has been investigated, and the turbulence normalization method introduced in a draft for the second edition of the IEC 61400-12-1 standard has been evaluated. Experimental data has been collected from a wind turbine test site at the coast of Norway over a 10 month period from a Windcube v2 ground-lidar, and a 3MW wind turbine, and the effect of turbulence intensity on power curves and annual energy production (AEP) has been studied. The range of turbulence conditions experienced at this coastal site has a significant influence on the power curve and calculated AEP. The measured dataset was split into two subsets featuring different turbulence conditions, and AEP was then calculated for each of these based on power curves from the opposite subset. Using the zero turbulence power curve method the difference in calculated AEP is reduced by 50% compared to using traditional power curves. The method only considers the effect of 10-minute averaging and not the direct influence of turbulence on rotor aerodynamics.