Reattachment Of Separated Flow Research Articles

Requirements for partial turbulence simulations using nondimensional turbulence energy contributions

Quantifying turbulence effects is crucial for understanding building aerodynamics and for ensuring accurate wind tunnel test methods. This is especially important in wind tunnel methods that require post-experiment adjustments because approximate wind fields are used, such as the Partial Turbulence Simulation (PTS) approach. Understanding and analyzing these effects enables load adjustments since the PTS method only requires matching the high frequency portions of the upstream spectra of the longitudinal velocity component in model and full-scale. However, the limits for which the PTS method is applicable are unclear in terms of the allowable range of wind field characteristics that can be used in the wind tunnel simulation. To address this, the paper utilizes two nondimensional parameters, one representing the small-scale turbulence energy, ES, and the other the large-scale turbulence energy, EL, to elaborate the aerodynamic effects of turbulence intensity and integral length scales in the upstream wind. The results show that the maximum allowable mismatch ratio of integral length scales and of Jensen numbers between model and full-scale simulations depend on the target small-scale turbulence energy and the maximum allowable deviation of small-scale energy. By quantifying the effects of ES and EL on area-averaged pressure coefficients, the allowable limits are identified for wind tunnel test parameters that lead to negligible differences in the resultant pressure coefficient statistics in regions of separated-reattaching flow on the roof of a low-rise building.

Performance and mechanism of adaptive pitch control in a floating vertical axis wind turbine with a surge motion

Floating vertical axis wind turbines are bearing complex aerodynamic fatigue loads due to the movement of the platform and the reciprocating variations in the angle of attack and the blade relative wind speed. To explore the effect of the surge motion of the platform and verify the performance and acting mechanism of adaptive pitch control on reducing the fatigue loads and improving the power coefficient, a floating vertical axis wind turbine with three motion forms (non-surge, pure surge, and surge-adaptive pitch) is investigated in the condition of a low tip speed ratio with a high incoming wind speed. The results show that the surge motion enhances the fatigue loads, with the power coefficient less affected. The adaptive pitch control reduces the enhanced fatigue loads and improves the power coefficient by nearly twice. Vorticity and pressure distributions verify that the fluctuation ranges of the angle of attack and the blade relative wind speed are expanded by the surge motion, and the angle of attack is reduced by the adaptive pitch control especially in the upwind section, with the flow separation mitigated. However, the effects of the surge motion and the pitch variation are also influenced by the shedding vortex and the flow separation reattachment. For the upwind part of the blade aerodynamic forces, with the surge amplitude increase or the surge period decrease, the global amplitudes are increased gradually by the surge motion and then reduced by the pitch control, with the load reduction performance enhanced gradually.

Gust effect factors for regions of separated flow around rigid low-, mid-, and high-rise buildings

This paper examines the gust effect factor model for regions of separated flow on rigid buildings. Boundary layer wind tunnel data for a total of 58 buildings, with heights ranging from low-rise to high-rise were analyzed. The results show that the characteristics of separated flow determine the statistical distribution of surface pressures. It was found that the reattachment of separated flows onto building surfaces reduces the skewness and kurtosis of area-averaged surface pressures, causing them to follow the Gaussian distribution more closely. However, the pressures tend to have non-Gaussian distributions for surfaces without flow reattachment. Strong Karman-like vortex shedding increases aerodynamic admittance functions around the Strouhal number, along with the peak factors and gust effect factors of area-averaged side wall pressures. For surfaces with flow reattachment, Solari's (1993a) model predicts the gust effect factors with reasonable accuracy, particularly for uplift on low-rise buildings, mid-rise buildings with height-to-width ratios, H/W = 1 and high-rise buildings with height-to-width ratios, H/W > 4, and length-to-breadth ratios, L/B > 2. For separated flows without reattachment on the building, particularly for H/W > 4 and L/B < 2, periodic Karman-like vortex shedding tends to cause inaccuracies in the gust effect factor model, as might be expected.

Enhanced prediction of three-dimensional finite iced wing separated flow near stall

Icing on three-dimensional wings causes severe flow separation near stall. Standard improved delayed detached eddy simulation (IDDES) is unable to correctly predict the separating-reattaching flow due to its inability to accurately resolve the Kelvin-Helmholtz instability. In this study, a shear layer adapted subgrid length scale is applied to enhance the IDDES prediction of the flow around a finite NACA 0012 wing with leading edge horn ice. It shows that using the new length scale contributes to a more accurate prediction of the separated shear layer (SSL). The predicted reattachment occurs earlier as one moves towards either end of the wing, and the computed surface pressure distributions agree well with the experimental measurements. In contrast, standard IDDES severely elongates surface pressure plateaus. For instantaneous flow, the new length scale helps to correctly resolve the rollup and subsequent pairing of vortical structures due to its small values in the initial SSL. The vortical motion frequencies increase when moving towards the wing tip due to the downwash effect of the tip vortex. In comparison, the excessive eddy viscosity levels from the standard IDDES delay the rollup of spanwise structures and give rise to “overcoherent” structures.

Transient Characteristics of Three-Dimensional Flow in a Centrifugal Impeller Perturbed by Simple Pre-Swirl Inflow

The pre-swirl inflow generated by guide vanes could improve the hydrodynamic performances of centrifugal pumps as long as the inflow matches the patterns of internal flow of the impeller. In this work, we present a numerical investigation on the internal flow in a centrifugal impeller subjected to inflow artificially constructed with simple pre-swirling; unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations are performed at the designed flow rate with five values of rotating velocity of the inflow, i.e., Urot/Uref = −0.5, −0.3, 0.0, 0.3 and 0.5, where Urot and Uref denote the rotating and normal velocity component at the entrance of the inflow tube, respectively. The primary objective of this work is to reveal the three-dimensional characteristics of internal flow of the impeller as influenced by the superimposed pre-swirl inflow, and to identify the propagation of inflow within the impeller. The numerical data are presented and analyzed in terms of the streamline fields, the distributions of various velocity components along the circumferential and axial directions, the pressure distribution and limiting streamlines on the surfaces of a blade. Numerical results reveal that separation occurs around the leading edge of the blades and occasionally at the trailing edge, and the internal flow is more uniform in the central region of the channels. A noticeable fluctuation of both radial and circumferential velocities is observed at the outlet of the impeller as it is subjected to counter-rotating inflow, and the greatest fluctuation is close to the hub instead of the middle channel and shroud as for the co-rotating inflow. The boundary layer flow of suction surface is more sensitive to the inflow; occasional small-scale separation bubble occurs on the suction surface around the leading edge for some blades, and reattachment of separated flow is reduced for the counter-rotating inflow.

Enhanced Prediction of Three-Dimensional Finite Iced Wing Separated Flow Near Stall

Icing on three-dimensional wings causes severe flow separation near stall. Standard improved delayed detached eddy simulation (IDDES) is unable to correctly predict the separating reattaching flow due to its inability to accurately resolve the Kelvin-Helmholtz instability. In this study, a shear layer adapted subgrid length scale is applied to enhance the IDDES prediction of the flow around a finite NACA (National Advisory Committee for Aeronautics) 0012 wing with leading edge horn ice. It is found that applying the new length scale contributes to a more accurate prediction of the separated shear layer (SSL). The reattachment occurs earlier as one moves towards either end of the wing due to the downwash effect of the wing tip vortex or the influence of end-wall flow. Consequently, the computed surface pressure distributions agree well with the experimental measurements. In contrast, standard IDDES severely elongates surface pressure plateaus. For instantaneous flow, the new length scale helps correctly resolve the rollup and subsequent pairing of vortical structures due to its small values in the initial SSL. The computed Strouhal numbers of vortical motions are approximately 0.2 in the initial SSL based on the vorticity thickness and 0.1 around the reattachment based on the separation bubble length. Both frequencies increase when moving towards the wing tip due to the downwash effect of the tip vortex. In comparison, the excessive eddy viscosity levels from the standard IDDES severely delay the rollup of spanwise structures and give rise to "overcoherent" structures.

Probe influence on total pressure measurements in the zone of supersonic laminar separated flow reattachment

Probe influence on total pressure measurements in the zone of supersonic laminar separated flow reattachment

Study on the effect of separation and reattachment flow between blades on fan noise

With the growth of the EV/HV market, the main cause of cabin noise has changed from engine driving sound to air conditioner noise. The blower noise is the largest in the air conditioner noise, and the noise reduction is urgent. Separated and reattached flows between fan blades are considered to be the main sources of blower noise. In the past, we tried to reduce the noise by reducing the separation. This time, the blade shape to further reduce the separation was produced and evaluated. As a result, the noise was greatly reduced, but a new problem was found that there was a flow velocity condition in which the noise increased despite the small separation. Therefore, we visualized the flow between blades by PIV, investigated the state of separated and reattached flow in detail, and investigated the factors related to noise increase and decrease by measuring noise and pressure fluctuation of blade surface simultaneously. As a result, it was found that the noise generation condition in the separation reattachment flow between blades is not only the size of separation but also the distance of separation shear layer from blade surface and the strength of vortex generated in shear layer.

Effects of the angle of attack on the aerodynamic characteristics of a streamlined box girder

This paper presents experimental and numerical approaches to systematically investigate the aerodynamic characteristics of streamlined box girder at angles of attack (AoA) of up to 12°. The effects of AoA on the pressure characteristics and total forces are first studied through wind tunnel experiments. Based on three-dimensional large-eddy simulations (3D LES), the flow topology is investigated to study the influence mechanism of AoAs. The results show that the flow topology around the section can be divided into three types: (i) completely attached flow pattern as |α| ≤ 4°; (ii) separated-reattached flow pattern as 6°≤|α|≤8°; (iii) fully separated flow pattern as |α| ≥ 10°. The combination of experimental and numerical results presents a deep understanding of the aerodynamic characteristics of a streamlined box girder and reveals the relationships between the pressure features and flow topology. The pressure characteristics and total forces are very sensitive to the flow separation and reattachment. At small AoAs, the flow attaches to the bridge section, the mean and RMS pressure, total force, as well as streamwise correlations change slightly with AoAs. Besides, the shear layer is a laminar pattern and has less impact on the span-wise correlations. Moreover, the vortices in the wake remain successive in the span-wise direction, and the Strouhal numbers are larger. On the other hand, with the increase of AoAs, large separation bubbles can be observed around the bridge section. Especially for fully separated flow pattern as |α| ≥ 10°, the shear layer no longer attaches to the bridge section and the separation bubble is longer than the section width. Accordingly, the mean and RMS pressure, total force, as well as streamwise correlations increase significantly. Besides, the shear layer is a turbulent pattern, and the turbulent fluctuations in wake become complex and less organized in the span-wise direction, then the St is lower than others.

Cooling of an isothermal surface having a cavity component by using CuO-water nano-jet

PurposeThe purpose of this study is to numerically analyze the convective heat transfer features for cooling of an isothermal surface with a cavity-like portion by using CuO-water nano jet. Jet impingement cooling of curved surfaces plays an important role in practical applications. As compared to flat surfaces, fluid flow and convective heat transfer features with jet impingement cooling of a curved surface becomes more complex with additional formation of the vortices and their interaction in the jet wall region. As flow separation and reattachment may appear in a wide range of thermal engineering applications such as electronic cooling, combustors and solar power, jet impingement cooling of a surface which has a geometry with potential separation regions is important from the practical point of view.Design/methodology/approachNumerical simulations were performed with a finite volume-based solver. The study was performed for various values of the Reynolds number (between 100 and 400), length of the cavity (between 5 w and 40 w), height of the cavity (between w and 5w) and solid nano-particle volume fraction (between 0 and 4 per cent). Artificial neural network modeling was used to obtain a correlation for the average Nusselt number, which can be used to obtain fast and accurate predictions.FindingsIt was observed that cavity geometrical parameters of the cooling surface can be adjusted to change the flow field and convective heat transfer features. When the cavity length is low, significant contribution of the inclined wall of the cavity on the average Nusselt number is achieved. As the cavity length and height increase, the average Nusselt number, respectively, reduce and slightly enhance. At the highest value of cavity height, significant changes in the convective flow features are obtained. By using nanofluids instead of water, enhancement of average heat transfer in the range of 35-46 per cent is obtained at the highest particle volume fraction.Originality/valueIn this study, jet impingement cooling of an isothermal surface which has a cavity-like portion was considered with nanofluids. Addition of this portion to the impingement surface has the potential to produce additional vortices which affects the fluid flow and convective features in the jet impingement heat transfer. This geometry has the forward-facing step for the wall jet region with flow separation reattachment in the region. Based on the above literature survey and to the best of the authors’ knowledge, jet impingement cooling for such a geometry has never been reported in the literature despite its importance in practical thermal engineering applications. The results of this study may be useful for design and optimization of such systems and to obtain best performance in terms of fluid flow and heat transfer characteristics.

Simulation of NACA0015 flow separation control by burst-mode plasma actuation

Dielectric Barrier Discharge (DBD) plasma actuators can generate trains of vortices under burst mode actuation, which is very different from the wall jet flow under steady operation. To clarify the mechanism of DBD under burst mode actuation on separation control, the improved simulation model of burst-mode plasma actuation was developed and validated, and then, the flow field of NACA0015 under 12° and 14° of stall was numerically simulated and compared with the experimental result simultaneously. The results show that the DBD plasma actuator under burst mode actuation is very efficient for separation flow control, and the ability of separation flow control under F+ = 10 is more efficient than that under F+ = 0.3–5 investigated. The mechanism of burst mode actuation proposed that trains of vortices shed by burst mode actuation could enhance the momentum transfer between the boundary layer and the main flow, and the vortex of large scale shed from the leading edge could merge with the vortices of small scale shed by the DBD plasma actuator with burst mode actuation, which results in the reattachment of separated flow.

Numerical Study on Turbulent Separation Reattachment Flow in Pipe Bends with Different Small Curvature Ratio

A turbulent separation flow is one of the most complex flows in existence. Flow separation and reattachment phenomena take place on the lower curved wall under high-pressure gradient for high Reynolds number in different pipe bends. The present study deals with the numerical simulation of the turbulent separation reattachment flow under high Reynolds number in different curvature ratios. Through making use of k–e turbulence model, the turbulent single-phase flow via a 90-degree bend pipe is studied numerically. A more precise research has been conducted to discover the impact of Reynolds number (Re) as well as curvature ratio (Rc/D) on the reattachment and separation of flow inside the pipe bend. The separation region and the separation–reattachment points are reported here. Additionally, mean statistics of the primary and secondary velocity field flows in diverse sections and the occurrence of Dean vortices have been shown. It has been observed that bends with low curvature ratio enable flow separation to be clearly visualized. Distribution of velocity depicted that the secondary motion is clearly encouraged by the progress of fluid from the inner to the outer wall region of the bend resulting in the separation of flow. In general, this numerical study analyses the flow separation phenomenon and predicts the separation and reattachment locations in 90° bend pipe for different Reynolds numbers and bend curvatures.

Heat transfer and fluid flow behaviors in a tube with modified wire coils

Experimental turbulent heat transfer and flow resistance behaviors in a plain tube inserted with wire coils (WCs) were investigated by using air as working fluid. The thermal and pressure drop tests were performed respectively under uniform constant wall heat flux conditions and iso-thermal conditions with Reynolds number (Re) ranging from 6000 to 20,000. The various arrangements in WCs including WCs with uniform pitch (WCs-UP) at p/d = 0.172, 0.345, 0.517, 0.690, 0.862 and 1.034, WCs with varying pitch (WCs-VP) at p/d = 0.172–0.345, 0.172–0.517, 0.345–0.517, 0.172–0.345–0.517, 0.172–0.690–0.690 and 0.172–0.690 and WCs with gradually varying width (WCs-GVW) at w/d = 0.552–0.897–0.552 were studied. Effects of those parameters on heat transfer and flow characteristics in terms of Nusselt number (Nu), friction factor (f) and performance evaluation criterion (PEC) were investigated. The obtained results show that the WCs augment considerably the heat transfer and pressure drop over the plain tube and the Nu in WCs gives the range of 1.46–2.49 times that of the plain tube and the increased f ratio is in the range of 8.36–18.62, depending on varying Re and arrangements. The interesting heat transfer and flow behaviors are found in the WCs-UP with the largest Nu and highest f obtained respectively by p/d = 0.690 and p/d = 0.345 cases. The use of WCs-VP presents superior Nu than the WCs-UP with p/d = 0.172 while the adoption of WCs-GVW yields higher Nu than its counterpart of the WCs-UP with p/d = 0.345, however, both of these modified WCs exhibit lower Nu than the WCs-UP with p/d = 0.517. Due to the compound heat transfer and fluid flow mechanisms governed by the flow separation-reattachment, the second tube effect and the swirl flows, the WCs-UP with p/d = 1.034 gives the largest PEC of about 1.14 under considered operating and geometry conditions in the light of competition between heat transfer improvement and pressure drop enhancement. In addition, the use of the WCs-VP and WCs-GVW give no further improvement in PEC compared with the WCs-UP. Finally, the Nu and f empirical correlations within ±4.0% and ±7.0% from experimental values are constructed and agree well with some published data.

Effects of turbulence on the mean pressure field in the separated-reattaching flow above a low-rise building

The effects of upstream turbulence in the atmospheric boundary layer flow on the mean surface pressure distribution within the separated flow above a typical low-rise building roof are investigated experimentally. Time-averaged Navier-Stokes equations are used to evaluate the pressure gradients from planar particle image velocimetry data. The pressure fields are reconstructed by integrating the pressure gradients using an analytic interpolation approach. This reconstruction approach is validated by successfully matching the reconstructed pressure to Bernoulli's equation along a streamline far from the body and with pressure measurements on the surface of the body. Through this process, the mean pressure field can be directly explained from the mean velocity and turbulence fields near the roof. For high turbulence intensity levels, the maximum suction coefficient on the roof surface was found to be increased. Such increased magnitudes are directly related to the reduced size of mean separation bubble in higher turbulence, more rapid variation of the velocity magnitude near the leading edge, and enhanced variation of the turbulence stresses. On the other hand, a higher rate of surface pressure recovery is found in the leeward portion of the separation bubble, which is mainly due to the more rapid variation of the turbulence stresses.

Numerical simulation of supersonic separating-reattaching flow through RANS

The present work reports turbulent supersonic flow subjected to separation-reattachment computed through Reynolds Averaged Navier-Stokes (RANS) based models. The modified density based solver, rhoCentralFoam in OpenFOAM framework, is employed for the flow simulation over a backward facing step and a rectangular cavity. The predictions by the different RANS models show good agreement with the empirical and numerical observations. Qualitatively, mean flow features such as boundary layer separation, re-circulation, and expansion fan and re-attachment shock are predicted with acceptable accuracy; while the quantitative assessment reveals the limitation of different models.

Flow separation Reattachment over an Airfoil using Streamwise Vortex by DBDPA

Flow separation Reattachment over an Airfoil using Streamwise Vortex by DBDPA

Construction of Burst Mode and Velocity Measurement System aimed at Flow Separation Reattachment over a Windmill Airfoil using DBDPA

Construction of Burst Mode and Velocity Measurement System aimed at Flow Separation Reattachment over a Windmill Airfoil using DBDPA

Effect of side ratio on fluid flow and heat transfer from rectangular cylinders using the PANS method

A computational study of heat transfer from rectangular cylinders is carried out. Rectangular cylinders are distinguished based on the ratio of the length of streamwise face to the height of the cross-stream face (side ratio, R). The simulations were performed to understand the heat transfer in a flow field comprising separation, reattachment, vortex shedding and stagnation. The Partially-Averaged Navier–Stokes (PANS) modeling approach is used to solve the turbulent flow physics associated and the wall resolve approach is used for the near wall treatment because of the flow separation involved. The simulations were performed using a finite volume based opensource software, OpenFOAM, at Reynolds number (Re)=22,000 for rectangular cylinder at constant temperature kept in an air stream. Two critical side ratios were obtained, R=0.62 and 3.0. At R=0.62, the maximum value of the drag coefficient (Cd)=2.681 was observed which gradually reduced by 54% at R=4.0. The base pressure coefficient and global Nusselt number also attained the maximum value at R=0.62 and from R=2.5 to 3.0 a sharp discontinuous increase by 140% in the Strouhal number was observed. At R=0.62, it was observed that the separated flow reattaches at the trailing edge after rolling over the side face and therefore increases the overall Nusselt number. The phase averaging was also performed to analyze the unsteady behavior of heat transfer.

215 数値解析による翼間はく離/再付着流れと騒音発生要因に関する研究(OS3-1 乱流現象の実験とシミュレーション(1),OS3 乱流現象の実験とシミュレーション)

215 数値解析による翼間はく離/再付着流れと騒音発生要因に関する研究(OS3-1 乱流現象の実験とシミュレーション(1),OS3 乱流現象の実験とシミュレーション)

P021 低Reynolds数における周期流中でのNACA0012翼表面の剥離/再付着流れ(メカボケーション学生研究発表セッション)

P021 低Reynolds数における周期流中でのNACA0012翼表面の剥離/再付着流れ(メカボケーション学生研究発表セッション)