Boundary Layer Separation Point Research Articles

An investigation of airfoil dual acoustic feedback mechanisms at low-to-moderate Reynolds number

An experimental investigation was performed in an anechoic wind tunnel involving acoustic measurements, hot-wire anemometry and surface flow visualisation techniques to investigate airfoil tonal noise generating mechanisms. Tests were conducted using a NACA 0012 airfoil at corrected angles of attack of 0° and 1.58° and Reynolds numbers of 50,000 to 150,000. A dual acoustic feedback model is presented, where feedback processes act independently on the airfoil pressure and suction surfaces between the point of boundary layer separation and the trailing edge. It is proposed that the tones generated on both airfoil surfaces, with the same or similar frequencies on each surface, interfere constructively. The primary tone possesses near exact frequencies on both surfaces, whereas the secondary tones have larger differences in frequencies between both surfaces, thus explaining their relative magnitudes based on acoustic superposition. This model provides a better comparison with the experimentally obtained tonal frequencies than the existing feedback models. Despite this agreement, the feedback model cannot perfectly predict the acoustic tones as the tones are not perfectly equispaced. An empirical feedback length is calculated by reverse engineering an acoustic feedback length scale by using the recorded primary tone as an input that also minimises the secondary tone prediction errors. This empirical length closely matches the dual acoustic feedback model presented in this paper.

PIV analysis of near-wake flow patterns of an ice-accreted bridge cable in low and moderately turbulent wind

The study contributes to the explanation of flow behaviour and the basis of phenomena existing during two-dimensional airflow around an ice-accreted cylinder, representing the section model of a bridge cable. The geometrical features and flow characteristics of the near-wake flow patterns of the iced cable were investigated within the Reynolds number range of 2.2·104–6.4·104 in low and moderately turbulent flow. The experimental procedure was conducted to create ice accretion on the cylindrical model. The shape of the ice was registered using a photogrammetry method. For the aerodynamic investigations the ice-accreted model was reproduced at a smaller scale by means of 3D printing. Representative snapshots of the flow field behind the stationary horizontal simulated iced cable were digitally analyzed by the Particle Image Velocimetry technique. The flow visualization provided quantitative data about the velocity and the direction of flow streams within both the near-wake and the sidewise regions of distributed flow around the model. Evidence of the existence of the vortex excitation process was obtained at three principal angles of attack. The characteristics of the vortex street and the location of the flow boundary layer separation points were recognized. The obtained results were compared with results for a smooth cylinder.

Lagrangian analysis of mass transport and its influence on the lift enhancement in a flow over the airfoil with a synthetic jet

The mass transport and lift enhancement in flows over the NACA0015 airfoil with a synthetic jet (SJ) are studied numerically using Lagrangian coherent structures (LCSs). Trajectories of synthetic particles belonging to 9 different groups in a vicinity of the airfoil are tracked to analyze the transport. The influence of mass transport on the aerodynamic performance of the airfoil and on the surface pressure coefficient distribution is studied. The difference in transport properties for the airfoil with and without the SJ is analyzed to understand the reasons of the lift enhancement. The results show that the LCSs could be successfully used to describe transport and mixing in the complex flows in the blowing and suction phases. More, the fluid originating in the upper reaches is continuously sucked out of the flow field in the suction phase, then the jet injected by the jet slot moves to the lower reaches of the jet slot in the blowing phase. Compared with the original airfoil, the controlled airfoil could bring about the fluid adhering to the surface moving constantly and the lift coefficient improved apparently, whether in the suction or blowing phase. Resulting from the SJ, the high momentum fluid is injected into the flow field near the airfoil. Therefore, the boundary layer separation point of the airfoil is pushed downstream, and the area of the flow separation zone is greatly reduced. Generally, utilizing LCS to study mass transport process can provide a new perspective for the study of the SJ.

Accelerated laminar flow near the heated permeable surface

The paper reports on numerical simulation of an accelerated laminar boundary layer over the heated permeable wall. Xenon was used as a working gas. The finite difference method was applied to solve the system of laminar boundary layer equations with variable gas properties. It has been shown that the gas injection in the mentioned conditions increases maximal velocity within the boundary layer and decreases skin-friction coefficient and thermal Stanton number. On the contrary, gas suction decreases an overshoot phenomenon down to its vanishing and increases skin-friction coefficient and thermal Stanton number up to the value at asymptotic suction. The paper considers a separation of thermal boundary layer at gas injection. The proposed conventional point of this separation is situated farther from the leading edge than the separation point of the dynamic boundary layer.

Effects of radiation and magnetic field on mixed convection flow of non-Newtonian power-law fluids across a cylinder in the presence of chemical reaction

The effects of thermal radiation, magnetic field and chemical reaction on momentum, heat and mass transfer of mixed convection flow of a non-Newtonian power-law fluid across a horizontal cylinder are presented. Governing equations have been reduced to a set of dimensionless equations using appropriate transformations. The resulting equations are solved employing an implicit finite difference method up to the point of boundary layer separation. For the non-Newtonian fluids, the Nusselt number and the Sherwood number demonstrate completely different characteristics from the Newtonian fluids near the front stagnation point. The magnetic field parameter produces lower skin friction, heat transfer, and mass transfer, whereas these are increased for the thermo-solutal parameter and the mixed convection parameter. The skin friction and the Nusselt number are found to decrease and the Sherwood number increases with an increase of the Schmidt number and the chemical reaction parameter. Moreover, the conduction-radiation parameter generates higher skin friction and mass transfer and lower heat transfer rate.

Performance of divergent-chimney solar power plants

Simulations of divergent-chimney solar power plants (DSPPs) are conducted, and the DSPP performance studied by changing chimney outlet-to-inlet area ratios (COAR, representing the degree of divergence) over a wide range of values. Study method involves the use of total pressure potential (TPP) which consists of buoyancy and static pressure recovery in which an effective pressure potential recovery coefficient (EPPRC) is employed. Results show when the COAR is large enough, the boundary layer separation (BLS), flow stall and backflow will occur, vortex be formed, and a part of flow area be blocked. Due to the backflow from the ambient cool air, the temperature greatly decreases above the BLS point, possibly causing the reduction of buoyancy of the divergent chimney. With COAR increasing, the TPP initially increases and reaches a maximum for COAR = 8.7 then decreases. The mass flow rate and the power output have the same variational trend, while the collector temperature rise has the inverse variational trend. A maximum power of 231.7 kW is attained at COAR = 8.7, which is 11.9 times as high as that for COAR = 1. Before the occurrence of flow stall, the EPPRC decreases slowly due to the gently thickening of boundary layer and is higher than 0.91. While after the occurrence of flow stall, mainly due to the vortex and backflow the EPPRC greatly decreases and is found to decrease gradually with COAR. The effective ground heat flux increases the power output, but has little influence on the characteristics of the flow stall in the chimney, specifically the EPPRC.

Flow-induced vibration of a locally rough cylinder with two symmetrical strips attached on its surface: Effect of the location and shape of strips

The flow induced vibration of a circular cylinder selectively roughened with two strips is numerically investigated using a two-way fluid-structure interaction (FSI) approach at high Reynolds numbers (30480 ≤ Re ≤ 304800). The main purpose of the study is to evaluate the effect of the location and shape of strips on the vibration response. The numerical model is validated with the experimental and numerical results reported in previous studies. The results indicate that the response is sensitive to the placement angle and shape of strips. The 12 in. cylinder with rectangular strips at α = 20° has the similar response trend as the 3.5 in. cylinder. The gap between VIV and galloping is filled by placing strips in the front surface of the cylinder. Hard galloping zone 1 (HG1) appears at α = 20° and 60° where the amplitude grows quickly after the onset of galloping, and hard galloping zone 2 (HG2) appears at α = 0° and 74° where the amplitude grows relatively slowly. Among them, placing strips at α = 20° has the best performance in enhancing vibration. However, the vibration is suppressed by placing the rectangular strips at α = 120°, where the boundary layer separation point transfers to the strip surface. The galloping is observed using trapezoid I and II strips, while it is not achieved by placing triangular or arc strips. The changes in turbulent intensity, vortex shedding mode and wake width reflect the transition from VIV to galloping.

Lift on a steady 2-D symmetric airfoil in viscous uniform shear flow

We present an investigation into the influence of upstream shear on the viscous flow around a steady two-dimensional (2-D) symmetric airfoil at zero angle of attack, and the corresponding loads. In this computational study, we consider the NACA 0012 airfoil at a chord Reynolds number $1.2\times 10^{4}$ in an approach flow with uniform positive shear with non-dimensional shear rate varying in the range 0.0–1.0. Results show that the lift force is negative, in the opposite direction to the prediction from Tsien’s inviscid theory for lift generation in the presence of positive shear. A hypothesis is presented to explain the observed sign of the lift force on the basis of the asymmetry in boundary layer development on the upper and lower surfaces of the airfoil, which creates an effective airfoil shape with negative camber. The resulting scaling of the viscous effect with shear rate and Reynolds number is provided. The location of the leading edge stagnation point moves increasingly farther back along the airfoil’s upper surface with increased shear rate, a behaviour consistent with a negatively cambered airfoil. Furthermore, the symmetry in the location of the boundary layer separation point on the airfoil’s upper and lower surfaces in uniform flow is broken under the imposed shear, and the wake vortical structures exhibit more asymmetry with increasing shear rate.

Buzz Flows in an External-Compression Inlet with Partially Isentropic Compression

A rectangular external-compression supersonic inlet with partially isentropic compression is experimentally studied at a freestream Mach number of 2.0. During the test, the inlet operates at the little buzz state after going through the supercritical state and the steady subcritical state. A striking feature of the little buzz state is that both the most upstream and downstream positions of the unsteady separation point of the forebody boundary layer take on a linear variation trend with the increase of throttling ratio, yielding an intensifying flow oscillation. Then, the mixed buzz phenomenon that the little and big buzz coexist appears. As the throttling ratio further rises, the occurrence probability of little buzz is reduced, and finally the big buzz state takes place. In this state, two unique transient flow patterns with the inflow and supersonic backflow coexisting at the duct entrance can be observed. Different from the prevailing viewpoint, the present little buzz belongs to the Dailey instability and shares a common origin with the big buzz in essence. Furthermore, a theoretical analysis based on Fisher’s criterion indicates that a single shear layer from a -shaped terminal shock with an upstream Mach number below 1.60 is unable to trigger the Ferri instability of a supersonic inlet.

Vortex induced vibration response and energy harvesting of a marine riser attached by a free-to-rotate impeller

The vortex induced vibration response of a riser attached by a free-to-rotate impeller and the rotation response of the impeller are investigated numerically by computational fluid dynamics models in order to examine the feasibility of energy harvesting. Unsteady Reynolds- Averaged-Navier-Stokes equations coupling with a shear stress transport k-ω turbulence model are employed to calculate the flow field with Reynolds number ranging from 30512 to 488567, while an improved fourth order Runge-Kutta method is adopted to capture the motion of the system. The results indicate that the boundary layer separation points move from the riser surface to the tips of the impeller. The classical three branches of amplitude response are found in the vibration response of a riser with an impeller. However, the rotation of impeller delays the appearance of the lock-in phenomenon to a larger incoming flow rate. Although the attachment of impeller does not enhance the cross-flow vibration, the enhancement of the in-line vibration is apparent. The energy harvesting should be based on the premise of the riser’s security service. Therefore, the two goals, vibration suppression and high energy extraction, are achieved simultaneously when the reduced velocity is larger than 9.85 for a riser with a free-to-rotate impeller.

Unsteady Magnetohydrodynamic Mixed Convection Flow of Micropolar Fluid Past a Permeable Sphere

In this paper, the flow and heat transfer characteristics for unsteady mixed convection flow of an electrically conducting micropolar fluid past a permeable sphere are investigated in detail. The effects of thermal radiation and heat generation or absorption are also considered. The governing equations are transformed into nonlinear ordinary differential equations using new coordinate transformations. Solutions are obtained by employing an implicit finite difference method. Numerical results can predict the boundary-layer characteristics even after the point of boundary-layer separation. The remarkable findings are that the mixed convection parameter and the fluid suction through the porous wall considerably enhance the friction factor, couple stress, and Nusselt number; whereas they are found to decrease with the magnetic field parameter and the fluid injection. Due to an increase of the heat generation parameter and the conduction–radiation parameter, the friction factor and the couple stress increase but the Nusselt number decreases. However, the heat absorption parameter leads to the converse characteristics.

The influence of heat radiation on mixed convection boundary layer flow of a viscoelastic fluid over a circular cylinder with constant surface temperature

The influence of mixed convection boundary layer flow of a viscoelastic fluid over an isothermal horizontal circular cylinder has been analyzed. The boundary layer equations governing the problem are reduced to dimensionless nonlinear partial differential equations and then solved numerically using Keller-box method. Skin friction coefficient and Nusselt number are emphasized specifically. These quantities are displayed against curvature parameter. Effects of mixed convection parameter and radiation-conduction parameter on skin friction coefficient and Nusselt number are illustrated through graphs and table. The boundary layer separation points along the surface of cylinder are also calculated with/without radiation, and a comparison is shown. The presence of radiation helps to reduce the skin friction coefficient in opposing flow case and enhances it for assisting flow case. The increase in value of radiation-conduction parameter helps increase the value of skin friction coefficient and Nusselt number for viscoelastic fluids. The boundary layer separation delays due to thermal radiation.

Simulation of a Round Steel Charge Heating by a Pulsatory Disturbed Flue Gas Stream Using the Fluent Software

The paper presents a numerical modelling of a round steel charge heating by a pulsatory disturbed flue gas stream. The problem, which was decided to be modelled using modern tools for numerical solutions, is the performance of calculations of the surface temperature distribution on a cross-section of a tube 64mm in outside diameter and with the wall 10mm thick. Boundary conditions calculated on the basis of data obtained from experimental studies were used for computational purposes. The geometrical shape caused that at different points on the cylinder circumference the heat exchange was proceeding at various intensities, which resulted in a diversified temperature distribution on the cross-section. The knowledge of the impact of the introduced pulsatory disturbances and of the location of boundary-layer separation point in a significant way can shorten the process of low-temperature heating and by appropriate scheme of the round charge rotation can ensure a uniform distribution of the temperature field on its circumference.

Experimental Investigation of End PlateEffects on the Vertical Axis Wind TurbineAirfoil Blade

An experimental investigation of the variation of lift coefficient at low wind speeds comparative to the operation of a vertical axis wind turbine operation was undertaken. Data was recorded at angle of attack for±0, 5, -5, 10, -10, 15, -15 degrees and the wind speed was varied for Reynolds number ranging from 3.32 x105 to 9.64 x 105. Pressure taps on the airfoil skin collected the stagnation pressures which were converted to lift coefficient values. Experiments were conducted with and without end plate attachment to the MI-VAWT1 airfoil.Results indicated that the addition of the endplates increased the lift coefficient at all angle of attack over the range of Reynolds number tested. Similar trends were observed with and without endplate.The flow regime boundary layer separation point advanced toward the leading-edge with an increase in the angle of attack up to ±10o.As the angle of attack increased above ±10o the stall angle was approached. This resulted in the separation bubble bursting, which created a suction peak and the lift coefficient decreased.

The Influence of Grooved Surface and Liquid Properties on Vortices Formation in Vicinity of Immersed Cylinders

The paper is dedicated to the experimental and numerical investigations of the viscous and viscoelastic flows around immersed cylinders with smooth and grooved surfaces. The study analyses the influences of the cylinders surface quality on the vortical structures and the wake developed downstream in a 2D open channel. The analyzed flow is in the domain of the free surface weakly turbulence regime under subcritical conditions. Numerical simulations are performed with turbulent solvers implemented in Ansys Fluent, using the VOF code for the calculation of the free surface geometry. The numerical results and visualizations are corroborated to determine the effect of the grooved surface on the drag coefficient and the location of the boundary layer separation points. The investigations emphasize also the qualitative changes of the flow spectrum induced by the Reynolds number magnitude and the presence of elasticity within the viscous fluid.

Numerical study on vortex-induced vibration responses of a circular cylinder attached by a free-to-rotate dartlike overlay

In the present paper, the two-degree-of-freedom vibration responses of a circular cylinder attached by a free-to-rotate dartlike overlay with Reynolds number ranging from 1715.9 to 6851.6 are modeled in two dimensions by using the commercial CFD code “Fluent”. The rotation of the dartlike overlay can disturb the flow around the cylinder and simultaneously convert hydrokinetic energy to mechanical energy. The URANS equations and SST k-ω turbulence model are employed to calculate the flow field, while a fourth-order Runge–Kutta method is applied to evaluate the motion of the system. It is found that the overlay rotates counterclockwise under the drive of fluid flow, resulting in 2S wake mode throughout the reduced velocity range. The boundary layer separation points move to the four tips of the overlay. Due to the shearing action of rotating overlay, small vortices are cut off from the vortices shedding from overlay tips. The rotation of overlay speeds up the migration of the vortices to the downstream. However, as the reduced velocity is larger than 6.7, the vibration of the system is enhanced. So this free-to-rate overlay can be used as power generation device at high reduced velocity.

A study on the thrust vector control using a bypass flow passage

Bypass actuators are both indispensable and critical components for thrust vector control of supersonic vehicles. Detailed flow features concerned with bypass flow play a key role in thrust vectoring systems. An analytical model was created in supersonic nozzle with bypass flow passage. Meanwhile, a computational fluid dynamics-based study was conducted for studying this phenomenon. To validate the analytical and numerical models, comparisons with experimental result were performed. Good agreement was observed in overall region. Analytical and numerical pressure distributions under different injection positions were compared. The distance between separation point and injection position becomes smaller as the secondary injection position moves upstream. This is because of the reducing of bypass flow rate. The effects of expanded ratio and mass flow rate were adequately researched and tested. Significant variation of system thrust ratio and specific impulse takes place in the over-expanded conditions. It is indicated that the secondary flow produces great effects for over-expanded conditions. The separation point of boundary layer moves upstream as bypass flow rate increases. System thrust ratio and specific impulse will reduce if bypass flow rate increases.

Effect of the turbulent viscosity relaxation time on the modeling of nozzle and jet flows

Certain modifications of three-equation turbulence models are proposed. They are intended for increasing the accuracy of the calculations of turbulent flows in nozzles with boundary layer separation and in supersonic jets with complicated shock wave structures. Basing on the idea of the inclusion of flow prehistory in terms of an additional relaxation equation for nonequilibrium turbulent viscosity we propose three modifications of the k-ω-µ t model based on the k-ω model and a version of the k- ɛ-µ t turbulence model. In these modifications we introduce an additional dependence of the nonequilibrium turbulent viscosity relaxation time on different physical parameters which can be important near the point of boundary layer separation from the nozzle wall, such as viscous effects and effects of large gradients of the mean velocity and the kinetic energy of turbulence (turbulent pressure). The comparison of the results of the calculations with the experimental data shows that all the proposed versions of the three-equation models make it possible to improve the accuracy of the calculations of turbulent flows in nozzles and jets.

Mixed convection flow from a horizontal circular cylinder embedded in a porous medium filled by a nanofluid: Buongiorno–Darcy model

Steady mixed convection boundary layer flow from an isothermal horizontal circular cylinder embedded in a porous medium filled with a nanofluid has been studied for both cases of a heated and cooled cylinder using the Buongiorno–Darcy mathematical nanofluid model. The resulting system of nonlinear partial differential equations is solved numerically using an implicit finite-difference scheme. The solutions for the flow and heat transfer characteristics are evaluated numerically for various values of the governing parameters, namely the constant mixed convection parameter λ, the traditional Lewis number Le, the buoyancy ratio parameter Nr, the Brownian motion parameter Nb and the thermophoresis parameter Nt. It is found that in the present case of the porous medium flow, the separation is always suppressed at negative values of λ. When λ changes from −2.1 to 0, one has a “heating” of the cylinder, but a heating in the negative range of λ (λ < 0). However, for a clear (Newtonian) fluid, Merkin (1977) found that heating the cylinder (λ > 0) delays the separation of the boundary layer and if the cylinder is hot enough (large values of λ > 0), then it is suppressed completely at a positive value of λ, somewhere between 0.88 and 0.89. On the other hand, cooling the cylinder (λ < 0) brings the boundary layer separation point nearer to the lower stagnation point and for a sufficiently cold cylinder (large values of λ < 0) there will not be a boundary layer on the cylinder.

Modelling of fluidelastic instability in a square inline tube array including the boundary layer effect

Flow-induced vibration (FIV) is a design concern in many engineering applications such as tube bundles in heat exchangers. When FIV materializes, it often results in fatigue and/or fretting wear of the tubes, leading to their failure. Three cross-flow excitation mechanisms are responsible for such failures: random turbulence excitation, Strouhal periodicity, and fluidelastic instability. Of these three mechanisms, fluidelastic instability has the greatest potential for destruction. Because of this, a large amount of research has been conducted to understand and predict this mechanism. This paper presents a time domain model to predict the fluidelastic instability forces in a tube array. The proposed model accounts for temporal variations in the flow separation. The unsteady boundary layer is solved numerically and coupled with the structure model and the far field flow model. It is found that including the boundary layer effect results in a lower stability threshold. This is primarily due to a larger fluidelastic force effect on the tube. The increase in the fluidelastic effect is attributed to the phase difference between the boundary layer separation point motion and the tube motion. It is also observed that a non-linear limit cycle is predicted by the proposed model.