Boundary Layer Separation Point Research Articles

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.

Magnus effect on a rotating soccer ball at high Reynolds numbers

The Magnus effect on a prototype model soccer ball rotating perpendicular to the flow direction at Reynolds numbers in the range of 0.96×105<ReD<4.62×105 was investigated by means of aerodynamic force measurements and of a flow field survey. Experiments were performed using a rear sting support where the soccer ball was split into two halves that were driven by a motor inside of them. In the non-rotating state the variation of force coefficients with Reynolds number and boundary layer separation points are within the range that is found for real soccer balls. In the rotating-state, considerable changes of the mean force coefficients with Reynolds number ReD and spin parameter SP occurred, which can be attributed to the altered boundary layer separation. These changes also lead to significant changes of size and deflection of the wake zones in the lateral direction. A negative Magnus effect occurs in the critical Reynolds number range. Positive Magnus force is induced when the boundary layer is either laminar or turbulent on both sides of the rotating model soccer ball.

Numerical simulations of droplet train and free surface jet impingement

The cooling behavior of the impingement of a droplet train, and free surface jets over a heated and pre-wetted surface is explored employing an Algebraic Volume-of-Fluid methodology. The code is based on a modified version of the two-phase numerical solver interFoam (OpenFOAM) (Trujillo and Lewis, 2012). Two versions of the free surface jet are studied. The first consists of a fully-developed profile exiting the nozzle, and the second is characterized by a uniform velocity distribution. Results show that both jet configurations have higher cooling performance than the droplet train locally and globally, with the fully-developed case being the most effective of the two jet arrangements. Locally, the performance is measured by radial profiles of the boundary-layer-displacement thickness and heat transfer coefficient. Globally, the cooling effectiveness is directly proportional to the surface area that resides within the high-convection region, i.e. before the boundary layer separation point. On a temporal basis, the liquid film within the impingement region of the droplet train exhibits pronounced variations in velocity magnitude and film thickness. This is directly attributed to the nature of continuous droplet impacts affecting the impingement region, and gives rise to an unsteady cooling and heating of the fluid near the wall. In contrast for the jets, the film and the corresponding free surface are nearly steady with only minor perturbations.

Numerical investigation on feedback control of flow around an oscillating hydrofoil by Lorentz force

In order to improve the hydrodynamic characteristics of a hydrofoil (NACA0012), this paper investigates an oscillating hydrofoil immersed in seawater (an electrically poorly conducting fluid) with feedback control of electromagnetic force (Lorentz force). This method is used in the iterative process, by forecasting the location of boundary layer separation points and attack angle at the next time step and figuring out the optimal force distribution function based on these parameters, then returns to the current time step and applies the optimal force onto the leeside to control the flow separation. Based on the basic flow governing equations, the flow field structures, lift evolutions and energy consumptions (the input impulse of Lorentz force) have been numerically investigated. Numerical results show that with this control, the flow separation could be fully suppressed. Meanwhile, the lift increases dramatically and oscillation is suppressed successfully. Furthermore, under similar lift improvement and control effects, the feedback control optimal ratio is 72.58%.

Wake flow stabilization with DBD plasma actuators for low Re numbers

We propose to study the stability properties of a wake air flow forced by electrohydrodynamic (EHD) actuators. EHD actuators can add momentum to the flow around an object in regions close to the wall. Hence, we adopt the hypothesis of the forcing actuation modeled as a slip-wall condition.We study the flow around a cylinder modified by EHD actuators, dielectric barrier discharge (DBD) type, by means of particle image velocimetry (PIV) measures. For the sake of simplicity we observed flows at low Reynolds(Re) numbers (∼ 200) where the flow is mainly two-dimensional. As the forcing frequencies, characteristic of DBD, are much higher than the natural shedding frequency of the flow (kHz vs. Hz), we consider in this work only the forcing actuation as stationary. The actuators are disposed symmetrically near the boundary layer separation point. Thus, flow symmetry is conserved, as it remains weakly non parallel. This aspect allows us to perform stability analysis of the flow and to determine its characteristics which are governed by a strong mean flow correction. We also present results on the global mode evolution under forcing.The main motivation of this work is to optimize the forcing on the flow, leading to a more effective energy consumption of EHD actuators.

A numerical study of the wake behind a blunt‐trailing‐edged body. Part 2: the topology of the flow

SUMMARYThis project focuses on the wake behind a two‐dimensional blunt‐trailing‐edged body. The body has an elliptical nose followed by a straight section that ends in a blunt base. For this geometry, the boundary layer separation points are defined and fixed. This permits a better assessment of the vital influence of the boundary layers upon the wake. It is hoped that the study of this special geometry will contribute to a better understanding of the wake flow. The main aim of this article is to study the topology of the wake by means of direct numerical simulation (DNS). The idea is to mimic some experimental work techniques. In an experiment, by means of proper synchronization, one is able to arrive at the mean‐in‐the‐phase fields. In the present work, similar data are obtained through the use of numerical tools. The topology of the wake flow in terms of the movement of the coherent structures, fields of Reynolds stresses, position of saddles and centers, is obtained for certain values of the body's aspect ratio. A comparison with the circular cylinder experimental data is also provided. An important result is the finding of some specific stress distributions around the main structures. The stress level lines are a hint of where the energy transfer of the mean flow to the turbulence is more intense. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Numerical Simulation on Plasma Circulation Control Airfoil

A novel plasma circulation control technique is proposed to overcome the disadvantage of conventional jet circulation control about the requirement for air source. By employing the plasma actuator on the trailing edge of the airfoil, the plasma-induced wall jet is tangential to the airfoil surface and served as Coanda blowing jet, which makes the trailing-edge detachment point migrate to the lower surface of the airfoil. In this way, the circulation and the lift of airfoil are directly controlled by the plasma actuator. The effectiveness of plasma circulation control on the aerodynamic characteristics improvement of NCCR 1510-7067N airfoil is demonstrated by solving the Reynolds-averaged Navier―Stokes equations. The plasma actuator is modeled with a phenomenological model by adding the body force source terms to the governing equations. The results indicate that the efficiency of plasma circulation control for lift augmentation is superior to that of conventional jet circulation control method. The lift augmentation efficiency after the location optimization reaches ΔC L /C μ = 134.86, which is much higher than the maximum lift augmentation efficiency 80 for conventional jet circulation control. The optimal location for the plasma actuator in plasma circulation control is downstream of the boundary-layer separation point with proper distance, where the plasma actuator makes the separated shear layer reattach to the airfoil surface and delays the trailing-edge stagnation by Coanda effect.

Mixed Convection Boundary Layer Flow from a Horizontal Circular Cylinder Embedded in a Porous Medium Filled with a Nanofluid

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. 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 nanoparticle volume fraction φ and the mixed convection parameter λ. Three different types of nanoparticles are considered, namely Cu, Al2O3 and TiO2. It is found that for each particular nanoparticle, as the nanoparticle volume fraction φ increases, the magnitude of the skin friction coefficient decreases, and this leads to an increase in the value of the mixed convection parameter λ which first produces no separation. On the other hand, it is also found that of all the three types of nanoparticles considered, for any fixed values of φ and λ, the nanoparticle Cu gives the largest values of the skin friction coefficient followed by TiO2 and Al2O3. Finally, it is worth mentioning that heating the cylinder (λ > 0) delays separation of the boundary layer and if the cylinder is hot enough (large values of λ > 0), then it is suppressed completely. 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.

The influence of the magnetic field on the ionized gas flow adjacent to the porous wall

This paper studies the influence of the magnetic field on the planar laminar steady flow of the ionized gas in the boundary layer. The present outer magnetic field is homogenous and perpendicular to the body within the fluid. The gas of the same physical characteristics as the gas in the main flow is injected (ejected) through the contour of the body. The governing boundary layer equations for different forms of the electroconductivity variation law are transformed, brought to a generalized form and solved numerically in a four-parametric approximation. It has been determined that the magnetic field, through the magnetic parameter, has a great influence on certain quantities and characteristics of the boundary layer. It has also been shown that this parameter has an especially significant influence on the non-dimensional friction function, and hence the boundary layer separation point.

Design of airfoils providing the absence of separation in a compressible flow in a specified range of angles of attack

A problem of modification of the classical airfoils that ensure the absence of separation in a subsonic ideal-gas flow in a specified range of angles of attack is solved by a numerical-analytical method based on the quasi-solution of inverse boundary-value problems of aerohydrodynamics and Karman-Jiang formulas. Loitsyanskii’s criterion of the non-separated flow is used to determine the boundary-layer separation point.

Rotor and Subrotor Dynamics in the Lee of Three-Dimensional Terrain

Abstract The internal structure and dynamics of rotors that form in the lee of topographic ridges are explored using a series of high-resolution eddy-resolving numerical simulations. Surface friction generates a sheet of horizontal vorticity along the lee slope that is lifted aloft by the mountain lee wave at the boundary layer separation point. Parallel-shear instability breaks this vortex sheet into small intense vortices or subrotors. The strength and evolution of the subrotors and the internal structure of the main large-scale rotor are substantially different in 2D and 3D simulations. In 2D, the subrotors are less intense and are ultimately entrained into the larger-scale rotor circulation, where they dissipate and contribute their vorticity toward the maintenance of the main rotor. In 3D, even for flow over a uniform infinitely long barrier, the subrotors are more intense, and primarily are simply swept downstream past the main rotor along the interface between that rotor and the surrounding lee wave. The average vorticity within the interior of the main rotor is much weaker and the flow is more chaotic. When an isolated peak is added to a 3D ridge, systematic along-ridge velocity perturbations create regions of preferential vortex stretching at the leading edge of the rotor. Subrotors passing through such regions are intensified by stretching and may develop values of the ridge-parallel vorticity component well in excess of those in the parent, shear-generated vortex sheet. Because of their intensity, such subrotor circulations likely pose the greatest hazard to aviation.

Supersonic flow past a wedge on a flat plate. Laminar boundary layer separation and reattachment

Supersonic laminar flow past a two-dimensional “flat-plate/wedge“ configuration is numerically investigated. The pressures at the boundary layer separation and reattachment points are calculated over wide Mach and Reynolds number ranges. The minimum angles of the wedge surface inclination at which a return flow occurs are determined. The results are presented in the form of generalized Mach-number-dependences of the theoretical pressure on the wedge surface initiating boundary layer separation and the pressure at the boundary layer reattachment point.

Detection of the boundary-layer separation point based on the flexible sensor arrays

Microelectromechanical systems plays an important role for studying gaseous or liquid fluid flows experimentally due to the dimensions of the device on the μm-scale. For aerodynamic applications, the detection of the location of the separation point of the boundary layer is very important, which is used for active flow control and smart skin adaptive control system. This paper presents the design and integration methods of a flexible polymide substrate micro thermal sensor array, sensor array driver circuit, dynamic data acquisition circuit, and criterion algorithm of the boundary layer separation point. The above algorithms were used to analyze the NACA0012 wind tunnel test data at flow conditions of the velocity 40 m/s and AOA 16°. The result demonstrated the above system can fulfill the dynamic and real-time of the adaptive control system.

Two-dimensional numerical simulations of shock waves in micro convergent–divergent nozzles

The steady two-dimensional Navier–Stokes equations with the slip wall boundary conditions were used to simulate the supersonic flow in micro convergent–divergent nozzles. It is observed that shock waves can take place inside or outside of the micronozzles under the earth environment. For the over-expanded flows, there is a boundary layer separation point, downstream of which a wave interface separates the viscous boundary layer with back air flow and the inviscid core flow. The oblique shock wave is followed by the bow shock and shock diamond. The viscous boundary layer thickness relative to the whole nozzle width on the exit plane is increased but attains the maximum value around of 0.5 and oscillates against this value with the continuous increasing of the nozzle upstream pressures. The viscous effect either changes the normal shock waves outside of the nozzle for the inviscid flow to the oblique shock waves inside the nozzle, or transfers the expansion jet flow without shock waves for the inviscid flow to the oblique shock waves outside of the nozzle.

Detecting boundary-layer separation point with a micro shear stress sensor array

Knowledge of wall shear stress is essential for understanding the dynamics of fluid flow, and its measurement holds great importance for investigating and controlling near wall turbulence and flow separation in aerodynamic control. Based on the well-established principle of thermal anemometry, this paper describes an array of thin film sensors on a flexible polyimide substrate. Then, Computational Fluid Dynamics (CFD) simulations were performed to estimate the corresponding shear stress distribution within the column cylinder aerofoil at flow conditions is the velocity 17.3 m/s. In wind tunnel tests, by disposed the two sensor arrays output signals by the sensors output voltages means and RMS values, the result proved these flexible arrays could detect the location of the fluid separation point roughly.

METHOD AND SIMULATION OF THE USE OF MICRO SHEAR STRESS SENSOR ARRAY IN DETECTING BOUNDARY-LAYER SEPARATION POINT

At the boundary layer's separation point, the mean of the shear stress drops to a small value while its fluctuation increases dramatically. Based on the thermal method, we can fabricate a MEMS-based shear stress sensor array to bend with the curved surface, which can measure the shear stress profile of the boundary layer. This paper presents two methods, mean and RMS of the shear stress difference max value and the second order of the array signals difference algorithm, to calculate the location of the flow separation point. Through combination of the two methods and analyzing the 2D circular column CFD simulation data, the position of the separation point can be determined accurately.

Hydrodynamics and mass-transfer-limited current distribution for a submerged stationary hemispherical electrode under jet impingement

The stationary hemispherical electrode under a submerged impinging jet is suggested as an experimental system that is amenable to in situ observation and has a uniform current distribution below the mass-transfer-limited value. The present work provides solutions for the steady-state boundary-layer hydrodynamics and convective diffusion. The observation of a boundary layer separation in the present work suggests that the model for convective diffusion impedance for this system must account for both the circulation region which follows the point of boundary layer separation and the boundary layer region itself. The present work is intended to provide a foundation for the design of stationary hemispherical electrode systems and for development of models for the corresponding impedance response.

The Brinkman model for the mixed convection boundary layer flow past a horizontal circular cylinder in a porous medium

The Brinkman model is used for the theoretical study of the mixed convection boundary layer flow past a horizontal circular cylinder with a constant surface temperature and embedded in a fluid-saturated porous medium in a stream flowing vertically upwards. Both the cases of a heated (assisting flow) and a cooled (opposing flow) cylinder are considered. It is shown that there are two governing dimensionless parameters, which are related to thermal and viscous effects. These are the Darcy–Brinkman parameter Γ and the mixed convection parameter λ. It is shown that for Γ=0 the problem reduces to the similarity Darcy’s model, while for Γ≠0 the governing equations are non-similar and they have been solved numerically using the Keller-box method. It is found that heating the cylinder ( λ>0) delays separation of the boundary layer and can, if the cylinder is warm enough (large values of λ>0), suppress it completely. On the other hand, cooling the cylinder ( λ<0) brings the boundary layer separation point nearer to the lower stagnation point and for sufficiently cold cylinder (large values of λ<0) there will not be a boundary layer on the cylinder. A complete physical description of the problem is presented throughout the analysis. Some results were given in the form of tables. Such tables are very important and they can serve as a reference against which other exact or approximate solutions can be compared in the future.

A Novel Method of Detecting Boundary-Layer Separation at Near-Sonic Velocities

A novel method of detecting the boundary-layer separation point based on measuring the static-pressure pulsations is proposed. With reference to the example of a tapped yawed wing, criteria for the occurrence and development of separation at near-sonic velocities are found using this method.

Experimental study of wind-turbine airfoil aerodynamics in high turbulence

Wind turbines very often have to operate in high turbulence related, for example, with lower layers atmospheric turbulence or wakes of other wind turbines. Most available data on airfoil aerodynamics concerns mainly aeronautical applications, which are characterized by a low level of turbulence (generally less than 1%) and low angles of attack. This paper presents wind tunnel test data for the aerodynamic properties—lift, drag, pitching moment, pressure distributions—of an airfoil used on a wind turbine when subjected to incident flow turbulence levels of 0.5–16% and placed at angles of attack up to 90°. The results show that the aerodynamic behavior of the airfoil can be strongly affected by the turbulence level both qualitatively and quantitatively. This effect is especially evidenced in the angle of attack range corresponding to airfoil stall, as the boundary layer separation point advances along the leeward surface of the airfoil.