Location Of Separation Point Research Articles

Finite Volume Study of Flow Separation Phenomena for Steady Flow over a Circular Cylinder at Low Reynolds Number

Flow separation is one of the major distinguishing phenomena between the potential flow and actual fluid flow and circular cylinder has long been considered as the simplest form of bluff body for analyzing flow separation characteristics. Here in this article, different flow separation phenomena such as onset of separation, angular position of separation point, size of separation bubble, drag due to the separation and effect of blockage ratio on position of separation point for steady flow over a smooth circular cylinder are studied numerically using finite volume method of solving the Navier-Stokes equations at very low Reynolds number up to 50. The trends of change of these phenomena with increasing Reynolds number are expressed in form of some empirical equations and compared with results of previously published literatures. The separation starts at Reynolds number as low as 2.0 and the starting-separation angle is 146.557o from the upstream stagnation point and with increasing Reynolds number, the point of separation travels upstream while the separation bubble length increases with increasing Reynolds number. Profound effect of the blockage ratio on the location of separation point is evident in this numerical analysis.

Flow and heat transfer characteristics around an elliptic cylinder placed in front of a curved plate

An experimental investigation has been conducted to clarify heat transfer characteristics and flow behaviors around an elliptic cylinder. Also, flow visualization was carried out to clarify the flow patterns around the cylinder. The elliptic cylinder examined has an axis ratio of 1:2.17, was placed in the focus of parabolic plate. The test fluid is air and the Reynolds number based on the major axis length, c, ranged from 5 x 103 to 3 x 104. The angle of attack (?) was changed from 0? to 90? at 15? interval. It is found that the pressure distribution, form drag, location of separation point, and heat transfer coefficient depend strongly upon the angle of attack. Over the Reynolds number range examined, the mean heat transfer coefficient is at its highest at ? = 60? - 90?. The values of heat transfer coefficient in the case of free cylinder are higher than those for cylinder/plate combination at all angles of attack and Reynolds number range examined.

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%.

The effect of entrance radii on intraglottal pressure distributions in the divergent glottis

Modeling laryngeal aerodynamics requires specification of the glottal geometry. Changing the glottal exit radius alters the intraglottal pressure distributions in the converging glottis [Scherer et al., J. Acoust. Soc. Am. 110, 2267-2269 (2001)]. This study examined the effects of the glottal entrance radius on the intraglottal pressure distributions for divergent angles of 5°, 10°, 20°, 30°, and 40°. Glottal airflow and minimal glottal diameter were held constant at 73.2 cm(3)/s and 0.02 cm, respectively. The computational code FLUENT was used to obtain the pressure distributions. Results suggest that a smaller glottal entrance radius tends to (1) lower the transglottal pressure (reduce glottal flow resistance), although this is angle dependent, (2) make the pressure dip near the glottal entrance more negative in value, (3) increase the slope of the pressure distribution just upstream of the glottal entrance, and (4) make the initial pressure recovery (rise) in the glottis steeper. A general empirical equation for transglottal pressure as a function of radius, angle, and separation point location is offered. These results suggest that glottal entrance curvature for the divergent glottis significantly affects the driving pressures on the vocal folds, and needs to be well specified when building computational and physical models.

A comparative study of turbulence models performance for separating flow in a planar asymmetric diffuser

This paper presents a computational study of the separated flow in a planar asymmetric diffuser. The steady RANS equations for turbulent incompressible fluid flow and six turbulence closures are used in the present study. The commercial software code, FLUENT 6.3.26, was used for solving the set of governing equations using various turbulence models. Five of the used turbulence models are available directly in the code while the v 2– f turbulence model was implemented via user defined scalars (UDS) and user defined functions (UDF). A series of computational analysis is performed to assess the performance of turbulence models at different grid density. The results show that the standard k– ω, SST k– ω and v 2– f models clearly performed better than other models when an adverse pressure gradient was present. The RSM model shows an acceptable agreement with the velocity and turbulent kinetic energy profiles but it failed to predict the location of separation and attachment points. The standard k– ε and the low-Re k– ε delivered very poor results.

Flow past a square cylinder with an angle of incidence

A parametric study has been carried out to elucidate the characteristics of flow past a square cylinder inclined with respect to the main flow in the laminar flow regime. Reynolds number and angle of incidence are the key parameters which determine the flow characteristics. Location of separation point is greatly affected by angle of incidence, thus determining the flow field around the square cylinder. The critical Reynolds number for periodic vortex shedding at each angle of incidence considered is obtained by using Stuart–Landau equation. Attempt is made to classify the related flow patterns from a topological point of view, resulting in three distinct patterns in total. A comprehensive analysis of the effects of Reynolds number and angle of incidence on flow-induced forces on the square cylinder is presented. Collecting all the results obtained, contour diagrams of force and moment coefficients, Strouhal number, rms of lift-coefficient fluctuation, as well as a flow-pattern diagram are proposed for the ranges of the two parameters considered in the current investigation. Finally, a Floquet stability analysis is presented to detect the onset of the secondary instability leading to three-dimensional flow. The proposed diagrams and the Floquet stability analysis shed light on better physical understanding of the flow past a square cylinder, which should be useful in many engineering applications.

Linear stability analysis of two‐layer rectilinear flow in slot coating

AbstractTwo‐layer coating occurs in many products. Ideally, the liquids are deposited onto the substrate simultaneously. In the case of two‐layer slot coating, the interlayer between the coating liquids is subjected to enormous shearing. This may lead to flow instabilities that ruin the product. It is important to map the regions of the parameter space at which the flow is unstable. Most of the stability analyses of two‐layer rectilinear flow consider the position of the interlayer as an independent parameter. Classical results cannot be applied directly in coating flows. We present a linear stability analysis of two‐layer rectilinear flow considering the flow rates as an independent parameter. The predicted neutral‐stability curves define the region of stable flow as a function of the operating parameters. The range of coating operating conditions is restricted further, when the condition for the desirable interlayer separation point location are considered together with the stability condition. © 2010 American Institute of Chemical Engineers AIChE J, 2010

An experimental investigation of velocity fields in divergent glottal models of the human vocal tract

In speech, sound production arises from fluid-structure interactions within the larynx as well as viscous flow phenomena that is most likely to occur during the divergent orientation of the vocal folds. Of particular interest are the flow mechanisms that influence the location of flow separation points on the vocal folds walls. Physiologically scaled pulsatile flow fields in 7.5 times real size static divergent glottal models were investigated. Three divergence angles were investigated using phase-averaged particle image velocimetry (PIV). The pulsatile glottal jet exhibited a bi-modal stability toward both glottal walls, although there was a significant amount of variance in the angle the jet deflected from the midline. The attachment of the Coanda effect to the glottal model walls occurred when the pulsatile velocity was a maximum, and the acceleration of the waveform was zero. The location of the separation and reattachment points of the flow from the glottal models was a function of the velocity waveform and divergence angle. Acoustic analogies show that a dipole sound source contribution arising from the fluid interaction (Coanda jet) with the vocal fold walls is expected. [Work funded by NIH Grant RO1 DC03577.]

Thermofluiddynamic experiments with a heated and rotating circular cylinder in crossflow

Two-dimensional boundary layer profiles of heated and rotating circular cylinders in crossflow were investigated for subcritical freestream-Reynolds-numbersRe = 3.4 · 104 andRe = 4.8 · 104. The peripheral speed of the cylinder surface corresponds to velocity ratios 0 ≦ ξ ≦ 2(α, circular/freestream-velocity). Special attention was focused on the location of separation points, which was determined as a function of the velocity ratio a whereby a strong influence of wall temperature was revealed.

Numerical Simulation of Side Loads in an Ideal Truncated Nozzle

The side loads induced by unsymmetrical and unsteady separation of the flow taking place in the nozzle extension during launch are a very important limiting factor for the performance of a rocket engine. The onset of suchloads is not yet a fully understood phenomenon because only a limited amount of experimental data is available due to test difficulties. A numerical study of the three-dimensional overexpanded nozzle flow during stationary operations has been undertaken to try to understand the origin of this phenomenon. The flow separation in an truncated ideal contoured nozzle is investigated together with the resulting side loads. Comparisons with experimental data are given. The numerical simulation relies on the resolution of the three-dimensional unsteady Reynolds-averaged Navier-Stokes equations. An algebraic turbulence model based on Baldwin-Lomax and Goldberg's backflow correction has been implemented in a three-zone formulation adapted to the flow topology of interest. The main features of the flowfield, side-loads mean value, and the separation point location are well estimated. With regard to the separation point location, the proposed method gives better results than the most commonly used semi-empirical criteria. Unsteady characteristics of the flowfield are presented.

極超音速流中に置かれた凹壁面上の剥離に関する研究

To clarify the action of the rudder of HSTs, a hypersonic concave flow separation was studied. In hypersonic flows the rudder can not act efficiently because of the boundary layer separation, so that studies of that are needed. In previous studies, the hypersonic concave flow separation was studied to clarify the mechanisms of the separation region. But, the relation between the separation point location and the rudder angle was not cleared. Therefore, experimental and analytical studies were carried out to clarify this relation. The first, the flow visualization experiment by schlieren method was carried out. Then the transfer of the separation point due to the variation of the rudder angle was observed. Next, the separation point transfer in the boundary layer was calculated analytically by potential flow theory. The analytical result agreed well with the experimental observation. Consequently, the correlation between the separation point transfer and the rudder angle variation was confirmed.

Numerical investigation of separated nozzle flows

A numerical study of axisymmetric overexpanded nozzle is presented. The flow structure of the startup and throttle-down processes are examined. During the impulsive startup process, observed flow features include the Mach disk, separation shock, Mach stem, vortex core, contact surface, slip stream, initial shock front, and shocklet. Also the movement of the Mach disk is not monotonical in the downstream direction. For a range of pressure ratios, hysteresis phenomenon occurs; different solutions were obtained depending on different processes. Three types of flow structures were observed. The location of separation point and the lower end turning point of hysteresis are closely predicted. A high peak of pressure is associated with the nozzle flow reattachment. The reversed vortical structure and affects engine performance.

The control of vortex shedding behind heated circular cylinders at low Reynolds numbers

We report some experiments undertaken to investigate the control of vortex shedding behind electrically heated cylinders at low Reynolds numbers owing to the heat input brought to the cylinder. In airflow, depending on the Reynolds number value, complete suppression or modification of the vortex shedding phenomenon can be achieved with increase of heat input. Experimental results suggest that this control could result of a slight change of the separation point location due to the increase of the dynamic viscosity of the fluid.

Finite Element Solution of Laminar Combined Convection From a Sphere

The purpose of this investigation is to study the flow and heat transfer characteristics of laminar combined convection from an isothermal sphere. The full Navier–Stokes and energy equations are solved by a finite element method. The variations of surface shear stress, pressure, and Nusselt number are obtained over the entire surface of the sphere including the zone beyond the separation point. The predicted values of average Nusselt number, location of separation point, total drag shear force, and some plots of the flow and temperature fields are also presented. The results are compared with those of previous numerical and experimental investigations and the agreement is satisfactory.

A Comparison of Three Critical Flow Venturi Designs

Three critical flow venturi designs have been examined from the standpoint of their sensitivity to initial boundary-layer thickness, inlet flow nonuniformity, separation, and transition point location. These sensitivity studies were carried out with a finite-difference computer code which represents a coupled solution of the inviscid potential core and the viscous boundary layer. Since it makes good sense to select a design that is least sensitive to these effects, it is expected that this information will be of interest to potential users of these devices. Differences between the sensitivities of the various designs were found to be small; however, a slight advantage is seen in designs which employ a circular arc entrance section and a circular arc throat.

Analytical Model of Axisymmetric Af terbody Flow Separation

A practical engineering approach to the prediction of pressure distribution and boundary layer separation point location on afterbodies in subsonic flow is developed. Experimental data are reviewed and the inadequacy of currently available separation prediction methods is demonstrated. A control volume technique is developed as an alternative and is shown to have merit. The separation bubble is then modeled and another control volume technique is developed to predict its outer boundary. Finally, these two developments are combined with a conventional inviscid flowfield calculation and a boundary-layer analysis to produce an iterative procedure to predict pressure distribution and separation point location on an afterbody given only body shape and freestream flow condition.

Flow Separation on a Vibrating Circular Cylinder

The location of the boundary-layer separation point on a circular cylinder has been investigated experimentally for a stationary cylinder and for a cylinder which is performing vortex-excited oscillations. The results for the stationary cylinder are in close agreement with published data. For the vibrating cylinder, the range of angular displacement of the separation point was found to depend upon the vibration amplitude and frequency, and the peak displacement occurred at a different frequency from that which gave the peak cylinder amplitude. As the frequency of the wake motion increases beyond that which gives that maximum range in the location of the separation point, the range of the motion of the separation point decreases. For given cylinder and wake frequencies, the higher the cylinder amplitude, the greater the motion of the wake. The phase shift of the actual wake was found to be close to 180 deg behind that of an ideal wake which aligns itself behind the relative velocity vector of the flow. The phase shift was found to be independent of cyclinder amplitude.

Numerical studies of the laminar boundary layer for Mach numbers up to 15

A comprehensive set of exact solutions to the first-order boundary-layer equations has been computed using the finite difference computer programme of Sells, with and without wall cooling. The effects of Prandtl number, wall cooling and Mach number on separation point location were studied. Values of displacement thickness, skin friction coefficient and Stanton number are displayed graphically for the supersonic flow over a circular concave arc, for a subsonic cooled cylinder and for the case of a linearly retarded velocity distribution. The influence of pressure gradient on recovery factor was studied. Velocity and temperature profiles are shown for four cold wall cases. The exact computer results show the errors in many of the more approximate methods available for the case whereUe=U∞(1 -X/L). The importance of second-order effects and the applicability of a first-order solution are discussed briefly.

Heat Transfer to a Vaporizing, Ablating Surface

must be found. In the simple case of separation ahead of a step, the reattachment point is fixed, and the oblique-shock theory gives the required second relation between separation-point location and pressure rise. For most other problems, however, the reattachment point is not known. It is, therefore, necessary to find a relationship between reattachment-point location and pressure rise at reattachment. Further experimental work is needed to define such a relation. An expression similar to the present semiempirical relation has been proposed by Guman. His expression, however, uses a Reynolds Number based on the distance to the point where the disturbance would intersect the boundary layer if there were no interaction. It, therefore, gives the pressure rise for given flow conditions directly without requiring local expressions for separation-point or reattachment-point locations. An empirical constant is used to fit the equation to experimental data for a given body shape. Guman's equation can be written