Ratio Of Free Stream Velocity Research Articles

Flow structure on a rotating wing: Effect of steady incident flow

The flow structure along a rotating wing in steady incident flow is compared to the structure on a rotating wing in quiescent fluid, in order to clarify the effect of advance ratio J (ratio of free-stream velocity to tip velocity of wing). Stereoscopic particle image velocimetry leads to patterns of vorticity, velocity, and Q-criterion (constant values of the second invariant of the velocity gradient tensor), as well as streamlines, which allow identification of critical points of the flow. The effective angle of attack is held constant over the range of J, and the wing rotates from rest to a large angle that corresponds to attainment of the asymptotic state of the flow structure. Prior to the onset of motion, the wing is at high angle of attack and the steady incident flow yields a fully stalled state along the wing. After the onset of rotation, the stalled region quickly gives rise to a stable leading edge vortex. Throughout the rotation maneuver, the development of the flow structure in the leading edge region is relatively insensitive to the value of J. In the trailing-edge region, however, the structure of the shed vorticity layer is strongly dependent on the value of J. Further insight into the effects of J is provided by three-dimensional patterns of spanwise-oriented vorticity, spanwise velocity, and Q-criterion.

LES of the adverse-pressure gradient turbulent boundary layer

We describe large-eddy simulations (LES) of the flat-plate turbulent boundary layer in the presence of an adverse pressure gradient. The stretched-vortex subgrid-scale model is used in the domain of the flow coupled to a wall model that explicitly accounts for the presence of a finite pressure gradient. The LES are designed to match recent experiments conducted at the University of Melbourne wind tunnel where a plate section with zero pressure gradient is followed by section with constant adverse pressure gradient. First, LES are described at Reynolds numbers based on the local free-stream velocity and the local momentum thickness in the range 6560–13,900 chosen to match the experimental conditions. This is followed by a discussion of further LES at Reynolds numbers at approximately 10 times and 100 times these values, which are well out of range of present day direct numerical simulation and wall-resolved LES. For the lower Reynolds number runs, mean velocity profiles, one-point turbulent statistics of the velocity fluctuations, skin friction and the Clauser and acceleration parameters along the streamwise, adverse pressure-gradient domain are compared to the experimental measurements. For the full range of LES, the relationship of the skin-friction coefficient, in the form of the ratio of the local free-stream velocity to the local friction velocity, to both Reynolds number and the Clauser parameter is explored. At large Reynolds numbers, a region of collapse is found that is well described by a simple log-like empirical relationship over two orders of magnitude. This is expected to be useful for constant adverse-pressure gradient flows. It is concluded that the present adverse pressure gradient boundary layers are far from an equilibrium state.

Unsteady thermal radiation mixed convection flow from a moving vertical plate in a parallel free stream: Effect of Newtonian heating

An unsteady mixed convection flow over a moving vertical plate in a parallel free stream is considered to investigate the combined effects of buoyancy force and thermal diffusion in presence of Newtonian heating in which heat transfer rate from the surface is proportional to the local temperature and thermal radiation effects. It is assumed that the unsteadiness is caused by the time dependent free stream velocity as well as by the moving plate velocity. The governing boundary layer equations with boundary conditions are transformed into a non-dimensional form by a group of non-similar transformations. The resulting system of coupled non-linear partial differential equations is solved by an implicit finite difference scheme in combination with the quasi-linearization technique. Computations are performed and representative set is displayed graphically to illustrate the influence of the mixed convection parameter (λ), Prandtl number (Pr), the ratio of free stream velocity to the composite reference velocity (ɛ), stream wise co ordinate (ξ) and thermal radiation parameter (R) on the velocity and temperature profiles. The numerical results for the local skin-friction coefficient (ReL1/2Cf) and wall temperature (GW(ξ,0)) are also presented. The numerical results are presented in graphs revealing some interesting features of flow and heat transfer phenomena in this study. Present results are also compared with previously published works and are found to be in excellent agreement.

Theoretical and experimental investigation on aero-optical effect for a compressible mixing layer

Using dimensional analysis and experimental technique, the aero-optical effect for a compressible mixing layer around optical side window is investigated. According to the theoretical analysis, we find that time-averaged boresight error increases with the increase of convective Mach number (Mc). Moreover, we also present the relations between time-averaged boresight error (BSE) and Reynolds number, shear stress at interface between mixing layer and free stream, free stream velocity ratio and density ratio. The experimental results from a thin beam light passing through compressible mixing layer verify the theoretical result between time-averaged BSE and Mc.

Wave model of turbulence in a mixing layer

Possibilities of using a phenomenological model of turbulence based on the wave approach to describe the mixing layer flow have been estimated. In a single-mode approximation, the mean flow field and the dependence of the mixing layer growth rate on the free stream velocity ratio have been determined. The results show evidence for adequacy of the proposed wave model for the description of a turbulent mixing layer.

0418 Performance of a High-order LES Model Related to the Effect of Jet on Airfoil profile change(OS4-4 流体機械の研究開発とそれに関連した複雑流動現象,OS4 流体機械の研究開発とそれに関連した複雑流動現象)

Recent aircraft morphing program envisions an aircraft without conventional control surfaces. Instead of moving control surfaces, the vehicle control systems may be implemented with a combination of propulsive forces, micro-surface-effectors, and fluidic devices dynamically operated by an intelligent flight control system. In the present work, first of all, the accuracy of a high-order LES turbulent model, developed by the author is verified by applying it to predict the experimental drag and lift coefficient for (NACA4412) airfoil. Finally, the work described in this paper focused on the aerodynamic virtual shaping of an (NACA0012) symmetric airfoil at zero angle of attack, using only a thin jet without additional fixtures. The effects of virtual shaping are investigated systematically with jet actuation at different chord-wise locations, and different ratio of jet and free-stream velocity. The jet was used to alter the aerodynamic shape of a symmetric airfoil to enhance lift and/or to reduce drag.

Nonlinear MHD Flow and Heat Transfer over a Power-Law Stretching Plate with Free Stream Pressure Gradient and Viscous Dissipation in Presence of Variable Thermal Diffusivity

The numerical analysis is carried out on steady nonlinear magnetohydrodynamic flow and heat transfer over a power-law stretching plate with free stream pressure gradient and viscous dissipation in the presence of variable thermal diffusivity and variable transverse magnetic field. The thermal diffusivity is assumed to vary as linear functions of temperature. Using similarity transformation, the nonlinear partial differential equations are converted into nonlinear ordinary differential equations. Numerical solutions of these equations are obtained with the help of Runge-Kutta shooting method. The effects of magnetic field, stretching parameter, Prandtl number, free stream velocity ratio, Eckert number and thermal diffusivity parameter on skin friction, velocity, rate of heat transfer and temperature are thoroughly discussed. It is found that the effect of magnetic field is to decrease the velocity and enhance the temperature.

Effects of variable properties on MHD heat and mass transfer flow near a stagnation point towards a stretching sheet in a porous medium with thermal radiation

The effect of variable viscosity and thermal conductivity on steady magnetohydrodynamic (MHD) heat and mass transfer flow of viscous and incompressible fluid near a stagnation point towards a permeable stretching sheet embedded in a porous medium are presented, taking into account thermal radiation and internal heat genberation/absorbtion. The stretching velocity and the ambient fluid velocity are assumed to vary linearly with the distance from the stagnation point. The Rosseland approximation is used to describe the radiative heat flux in the energy equation. The governing fundamental equations are first transformed into a system of ordinary differential equations using a scaling group of transformations and are solved numerically by using the fourth-order Rung—Kutta method with the shooting technique. A comparison with previously published work has been carried out and the results are found to be in good agreement. The results are analyzed for the effect of different physical parameters, such as the variable viscosity and thermal conductivity, the ratio of free stream velocity to stretching velocity, the magnetic field, the porosity, the radiation and suction/injection on the flow, and the heat and mass transfer characteristics. The results indicate that the inclusion of variable viscosity and thermal conductivity into the fluids of light and medium molecular weight is able to change the boundary-layer behavior for all values of the velocity ratio parameter λ except for λ = 1. In addition, the imposition of fluid suction increases both the rate of heat and mass transfer, whereas fluid injection shows the opposite effect.

Turbulent asymptotic suction boundary layers studied by simulation

The turbulent asymptotic suction boundary layer (ASBL) is studied using numerical simulations. Uniform suction is applied on the wall in order to compensate for the momentum loss inflicted by the wall friction. Four Reynolds numbers, defined as the ratio of free-stream velocity and suction rate, Re = 333, 400 and 500, are considered, whereas Re = 280 relaminarised. In agreement with previous studies, suction causes the fluctuation intensities to decrease, and the near-wall anisotropy to increase. The shape of the mean velocity profile is considerably changed yielding a decreased slope in the overlap region. It is shown that even for moderate suction rates large values for the friction Reynolds number Reτ = δ99+ are obtained; at Re = 333 a value of Reτ = 1900 is reached and Re = 400 yields Reτ = 5700. Artificially using smaller computational domains, limiting the size of the largest turbulent structures, gives unexpected results: The mean velocity profile starts to show a distinct wake region which only disappears for large enough domains. Moreover, the boundary layer thickness δ99 strongly depends on the chosen domain size. Spectral maps of the flow are analysed, showing an outer peak appearing at a spanwise size of about 0.6δ99, albeit with considerably lower amplitude compared to cases without suction. Visualisations of the flow are also discussed.

The influence of thermal radiation on MHD station-point flow past a stretching sheet with heat generation

This letter is concerned with the plane and axisymmetric stagnation-point flows and heat transfer of an electrically-conducting fluid past a stretching sheet in the presence of the thermal radiation and heat generation or absorption. The analytical solutions for the velocity distribution and dimensionless temperature profiles are obtained for the various values of the ratio of free stream velocity and stretching velocity, heat source parameter, Prandtl number, thermal radiation parameter, the suction and injection velocity parameter and magnetic parameter and dimensionality index in the series form with the help of homotopy analysis method (HAM). Convergence of the series is explicitly discussed. In addition, shear stress and heat flux at the surface are calculated.

Hydrodynamic plane and axisymmetric slip stagnation-point flow with thermal radiation and temperature jump

This work focuses on the steady boundary layer flow and heat transfer near the forward stagnation point of plane and axisymmetric sheet towards a stretching sheet with velocity slip and temperature jump. The resulting nonlinear partial differential equations are reduced to the system of nonlinear ordinary differential equations by means of similarity transformations. The analytical solutions for the velocity and temperature distributions are obtained for the various values of the ratio of free stream velocity and stretching velocity, velocity slip parameter, magnetic parameter, the suction parameter, temperature jump parameter, Prandtl number, the radiation parameter and dimensionality index parameter in the series forms with the help of homotopy analysis method. Convergence of the series is explicitly discussed. The flow and shear stresses depend heavily on the velocity slip parameter. The temperature gradient at the wall increases with velocity slip parameter, temperature jump factor and decreased thermal radiation.

Numerical Investigation of Inlet Distortion on a Wing-Embedded Lift Fan

The inlet distortion arising on a generic fan-in-wing wind-tunnel model is investigated by means of unsteady Reynolds-averaged Navier–Stokes simulations. Flow separation occurs on the inlet lip and generates a separation bubble above the rotor blades. A distortion of the total pressure distribution at the fan inlet significantly influences the blade loading. The strong flow-velocity gradient over the inlet lip induces an increase of the axial-velocity magnitude, resulting in a reduction of incidence angle on the rotor blade. In the bubble-core region, the low axialvelocity magnitude enhances the blade incidence angle and provokes blade tip stall. Steady-state calculations, using an actuator disk approach, are also conducted to identify the key parameters affecting lip boundary-layer separation. The ratio of freestream velocity to fan jet velocity and the inlet-lip radius affect the total pressure distribution bymodifying the location of the flow separation. The angle of attack on the wing has a negligible impact on the inlet distortion. An inlet lip with a large radius at its front part suppresses the lip separation and considerably reduces the total pressure distortion. Injecting a jet over the inlet lip can be a solution to actively control the lip flow.

Scaling Transformations for Boundary Layer Flow near the Stagnation-Point on a Heated Permeable Stretching Surface in a Porous Medium Saturated with a Nanofluid and Heat Generation/Absorption Effects

In this article, a similarity solution of the steady boundary layer flow near the stagnation-point flow on a permeable stretching sheet in a porous medium saturated with a nanofluid and in the presence of internal heat generation/absorption is theoretically studied. The governing partial differential equations with the corresponding boundary conditions are reduced to a set of ordinary differential equations with the appropriate boundary conditions via Lie-group analysis. Copper (Cu) with water as its base fluid has been considered and representative results have been obtained for the nanoparticle volume fraction parameter \({\phi}\) in the range \({0\leq \phi \leq 0.2}\) with the Prandtl number of Pr = 6.8 for the water working fluid. Velocity and temperature profiles as well as the skin friction coefficient and the local Nusselt number are determined numerically. The influence of pertinent parameters such as nanofluid volume fraction parameter, the ratio of free stream velocity and stretching velocity parameter, the permeability parameter, suction/blowing parameter, and heat source/sink parameter on the flow and heat transfer characteristics is discussed. Comparisons with published results are also presented. It is shown that the inclusion of a nanoparticle into the base fluid of this problem is capable to change the flow pattern.

Heat transfer on a power law stretching sheet subjected to free stream pressure gradient

The present work deals with the numerical study of temperature distribution in the laminar boundary layer driven by the stretching boundary surface subjected to pressure gradient. The similarity transformation obeying the same power law based on composite reference velocity (union of velocities of the stretching boundary and free stream) has been employed that leads to a single set of equations, irrespective of the condition whether U w > U ∞ or U w < U ∞, containing three parameters: β measuring the stretch rate of the moving boundary, ε is the ratio of free stream velocity to composite reference velocity and Pr is the Prandtl number of the ambient fluid. The numerical solutions of the thermal boundary layer equations are obtained for three Prandtl numbers 0.7, 1.0 and 10 for 0 ⩽ ε ⩽ 1 and for 0 ⩽ β ⩽ 2. The heat transfer coefficient show appreciable dependence on the ratio of free stream velocity to union of velocities of the stretching surface boundary and free stream.

Unsteady mixed convection flow from a moving vertical plate in a parallel free stream: Influence of heat generation or absorption

An unsteady mixed convection flow over a moving vertical plate in a parallel free stream is considered to investigate the combined effects of buoyancy force and thermal diffusion in presence of heat generation or absorption. The unsteadiness is introduced by the time dependent free stream velocity as well as by the moving plate velocity. The governing boundary layer equations are transformed into a non-dimensional form by a special group of non-similar transformations. The resulting system of coupled non-linear partial differential equations is solved by an implicit finite difference scheme in combination with the quasi-linearization technique. Computations are performed and numerical results are displayed graphically to illustrate the influence of the buoyancy (mixed convection) parameter, Prandtl number, the ratio of free stream velocity to the composite reference velocity and heat generation or absorption parameter on the velocity and temperature profiles. The numerical results for the local skin-friction coefficient and local Nusselt number are also presented. Present results are compared with previously published work and are found to be in excellent agreement. It is found that in presence of buoyancy force ( λ > 0), the velocity profile exhibits velocity overshoot 80% more for lower Prandtl number ( Pr = 0.7) as compared to the magnitude of the velocity overshoot for higher Prandtl number ( Pr = 7.0).

Experimental Investigation of Flow Separation Control Using an Array of Synthetic Jets

An experimental study of flow control using an array of three synthetic jets has been undertaken in a separated laminar flow over a flap in water. PIV was employed to obtain the information about the extent of flow separation delay at different synthetic jet operating conditions. Laser induced florescence (LI F) flow visualisation technique was used to reveal the characteristics of the vortical structures produced by the synthetic jets which result in a delay of separation. It was obser ved that the flow separation delay is typically associated with the presence of two strea ks or three streaks of high streamwise velocity in the separating flow. Based on the resul ts from the present experiment, a parameter map indicating the flow patterns observed at different synthetic jet operating conditions is produced. In addition, a contour map showing the separation control effectiveness at the corresponding conditions is al so obtained. It was found that the twostreak flow pattern is mostly produced at a jet-to- freestream velocity ratio (VR) between 0.3 and 0.5 whereas the three-streak pattern occurs at 0.5<VR<1.5 at medium dimensionless frequencies. The LIF images reveal th at the two-streak flow pattern is caused by the legs of the hairpin vortices produced by synthetic jets whereas the threestreak pattern is caused by the longitudinal struct ures induced by the trailing legs of tilted vortex ring type structures. For the experimental c onditions tested in this study, operating the synthetic jets at a dimensionless stroke length around 2 and a VR around 0.5, which generate vortical structures with a streamwise spac ing not larger than the local boundary layer thickness before separation, would deliver th e best flow control effect with the least energy consumption.

Interaction of Synthetic Jet with Boundary Layer Using Microscopic Particle Image Velocimetry

The aerodynamic interaction between an unsteady, inclined synthetic jet and a crossflow boundary layer was studied as a precursor toward applying active flow control concepts for rotor applications, such as dynamic stall control and fuselage drag reduction. Because the flowfield offered numerous challenges from a measurement perspective, several experiments were carried out using a phase-locked, two-dimensional microscopic particle image velocity technique in a building block approach, by adding one complexity after another. The procedure began with boundary layer measurements made on a simple flat plate using the microscopic particle image velocity technique. Velocity measurements were made deep in the viscous sublayer, as close as 20μm from the surface. Following this, the synthetic jet actuator was characterized while operating in quiescent air as well as in crossflow. The results showed that the evolution of the synthetic jet in crossflow was substantially different from its evolution in quiescent air, suggesting that any flow physics or performance prediction (for example, the depth of penetration of the jet into the boundary layer) made based on the quiescent flow conditions may not be applicable in crossflow. All the momentum added to the boundary layer had its source from the synthetic jet actuator, and the penetration of the jet was limited to the viscous sublayer and log layer; the outer layer was unaffected, despite using a jet to freestream velocity ratio of four. Significant effort was also made to validate the microscopic particle image velocity technique and evaluate its capability to accurately resolve such a complex flowfield. To this end, microscopic particle image velocity measurements were compared with hot-wire measurements made on a simple steady jet, as well as an unsteady, periodic synthetic jet. Excellent correlation was found between the two techniques, validating microscopic particle image velocity measurements.

Analytical solution of magnetohydrodynamic stagnation-point flow of a power-law fluid towards a stretching surface

A new kind of analytic technique, namely the homotopy analysis method (HAM), is employed to give an explicit analytical solution of the steady two-dimensional stagnation-point flow of an electrically conducting power-law fluid over a stretching surface when the surface is stretched in its own plane with a velocity proportional to the distance from the stagnation-point. A uniform transverse magnetic field is applied normal to the surface. An explicit analytical solution is given by recursive formulae for the first-order power-law (Newtonian) fluid when the ratio of free stream velocity and stretching velocity is not equal to unity. For second and real order power-law fluids, an analytical approach is proposed for magnetic field parameter in a quite large range. All of our analytical results agree well with numerical results. The results obtained by HAM suggest that the solution of the problem under consideration converges.

Nonsimilar, laminar, steady, electrically-conducting forced convection liquid metal boundary layer flow with induced magnetic field effects

A nonsimilar steady laminar boundary layer model is described for the hydromagnetic convection flow of a Newtonian, electrically-conducting liquid metal past a translating, non-conducting plate with a magnetic field aligned with the plate direction. The non-dimensional boundary layer equations are solved with the Sparrow–Quack–Boerner local nonsimilarity method (LNM). An increase in magnetic Prandtl number ( Pr m ) is found to strongly enhance wall heat transfer rate ( Nu x Re x − 1 / 2 ), velocity ( f ′ ) and induced magnetic field function ( g), but exerts negligible influence on the temperature ( θ) in the boundary layer. A rise in magnetic force number ( β) increases velocity, f ′ , shear stress function, f ″ , and wall heat transfer gradient, i.e. Nu x Re x − 1 / 2 , but reduces magnetic field function, g and temperature, θ. Increasing ordinary Prandtl number ( Pr), decreases temperature, θ, but increases wall heat transfer rate ( Nu x Re x − 1 / 2 ). An increase in wall to free stream velocity ratio parameter, ζ, increases flow velocity, f ′ , and induced magnetic field gradient, g ′ for small ξ but reduces g ′ for larger ξ, and also boosts the wall temperature gradient, Nu x Re x − 1 / 2 . The model has potential applications in astronautical magneto-thermo-aerodynamics, nuclear reactor channel flow control with magnetic fields and MHD (magnetohydrodynamic) energy generators.

Magnetohydrodynamic stagnation-point flow of a power-law fluid towards a stretching surface

Steady two-dimensional stagnation-point flow of an electrically conducting power-law fluid over a stretching surface is investigated when the surface is stretched in its own plane with a velocity proportional to the distance from the stagnation-point. We have discussed the uniqueness of the solution except when the ratio of free stream velocity and stretching velocity is equal to 1. The effect of magnetic field on the flow characteristic is explored numerically and it is concluded that the velocity at a point decreases/increases with increase in the magnetic field when the free stream velocity is less/greater than the stretching velocity. It is further observed that for a given value of magnetic parameter M, the dimensionless shear stress coefficient | F ″ ( 0 ) | increases with increase in power-law index n when the value of the ratio of free stream velocity and stretching velocity is close to 1 but not equal to 1. But when the value of this ratio further differs from 1, the variation of | F ″ ( 0 ) | with n is non-monotonic.