Delay Boundary Layer Separation Research Articles

Numerical investigation of the dynamic stall reduction on the UAS-S45 aerofoil using the optimised aerofoil method

AbstractThis paper investigates the effect of the optimised morphing leading edge (MLE) and the morphing trailing edge (MTE) on dynamic stall vortices (DSV) for a pitching aerofoil through numerical simulations. In the first stage of the methodology, the optimisation of the UAS-S45 aerofoil was performed using a morphing optimisation framework. The mathematical model used Bezier-Parsec parametrisation, and the particle swarm optimisation algorithm was coupled with a pattern search with the aim of designing an aerodynamically efficient UAS-45 aerofoil. The $\gamma - R{e_\theta }$ transition turbulence model was firstly applied to predict the laminar to turbulent flow transition. The morphing aerofoil increased the overall aerodynamic performances while delaying boundary layer separation. Secondly, the unsteady analysis of the UAS-S45 aerofoil and its morphing configurations was carried out and the unsteady flow field and aerodynamic forces were analysed at the Reynolds number of 2.4 × 106 and five different reduced frequencies of k = 0.05, 0.08, 1.2, 1.6 and 2.0. The lift ( ${C_L})$ , drag ( ${C_D})$ and moment ( ${C_M})\;$ coefficients variations with the angle-of-attack of the reference and morphing aerofoils were compared. It was found that a higher reduced frequencies of 1.2 to 2 stabilised the leading-edge vortex that provided its lift variation in the dynamic stall phase. The maximum lift $\left( {{C_{L,max}}} \right)$ and drag $\left( {{C_{D,max}}} \right)\;$ coefficients and the stall angles of attack are evaluated for all studied reduced frequencies. The numerical results have shown that the new radius of curvature of the MLE aerofoil can minimise the streamwise adverse pressure gradient and prevent significant flow separation and suppress the formation of the DSV. Furthermore, it was shown that the morphing aerofoil delayed the stall angle-of-attack by 14.26% with respect to the reference aerofoil, and that the ${C_{L,max}}\;$ of the aerofoil increased from 2.49 to 3.04. However, while the MTE aerofoil was found to increase the overall lift coefficient and the ${C_{L,max}}$ , it did not control the dynamic stall. Vorticity behaviour during DSV generation and detachment has shown that the MTE can change the vortices’ evolution and increase vorticity flux from the leading-edge shear layer, thus increasing DSV circulation. The conclusion that can be drawn from this study is that the fixed drooped morphing leading edge aerofoils have the potential to control the dynamic stall. These findings contribute to a better understanding of the flow analysis of morphing aerofoils in an unsteady flow.

Impacts of thermal radiation with nanoparticle aggregation and variable viscosity on unsteady bidirectional rotating stagnation point flow of nanofluid

Interest in nanofluids with nanoparticle-induced aggregation effects like solar-powered energy and crossflow heat transfer is growing in the modern industrial sector. The main goal of this research is to find out how nonlinear radiation, changes in viscosity, and aggregation affect the flow of ethylene glycol-based nanofluids in three dimensions. It is concerned with stagnation point flows that are not axisymmetric and that take place on stretching sheets. In accordance with the imposed hypotheses, the governing equations will be modelled. With the help of a similarity transformation, a complicated set of nonlinear partial differential equations can be turned into a simpler set of ordinary differential equations. This simplification facilitates the process of solving the equations. The algorithm will then be used to perform a numerical solution of the simplified set of equations. Runge-Kutta (RK-IV) and the shooting method are used to derive the numerical results. Different parameters and their effects are graphically displayed. As the concentration of nanoparticles in the skin increases, it appears that both the rate of heat transmission and the coefficient of friction increase. The heat transfer rate and the skin coefficient of friction inclination both increase as the instability parameter is increased. To the contrary, it was found that delaying boundary layer's separation by increasing stress rate of surrounding fluid was effective. Tables also compare the nanoparticles aggregation effects with and without radiation and show the variation in Nusselt numbers. High levels of agreement were found between the findings of the current study and those of a previous publication for the same instance, lending further support to the findings.

Stall Delay Characteristics Study of NACA 2412 Wing with Outward Dimples

Flow separation in an aircraft is one of the ground causes of decrease in the lift and increase in the pressure drag which eventually leads to loss of energy, vibrations in machinery and may cause structural failure of the aircraft. Many studies and research works have been carried out on delaying boundary layer separation. One of such works is the implementation of various active and passive devices on the surface of the wing. In this research work, we have used one such device called a passive dimple. By placing these dimples on the upper surface of the wing, a turbulent flow is induced which produces vortices, causing the boundary layer to stay attached to the surface of the wing to some extent and thereby increasing the stall angle of the aircraft. In this research work, we have placed outward dimples on the upper surface of the wing at various positions and ran simulations at different angle of attacks to observe the effects.

Theoretical breakthrough in the dynamics of a jet in a free-stream flow around a corner

For the first time, the laminar jet in a free-stream flow around a corner with/without a moving wall condition (as it assists the flow) is studied theoretically. The corner may contract the streamlines or expand them depending on the angle of inclination. In the context of incompressibility, it is presented a composite non-similar transformation of the Navier-Stokes (N-S) equations in Cartesian space to approximately decipher the nature of the boundary layer flow over the entire physical domain; i.e. 0 ≤ x, y < ∞. The transformed Partial Differential Equations (PDEs) are then solved by an in-house MATLAB code, which employs an implicit algorithm of a tridiagonal form with quasi-linearization technique. For a stationary wall, as the negative angle of the corner meets Falkner-Skan (F-S) limitation and expands beyond, the predictive model starts to show a critical location, which reveals directly the flow separation point. This critical location moves toward the jet origin as the angle of inclination becomes more negative. Here, these locations are extracted as a function of the inclination parameter in order to manifest the suitability of a jet to delay boundary layer separation. A similar scenario also applies to the moving wall case. In this case, it was recorded that a moving wall condition delays separation for a considerable distance. It should be pointed out that up to date, there is no theoretical investigation on this specific flow geometry despite the many associated applications; and the limited available reports on an analogous flow geometry are those from the Computational Fluid Dynamics (CFD) and on a case by case basis. More specifically, the present geometry is quite similar to the flow passing the way down to the trailing edge of an airfoil, with a jet generator mechanism such as Dielectric Barrier Discharge (DBD) plasma actuator to delay flow separation on the backside of the airfoil.Here, by the use of the theoretical non-similarity concept empowered by some recent advancements (see Jafarimoghaddam, 2020 and Jafarimoghaddam, 2021), the fundamental problem of a jet discharged in the lower boundary of a freestream flow over a wedge is successfully solved.This theoretical breakthrough suggests promising avenues for the future design of airfoils with moving parts/flaps as an innovative step forward to enhance maneuverability of the flying objects.

دراسة تجريبية لتأثير مولد الدوامة على الطبقة الحدودية للوحة مسطحة

This paper is dealing with an experimental study to show the influence of the geometric characteristics of the vortex generators VG son the thickness of the boundary layer (∂) and drag coefficients (CD) of the flat plate. Vortex generators work effectively on medium and high angles of attack, since they are "hidden" under the boundary layer and practically ineffective at low angles. The height of VGs relative to the thickness of the boundary layer enables us to study the efficacy of VGs in delaying boundary layer separation. The distance between two VGs also has an effect on the boundary layer if we take into account the interference between two pairs of VGs. The effect of the changing in (h- the height of vortex generator, d- the average distance between tow vortex generators) on the thickness of the flat plate boundary layer and the drag coefficients has been studied for triangular vortex generator. The measurements of the vortex generator have been changed to determine the optimum boundary layer thickness and the change in drag coefficients. An experiment was done at an average free stream velocity, (U∞,) of 28 m/s. The experiment was conducted in the wind tunnel UTAD-2 University (NAU) Kiev, Ukraine.

Heat Transport of Casson Nanofluid Flow over a Melting Riga Plate Embedded in a Porous Medium

This article investigates the flow of Casson nanofluid induced by a stretching Riga plate in the presence of a porous medium. The implication of the Riga plate is to generate electromagnetohydrodynamic force which influences the fluid speed, and as well applicable in delaying boundary layer separation. The complexity of the equations governing the problem is reduced using similarity transformation. The resulting coupled nonlinear ordinary differential equations are solved by employing Chebyshev collocation scheme (CCS) and validated with Galerkin weighted residual method (GWRM). The influence of parameters, such as modified Hartmann number and melting parameter, on the nanofluid flow, heat, and mass transfer is considered. Some of the major findings include that modified Hartmann number tends to increase nanofluid flow. Also, increasing the value of melting parameter is in favor of both velocity and nanoparticle volume fraction profiles but diminishes temperature profile. The application of this work can be found in polymer synthesis, metallic processing, and electromagnetic crucible systems.

Passive separation control of shortfin mako shark skin in a turbulent boundary layer

This experimental investigation discusses the effectiveness of shortfin mako shark skin to passively control turbulent boundary layer separation. Experiments were conducted in a water tunnel facility with the shark skin specimens mounted on a smooth flat plate subjected to an adverse pressure gradient. The two skin specimens have shark scales with different sizes, shapes, and bristling angles. The flank region scales (B2) are slender, tall, and can bristle at 50°, while the scales in the region between flank and dorsal fin (B1) are wide, short, and can bristle at 30°. The adverse pressure gradient on the plate was generated by a rotating cylinder. DPIV measurements indicate that unpainted scales B2 can fully control or delay boundary layer separation and reduce the fraction of time the flow is reversed. The bristling of the scales caused fluctuation in the skin friction curve and the single spike intensity of the normal stress very close to the wall. These capabilities are directly related to the scale bristling angle. In contrast, the low bristling angle of the scales from region B1 did not impede the reversing flow, and therefore separation was enhanced. The increase in Reynold’s stress fluctuations away from the wall indicated that the separation region increased in size due to vortex structures further from the wall. Once the bristling mechanism of the B2 scales was eliminated by painting over the skin, the scales functioned as a rough surface but still promoted separation delay. In addition, the streamwise Reynolds stress profile for the flow over the painted scales measured after separation occurred, along with the high intensity of the normal stress near the surface are both characteristics of a flow over a rough surface. In summary, the shark skin surface showed an ability to both delay and increase flow separation depending on the capabilities of the scale bristling to inhibit reversing flow.

Overlapping Multi-Domain Spectral Method for MHD Mixed Convection Slip Flow Over an Exponentially Decreasing Mainstream with Nonuniform Heat Source/Sink and Convective Boundary Conditions

Overlapping multi-domain bivariate spectral quasilinearization method is applied on magnetohydrodynamic mixed convection slip flow over an exponentially decreasing mainstream with convective boundary conditions and nonuniform heat source/sink effects. The method is employed in solving the transformed flow equations. The convergence properties and accuracy of the method are determined. The method gives highly accurate results after few iterations and using few grid points in each space subinterval and the entire interval. The use of minimal numbers of grid points at each subinterval minimizes the effects of round-off errors that can lead to instabilities. The accuracy increases as the number of overlapping subintervals increases. The accuracy improvement is achieved through making the coefficient matrices less dense. The effects of controlling parameters on the flow fields and physical quantities of interest are studied. Results show that increasing Biot number and nonuniform heat source/sink enhances the flow fields while reducing skin friction and heat transfer rate. The fluid properties improve with injection whereas the flow characteristics augment with suction. The considered exponentially decreasing external flows have particular applications in diverging channel flows. This study has practical significance in various boundary layer problems such as in controlling and delaying boundary layer separation on control surfaces and in suppressing recirculating bubbles.

Separation control and drag reduction for boat-tailed axisymmetric bodies through contoured transverse grooves

We describe the results of a numerical and experimental investigation aimed at assessing the performance of a control method to delay boundary layer separation consisting of the introduction on the surface of contoured transverse grooves, i.e. of small cavities with an appropriate shape orientated transverse to the incoming flow. The shape of the grooves and their depth – which must be significantly smaller than the thickness of the incoming boundary layer – are chosen so that the flow recirculations present within the grooves are steady and stable. This passive control strategy is applied to an axisymmetric bluff body with various rear boat tails, which are characterized by different degrees of flow separation. Variational multiscale large eddy simulations and wind tunnel tests are carried out. The Reynolds number, for both experiments and simulations, is $Re=u_{\infty }D/\unicode[STIX]{x1D708}=9.6\times 10^{4}$; due to the different incoming flow turbulence level, the boundary layer conditions before the boat tails are fully developed turbulent in the experiments and transitional in the simulations. In all cases, the introduction of one single axisymmetric groove in the lateral surface of the boat tails produces significant delay of the boundary layer separation, with consequent reduction of the pressure drag. Nonetheless, the wake dynamical structure remains qualitatively similar to the one typical of a blunt-based axisymmetric body, with quantitative variations that are consistent with the reduction in wake width caused by boat tailing and by the grooves. A few supplementary simulations show that the effect of the grooves is also robust to the variation of the geometrical parameters defining their shape. All the obtained data support the interpretation that the relaxation of the no-slip boundary condition for the flow surrounding the recirculation regions, with an appreciable velocity along their borders, is the physical mechanism responsible for the effectiveness of the present separation-control method.

CFD Studies on Triangular Micro-Vortex Generators in Flow Control

In the present study, the flow characteristics of the commercially used S809 wind turbine airfoil controlled with triangular counter-rotating micro-vortex generators at stall angle of attack of 15 degrees and 10 m/s, 15 m/s and 20 m/s (speed range used in the wind turbine applications) had been computationally investigated. In addition to the controlled airfoil, an uncontrolled airfoil was also studied for the comparison. The modelling and analysis had been carried out using incompressible, Reynolds Averaged Navier Stokes equation using Spalart-Allmaras one equation turbulence model. The numerical computations were performed with SIMPLE algorithm. The velocity profiles at different locations on the suction surface were plotted for both uncontrolled and controlled airfoils. The shear stresses exerted on the upper surface of the airfoil in both the configurations were also compared. It is found that the controlled airfoil, the shear stress distribution was greatly increased near to trailing edge of the airfoil revealing the superiority of vortex generators in increasing the efficiency of wind turbine by delaying boundary layer separation. The qualitative results of flow visualization in the spanwise direction also support the quantitative findings of velocity profiles and shear stress distribution.

Delaying Boundary Layer Separation in a Retarded Potential Flow

The phenomena of convective transport in the boundary layer flow carrying a dilute nanosize suspension of small particles past a stationary wedge is investigated in this communication. The nanofluid modeling is based on non-homogeneous nanoparticle concentration within the boundary layer along with variable thermophysical material properties. It is shown that for the present nanofluid modelling the self-similar solution also exists as it does for the pure fluid case. The self-similar solution is found to exist for the situation when the undisturbed potential flow is of the form \(u_e \left( x \right) =u_1 x^{m}\). A number of qualitatively distinct potential scenarios are predicted concerning the physics of flow and heat transfer. These include boundary layer separation; control or its delay, and heat transfer enhancement. In depth analysis of momentum transport across the boundary layer revealed interesting physics. Worth mentioning effect of nanoparticle is to (slightly) delay the boundary layer separation in the case of retarding potential flow. The values of m are calculated for which the separation starts and are found to depend upon the nanoparticle concentration. Heat transfer augmentation connected directly with the nanoparticle concentration is also an important conclusion of this study.

An experimental study on the effects of uniform injection through one perforated surface of a square cylinder on some aerodynamic parameters

It is well known that injection/suction (transpiration) through a perforated surface is an efficient way of influencing the characteristics of a turbulent boundary layer. Injection application creates a thicker boundary layer on a flat plate and it thus decreases drag. In aeronautical applications, suction is frequently used to delay boundary layer separation. This paper presents an experimental study on the effects of uniform injection through one perforated surface of a square cylinder on the pressure distribution and drag coefficient in a two-dimensional turbulent flow. For this purpose, surface pressure measurements around a square cylinder have been performed at three different Reynolds numbers in a wind tunnel. The parameters taken into account were injection rate, position of perforated surface (i.e., front, top, and rear), and pressure coefficient and drag coefficient. The results show that variation in pressure coefficient around the square cylinder and drag coefficient were influenced by the position of perforated surface and by injection rate.

縦渦によって乱された乱流境界層について

Measurements were made on the mean and fluctuating velocities in a turbulent boundary layer without pressure gradient, but with streamwise vortices introduced artificially by a series of vanetype vortex generators. Effect of the streamwise vortices is observed explicitly from spanwise variation of the profiles of mean and fluctuating velocities. In particular, the shape factor of the mean velocity profile (the ratio between the displacement and momentum thicknesses) exhibits values lower than that of the undisturbed flow in the majority of the spanwise sections, suggesting the tendency toward delaying boundary layer separation. The mean streamwise vorticity is mostly concentrated in the vicinity of the vortex core, and this is accounted for as produced by the anisotropy of the normal Reynolds stresses. In the adjoining spanwise section there exists also a region of counter-rotating streamwise vorticity, which is weaker in strength but inhibits the diffusion of the primary streamwise vorticity. Both the anisotropy of normal Reynolds stresses and the generation of counter streamwise vorticity appear to contribute to the mechanism by which the streamwise vorticity could persist for a long distance.