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Relevant analyses are presented on the base of the compressible vortex method for simulating the development of two or three co-rotating vortices with different characteristic Mach numbers. The paper aims to discuss this issue.

In addition to having vorticity and dilatation properties, the vortex particles also carry density, enthalpy, and entropy. Taking co-rotating vortices in two-dimensional unsteady compressible flow for an example, truncation of unbounded domains via a nonreflecting boundary condition was considered in order to make the method computationally efficient.

For two identical vortices, the effect of the vortex Mach number on merging process is not evident; if two vortices have the same circulation rather than different radiuses, the vorticity and dilatation fields of the vortex under a vortex Mach number will be absorbed by the vortex under a higher vortex Mach number. For three vortices, if the original arrangement of the vortices is changed, the evolvement of the vorticity and dilatation fields is different.

The paper reveals new mechanism of the three co-rotating vortices by a feasible compressible vortex method.

The purpose of this paper is to study the structure and dynamic development of a pair of co-rotating trailing vortices, during their formation, interaction and merging, using detailed experimental measurements of the velocity and vorticity fields.

The vortices were generated using two half wings (NACA0030) positioned at equal and opposite angles of attack at the entrance of the test section of an open-circuit, subsonic, wind tunnel. Velocity vector measurements were obtained at Re_c = 133,000, on cross-plane grids at several distances from the trailing edges of the wings, using an in-house developed four-sensor hot wire anemometer probe.

The results include cross-plane contour plots of the mean and fluctuating velocity as well as mean vorticity fields. Each of these variables is affected in a different way, providing complementary information on the development of the flow field. After shedding, the two vortices are swept along the stream-wise direction and spiral around each other, thereby developing a braid of two vortices, which then deforms the external flow field. Gradually, the interaction with the external flow field links both vortices together until the final merging and the formation of a new stable linear vortex emerges.

Trailing vortices have been rendered particularly important during the past decades, because of increasing traffic density of very heavy aircrafts and several plane “incidents”, which were attributed to the action of the vortex wake.

The presented results provide information on the evolution and merging of a pair of vortices formed by a closely spaced differential wing configuration. The vortices interact almost immediately after shedding as expected in flap–flap or flap–wing vortices interaction.

With aims to increase the aerodynamic efficiency of aerodynamic surfaces, study on flow control over these surfaces has gained importance. With the addition of flow control devices such as synthetic jets and vortex generators, the flow characteristics can be modified over the surface and, at the same time, enhance the performance of the body. One such flow control device is the tubercle. Inspired by the humpback whale’s flippers, these leading-edge serrations have improved the aerodynamic efficiency and the lift characteristics of airfoils and wings. This paper aims to discusses in detail the flow physics associated with tubercles and their effect on swept wings.

This study involves a series of experimental and numerical analyses that have been performed on four different wing configurations, with four different sweep angles corresponding to 0°, 10°, 20° and 30° at a low Reynolds number corresponding to Re_c=100,000.

Results indicate that the effect of tubercles diminishes with an increase in wing sweep. A significant performance enhancement was observed in the stall and post-stall regions. The addition of tubercles led to a smooth post-stall lift characteristic compared to the sudden loss in the lift with regular wings. Among the four different wings under observation, it was found that tubercles were most effective on the 0° configuration (no sweep), showing a 10.8% increment in maximum lift and a 38.5% increase in the average lift generated in the post-stall region. Tubercles were least effective on 30° configuration. Furthermore, with an increase in wing sweep, co-rotating vortices were distinctly observed rather than counter-rotating vortices.

While extensive numerical and experimental studies have been performed on straight wings with tubercles, studies on the tubercle effect on swept wings at low Reynolds number are minimal and mainly experimental in nature. This study uses numerical methods to explore the complex flow physics associated with tubercles and their implementation on swept wings. This study can be used as an introductory study to implement passive flow control devices in the low Reynolds number regime.

In finned-tube heat exchangers, the array of tubes generates three-dimensional vortices at fin-tube junctions. Theses vortices known as horseshoe vortex (HSV) system are responsible of flow mixing and heat transfer increase. The purpose of this paper is to focus on the effect of the fin spacing on the formation, the spatial evolution and dissipation of the HSV system at fin-tube junctions in a two-rows finned-tube heat exchanger. The global characterisation of the heat exchanger performance is also presented.

The flow structure is numerically analysed through the use of computational fluid dynamics tools. The different vortices of the HSV system are highlighted and quantitatively analysed at each fin-tube junction with vorticity, wall shear stress analysis and two-dimensional streamline plots around tubes.

The results show that the primary and secondary vortices of the HSV system have antagonistic behaviors with respect to the azimuthal angle variation. The optimum fin spacing ratio E/D that generates the most intense first primary vortex in the HSV system lies between 0.20 and 0.25. Similar observation are made on the thermalhydraulic performance of the heat exchanger as j/f exhibits a maximum value for a fin spacing ratio E/D=0.25.

A detailed URANS simulation shows that even if the flow remains steady in the core of the heat exchanger, unsteady behavior is noticed in the wake of the second tube.

In this study, the flow topology is quantitatively analysed in successive radial planes around heat exchanger tubes. The strong effect of the fin spacing on the HSV generation and dissipation is deeply analysed.

This study aims to provide an insight into the relationship between design parameters and thermal performance of plate fin heat sinks (PFHSs) incorporating longitudinal vortex generators (VGs) inside a PFHS channel.

A computational fluid dynamics model of a delta winglet pair VG mounted inside a PFHS geometry is detailed, and the model is validated by comparison with experimental data. The validated model is used to perform a virtual design of experiments study of the heat sink with bottom plate and vertical plate mounted VGs. Data from this study is used to regress a response surface enabling the influence of each of the assessed design variables on thermal performance and flow resistance to be determined.

The results of this study show that the thermal hydraulic performances of a PFHS with bottom plate mounted VG and vertical plate fin mounted VG are, respectively, 1.12 and 1.17 times higher than the baseline PFHS. Further, the performance variation of the heat sink with VG, relative to delta winglet’s arrangement (common flow up and common flow down), trailing edge gap length and Reynolds number were also evaluated and reported.

For the first time, performance characteristics of delta winglet VGs mounted inside the PFHS are evaluated against different design variables and a polynomial regression model is developed. The developed regression model and computed results can be used to design high performance PFHSs mounted with delta winglet VGs.

The purpose of this paper is to compare the effects of two different configurations of plasma streamwise vortex generators (PSVG), including comb-type and mesh-type in controlling flow. This is demonstrated on the NACA 0012 airfoil.

The investigations have been done experimentally at the various electric and aerodynamic conditions. The surface oil flow visualization method has been used to the better understanding of the flow physics and the interaction of the oncoming flow passing over the airfoil and the vortex generated by comb-type PSVG.

This paper demonstrates the potential capabilities of the mesh-type and comb-type PSVGs in controlling flow in unsteady operation. It was found that the vortex generated by the mesh-type PSVG in unsteady operation was an order of magnitude stronger than comb-type PSVG. The flow visualisation technic proved that only a part of the plasma actuator is effective in the condition that the actuator is installed only on a portion of the upper surface of the airfoil.

This paper experimentally confirms the capabilities of the mesh-type PSVG unsteady operation in compare with comb-type PSVG in controlling flow, whereby recommends using mesh-type PSVG in the leading edge in front of comb-type PSVG on the entire wingspan to prevent the stall.

This paper aims to focus on the temporal and spatial fourth‐order finite volume discretization of the incompressible form of the Navier‐Stokes equations on structured uniform grids. The main purpose of the paper is to assess the accuracy enhancement with the inclusion of a high‐order reconstruction of the point‐wise velocity field on a fourth‐order accurate numerical scheme for the solution of the unsteady incompressible Navier‐Stokes equations.

The present finite volume method uses a fractional time‐step for decoupling velocity and pressure. A Runge‐Kutta integration scheme is implemented for integrating the momentum equation along with a polynomial interpolation and Simpson formula for space‐integration. The formulation is based on step‐by‐step de‐averaging process applied to the velocity field.

The reconstruction of the point‐wise velocity field on a higher‐order basis is essential to obtain solutions that effectively stand for a fourth‐order approximation of the point‐wise one. Results are provided for the Taylor vortex decay problem and for co‐ and counter‐rotating vortices to assess the increase in accuracy promoted by the inclusion of the high‐order de‐averaging procedure.

High‐order reconstruction of the point‐wise velocity field should be considered in high‐order finite volume methods for the solution of the unsteady incompressible form of the Navier‐Stokes equations on structured grids.

The inclusion of a high‐order reconstruction of the point‐wise velocity field is a simple and effective method of enhancing the accuracy of a finite volume code for the computational fluid dynamics analysis.

The paper develops an improved version of a fourth‐order accurate finite volume projection method with the inclusion of a high‐order reconstruction step.

The stability of two‐dimensional natural convection of water near its density maximum (cold water) inside a vertical rectangular enclosure with an aspect ratio of eight is investigated via a series of direct numerical simulations. The simulations aim to clarify, under the influence of density inversion, the physical nature of the instability mechanism responsible for the laminar buoyancy‐driven flow transition from a steady state to an oscillatory state in the enclosure filled with cold water. Two values of the density inversion parameter, _m= 0.4 and 0.5, where the density inversion of cold water may exert strong influence on the flow, are considered in the present study. The results show that the transition from steady state to periodically oscillatory convection arises in the cold‐water‐filled enclosure through a Hopf bifurcation. The oscillatory convection in the water‐filled enclosure for both values of _m is found to feature an oscillatory multicellular structure within the contra‐rotating bicellular flow regions. A traveling wave motion accordingly results along the maximum density contour, which demarcates the contra‐rotating bicellular flows in the enclosure. For both cases the nature of transition into unsteadiness is found to be buoyancy‐driven. The critical Rayleigh number for the bifurcation at _m = 0.4 is found to be markedly higher than that at _m = 0.5.

The purpose of this paper is to consider a two‐dimensional, steady, mixed convection flow in an enclosure filled with a fluid‐saturated uniform porous medium. The left wall is moving down and the right wall is moving up and are maintained at cold and hot constant temperatures, respectively. The top and the bottom walls are fixed and are thermally insulated.

The governing equations are normalized and solved numerically with appropriate boundary conditions by finite‐volume approach using third‐order accurate scheme (deferred QUICK).

The study is conducted by varying the key parameters, i.e. Richardson number (Ri=Gr/Re²), Darcy number (Da=κ /H²) and Grashof number (Gr=gβ H³ Δ T/ν ²) and fixing Prandtl number at (Pr=0.71). A parametric study is conducted and a set of streamline and isotherm plots are presented. The average Nusselt number reaches a value of 1 asymptotically when the Ri is gradually increased for Gr up to 10³. The asymptotic value is 1.5 for Gr=10⁴. A heat transfer correlation is also presented.

The study of the mixed convection problem with lid‐driven flows in enclosures will be useful in cooling of electronic devices, lubrication technologies, chemical processing equipment, etc.

Under this heading are published regularly abstracts of all Reports and Memoranda of the Aeronautical Research Council, Reports and Technical Memoranda of the United States National Advisory Committee for Aeronautics and publications of other similar Research Bodies as issued.