Pressure and friction drag components contribute the most to the whole drag of different high-speed vehicles [1,2]. Drag reduction is one of the most popular directions of modern research in the field of fluid-mechanics, because it allows to reduce power consumption and amount of air pollutant emissions and make airplanes and other high-speed vehicles more efficient. Existing methods of friction reduction can purposefully influence a very complicated energy exchange mechanism in the near-wall vortical structure, with a wide range of scales and disturbances, localised in the wall neighbourhood. These methods are effective for streamlined bodies, elongated in the direction of flow development. Pressure drag reduction is rather associated with streamlined body shape optimisation. These two different techniques are not independent, and their interaction for some modes of streamline can be significantly non-linear. Sometimes, even quite a strong positive reduction of the friction drag component can be neutralised by its concomitant negative influence on pressure distribution and, as a result, some promising drag reduction technologies cannot be effectively implemented in engineering practice.
This paper deals with the results of the research conducted by the authors and is associated with one of the most effective methods of skin friction reduction, which is the distributed exchange of mass through the streamlined surface in the form of microblowing or/and suction (Fig. 1).
This technology is not new, and its mechanism has been known for a long time. However, about 30 years ago, the method of micro-injection through huge arrays of very small holes got a new impetus in Hwang’s research activity [3] and was named by him as microblowing. One of the objective reasons for this unexpected growth of interest in this promising technology is the significant progress made in the creation and fabrication of materials with the required permeability and reasonable cost. Different effective improvements of this technology and, in particular, microblowing through several permeable sections, placed laterally to the flow direction (Fig. 2A), have been studied by Kornilov and Boiko [4].
The goal of this paper is to analyse this technique’s efficiency as well propose and study the multi-slot microblowing through longitudinally placed permeable sections (Fig. 2B).
In addition, we propose an improved methodology for the experimentally based indirect drag coefficient estimation with the use of the wake flow parameters and by taking into account the possible presence of blowing/suction through some locally placed separable permeable sections.
This section represents the results of the numerical modelling of the influence of mass-exchange through the permeable sections of a streamlined surface of a rectangular wing section on the developing flow properties. The formalised problem statement consists in the numerical prediction of the most-relevant distributed characteristics of the flow around the rectangular wing section with an NACA0012 airfoil under the following conditions (reproducing the experiments performed by Kornilov [5]): chord
The computational domain ABCDEF (Fig. 3) had a C-topology, and its dimensions were determined as follows: the radius of the arc BCD and the lengths AB and DE are 6- and 8-times more than a chord c, respectively. The width EF corresponds to the span
To simulate the 3D flow around a high-speed vehicle, the following system of Reynolds-averaged Navier–Stokes (RANS) governing equations has been solved under the assumptions of stationary incompressible and predominantly turbulent air flow, completely corresponding to the formulated above-mentioned working conditions and with the use of the ANSYS Fluent 17.1:
Here
The boundary conditions were established by taking into account the motion of ground surface relative to the train body according to the ANSYS Fluent formalism and given as follows: external surface of the wing section G – velocity magnitude 0 m/s (wall); inlet faces (horizontal AB, DE, forward arc BCD) – velocity magnitude
According to the Boussinesq approach, the Reynolds stresses can be directly connected with the strain rate tensor components similar to the laminar case, but by the use of additional viscosity (so-called eddy- or turbulent viscosity
The turbulent viscosity
The mass transfer factor through the streamlined surface was taken into account by modifying the sources values in governing equations of continuity, momentum and turbulence model using specially developed user defined functions (UDF) that determine the permeable sections’ geometry and other parameters of mass-exchange through the streamlined surface. The obtained numerical solutions were additionally tested to implement the integral mass balance at the outer boundary of the computational domain and the wing streamlined surface.
The first step of verification of the proposed modelling methodology was done for the simplest flow, developing over the flat plate with the permeable insert (
The results of
As it follows from the presented comparisons with the experimental data [5], the elaborated formalised numerical procedure is able to reproduce the principal aspects of flow development behaviour in the case of laterally placed permeable slots (Fig. 2A). The systematic discrepancies between numerical predictions and experimental data (Fig. 6) can be explained by the different conditions of numerical modelling (unbounded free-stream) and experiment (influence of the walls of the wind tunnel 1 × 1 m2 testing section and presence of turbulators near the leading edge of the wing). At the same time, the numerical prediction does not allow to simulate the slow rate of relaxation of
The next step of study of the properties of mass-exchange through the streamlined surface is associated with arrays of longitudinally arranged permeable slots (Fig. 2B). We suppose that the arrays of longitudinal permeable slots allow to intensify turbulent vortical structures exchange in the lateral direction and, as a result, improve flow stability to stall. Moreover, due to creating the regular anisotropy of the boundary layer in the lateral direction, this modified blowing technique can potentially have some synergistic properties, which can give the additional benefits. Here, we shall demonstrate only the first attempts of this idea realisation. The span of wing section was separated into n subsections with permeable slots, occupying a half of the subsection length along the lateral coordinate. The developed UDF allows to change the slot number n, slot location, length along chord and width along span as well as mass-exchange intensity. The obtained results of an uneven, but at the same time regular distribution of the isolines of the longitudinal velocity component in the vicinity of the streamlined surface of a flat plate in the transversal z direction with respect to the incoming flow in the case of 11 longitudinal slots, uniformly installed along its span are illustrated in Fig. 7. In the absence of blowing through these longitudinal slots, the isolines should be strictly parallel to the wall, whereas the presence of blowing naturally leads to local displacement of the flow and its velocity isolines above the slots in the normal to the wall direction y, which is adequately reproduced by the developed numerical model.
The obtained regularity of the velocity isolines along the span of the streamlined surface (wing, etc.) creates favourable conditions for the formation of transverse inhomogeneity of the boundary layer in the near-wall region, which allows generating an appropriate regular vortex structure of longitudinal vortices with the desired frequency along span, which will actively influence the process of formation and the further development of turbulence. In addition, the three-dimensional wall structure of longitudinal vortices increases the level of spatiality of the boundary layer near the streamlined surface, which should also increase the resistance of the boundary layer to separation, which is also very important, since the blowing itself acts in the opposite way. All these factors make it possible to consider this flow control method as promising both for its stationary and non-stationary implementations, for example, for generating transverse travelling waves, which requires further research and will be the subject of further efforts application by the authors.
The obtained results demonstrate a quite good level of correlation between the behaviour of numerical predictions of pressure coefficient distribution along chord and the corresponding experimental data both for the reference configuration and in the cases of separate suction and combined suction-blowing.
Even a fairly intensive blowing from one side of airfoil (
Various complex combinations of blowing and suction intensity, slots orientation and placement can be promising drag-reduction techniques for different streamlined surfaces of aircraft. In addition, this technology has great potential for effective application in other high-speed vehicles such as submarines, bullet trains, etc.