Flow Separation Control of Backward-Facing Step Airfoil NACA0015 by Blowing Technique

The aim of this paper is to control the flow separation above backward-facing step (BFS) airfoil type NACA 0015 by blowing method. The flow field over airfoil has been studied both experimentally and computationally. The study was divided into two parts: a practical study through which NACA 0015 type with a backward -facing step (located at 44.4% c from leading edge) on the upper surface containing blowing holes parallel to the airfoil chord was used. The tests were done over two-dimensional airfoil in an open circuit suction subsonic wind tunnel with flow velocity 25m/s to obtain the pressure distribution coefficients. A numerical study was done by using ANSYS Fluent software version 16.0 on three models of NACA 0015, the first one has backward-facing step without blowing, the second with single blowing holes and the third have multi blowing holes technique. Both studies (experimental and numerical) were done at low Reynolds number (Re=4.4x10 5 ) and all models have chord length 0.27m.The experimental investigations and CFD simulations have been performed on the same geometry dimensions, it has been observed that the flow separation on the airfoil can be delayed by using velocity blowing (30m/s) on the upper surface. The multi blowing holes with velocity improved the aerodynamics properties.The multi blowing holes and single blowing hole thesame effect onpressure distribution


Introduction
The field of flow separation control has a great interest and a progress search for refinement in high lift device performance seen has been explored.This progress occurs because of an improvement in understanding fluid mechanics and the progress reached in computational techniques, the experimental work availability and the existing of flow control mechanisms nowadays.These mechanisms effect active and passive flow control techniques.Flow control over airfoils is mainly forced to be increasing the lift and decreasing the drag.Caused by the airfoil.This can be done by controlling the boundary and shear layer flows to reduce the separation region to the minimum over the suction surface of the airfoil [1,2].Saric et al. [3] studied computationally employ (LES) large eddy simulation, (DES) detached eddy simulation, and (T-RANS) transient Reynolds-averaged Navier-Stokes methods to limit turbulent flow above a backward-facing step(BFS) troubled at equal intervals by changing blowing∕ suction through a thin slit (0.05 H width) mounted at the step edge.A troubled frequency St=0.19 was used with a length decrease to 28.3% was compared to untroubled situation which has better agreement with computational methods.Uruba et al. [4] performed an experimental work of channel flow behind a backward-facing step by blowing/suction near the step.Blowing and suction were dominate causing reduction in the length of the separation zone down to one third of its value when no control in used.Two parameters are important for blowing (slot area and shape) in which small cross-section and serrated edge is the most operative.Interaction of the boundary layers near the edge of the step cause the three dimensions vortex structure near the crack to be existed.Mishriky et al [5] numerically investigate the introduction of a backward-facing step on the suction side of NACA 2412 airfoil at a high Reynolds number.To find the best position for the step, a study was done to know the position of the step on the lift coefficient, drag coefficient and critical angle of attack.Results showed that introducing such a suction side of NACA 2412 airfoil have on inversely effect on considerable properties.Hasan et al [6] Investigate experimentally a flow over a backward-facing step with laminar separation under controlled perturbation (Re=11000).The 'shear layer mode' and the 'step mode' was two distinct modes of instability found for the reattaching shear layer.The disturbance increased the rate of growth of the shear layer and the intensity of the turbulence and decreased the length of reattachment compared to the undisturbed flow.Flow visualization confirmed the division of the shear layer and showed the existence of a low-frequency flapping of the shear layer.Chun et al [7]

Exprimental Work
All experiments were made in an open circuit suction subsonic wind tunnel.The manufacturing of open wind tunnel (wide range of investigations into aerodynamics) by engineering technical college of Mosul.The tunnel has an open test section of 300mm by 300mm cross section and 500mm length.Maximum free stream wind velocity U∞ is about 36 m/s, and the chord Reynolds number is about 4.4x10 5 .The free stream is fixed at 25m/s for the present experiment.A rectangular wing with NACA 0015 airfoil (Wood with Aluminum) was used for the tests.Figure 1 shows the arrangement of the working section in wind tunnel.Wing chord was 270 mm; wingspan 297 mm and the angle of attack α can be varied continuously.
A model of the NACA 0015 airfoil has been built locally.Data for this section were taken from NACA's lists of wings section [8].Moreover, the Upper and lower static pressure taps coordinate has been listed in   The blowing system used in present study consists of multi holes, 8 discrete holes with 2mm diameter is distributed along the backward-facing step of the wing.The distance between the first center holes to the neighbor center is 30mm Figure 2.This distribution is used to investigate the effect of blowing method on the characteristics of pressure coefficients (Cp) and flow separation over the airfoil.Pressure coefficients have been measured and calculated by using surface pressure measurements technique that uses 13 pressure tapings over the suction surface of the wing.A multiple tube manometer was used to measure the surface pressure distribution.

Calculation of Pressure Coefficient
Many methods used to measure pressure coefficient Cp on the airfoil.In this work, the pressure coefficient, on the airfoil has been determined from measured pressure distribution over the airfoil's surface.The pressure distribution on the airfoil is expressed in dimensionless form by the pressure coefficient Cp as follow [10]: Where PO is the surface pressure measured at position i on the surface, P∞ is the pressure in the free stream,  is air density, and U∞ is the free-stream velocity known by:

Numerical Methodology
The CFD was the method of choice in the design of many aerospace, automotive and industrial components and processes in which fluid or gas flows play an important role.These programs can give results as close as the experimental methods.ANSYS Fluent software version 16.0 was used as a tool to design sections in the present study and to show the eff ect of control flow separation over backward-facing step (BFS) airfoil type NACA 0015 by blowing method.The steady flow field about airfoil with (BFS) has been simulated numerically by resolving the incompressible two-dimensional, Navier-Stokes equations on unstructured grids domain of complex shape .The simulated flow fields(Laminar model) are used to discuss the mechanisms of flow separation, and to explain the differences in separating performance between baseline case and active flow control using blowing technique.The procedures work are import the co-ordinates vertices (texts file to create the curve) of the airfoil [8]  Otherwise, BFS edge is divided into 10 divisions.The mesh boundaries were given set to the x and y velocity components, and the end boundary the property "pressure-outlet" to simulate the zero gauge pressure.The airfoil itself is given as wall properties.The Setting boundary conditions as shown in Table 2.
Table 2 Operating parameters.
The Reynolds number is specified as low sufficient to simulate an incompressible flow for comparison with experimental data.The Reynolds number is fixed at Re=4.As illustrated in Fig( 8-b) there is a clear improvement in pressure coefficient curve compare with the airfoil without BFS as in fig (8a).BFS with single blowing (α=0°).BFS with multi blowing (α=0°).

Conclusion
1-An improvement in the pressure coefficient for backward-facing step (BFS) airfoil using blowing technique.2-For the experimental results, the change in the pressure coefficient is noticeable when using the blowing technique especially at the location of the excitation (44.4% c) from leading edge.3-Blowing technique is most effective at small angles of attack (0 o , 5 o , 8 o ) and less influence at the angle of attack 10 o for the experimental study.

4-Separation point was delay when using
Blowing technique on the backward-facing step (BFS) airfoil.5-There is same change in the coefficient of the pressure when using single and a multi-hole blowing technique with velocity 30 m / s for backward-facing step (BFS) airfoil.

Figure 1 :
Figure 1:Backward-facing step (BFS) airfoil type NACA 0015 mounted inside wind tunnel with blowing method

. 2 .Figure 2 :
Figure 2:Schematic of Backward-facing step (BFS) airfoil type NACA 0015 with dimension in design modeler.The dimensions of the sketches are coinciding to the experimental airfoil with backward-facing step located at 44.4% c from leading edge.The grids utilized in this work are unstructured triangles grids and to ensure the computed aerodynamic results are independent of the size of the grid, the density of the grid increased until an insignificant difference was reached in the solution towards convergence Diyala Diyala Journal of Engineering Sciences Vol. 12, No. 01, Month 2019, pages 99-119 ISSN 1999-8716 DOI: 10.26367/DJES/VOL.12/NO.1/11eISSN 2616-6909 (three different meshes are tested).The outside boundary is of a C-topology type with front arc radius of 12.5c and wake distance of 20c.Create the geometry for the C-mesh domain by using sketcher toolbox and dimension tool.The mesh is constructed fine for the region of the boundary layer near the airfoil surface and coarser away from the airfoil (number of elements 50000).The surface edges of the upper airfoil have 50 divisions before and after the BFS up to (T.E.).
Figure 3 show experimental results of the distribution of pressure coefficient Cp on the upper surface Backward-facing step (BFS) airfoil type NACA 0015 with changes the angle of attack without blowing.At the beginning of the airfoil, Peak pressure shows the normal behavior of the pressure curve due to the acceleration of the flow.At the backward-facing step location the pressure coefficient distributions curve shows a decrease in the pressure value due to the sudden change in velocity (passive) and then increases over time due to the slow flow.

Figure 3 :Figure 4 :Figure 5
Figure 3: Pressure coefficient distributions Cp on the upper surface Backward-facing step (BFS) airfoil type NACA 0015 with changes the angle of attack without blowing Figure 4 illustrated the comparison between the experimental results of the pressure coefficient on

Figure 5 -Figure 6 6 :Figure 6 :
Figure 5-b: Velocity Vectors around the NACA 0015 airfoil at the angle of attack 0 deg captured from ANSYS Fluent Figure 6 shows the surface pressure coefficient distributions Cp measured around the airfoil without BFS, since the angle of attack changes from 5.0 deg to 20.0 deg.It was found that the surface pressure distribution on the upper surface of the airfoil varied significantly at different angles of attack _AOA_ 5.0 to 14 deg and remains unchanged until angle of attack 20 deg.The surface pressure coefficient profiles along the

Figure 8 -Figure 8 -Figure 9 :Figure 10 :
Figure 8-a: Pressure Coefficient Distributions around the NACA 0015 airfoil without BFS at the angle of attack 0 deg captured from ANSYS Fluent

Figure 12 -
Figure 12-a: pressure coefficient for NACA 0015 at angle of attack 0 deg

Diyala Diyala Journal of Engineering Sciences Vol. 12, No. 01, Month 2019, pages 99-119 ISSN 1999-8716 DOI: 10.26367/DJES/VOL.12/NO.1/11 eISSN 2616-6909 100
investigate a backward-facing step Excitations to separate the flow using a sine wave oscillatory jet to study flow over (BFS).A jet introduced from a slot near separation line.It is noticed an enhancement in shear layer grow the rate due to localized forced near the separation edge with small amount, which cause a roll-up vortex.Rice in higher rate of internment caused by large vortex in shear layer.Reduction in reattached length compered to unfree flow is noticed due to this parameter.

Table 1
a-Upper static pressure taps coordinate.