Control of Separation For NACA 2412 Airfoil At Different Angles of Attack Using Air Blowing

The study of th e separati on control using the jet blowing ba sed on t he computation of Reynol ds-average Navier-Stocks e quations is c arried out in this wor k. A numerical model based on collocated Finite Volume Method is developed to solve the governing e quations on a body-fitted grid, to com pute the performance of airfoil by using the blowing jet. Above of all, the performance of turbulence model is investigation. A revised k-ω model is proposed as the known turbulence mo dels perfor m well in r eproducing the flow of airfoil at pre-stall or stall angle of attack. The systematical investigation of the jet blowing is conducted on the NACA 2412 airfoil in the range of attack angle from 0 º to 30 º included up and beyond t he stall angle at range of Re=3.4*10 5 -1.7*10 6 . The influence of some parameters associated with using jet blowing, s uch as its location, a nd the speed ratio (Uj/U) strength on the performance of the NACA 2412 airfoil has also been studied. The result shows that the jet blowing is effective in controlling the separation at 0.3C and Uj/U=2. The large separation regions cannot be completely removed by t he jet blowing. However, the flow structure can be regularized. The lift coefficient of the control airfoil is also increased with th e angle of the attack. The experimental results are obtained on airfoil NACA 2412 at 0.3C blowing and Uj/U=2, the results are been good agreement with the computational results.

blowing location on the upper.surface of the airfoil on the aerodynamic characteristic of airfoil, influence of speed jet ratio on the aerodynamic characteristic of airfoil.
An experimental investigation has been done to validate the numerical approach of the boundary-layer separation phenomenon on the airfoil model (NACA 2412).This model was tested in the low speed wind tunnel.The use of mass transfer such as air blowing through jet holes located at the pressure side of the model was used to prevent or delay the flow separation.

Equations
In this section, we will summarize the basic equations governing fluid flow in turbulent, steady, incompressible flows.The derivation of these equations, details regarding the constitutive relations used and the various turbulence modeling assumptions employed can be found in several sources (e.g.White [7],Ferziger and Peric [8]).Employing indicial notation, the instantaneous form of continuity and momentum equations in cartesian coordinates can be written as follows: ( ) Two-equation model is the kω model, which will be presented here in the form given by Wilcox [6]: Where for the keqn.
for the εeqn.for the ω -eqn The various constants used in the standard model have the following values:

Transport Equations:
The conservative form of all fluid transport equations, including equations for scalar quantities can usefully be cast in one general form as shown below.

…… [7]
The term on the left hand side is the convective term, the first term on the right hand side is the diffusive term, and the last term is the source term.The source term includes both the source of and any other terms that cannot find a place in any of the convection or diffusion terms.
The full discretization of the transport equation on a nonorthogonal mesh can now be given.By approximating the convective fluxes with the central difference scheme, the steady transport equation can be discredited as:    It was a cambered airfoil of constant chord and made of laminated wood.The airfoil aspect ratio of 2.2, span and chord were 33 and 15 cm.respectively and the thickness to chord ratio was 0.12.This model was tested in the low speed wind tunnel of Re=3.4*10 5 Fig. (3).It was made to span the test-section of the wind tunnel, care being taken to seal the airfoil tips to the side walls of the tunnel and to achieve any incidence angle ( α ) of airfoil.The leading and trailing edges of the airfoil were made to form a circular arc.Raw of four blowing holes in the span wise direction were located on the upper surface of the airfoil.Each hole had 1.5 mm diameter with angle like incidence and the span wise distance is 10 cm between any two neighboring holes.The raw of holes is located in span wise direction at (x/c) =0.3 from the airfoil leading edge.
The holes geometry dimensions and located were taken as recommended by Ref. [9], and the holes were made to provide a blowing air.The models surface was coated with "cellar" resin against the absorption of oil which was used for flow visualization.In addition all surfaces were painted with a mattblack finish.Also a telly-surface measurement was made to assess the smoothness of the surfaces which were smoothed to avoid a slight roughness that might influence boundary-layer transition.
The method used to determine the separation position was the oil film technique, mixture of oil, (63% kerosene and 37% Paraffin), and this method was adopted to determine the separation position.

Result and Discussion:
Figures (4(a,b)) Shows the photos obtained from the experimental test.These two photos show clearly the luck of using the oil film technique for the two cases with and without blowing.The main point arised from these photos is that the experimental set up techniques and facilities gave acceptable results.
The experimental and computational results are obtained on airfoil at 0.3C blowing, Uj/U=2 and Re=3.4*10 5 in table (1), the results are been good agreement between them.The differences between the experiment and numerical simulation results over the NACA airfoil can also be attributed to other factors and errors which exist both on the experimental side and the numerical simulation side.On the experimental side, error in airfoil model, installation disturbance of measurement devices, interference between wind-tunnel wall and airfoil body, and freestream turbulence and boundarylayer trip effects can create errors in the measurements.On the numerical simulation side, turbulence models, grid density, and the limitations of two-dimensional simulation can produce computational inaccuracies.

(
) For the NACA 2412 and Re=1.7*10 6 at different blowing position as shown in Fig. (5,6,7,8), and blowing at α = 10, it seen that at blowing point (0.1C) the α max is larger than the other (increase from α = 14 to α = 24 at Uj/U =10), and when increase the Uj/U at the same position of blowing the C Lmax and α max increase.
As shown in Fig. ( 9), the perfect jet ratio is equal two when effect it on the (C L /C D ) max , because when increase the jet ratio the drag coefficient increase because the skin friction increase.
In Fig. ( 10) five different jet blowing (Uj/U=1, 2, 3, 4, 5) at α=14, and compare it with the baseline (without blowing).When jet blowing applied near and before the separation position (50% of chord), the separation is most effectively delayed and hence the separation bubble and circulation are much smaller than the other cases (at Uj/U=2).Conclusions 1-Navier-Stokes simulations are necessary for the flow control airfoils studies due to the complexity of the flow field and the strong viscous effects.The results indicate that this approach is an efficient and accurate way of modeling control airfoils with jet blowing.2-The jet blowing technology is a useful way of achieving very high lift at even zero angle of attack.It can also eliminate the vortex shedding in the trailing edge region, which is a potential noise source.The jet blowing as the flow control device is effective in controlling the stalled airfoil flow.The large separation region on stalled airfoil cannot be completely removed by the blowing, but instantaneous reattachment is possible.The stalled airfoil flow is sensitive to blowing jet ration (Uj/U) and its location, the best at the Uj/U=2 and 0.3C References [1]Ravindran S.S. ((Active Control of flow separation over an airfoil)), NASA/ TM-1999TM- -209838, 1999 …… [8]For a discretization of the twodimensional transport equation the individual coefficients of the A,B and c arrays are, the mesh is shown in figure with pdfFactory Pro trial version www.pdffactory.comEng.& Tech.Journal,Vol.28,No.16The mass flux source term Experimental work: An experimental investigation has been done to validate the numerical approach of the boundarylayer separation phenomenon on the airfoil model (NACA 2412) Fig. (2).

Figure ( 1 )Figure ( 7 )
Figure (1) A typical CV and the notation used for a Cartesian 2D grid

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