Effect of Roughness on airfoil

7/17/2019 Effect of Roughness on airfoil

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Applied Aerodynamics

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Some characteristic behaviors include low Cl/Cd ratio, anomalous behavior

such as sudden loss of Cl around Clmax values at low Reynolds number flow,

anomalous behavior near stall angle for higher Reynolds number flow.

Another difficulty with thick airfoils is that the minimum pressure is decreased

due to thickness. This results in a more severe adverse pressure gradient and

the need to start recovery sooner. If the maximum thickness point is specified,

the section with maximum thickness must recover from a given point with the

steepest possible gradient [2].

Problem definition:

Reynolds Number: 106 ; Thickness=35%

Aim:

To choose a thick airfoil, search for maximum Cl, lowest Cd and observe the sensitivity to roughness.

Theory:

Thick airfoils are chosen usually near the roots of tapered wings, propeller hubs, wind turbine hubs and

places in the airplane where the structural advantages of using a thick airfoil play a role. For instance, the design

of airfoils in rotor blades is a trade-off between airfoil performance including rotational effects and structural

requirements. Apart from providing more structural stiffness, the thick airfoils enable the blade designer to

reduce weight, giving a reduction of fatigue loads and costs. However, experimental tests [1] have shown that

after a thickness of 20%, they become so poor aerodynamically that other consideration do not justify their

usage. It is therefore interesting to study the aerodynamic characteristics of these kinds of airfoils.

Procedure:

Since the choice of airfoil was free, we chose to analyze two 5 digit NACA foils: 21035 and 25035

(Figures 3 and 4). The analysis was done in XFLR5.

Figure 3: NACA 21035 Figure 4: NACA 25035

Results:

Some key behaviors of these airfoils can be seen in the following graphs:

Figure 1 (above) Shows the sudden

transition in case of thick airfoils.

[2]

Figure 2 (Left) shows the Cl vs

Alpa plots from experimental data,

for various Reynolds Numbers. [1]




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Figure 5: Cl vs Cd

Figure 6: Cl vs Alpha

Figure 7: Cm vs Alpha

Figure 8: Cl/Cd vs Alpha




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Figure 9 : Cp Distribution

Discussions :

As it can be seen from the Cl vs Cd graph, the minimum value of Cd (At Cl=0) is found to be 0.0126.

From the Cl vs Alpha graph, we can see that at around 18 degrees, the Cl value stops increasing and becomes

more or less constant. It is strange because instead of dropping at stall like thin-medium airfoils, it continues tohold the same value or gradually increase.

The Cl/Cd vs Alpha graph shows us that the maximum value of Cl/Cd is not very high which depicts the

aerodynamic inefficiency of thick airfoils.

The pressure distribution has a sudden increase in gradient as expected and explained earlier, where the

transition takes place.

Effect of Roughness

Theory and Procedure:

The processes of boundary-layer separation and stall phenomena, which occur on the wind turbines

blade in the presence of surface roughness, are not fully understood. Since thick airfoils are used mostly for

blades of rotor, it is important to study the effects of roughness of surface on the aerodynamic performance of

the 2D foil since one of the most critical problems for wind turbine rotors is degradation of the performance, and

the unpredictability of stall due to dust accumulation on blade surface area.

The analysis was done using Javafoil. The effect of roughness on transition and drag is complex and cannot

be simulated accurately. [3] Even direct numerical simulation methods have difficulties simulating it. In Javafoil,

two effects of roughness are modeled: Laminar flow on a rough surface will be destabilized leading to premature transition

Laminar as well as turbulent flow on rough surfaces produce a higher skin friction drag.

Javafoil provides three options for surface roughness:

r0= perfectly smooth surface

r1= slightly rough surface

r2= NACA standard roughness

r3= dirty surface with spots of dirt, bugs and flies

The airfoil used was NACA 21035 which has already been analysed in the first part in XFLR5. Alpha was varied

from -3 to 40 degrees




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Results:

Figure10: Cl_max vs roughness Figure11: Cd_min vs roughness

Figure 12: Clvs Alpha

Figure 13: Cd vs Alpha

Figure 14: Cl/Cd vs Alpha




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Figure 15: Cl vs Cd

Discussions :

For the given Reynolds number, as can be seen in figure 10, Cl max value decreased consistently withincrease in roughness. Figure 11 shows the increase in Cd at zero lift coefficient.

For lower angles of attack, the Cl is not affected much. At higher angles of attack, Cl drops with

roughness. The stall angle moves very slightly to a lesser value. This change is not easy to observe here

because the thick airfoil stalls fairly smoothly at this Reynolds number so the curve is very gradual.

Surface roughness moves the transition point toward the leading edge and causes early trailing edge

turbulent separation, which results in reducing the effectiveness of the airfoil.

The flow apparently separated sooner for higher roughness.

Cl/Cd is reduced by a greater factor in each step when roughness is low, this factor decreases with

increase in roughness. In other words, performance of an airfoil is more sensitive to smaller roughness.

Cl/Cd decreases because Induced drag increases as the angle of attack increases. Therefore, since a

contaminated wing must fly at a higher angle of attack at a given airspeed to produce the required lift,

the induced drag generated at that airspeed will be higher than the induced drag of an uncontaminated

wing.

References:

[1] http://naca.central.cranfield.ac.uk/reports/1932/naca-report-391.pdf[2] http://adg.stanford.edu/aa241/airfoils/thicksections.html

[3] http://www.mh-aerotools.de/airfoils/java/JavaFoil%20Users%20Guide.pdf

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Effect of Roughness on airfoil