# Newsletter 2019.3 Index

# Experimental quantification of friction drag reduction effects on an airfoil using uniform blowing

Kaoruko ETO Keio University |
Yusuke KONDO Keio University |
Koji FUKAGATA Keio University |
Naoko TOKUGAWA Japan Aerospace Exploration Agency |

### Abstract

Effects of uniform blowing on an airfoil are investigated experimentally aiming at turbulent friction drag reduction. We use the 0.65 m×0.55 m low-turbulence wind tunnel in Japan Aerospace Exploration Agency (JAXA), and its schematic is shown in Fig. 1. The uniform blowing is applied on the rear part of the upper surface (i.e. suction side). In order to examine the control effects, velocity measurements on the airfoil by a hot-wire anemometry are conducted. The experiment is carried out at the Reynolds numbers based on the chord length of Re* _{c}* = 0.65 × 10

^{6}and 1.5 × 10

^{6}. Figure 2 shows the mean velocity profiles in the control region. It is found that the mean velocity profiles are shifted away from the airfoil surface. This result suggests that the velocity gradient on the airfoil surface is reduced by uniform blowing. We have also attempted a quantitative assessment of the friction drag reduction rate. This is not straightforward because the velocity in the surface proximity, which is needed for a quantification of friction drag, cannot be measured directly by the present hot-wire. Therefore, we rely on two methods based on the mean velocity profiles in the boundary layer: the modified Clauser-chart method (MCCM) and the method using the wall law with a pressure gradient (WL). In each assessment method, the fitting of the mean velocity to the modified Clauser chart and the law of the wall with a pressure gradient is conducted, respectively. The sample results are shown in Fig. 3. Through these assessments, it is found that these methods are consistent with each other and the local friction drag is suppressed effectively as shown in Fig. 4. From these investigations, it is confirmed that 21% - 66% of the local friction drag is reduced by this control; namely, the uniform blowing is found to be effective for friction drag reduction also on the airfoil.

### Key words

Active control, Drag reduction, Uniform blowing, Airfoil, Wind-tunnel experiment, Adverse pressure gradient

### Figures

Fig. 1: Schematic of the test section.

Fig. 2: Mean velocity profiles in the control region (*α* = 0°, Re* _{c}* = 0.65 × 10

^{6}):

(

*a*)

*x/c*= 0.65; (

*b*)

*x/c*= 0.70; (

*c*)

*x/c*= 0.75; (

*d*)

*x/c*= 0.80.

Fig. 3: Sample results of the velocity fitting in the quantitative assessment (*α* = 0°, Re*c* = 1.5 × 10^{6}, *x/c* = 0.70):

(*a*) Modified Clauser chart method; (*b*) Quantification using the wall law with pressure gradient.

Fig. 4: Skin friction coefficients *c _{f}* as a function of streamwise coordinate

*x.*