Enhanced outer peaks in turbulent boundary layer using uniform blowing at moderate Reynolds number

Abstract

Uniform blowing through a permeable surface acts as an active flow control method in wall bounded flows. Such control technique was investigated in a zero pressure gradient turbulent boundary layer over a flat plate. Measurement data were obtained with the help of Laser Doppler Anemometry technique. Besides the drag reduction characteristics of such flow control method, time averaged measurement of stream-wise and wall-normal velocity components was taken at Reynolds number based on momentum thickness ($R{e}_{\theta ,SBL}=U_{\mathrm{\infty }}\theta /\nu$) of 1100–3670. Due to the difference in surface condition with and without blowing, mean properties of the boundary condition at wall influence the flow properties when scaled with outer scaling properties. Enhanced turbulence is observed in Reynolds stresses using statistical analysis including the thickening of the boundary layer.

Keywords:

• Laser Doppler anemometry (LDA)
• Turbulent Boundary Layer (TBL)
• Turbulent Drag Reduction (TDR)
• Micro-Blowing Technique (MBT)

Acknowledgement

We would like to convey our sincere thanks to Dr. Ramis Örlü, KTH Royal Institute of Technology, Sweden and Prof. El-Sayed Zanoun, Brandenburg University of Technology, Germany for their insightful advice.

Disclosure statement

The authors declare that they have no conflict of interest.

Notes

1 Here, δ or boundary layer thickness represents the wall normal distance where streamwise velocity reaches 99% free stream velocity.

2 Very recently, Motuz [40] and Horn et al. [41] suggested that the staggered arrangement was also found to be as effective as discrete perforations in terms of blowing/suction cases. Therefore, Tailored Skin Single Duct (TSSD) design for Airbus A340-300 flight test also adopted staggered configuration (hole diameter to spanwise distance ratio ($R_b$) was kept at 1:2). Moreover, scale modification was done based on δ, this was discussed in detail from Krishnan et al. [42].

3 Total number of samples/(end${}_{time}$ – start${}_{time}$) was within 1000–11,000. Data validation rate was taken into consideration and was kept greater than 50%. This was obtained for each data points as a percentage of valid bursts/(valid bursts + invalid bursts).

4 Mach number (M) = free stream velocity (U${}_{\mathrm{\infty }}$)/speed of sound (c).

5 In order to realise uniform blowing surface as implemented in the numerical studies, requires finite surfaces with holes drilled at uniform distance. Therefore, one has to implement such surfaces in the experimental studies for an wide range of Reynolds number, e.g. MBT surfaces used in the present experiment.

6 Ideal value means calculations without considering mechanical losses in the pumping process. It should be noted that the S and G from Kametani and Fukagata [10] are not as the same definition from the data from the present data. Gain is significantly higher for the former due to the fact that, Kametani and Fukagata [10] used global friction coefficient, moreover, they have also concluded that control performance indices (G) should be much less when deviated from the ideal values. Instead experimental results are from local skin friction coefficient and input work from unit area. DNS performed in this case ($R{e}_{\tau ,SBL}=160$) was way smaller than the experimental data presented here.

Additional information

Funding

The work was financially supported by the Graduate Research School, Germany (35010) under the project stochastic methods and sub-project 4 - (Flow control of a flat plate turbulent boundary layer flow through micro blowing under stochastic forcing).