A near-wall control approach to suppress the log-law energy containing structures to obtain frictional drag reduction at relatively high Reynolds numbers
Creators: name:  A. ALTINTAS 
name:   L. DAVIDSON 
name:   S. PENG 
Publisher: name: Proceedings of  the 9th European Conference for Aerospace Sciences. Lille, France, 27 June - 1 July, 2022
Year: 2022
Subjects: FLOW CONTROL
AEROSPACE

description: The energy consumption in an aircraft is mainly due to the aerodynamic drag force that opposes an aircrafts motion through the air. Almost 50% of total drag is due to the viscous drag, which is directly related to the friction drag of the aircraft caused by the interaction of the turbulent boundary layer flow with the aircraft surface. Studies on the aircraft and turbulent boundary layer interactions, together with developments of advanced flow control technologies, can effectively reduce more than 40% of the viscous drag, which is equivalent to about 15% of the total drag and has, therefore, major implications to the energy consumption of commercial aircraft. Saving 1% of fuel on a Boeing 737 - 800 would result in a 100 metric tons yearly fuel reduction. It would also decrease the emission of pollutants by 318.7 tons of CO2 , 123.9 tons of H2O, 2.122 tons of NOx , 98 kg of SO2 and 56 kg of CO. Hence, even a small drag reduction gives significant benefits [1]. Many studies have addressed the existence of large-scale (LS) motions that are very important for wall turbulence at high Reynolds numbers [2, 3]. The streamwise lengths of the large eddies can be in an order of 10-20 boundary-layer thicknesses in the logarithmic region of wall-bounded flows [4]. These large eddies involve mostly streamwise velocity fluctuations and contain most of the streamwise kinetic energy. Many subsequent studies have reported that log-law LS motions strongly influence near-wall turbulent structures [3,5]. Therefore, to control the log-law large-scale structures is important to obtain frictional drag reduction at high Reynolds numbers. However, the general approach to control those log-law structures is to apply a flow control method on those relatively high wall-normal locations; this is not a feasible method when it comes to real-world applications. In this study, we suppressed positive wall-normal velocity near the wall, at y+ 20. By doing so we aimed to suppress the formation or interaction of organized flow structures in the log-law region. This study is performed for a frictional Reynolds number of Re? = 550, it is also supported with a higher Reynolds number, Re? = 800. A high amount of drag reduction is observed for the controlled case (approximately 35% and 40% for Re? = 550 and Re? = 800, respectively) that is related to the absence of the streak formations in the log-law region. Then we compared the streak formations for controlled and uncontrolled cases at the inner and outer kinetic energy peak locations. We observed those streak formations are reduced/disappeared in the log-law. We support our findings with two-point correlations and streamwise velocity contour plots. We quantify the difference in the effect of the log-law influence on the wall-turbulence for the controlled and uncontrolled applied cases by amplitude modulation analysis. Modulation effect is one of the two commonly accepted effects of the outer LS structures on near-wall turbulent fluctuations. Amplitude modulation analysis is a method to understand how LS motions amplify or attenuate the turbulent structures near the wall. We compare the amplitude modulation for the uncontrolled and the controlled cases for both Re? = 550 and Re? = 800, and we obtain a much lower correlation for the controlled cases compared to uncontrolled cases.