Manipulation Of Large Scales Via A Spanwise Array Of Wall-Normal Jets In A Turbulent Boundary Layer

Abstract

In recent years, research for turbulent boundary layer (TBL) has shifted towards developing novel approaches to reduce drag. This shift is driven by new industrial standards emphasizing fuel-efficient and environmentally friendly air transportation. As wall-bounded turbulence plays a significant role in the overall drag experienced by an aircraft, any understanding and control authority on associated flow structures becomes vital for more optimized and efficient flow control techniques to reduce drag. In this context, conventional passive flow control techniques have been widely used as they are simple and do not require any external power supply. However, active flow control techniques have demonstrated the potential for greater effectiveness, which offers more substantial drag reduction. Existing active control methods, such as wall oscillation, traveling wave, blowing, and suction, have been previously studied to reduce the viscous drag in the TBL. However, the efficacy of such control strategies tuned to outer layer large-scale motions (LSMs) at higher Reynolds number (Re) flows has emerged as a promising avenue for achieving significant drag reduction. In contrast, control techniques targeting the inner layer small-scales encounter limitations due to restricted control authority and the difficulties associated with miniaturizing hardware at higher Re flows.

LSMs convecting in the logarithmic region (log region) of TBL play a critical role in the dynamics of wall-bounded turbulence, as they carry a significant portion of the turbulent kinetic energy (TKE) [Abbassi et al., 2017]. The active wall oscillation technique tuned to the frequency of the LSMs has proven to be energy-efficient, as drag reduction increases with the frictional Reynolds number (Reτ ) and the contribution of LSMs to the TKE and wall shear stress increases with the Reτ [Marusic et al., 2021]. In this thesis work, the receptivity of LSMs to active large-scale control strategy is assessed via a spanwise array of wall-normal jets in a TBL. Multiple wall-normal jets manipulate the flow over a flat plate by introducing a spanwise traveling wave, which aims to mimic wall oscillation tuned to the large scales in the log region of the TBL.

To assess the efficacy of the suggested control strategy with spatial and temporal tuning, particle image velocimetry (PIV) and hot-wire anemometry (HWA) are employed to quantify second-order turbulence statistics and analyze the organization of the coherent patterns introduced in the flow via two-point correlations. In addition, the effect of different control cases derived from different tuning and control strategies is analyzed at Reτ = 2227. The TKE production plots show that control cases targeting the LSMs effectively attenuate the TKE production near the wall. Furthermore, based on the analysis of the spectral energy plot of streamwise velocity fluctuations, it is observed that the energy content associated with larger length scales convecting in the middle of the log region decreases in the controlled case compared to the uncontrolled base case. This finding suggests that certain control cases notably impact the organization of LSM and second-order turbulence statistics far downstream of the actuation point. Additionally, the reduction in TKE production near the wall represents a reduction in wall shear stress and viscous drag. However, additional research must be conducted in the future based on the current findings to reach a definitive conclusion regarding the efficacy of individual control cases.