Abstract
The fundamental mechanisms of a hitherto unstudied approach to control the crossflow-induced transition in a three-dimensional boundary layer employing unsteady control vortices are investigated by means of direct numerical simulations. Using a spanwise row of blowing/suction or volume-force actuators, subcritical travelling crossflow vortex modes are excited to impose a stabilizing (upstream) flow deformation (UFD). Volume forcing mimics the effects of alternating current plasma actuators driven by a low-frequency sinusoidal signal. In this case the axes of the actuators are aligned with the wave crests of the desired travelling mode to maximize receptivity and abate the influence of other unwanted, misaligned modes. The resulting travelling crossflow vortices generate a beneficial mean-flow distortion reducing the amplification rate of naturally occurring steady or unsteady crossflow modes without invoking significant secondary instabilities. It is found that the stabilizing effect achieved by travelling control modes is somewhat weaker than that achieved by the steady modes in the classical UFD method. However, the energy requirements for unsteady-UFD plasma actuators would be significantly lower than for steady UFD because the approach makes full use of the inherent unsteadiness of the plasma-induced volume force with alternating-current-driven actuators. Also, the input control amplitude can be lower since unsteady crossflow vortex modes grow stronger in the flow.
Highlights
Increasing energy efficiency by reducing aerodynamic drag is a challenging task for the design and manufacture of future aircraft, wind turbines and turbomachinery
An unsteady upstream flow deformation (UFD) technique to delay the CF-induced transition in a three-dimensional swept-wing-type boundary layer has been investigated by direct numerical simulations (DNS)
The volume force is varied by a low-frequency sinusoidal modulating signal, imitating the inherent unsteadiness of the plasma actuation, and has a small steady part
Summary
Increasing energy efficiency by reducing aerodynamic drag is a challenging task for the design and manufacture of future aircraft, wind turbines and turbomachinery. The same concept, named upstream flow deformation (UFD) and not necessarily based on roughness elements, was proposed by Wassermann & Kloker (2002) using direct numerical simulations (DNS) Their detailed analysis showed that the three-dimensional part of the control vortices weakens mainly the receptivity, whereas the mean-flow distortion reduces the growth rate of amplified modes. Similar to the application of homogeneous suction, Dörr & Kloker (2015b) and Chernyshev et al (2016) numerically investigated plasma actuators to stabilize a three-dimensional boundary-layer flow by base-flow manipulation: the plasma actuators are used to reduce the basic CF velocity and the growth of both steady and unsteady primary CF instabilities.
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