Zero mass jets for turbulent flow drag reduction

Drag reduction has always been a very important research topic in aircraft design area. Over many decades, a lot of passive and active control methods have been developed to reduce drag. Since the friction drag takes a big percentage in the total drag of a commercial aeroplane which flies under a turbulent condition, the industry has inclined to reduce the friction drag in turbulent flow. Practical approaches are being found to be applied to the next generation of aeroplanes.

This research focuses on a novel method of active flow control that can be used to reduce friction drag in a fully developed turbulent flow. The method is named as 'zero mass jets' which can be achieved by using a pair of synthetic jets. Since it is new, the method needs to be checked whether it can obtain drag reduction or not. The fully developed turbulent channel flow at a low Reynolds number is a good case to investigate the above method. The geometry is simple to generate a structured mesh. The Reynolds number is low to do direct numerical simulations to obtain detailed flow structures and study the turbulence phenomena.

Direct numerical simulations are often carried out by high order solvers. In our group, an in-house solver has already been developed for many years. Various fluid researches were investigated using our solver, including open boundary layer flow, synthetic jets, flow separations, and aerofoil designs. The solver can do RANS and DES by solving the compressible Navier-Stokes equations with TVD schemes and unstructured meshes. In order to do DNS, a high order method is added in the solver called 'SHEFFlow' to reduce the numerical dissipation in this research.

The turbulent flow is directly simulated by SHEFFlow. Since the flow is turbulent and periodically oscillated by zero mass jets, the obtained data is also complicated to be analysed. Useful information should be figured out by statistic tools. A triple decomposition is employed to decompose the velocity fluctuations into periodic characters and purely turbulent quantities. The Reynolds shear stresses and kinetic energy are studied by quadrant analysis and power spectral density, respectively. Using various analysis methods, the turbulence flow controlled by zero mass jets is studied in detail.

This novel method of active flow control is approved to be able to significantly reduce the friction drag through the study of zero mass jets. It's proved that this method has the potential to be applied on an aeroplane to improve the performance in the future.