Experimental Investigation into Symmetric and Asymmetric Turbulent Wakes

The idea of self – preservation in turbulent flows began during the beginning of the 20th century. Self – preservation is said to occur when the profiles of velocity (or any other quantity) can be brought into congruence by simple scale factors that depend on only one of the variables. A recent work “Identification of Any Aircraft by its Unique Turbulent Wake Signature” from Mark Garnet and Aaron Altman (University of Dayton) tried to challenge the aspect of the self – preservation theory making a turbulent model able to reconstruct the Reynolds Stress distribution only starting with the knowledge of certain parameter: Momentum thickness (via drag coefficient), velocity, and geometrical characteristics such as span and characteristic horizontal length of the investigated models.
All the experiments were conducted at University of Dayton “Kettering Labs” through an exchange program between the University of Dayton and the “Fondazione Comunità Domenico Tardini – Villa Nazareth”
First, a brief review of this theoretical foundation will be discussed wherein the drag coefficient can be used to estimate either a Reynolds Stress distribution or (indirectly) a model of the velocity profile of the turbulent wake.
Experimentally, initially the Reynolds stress distribution downstream of a circular cylinder is determined using Particle Image Velocimetry (PIV) in the University of Dayton Low Speed Wind Tunnel (UD – LSWT) and those results compared to the predicted Reynolds Stress distribution calculated from the drag coefficient obtained through direct force measurement. Subsequently, the Reynolds stress distribution downstream of a moderate aspect ratio wing obtained through PIV is similarly compared to a predicted Reynolds Stress distribution derived through the drag coefficient obtained from direct force measurement. Finally, a small remote controlled twin engine aircraft is tested with wind-milling propellers and its drag measured from a force balance is used to generate the expected Reynolds stress distribution. This distribution is then compared to the Reynolds stress distribution obtained from PIV downstream of the model where the wake had not yet achieved a self-preserved state. This prevented a complete demonstration of practical feasibility of the technique; however, several key underlying assumptions of the method are validated.