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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.

Mostra/Nascondi contenuto.
Chapter 1 – Analysis of Turbulent Wakes 12 Analysis of Turbulent Wakes 1.1 Introduction to Turbulent Flows Most fluid motion presented in nature or in engineering applications, is dominated by turbulent motion. There are many opportunities to observe, turbulent flows in our everyday surrounding: a smoke coming from a chimney, a water in a river or in a waterfall, the buffeting of a strong wind, or a wake generated by any object flying in clean air [1]. Also, If we observe fluid motion within a straight pipe, it will be noted that the shape of the velocity curve (the velocity profile across any given section of the pipe) depends upon whether the flow is laminar or turbulent as shown in fig. n° 1.1.1. If the flow in a pipe is laminar, the velocity distribution at a cross section will be parabolic in shape with the maximum velocity at the center being about twice the average velocity in the pipe. In fully turbulent flow, a fairly flat velocity distribution exists across the section of pipe [2, 3]. One of the most important parameters needed to describe this behavior, is the Reynolds number. This parameter is given by the relation (Reynolds 1894):  D U   Re (1.1.1) where U and D are characteristic velocity and length scale of the flow, and  is the kinematic viscosity of the fluid considered. D could be the diameter of any

Laurea liv.II (specialistica)

Facoltà: Ingegneria

Autore: Gabriele Ganci Contatta »

Composta da 133 pagine.

 

Questa tesi ha raggiunto 105 click dal 01/02/2012.

Disponibile in PDF, la consultazione è esclusivamente in formato digitale.