Modification of Prandtl Wind Tunnel

Wind tunnels are devices that enable researchers to study the flow over objects of interest, the forces acting on them and their interaction with the flow, which is nowadays playing an increasingly important role due to noise pollution. Since the first closed circuit wind tunnel with variable cross-section was built in Göttingen, its Prandtl configuration has little change. The wind tunnel with Prandtl configuration has four corners and vanes, more than 50% of the total pressure loss are caused by the corners and vanes. How to reduce the total pressure loss is a world class problem in the wind tunnel design. This study attempts to propose a novel configuration of wind tunnel, where the corners have been replaced by semi-circular tunnel. Sun wind tunnel 2 has only two corners and vanes, while Sun wind tunnel 1 has no corners and vanes at all. It is expected the new wind tunnel can reduce the total pressure loss from 50% to 10%.


Introduction
A wind tunnel is a tool used in aerodynamic research to study the effects of air moving past solid objects.Wind tunnel enable researchers to study the flow over objects of interest, the forces acting on them and their interaction with the flow.It has broad applications from aerospace to industrial, from high rise building to automobiles and even from sports and art animations as well.It is easy to be intrigued by application that pervades so many aspects of peoples' daily lives It was F. H. Wenham (1824-1908) who invented the first enclosed wind tunnel in 1871.K. Tsiolkovsky built an open-section wind tunnel with a centrifugal blower in 1897.The Wright brothers' use of a simple wind tunnel in 1901 to study the effects of airflow over various shapes while developing their Wright Flyer was in some ways revolutionary.In France, G. Eiffel (1832Eiffel ( -1923) ) built his first open-return wind tunnel in 1909 shown in Fig. 1.In 1909 Ludwig Prandtl Prandtl et al. (1969) in G ẗtingen built a closed circuit wind tunnel with various cross-section shown in Fig. 2. Since then the two configurations shown in Fig. 3 are coexisting in the wind tunnel design, while the Prandtl configuration dominates the wind design due to its advantages.In this article, we will focus on the design of Prandtl wind tunnel.
This article revisited Prandtl wind tunnel and propose a novel configuration.To make the paper self-contained, the paper is organised as follows.After this introduction, in Section 2 the Prandtl wind tunnel is highlighted with discussion of its both advantages and disadvantages; in Section 3, the main sources of total pressure loss is discussed from perspectives of turbulent flow separation around corners and induced drag of vanes.In

Prandtl wind tunnel
The general layout of the Prandtl wind tunnel is shown in Figure 4 below Prandtl et al. (1969).and relevant perspective drawing is shown in 4, Many of the large research wind tunnels are closed return tunnels.Main components of the wind tunnel are consist of: the contraction zone and the settling chamber.The

Main source of total pressure loss
Prandtl closed circuit wind tunnels require having four corners, which are responsible for more than 50% of the total pressure loss.The most critical contribution comes from the corner 1 because it introduces about 34% of the total pressure loss (ed).Curve of the corners cause the fluid particle acceleration.Assuming the flow inside a wind tunnel is steady flow, ie., ∂V /∂t = 0, thus V = V (s), and the acceleration in the s-direction becomes a s = dV dt = V dV ds , where V = ds/dt if we are following a fluid particle as it moves along a streamline.Therefore, acceleration in steady flow is due to the change of velocity with position.The accelerated fluid will cause velocity gradient that make the formation of circulating fluid structures, called vortices Cengel & Cimbala (2010).
To reduce the pressure loss and to improve the flow quality at the exit, corner vanes must be added.The vanes and corners will cause the flow severation (shown in Figure 7).The important consequence of flow severation is the formation of vortices, in the wake region.Those vortices will eventually transit to turbulence as the increase of flow velocity and acceleration.
The mechanism behind of the total pressure loss is due to the energy dissipation in turbulent flow generated by the corners.The turbulent flow is within the line of α 1 and α 2 , for flow around a right angle are α 1 = 5 0 , α 2 = 10 0 .We may menyion that the difference between the fluid pressures on the two sides of the turbulent region is very small.For example, in flow around a right angle it is found that p 1 − p 2 = 0.003ρU 2 , where U is the velocity of mean stream (along AO).p 1 the pressure in that stream and p 2 the pressure in the stream along BO ( §36, Landau & Lifshitz (2012)).
Another sources of total pressure loss is the induced drag of vanes.The induced drag is formed by the drag due to the dissipation of energy in the thin turbulent wake.Since the induced drag is proportional to the surface area, therefore more vanes will have more induced drag, which leads more total pressure loss ( §47, Landau & Lifshitz ( 2012)).
The vortices will forms a Karmán vortex which will make the vanes oscillating and generates noise pollution How to reduce the total pressure loss is a world class problem in the design of wind tunnel.

New configuration of closed circuit wind tunnel
In order to reduce the total pressure loss and acoustic noise level, it would be a natural attempts to remove corners, namely to replace corners by semi-circular closed.We like

Conclusions
This study has attempted to propose a novel design of wind tunnel, which can reduce the total pressure and energy loss, and acoustic noise level as well.This is a first part of my research on wind tunnel design.In the future, we will focus on simulation of the proposed wind tunnel Figure 9 and Figure 10, and will build one wind tunnel if financial supports are available and feasible based on Sun Wind Tunnel 1 in Figure 9.

Figure 1 :
Figure 1: Eiffel wind tunnel (open circuit) in Paris(1909).A cross-section drawing of the wind tunnel with the 2-meter test section in the Eiffel Laboratory at AuteuilDamljanović (2012)