When dealing with flowing fluids, it is often desirable to reduce the non-uniformity and turbulence in the flowing fluid. Generally speaking, any structure placed in a fluid will cause some turbulence when the fluid begins to flow. In addition, as the speed of a flowing fluid increases, the effects of the turbulence become more pronounced. For example, when testing aircraft in a wind tunnel, it is desirable to produce flowing air having a uniform flow with very low amount of inherent isotropic turbulence. Producing such a low turbulence fluid flow allows the users of the wind tunnel to more precisely monitor the effect of the flowing air in the tunnel on a model placed in the tunnel. If there is a large amount of turbulence in the flowing air in the tunnel, the turbulence will mask some of the effects of the model's structure on the fluid flow and vice versa.
The design of a flow straightening and Turbulence Reduction System (TRS) such as a wire-mesh screen for a wind tunnel application is based on a desire to maximize the attenuation of the flow unsteadiness and non-uniformity in the stilling chamber of the wind tunnel. To meet the very stringent flow uniformity and turbulence requirements in the test section where the test subject is positioned, mesh screens are integrated into the stilling chamber design. The stilling chamber is typically designed for operation at relatively lower flow speeds that have sub-critical Reynolds numbers based on the diameter of the fine wire used to create the screens. A Reynolds number is a non-dimensional number used to describe the turbulent nature of a flowing fluid. The Reynolds number of a flowing fluid is calculated as R=ρVD/μ where p is equal to the fluid density, V is equal to the velocity of the flowing fluid, D is equal to the characteristic distance or diameter, and μ is equal to the viscosity of the fluid. At Reynolds numbers larger than about 40 and higher, circular, cylindrical bodies, such as wire, positioned in a cross flow develop unsteady viscous wakes. The viscous wake is convectively unstable and rolls into large-scale coherent rotational flow regions. At higher Reynolds numbers, this is called the Karman vortex street and is composed of positive and negative vorticity that is present in concentrated patterns. These vortices develop higher frequency instabilities and eventually decay into turbulence.
The design of a Turbulence Reduction System (TRS) for most wind tunnels is based not only on modifying and reducing turbulence generated in the flow circuit by turbulence producing sources such as fans, heaters, coolers, turning vanes etc., but also on insuring that the turbulence generated by the TRS itself is small or negligible. TRS is generally formed by combination of perforated plates, honeycombs, and screens. Power losses associated with the performance of the TRS are proportional to the square of flow velocity through them. To reduce power consumed by TRS and to meet flow uniformity requirements, the stilling chamber is made as large as practically possible to lower the velocity in the chamber and the screen wire diameter is selected to be very small to achieve sub-critical Reynolds numbers. For large wind tunnels, there are at least two problems associated with this methodology. The first problem is that a wire screen extended over a large span does not retain its intended planar shape. The screen is deformed into a somewhat spherical shape that changes the flow direction in proportion to the local inclination of the screen relative to the flow upstream of the screen. This deformation is the result of a pressure drop that occurs when the fluid flows through the screen. The flow downstream of the deformed screen is, therefore, non-uniform and produces flow angularity in the test section. The second problem is the large required size of the stilling chamber necessary to produce the contraction ratio that is essential to reducing flow stream turbulence close to the desired value.
On the basis of both cost and mechanical considerations, the stilling chamber size requirement is especially limiting for the design of high Reynolds' number facilities which are often pressurized. Therefore, in high Reynolds number facilities, the stilling chamber is typically smaller in diameter, has a lower contraction ratio, and has a higher flow velocity for a given test section velocity than a non-pressurized tunnel of the same test section size. This places two contradictory requirements on the screens. The wire diameter must be kept small to operate at sub-critical Reynolds numbers. However, the wire diameter must be increased to reduce deformation due to increased loads on the screen and in turn, increases the turbulence level in the test section and requires a longer, and more expensive, contraction section to provide for increased decay of turbulence exiting the screen.
Thus, the current methods for reducing turbulence in a wind tunnel are to make the walls of the tunnel as smooth as possible and to place a screen across the fluid flow in the wind tunnel. Unfortunately, the smoothness of the walls does not help reduce turbulence in the center of the fluid flow and the screens tend to bend and produce turbulent vortices at higher flow velocities. Therefore, an improved structure and method for reducing turbulence in a wind tunnel are needed.
There are many other instances where it is desirable to decrease the turbulence and increase the flow uniformity of a fast flowing fluid. For example, with regard to engines, it is desirable to manage the flow of gas and air into the engine to increase the efficiency and the uniformity of the performance of the engine. This is typically accomplished by injecting the gas into the combustion chamber with a nozzle that creates a fairly uniform mixture of gas and air. In addition, valves are used to precisely control the amounts of gas and air injected into the engine. However, there are currently no methods for reducing the internal turbulence in the flowing fuel and air mixture itself. Turbulence in these mixtures results in small changes in the amount of fuel or air provided to the engine and in the uniformity of fuel air mixture. This variability in flow uniformity reduces the engines' overall performance and efficiency. Therefore, there is a need in the prior art for a method and structure for more effectively reducing turbulence and insuring flow uniformity in an engine.