The Centre for International Cooperation in Long Pipe Experiments (CICLoPE) is a research laboratory which allows high resolution turbulent-fluctuation and detailed flow structure measurements. The laboratory is operated with the idea of gathering world-leading scientists in the field of turbulence research to make decisive breakthroughs in the fundamental issues of high Reynolds number turbulence.
The centre was founded and is presently lead by a group of different universities and research centres: the University of Bologna, the International Centre of Theoretical Physics in Trieste, the Royal Institute of Technology, Illinois Institute of Technology and École Polytechnique Fédérale de Lausanne, the University of Rome “La Sapienza” and the Princeton University.
The laboratory is located beside the old factory of the Caproni Industry, which is basically a tunnel complex excavated. In 2006 the tunnels were given by the Air Force to the University of Bologna specifically for turbulence studies. The complex comprises two 130 m long tunnels with a diameter of about 9 m each. The stability of the ambient conditions, viz. temperature, humidity; the complete absence of any background noise (vibration, electrical noise, etc.) are the characteristics making Predappio an ideal site for this laboratory.
(Long Pipe Facility)
The Long Pipe facility is basically a closed loop wind tunnel operating with air at atmospheric pressure. A closed loop allows us to control the flow characteristics in terms of velocity, temperature and humidity. The layout resembles an ordinary wind tunnel where the main difference is the long test section, which gives most of the friction losses. The design of the various aerodynamic components (corners, screens, contraction) is inspired to some of the best existing research wind-tunnels (e.g. MTL of KTH Stockholm). The Long Pipe consists of a 111.5 m long tube with an inner diameter of 0.9 m. The pipe is made of 22 modular elements (approx. 5 m long) made of coarbon fibers, held by precision positioning elements.
Reynolds number range
The definition of the Reynolds number shows that various possibilities exist to obtain high Re: the velocity (U) should be high (but not so high that compressibility effects come into play), the density can be increased (by for instance pressurizing an air flow facility) or the physical dimensions (L) should be large. On the other hand, for the type of studies aimed at, it is necessary to have a facility where the spatial size of the smallest scale is sufficiently large to be resolvable by available measurement techniques. A measure of the smallest scale is the viscous length scale, l*, and it can be easily shown that for a pipe flow experiment with high Reynolds number, in order to keep large (and measurable) scales, l*, the diameter of the facility should be chosen as large as possible. Concerning the geometry to be used in the experiment, a unique advantage of the pipe flow compared to all the other canonical cases is that the wall shear stress can be determined directly from the pressure drop along the pipe, which can be measured accurately.
Over the years, there have been a number of pipe flow experiments reported in the literature, but they are mainly at low Reynolds numbers and do not fulfil the minimum Reynolds number needed to achieve a reasonable scale separation. In the Princeton “superpipe”, high Reynolds number is obtained through highly pressurizing the air used, reaching Reynolds numbers R+ of up to 500000. Although the achievable Reynolds numbers in the “superpipe” are extremely high for a laboratory experiment, it is at the expense of a very small viscous length scale, i.e., for R+=500000 the viscous length l* is only slightly above 0.1 mm. The scientific results from the “superpipe” include measurements of the mean velocity distribution and new information on the skin friction variation for both smooth and rough surfaces. However, due to the spatial resolution limitations, turbulent statistics are obtained using several correction schemes to compensate insufficient resolution.
In the figure below, we show the operative range of CICLOPE in the parameter space defined by R+= R/l* (horizontal axis) and viscous units l*. Comparing this with the range of the other existing facility, we see that CICLOPE covers an unexplored region of this parameter space, defined by a minimum Reynolds number (R+>10000) and size of viscous scales (l*>10 µm). The other experiments include air, compressed air and water as flow medium. As can be seen, most experiments cover the low-Reynolds number range. The “superpipe” covers a wide range of Reynolds numbers, but with much smaller size of the viscous scales. The Long Pipe in CICLoPE keeps the viscous units well above the 10 µm threshold all across its operative range. Therefore, standard hot-wire probes can be used to acquire turbulent statistcis with no need of a posteriori corrections.
The main part of the set-up is the long pipe, which consists of a 111.5m long tube of diameter 0.9m. The pipe is made of 22 modular elements, each 5m long plus one module of 1.5 m. The modules, made of carbon fiber, are built with extremely high precision to guarantee a smooth inner surface and perfect alignment (see table 1). Each module has 4 access holes of 15 cm diameter at the downstream end to introduce measurement devices or gain optical access along the whole length of the pipe. The main test section is a 1.5m long element at the end of the pipe. This section can be easily removed and replaced with a custom-made section specific to the type of experiment to perform. This offer the flexibility to study several particular flows such as: the effect of non-smooth walls; non-isothermal conditions; the evolution of various non-equilibrium flows; flows with particulates etc, without re-design of the entire pipe. Pressure can be equalized anywhere in along the pipe length, and customized flow manipulators (grids, active elements, etc.) can be installed at pipe inlet.