Center for International Cooperation in Long Pipe Experiments

• Large scale wind tunnel (0.9m in diameter) to generate turbulent scales fully resolvable with currently available measurement techniques;
• High Reynolds number;
• Fully developed turbulence in a “naturally” generated flow (120m long, L/D > 110);
• Possibility to insert generating “passive” or “active” grids;
• Full and easy access to the test section, and to several other places along the pipe development;
• Highly controlled environmental conditions (no background noise, stable temperature and humidity).

The infrastructure allows the worldwide best space and time resolved measurements in turbulent pipe flows. The moderately high Reynolds number is obtained by keeping a large pipe diameter and is sufficiently high to reproduce “real” applications. Since the test section is at ambient pressure the facility is perfectly suited for testing and optimizing novel measurement techniques and may be easily adopted to study various aspects of turbulent flows, e.g. pressure gradients, wall roughness and friction control.

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 Ecole 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.

The main facility that will be used (the “Long Pipe”, LP) is basically a closed loop wind tunnel ope­rating with air at atmospheric pressure. A closed loop has been chosen since it will allow the flow characteristics to be accurately controlled 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. Many of the various aerodynamic components are the same as those for an ordinary wind tunnel (diffusers, screens, contraction, etc.). The long pipe consists of a 115 m long tube with an inner diameter of 0.9 m and will be made of 5 m long carbon fibre modules held by precision positioning elements.

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 criteria above. In the Princeton “su­perpipe” the 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, few reliable turbulence data have been published.

In the figure below the range of various pipe experiments in terms of the viscous length scale as function of the Reynolds number is plotted. The experiments include air, compressed air and water as flow medium. As can be seen, most experiments are for low Reynolds numbers. The “super­pipe” covers a wide range of Reynolds numbers but does not satisfy both parameters simultane­ously. Although the Long Pipe in CICLoPE has a much smaller Reynolds number range than the “superpipe” it is designed such that the scales within that range will be large enough to be reason­ably well resolved by present measurement techniques and therefore it will offer the scientific users the possibility to fully characterize the flow characteristics and the physical phenomena involved.

 

The Long Pipe is designed to provide a test rig for at least twenty years of basic research in this field. It will also offer the external users the possibility for extensions with more direct impact on applications, such as the study of the effect of non-smooth walls or non-isothermal conditions, the evolution of various non-equilibrium flows, and of flows with some particulates.

The laboratory is presently under construction. Ventilation, electrical and mechanical systems will be positioned and tested before the end of this year. Most of the “Long Pipe” elements have been  ready to be installed afterwards. The “Long carbon fibre Pipe” is on the process to be acquired through European Competition. We estimate to be able to place the order before the end of this year. The facility is going to be completed and assembled by the end of year 2012. The classic configuration should guarantee standard time for the set-up and flow quality assessment. The ac­cess will be available to external users by January 2014.