A water tunnel is an experimental facility used for testing the hydrodynamic behavior of submerged bodies in flowing water. It is very similar to a recirculating wind tunnel

Wind tunnel

A wind tunnel is a research tool used in aerodynamic research to study the effects of air moving past solid objects.-Theory of operation:Wind tunnels were first proposed as a means of studying vehicles in free flight...

 but with water as the working fluid, and related phenomena are investigated, such as measuring the forces on scale models of submarines or lift and drag on hydrofoil

Hydrofoil

A hydrofoil is a foil which operates in water. They are similar in appearance and purpose to airfoils.Hydrofoils can be artificial, such as the rudder or keel on a boat, the diving planes on a submarine, a surfboard fin, or occur naturally, as with fish fins, the flippers of aquatic mammals, the...

s. Water tunnels are sometimes used in place of wind tunnels to perform measurements because techniques like Particle image velocimetry

Particle image velocimetry

Particle image velocimetry is an optical method of flow visualization used in education and research. It is used to obtain instantaneous velocity measurements and related properties in fluids...

 (PIV) are easier to implement in water. For many cases as long as the Reynolds number is equivalent, the results are valid, whether a submerged water vehicle model is tested in air or an aerial vehicle is tested in water. For low Reynolds number flows, tunnels can be made to run oil instead of water. The advantage is that the increased kinematic viscosity will allow the flow to be a faster speed (and thus easier to maintain stably) for a lower Reynolds number.

Whereas in wind tunnels the driving force is usually sophisticated multiblade propellers with adjustable blade pitch

Blade pitch

Blade pitch or simply pitch refers to turning the angle of attack of the blades of a propeller or helicopter rotor into or out of the wind to control the production or absorption of power. Wind turbines use this to adjust the rotation speed and the generated power...

, in water and oil tunnels the fluid is circulated with pumps, effectively using a net pressure head

Pressure head

Pressure head is a term used in fluid mechanics to represent the internal energy of a fluid due to the pressure exerted on its container. It may also be called static pressure head or simply static head...

 difference to move the fluid rather than imparting momentum

Momentum

In classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...

 on it directly. Thus the return section of water and oil tunnels does not need any flow management; typically it is just a pipe sized for the pump and desired flow speeds. The upstream section of a water tunnels generally consists of a pipe (outlet from the pump) with several holes along its side and with the end open followed by a series of coarse and fine screens to even the flow before the contraction into the test section. Wind tunnels may also have screens before the contraction, but in water tunnels they may be as fine as the screen used in window openings and screen doors.

Additionally, many water tunnels are sealed and can reduce the internal static pressure

Pressure

Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

, to perform cavitation

Cavitation

Cavitation is the formation and then immediate implosion of cavities in a liquidi.e. small liquid-free zones that are the consequence of forces acting upon the liquid...

 studies. These are referred to as cavitation tunnels.

Methods

Because it is a high-speed phenomenon, a special procedure is needed to visualize cavitation. The propeller, attached to a dynamometer

Dynamometer

A dynamometer or "dyno" for short, is a device for measuring force, moment of force , or power. For example, the power produced by an engine, motor or other rotating prime mover can be calculated by simultaneously measuring torque and rotational speed .A dynamometer can also be used to determine...

, is placed in the inflow, and its thrust and torque is measured at different ratios of propeller speed (number of revolutions) to inflow velocity. A stroboscope

Stroboscope

A stroboscope, also known as a strobe, is an instrument used to make a cyclically moving object appear to be slow-moving, or stationary. The principle is used for the study of rotating, reciprocating, oscillating or vibrating objects...

 synchronized with the propeller speed "freezes" the cavitation bubble. By this, one can observe if the propeller would be damaged by cavitation. To ensure similarity

Similitude (model)

Similitude is a concept applicable to the testing of engineering models. A model is said to have similitude with the real application if the two share geometric similarity, kinematic similarity and dynamic similarity...

 to the full-scale propeller, the pressure is lowered, and the gas content of the water is controlled.

Often, a tunnel will be co-located with other experimental facilities at a Ship model basin

Ship model basin

A ship model basin may be defined as one of two separate yet related entities, namely:* a physical basin or tank used to carry out hydrodynamic tests with ship models, for the purpose of designing a new ship, or refining the design of a ship to improve the ship's performance at sea;* the...

.

Canada

• National Research Council—Institute for Ocean Technology Cavitation Tunnel, St. Johns, Newfoundland.

France

• "Tunnel de Cavitation" Ecole Navale, Lanveoc
• "Grand Tunnel Hydrodynamique" Bassin d'Essais des Carènes, Val de Reuil

Germany

• Multiple cavitation tunnels at the Versuchsanstalt für Wasserbau und Schiffbau, Berlin
• Large Cavitation tunnel at Hamburg Ship Model Basin, Hamburg
• Multiple cavitation tunnels at the Oskar von Miller Institut, Technical University of Munich

India

• Fluid Control Research Institute, Palakkad,Kerala. http://www.fcriindia.com
• Cavitation Tunnel of the Naval Science and Technology Labs at Visakhapatnam.

Iran

• Applied Hydrodynamics Laboratory, Iran University of Science and Technology, Narmak, Tehran.
• Marine Engineering Laboratory, Sharif University of Technology, Azadi Av., Tehran.

The Netherlands

• Large Cavitation Tunnel and High Speed Cavitation Tunnel at Maritime Research Institute Netherlands
  MARIN (research)
  MARIN, the Maritime Research Institute Netherlands, is one of the leading institutes in the world for hydrodynamic research and maritime technology. The services incorporate a unique combination of simulation, model testing, full-scale measurements and training programmes. MARIN provides services...
  
   in Wageningen
  Wageningen
  ' is a municipality and a historical town in the central Netherlands, in the province of Gelderland. It is famous for Wageningen University, which specializes in life sciences. The city has 37,414 inhabitants , of which many thousands are students...
  
  

Norway

• "Cavitation Lab" NTNU, The Norwegian University of Science and Technology
  Norwegian University of Science and Technology
  The Norwegian University of Science and Technology , commonly known as NTNU, is located in Trondheim. NTNU is the second largest of the eight universities in Norway, and, as its name suggests, has the main national responsibility for higher education in engineering and technology...
  
  , Trondheim

Spain

• CEHIPAR (Canal de Experiencias Hidrodinámicas de El Pardo), http://www.cehipar.es/cehiparweb/index.php?lang=english, El Pardo (Madrid), Spain.

Serbia

• The Large Cavitation Tunnel at Military Technical Institute, Belgrade, Serbia
  

United Kingdom

• "Emerson Cavitation Tunnel
  Emerson Cavitation Tunnel
  The Emerson Cavitation Tunnel is a propeller testing facility based at Newcastle University, UK. The tunnel is based in the department of Marine Science and Technology, and is currently the second largest cavitation tunnel in the UK, the largest being that owned by the Ministry of Defence at...
  
  ," University of Newcastle upon Tyne
  University of Newcastle upon Tyne
  Newcastle University is a major research-intensive university located in Newcastle upon Tyne in the north-east of England. It was established as a School of Medicine and Surgery in 1834 and became the University of Newcastle upon Tyne by an Act of Parliament in August 1963. Newcastle University is...
  
  .

United States

• The Garfield Thomas Water Tunnel The Pennsylvania State University
  Pennsylvania State University
  The Pennsylvania State University, commonly referred to as Penn State or PSU, is a public research university with campuses and facilities throughout the state of Pennsylvania, United States. Founded in 1855, the university has a threefold mission of teaching, research, and public service...
  
  , State College, PA
• The William B. Morgan Large Cavitation Channel, Memphis, TN

External links

• "Under Water Speed Way..." Popular Mechanics, July 1949
• Description of a commercial water tunnel showing the inlet baffle, flow conditioning
  Flow conditioning
  Flow conditioning ensures that the “real world” environment closely resembles the “laboratory” environment for proper performance of inferential flowmeters like orifice, turbine, coriolis, ultrasonic etc.- Types of Flow :...
  
   screens, and converging section.
• Contains a nice picture of a water tunnel with PVC pipe and turbopump.
• These show the clear difference between a recirculating wind tunnel and a water tunnel.
• A miniature water tunnel whose return section is simply a reservoir tub with a sump pump
  Sump pump
  A sump pump is a pump used to remove water that has accumulated in a water collecting sump basin, commonly found in the basement of homes. The water may enter via the perimeter drains of a basement waterproofing system, funneling into the basin or because of rain or natural ground water, if the...
  
  . Flow rate is controlled by constricting the flow on the outlet of the pump with a valve.