Technical report | An Assessment of the Usefulness of Water Tunnels for Aerodynamic Investigations
Water tunnels are emerging as a possible useful alternative to small low-speed wind tunnels for an expanded range of aerodynamic testing. In this report, an assessment is made regarding the extent to which water tunnels can be used for such testing. It was found that their suitability for testing given models needs to be assessed on a case-by-case basis. For conventional tests on aircraft, such as force and moment measurements, they compare unfavourably with similar-sized wind tunnels, due to a mismatch in Reynolds numbers. Water tunnels are generally better suited to carrying out fundamental research than they are for applied aerodynamics testing. However, they are very useful as part of a large research program, by helping establish the testing schedule for large wind tunnels. In flow situations that are insensitive to Reynolds number, or where a test Reynolds number is close to that of a full-size vehicle, water tunnels should be regarded as the preferred option for experimental aerodynamics. Such examples include micro air vehicles, high-rate dynamic testing, and high-sweep sharp-edge configurations. Water tunnels are also very useful for providing validation data for computational-fluid-dynamics analyses of a flow. An earlier version of this work was prepared for the TTCP TR-AER-TP5 Panel in August 2010.
Water tunnels have some definite advantages over wind tunnels for aerodynamic testing of models. They are cheaper to build and operate, and simplified models can be easily modified to suit new testing requirements as a program proceeds. Water tunnels are ideally suited to flow-visualisation studies, giving an insight into the physics of the flow. With recent advances in measurement technology, they can now also be used to measure the very small flow-induced loads on models. Using water tunnels, it is possible to simultaneously measure loads and visualise high-quality off-surface flows, so that the loads and images can be correlated directly. Water tunnels are also better suited to modern laser-based flowfield-diagnostic methods, such as Particle Image Velocimetry. They are particularly amenable to dynamic testing because of the low rotation rates of models in water.
As with all testing in tunnels, similarity conditions ideally have to be satisfied to ensure that loads measured on models in tunnels can be scaled to correspond to full-size operation of a vehicle. Reynolds-number similarity is by far the most important similarity condition that must be satisfied, at least for low-velocity testing. Due to the small models and low free-stream velocities generally used in water tunnels, Reynolds numbers are typically at least three orders of magnitude less than those for full-size vehicles, such as aircraft. Therefore, water-tunnel data are only useful if this difference has no appreciable effects on flow patterns and the associated loads on models. Water tunnels are not useful for investigating flow problems at high Mach numbers, so the question of the usefulness of water tunnels for such flows was not addressed. The investigation is confined to low-subsonic flow regimes.
Although there are some definite advantages in using water tunnels rather than wind tunnels for aerodynamic investigations, the lack of Reynolds-number similarity is a major concern. There is little point using water tunnels for such investigations if the data are not meaningful. In this report, an assessment is made regarding whether water tunnels can now be used as a possible alternative to wind tunnels for an expanded range of aerodynamic testing.
It was found that the suitability of water tunnels for testing given models needs to be assessed on a case-by-case basis. Water tunnels will not overcome the need for large-scale, high-precision testing, and are at most marginally useful for producing reliable results in configuration aerodynamics, even in the conceptual design stage. For conventional aircraft testing, such as lift and drag measurements, they compare unfavourably with similar-sized wind tunnels, because of the mismatch in Reynolds numbers. Water tunnels are more of a fundamental research tool than an applied aerodynamics tool. However, they are very useful as a component of a large research program, by focusing the test matrix in large wind tunnels and providing validation for computational-fluid-dynamics analyses of the flow. In problems insensitive to Reynolds number, or where a test Reynolds number is close to that of a full-size vehicle, water tunnels should be regarded as the principal tool of experimental aerodynamics. Examples include micro air vehicles, high-rate dynamic testing, and high-sweep sharp-edge configurations.