Water tunnels are facilities designed to examine the efficiency of hydrofoils or other hydraulic structures when submerged in water within a controlled environment.
Near the air-water interface, the relative motion of both fluids affects the performance of the submerged object.
CFD simulation helps in flow visualization at different sections of the water tunnel with full-scale accuracy.
Water tunnel hydrodynamic testing validates the design and efficiency of hydrodynamic systems
What happens when an object is submerged completely or partially into a streamlined flow system? To what extent do the hydrodynamic forces and pressure act on it? How do these factors influence the design of hydrofoils? These are the key questions that need addressing when testing hydrodynamic scale models of boats or submarines.
When a body is submerged in a fluid—in this case, water—viscosity, Reynolds number, density, and other factors can have an effect on the drag or lift of the hydrofoil. Similar is the case for free-surface flow, where the performance of propellers can be affected due to the object moving too close to the surface of the water. The effect on the performance of the hydrodynamic system in such conditions can be examined with water tunnel hydrodynamic testing. Furthermore, with the help of CFD simulation, visualization of these studies can be made easier to ensure model optimization.
The Basic Functioning of a Water Tunnel in Hydrodynamic Testing
In functionality, a water tunnel is very similar to a wind tunnel except for the obvious difference in the fluid medium. Water tunnels are designed to examine the efficiency of hydrofoils or other hydraulic structures by facilitating the effective visualization of flow behavior in a controlled condition. The size of the water tunnel greatly varies depending on the scale of experimentation desired and the scale of the model or object being tested. The water contained in a tunnel is regulated with the help of high-capacity pumps, which makes the flow possible at different speed ranges.
The environment, or the sections of the tunnel, in which the experiment is carried out can be assessed manually, as the water tunnel is unpressurized. In the water tunnel, hydrodynamic testing for the flow patterns, pressure fluctuation, and boundary layer formation for different submerged bodies is examined in detail for high and low Reynolds numbers. Given the effectiveness of flow visualization, the water tunnel is also used in testing aerodynamic structures like airplanes.
In many cases, cavitation studies are required to investigate propeller function and the possibility of wear in ship hulls. The water tunnels are sealed for such assessment and termed cavitation tunnels, where the static pressure can be altered to study the potential cavitation phenomenon.
Water Tunnel Hydrodynamic Testing Near Air-Water Interface
For an object moving near the water surface in the free-surface water tunnel, hydrodynamics near the air-water interface are of special importance. Near the interface, the performance of the submerged object as well as the wave pattern is influenced by the relative motion of both fluids. The surface wave developed can interfere with the desired experimentation conditions to produce an error in the test results. This is exacerbated by the disturbance induced by water tunnel boundaries.
To account for these disturbances, the Froude number for the model and the water tunnel should be taken into account. For the flow pattern influenced by the presence of fluids with a different specific weight near the interface, the Froude number can be expressed as:
Note that V is the velocity of the relative motion, g is the gravity, and l is the length.
When the influence of channel dimensions are taken into account during flow pattern evaluation, the Froude number is:
Note that y is the depth of flow.
Given the velocity is the same in both conditions, we can simplify the above equations to:
This relation is important in the design of a water tunnel that meets all hydrodynamic testing requirements. To ensure the accuracy of hydrodynamic testing in terms of scale and test environment, further analysis can be done in terms of dynamic similitude.
The CFD Method of Hydrodynamic Testing
For a wide range of hydrodynamic (or aerodynamic) testing, it is often difficult to calculate the associated flow variables at different sections of the water tunnel. The complexity increases even more when more than one fluid is involved, such as when testing near the air-water interface. CFD simulation is helpful in flow visualization with full-scale accuracy of the water tunnel hydrodynamic testing.
Employing CFD in a high or low speed water tunnel means at different test sections, the flow behavior, velocity profile, pressure head, lift, and drag on reduced-scale models can be accurately simulated. This is crucial in identifying the need for potential modifications not only in the design of the submerged object but also in the water tunnel. By validating the associated flow data with high-fidelity simulation, efficient design of the hydrodynamic system can be ensured.
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