Instrumentation

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6 Component Force Balances

Force Measuring Element of Six Component Balance #1 and Vortex Generator Attachment

Three six component force balances have been developed by AMC to investigate the hydrodynamic performance of surface/underwater vehicles and their appendages. These typically may include remotely operated underwater vehicles, submarine appendages, sonar tow fish, sonar devices and active and passive control surfaces for manoeuvring or suppression of undesirable motion. These instruments enable the measurement of all six steady and unsteady components of the total hydrodynamic force/moment acting on a model. In addition they provide consoles for the mounting of models, adjustment of their incidence and a convenient means for penetration of tubing for pressure tappings and electrical connections for various sensors and devices integrated with the model. All three instruments are so called `mechanical balances' where the actions on the model are decoupled via an array of flexures and measured using six individual load cells. A fourth six component `strain gauge balance' has also been developed for internal force measurements with sting-mounted models - for which there is a separate description below. The balances may be either mounted vertically on top of the test section or horizontally through a side window. Models can be attached to the force balance either directly or via a sting mount. All the force balances have the same nominal load rating of maximum 2000N but different capabilities for particular testing. Force Balance #1 is a balance used for sting mounted models and has one manually adjustable rotational degree of freedom. Force Balance #2 is very similar to #1 except that it has automatic incidence adjustment and is especially suited to hydrofoil testing. Force Balance #3 has been designed for oscillating models (maximum 10Hz) and for where greater precision is required for drag measurements.

Propeller Dynamometers

The design of marine propellers is a complex process as generally they are located at the stern of the ship and operate in the separated and highly viscous wake flow. Propeller performance is investigated for steady and unsteady effects including those due to cavitation - and possibly the effects of hull pressure pulses and radiated noise. Steady performance is important for the ship making speed and matching the propeller with the propulsion machinery. Unsteady performance is investigated as it may lead to vibration, strength, fatigue, noise or machinery problems. While steady performance of propellers is predicted reasonably well with computational methods more complex flow problems such as unsteady flow and cavitation often need to be investigated experimentally. It is also common for the propeller to be tested with appendages such as rudders or control surfaces to account for mutual hydrodynamic interactions. A conventional right angle drive propeller dynamometer using a spiral bevel gear drive and shaft end thrust/torque transducer has already been developed by AMC. A new `silent' dynamometer is currently under development by AMC and the DSTO utilising direct electric motor drive, to eliminate gear noise, and a multi-component shaft end force transducer. The new dynamometer will also be able to be tilted and moved vertically in the test section.

Model sting mount including internal force balance and instrumentation

Models of control surfaces and other appendages are conveniently mounted on the walls of the test section via direct or force balance mounting. While models of underwater vehicles or devices that are required to be mounted in the centre of the test section are generally sting mounted - supported via a cantilevered beam from their downstream extremity. As part of a project for the investigation of flow about underwater vehicles a sting mount has been developed by the AMC and DSTO. The sting may be directly mounted to the tunnel wall downstream of the model or via a balance for force measurement. To minimise tare and interference correction of forces acting on the sting when balance mounted a shroud may also be fitted about the strut section of the sting. Whether directly mounted or connected to a force balance the sting has been designed with internal galleries for tubing and electrical connections for pressure tappings and sensors fitted to the model including an internal force balance. The internal force balance consists of a compact six component ATI Industrial Automation force transducer contained within a silicon filled canister to eliminate the effects of tunnel static pressure. The measurement side of the balance is connected to the model and the non-measurement side to the sting via brackets that permit discrete incidence adjustment of the model. The internal balance maximum load is approximately 660N.

3D Automated Traverse and 3D Fast Response Probe

3D Automatic Traverse Installed in Test Section

A range of experimental equipment and instrumentation has been developed for investigating the performance of under water bodies, propellers, waterjet propulsors, control surfaces and other appendages. This equipment typically involves the capability for force measurements and to adjust the position and orientation of the device under investigation in the tunnel test section. Often devices under test are also fitted with instrumentation for measurement of steady and unsteady velocities and pressures on or about the device. A range of probes, mutlplexers and traverse mechanisms have been developed for particular applications of flow field investigation. In many cases traverses and data acquisition are automated not only for improved productivity but also improved experimental investigation and to enable collection of data sets for comparison with computational fluid dynamics. To this end a more general capability for flow field investigation was conceived. Significant advances have been made, in this regard, with Laser instrumentation and the tunnel test section has been designed for this purpose with optical quality windows on three sides and limited optical access on the ceiling. Laser instrumentation is often completely non-intrusive with external traversing of optical equipment although probes and optics for light sheet formation can reside within the flow volume remote from the point of measurement. Plans are being made to acquire a system capable of measuring a range of flow properties including cavitation nuclei spectra. However, Laser instrumentation is specialised, expensive and there are complexities in its use. Therefore, it was decided that a complementary, relatively inexpensive and easy to use system using physical probes with flush wall mounted traversing also be developed by AMC and the DSTO. Typical applications for the system include wake velocity and turbulence surveys for propulsion studies, wake instabilities and vortex shedding and gathering of general data sets for comparison with CFD.

Typical physical probes used in the cavitation tunnel are pressure probes although hot film and other types may also be used. Although intrusive, physical probes have the advantage of continuous signals and no need for seeding of the flow with light scattering particles. The high dynamic pressure and relatively low frequency turbulent fluctuations in water compared with air also provide advantages in using pressure probes. Their ability to directly measure static pressure is also a particular advantage. On this basis it was decided that a three dimensional fast response pressure probe be developed, for use with the traverse, in addition to the existing range of one and two dimensional probes. This probe is designed to simultaneously measure three velocity and turbulence components and unsteady static pressure. To take full advantage of the probe capabilities the traverse and data acquisition system are fully automated. The traverse has been designed to replace a side window of the test section and other than the probe and its support be flush with the interior of the test section. The tunnel test section has three side windows which enables mounting at three discrete axial positions either side of the test section. Compatible model mounts enable an over lapping range of volumes about the model to be investigated. The probe consists of a four-hole head designed and manufactured by Turbulent Flow Instrumentation with stem imbedded transducer installation designed by AMC. The probe head tappings are connected via short tube lengths to the four stem imbedded high frequency pressure transducers. The short tube lengths eliminate the need for compensation of signals for tube response below approximately 1kHz.

Waterjet test loop

Over recent decades waterjet propulsion has been applied to increasingly larger and faster vessels and has proven a robust system. The success of the fast ferry industry is in part attributable to developments in waterjet propulsion. Waterjets as fitted to fast ferries consist of an inlet duct through which water is drawn into the pump. The opening of the inlet is flush with the bottom of the hull. The water having passed through the pump is then exhausted via a nozzle in the transom, thus propelling the vessel. The inlet duct must be an `S' shaped passage in order to draw water from the free-stream and deliver it parallel to the pump's axis. Flow within the inlet is complex and viscous in nature as a result of the necessarily complex geometry and ingestion of the hull boundary layer with a flush inlet. Several aspects of the flow may lead to serious problems of unsteadiness and cavitation within the inlet and non-uniformity at the pump face.

Waterjet Inlet Model Fitted to Test Section

This among other factors was the reason for incorporating a waterjet test capability in the development of the cavitation tunnel. The test loop consists of auxiliary duct work and devices in parallel with the main cavitation tunnel circuit that permit the extraction of water from the top wall of the tunnel test section through a waterjet test model and its return into the tunnel at a remote position. The waterjet models tested thus far have consisted only of an inlet (no pump or nozzle) made from acrylic to permit visualisation of flow phenomena including cavitation. Downstream of the acrylic inlet model is an instrumented pipe length to investigate flow properties at the notional waterjet pump face. A stationary dummy pump drive shaft extending approximately one inlet duct diameter downstream of the notional pump face is generally included. As the ingested hull boundary layer is a key factor in waterjet performance considerable efforts have been made to develop devices for generation of thickened boundary layers on the ceiling of the test section.

The loop has been designed to permit the testing of waterjet models up to the maximum speed and the extreme pressure ranges of the tunnel. Once the ingested flow has passed through the waterjet model it is diffused through machined ducts to conventional pipework. After diffusion the loop flow passes through two 90° bends. To attenuate secondary flows and non-uniformity resulting from the ingested flow and the 90° bends a flow conditioner is installed just downstream of the second bend. There is then seventeen diameters of vertical straight pipe length before passing through a flow meter. An electromagnetic flow meter was chosen over conventional differential pressure flow metering devices for improved precision and to avoid cavitation. Downstream of the flow meter there is sufficient pipe length that the 90° bend at the base of the vertical pipe run does not influence the flow meter. A further flow conditioner is installed in the horizontal pipe length to attenuate the effects of the bend on the pump inlet flow. The pump is located at the lowest point of the loop to avoid cavitation. An isolation valve is fitted on the pump discharge after which the flow is returned to the tunnel near the lower end of the second diffuser. The loop flow rate is controlled with a two quadrant variable speed alternating current electric motor using closed loop feedback from the electromagnetic flow meter.

Analog and digital photography

A conventional range of photographic equipment is available for still and stroboscopic photography using both analog and digital 35mm SLR cameras. Plans are currently underway for acquisition of a high speed digital camera and Laser lighting for basic cavitation studies including turbulent effects, cavitation inception and bubble dynamics.