DESCRIPTION

Practical demonstration and visualisation are essential elements of fluid flow study.

The S2 – Mobile Bed and Flow Visualisation Tanks are used in two principal fields of study. The first involves detailed investigation of mobile bed situations. These may be in relation to watercourses or civil engineering structures.

The second field involves two dimensional flow visualisation. This may be undertaken using the Ahlborn (c.1902) dust indicator technique or by any other suitable method of flow visualisation.

The tank is moulded from self coloured Oxford Blue (BS0105) glass fibre reinforced plastic with steel reinforcements to provide rigidity. It is manufactured in three sections; the inlet tank, the working section and the discharge reservoir tank. The sections are joined by flanged
connections and despatched from the factory as a complete assembly. A drop-tight adjustable overshot weir with upstream sand trap is accommodated within the discharge tank. The inlet tank features a perforated baffle plate which spreads the flow evenly across the width
of the table.

A removable glass sheet coloured blue on one side and white on the other, is provided to cover the sand bed when flow visualisation experiments are in progress. A pair of adjustable aluminium instrument rails are fitted to the top of the tank. These extend over the full length of the working section and one rail carries a positioning scale.

The depth gauge supplied is used to measure the water level and to map the contours of the sand bed produced during exercises.

It is provided with a stainless steel hook and point and incorporates a Vernier scale enabling levels to be determined accurately.

The gauge is designed for mounting on the instrument carrier assembly which can be positioned over any point of the working area. The main carrier (longitudinal traverse) is provided with a locking device and cursor to operate in conjunction with the instrument rail scale.
The sub-carriage (transverse traverse) operates on rails provided by the main carriage. There is again a transverse scale and locking device. The correct use of the apparatus is described in a comprehensive manual supplied with the tank.

Water flow is provided by a centrifugal pump made of materials which are selected for their corrosion resistance as is the pipework which includes an electrically operated flow regulating valve.
A GRP moulding carries the pump, control valve, rigid pipework and ‘in-line’ flowmeter.

The assembly is connected to the tank by flexible hoses. The motor starter and digital meter readout are mounted in a table top cabinet supplied complete with the required flexible cables.

The tank is available with optional working lengths:

Model S2-2M: working length 2m
Model S2-4M: working length 4m

EXPERIMENTAL CONTENT

Mobile bed experiments

• Two-dimensional flow patterns by the Ahlborn technique

          – Converging flow in a curved entry
          – Diverging flow from a nozzle
          – Flow around isolated bodies

               - Circular cylinder
               - Rectangular block
               - Tee shapes
               - Symmetrical aerofoil parallel to the flow
               - Symmetrical aerofoil at incidence
               - Diamond shape with and without suction

          – Arrays of cylinders
          – Free jets
          – A turbulent jet
          – A laminar jet
          – Confined zones of separated flow
          – Flow past a gate-slot
          – Elongated tee and step
          – Simulation of a hydraulic jump
          – Protection for a spillway toe

• Velocity distribution in duct flow

          – Flow in a smooth-walled pipe
          – Flow past a rough boundary

• Hydraulic analogy to compressible flow
• Three-dimensional flow patterns by the Ahlborn technique

          – Influence of normal forces on boundary layer flows

               - Decelerating flow near a corner
               - Boundary layer flow in a curved path

          – Simulations of practical three-dimensional flow patterns

              - Bridge piers and piles
              - Spurs (or groynes) and wall discontinuities
              - Tee junctions in irrigation systems
              - Meandering rivers

• Characteristics of meandering water courses
• Experimental investigation of erosion and deposition
• Boundary layer suction demonstration
• Boundary layer flows
• Experiments with loose boundaries

          – Bed movement near a circular cylinder
          – Alleviation of erosion around a cylinder
          – Bed movement near non-circular bridge piers
          – Bed movement near groynes and projections
          – Ripple formation in fine sand

• Hydraulic model studies in an Ahlborn tank

          – Operation of a gated weir
          – Cause of damage to an irrigation drop structure
          – Curved flow in a channel with a bend
          – Forces on moored ships
          – Circulation in a lock

• Unsteady flow patterns

         – Unsteady boundary layer growth of a cylinder
         – Eddy generation of an oscillating point of separation

               - Flow patterns near a separation point

         – Unsteady flows near stationary boundaries

               - Eddy shedding from a cylinder
               - Excitation of lateral standing waves in a flow channel

         – Flow-induced vibration of boundaries

               - Vibration of cylinders
               - Lateral vibration of a box

TECHNICAL SPECIFICATIONS

Working area: 2 m x 610mm or

    4 m x 610mm

Max water depth: 120mm

Thickness of sand bed: 60mm

Flow range: 0-3.6 litres/sec

Sump capacity: 300 litres

Accuracy of flow metering: ±1.5% of full scale deflection

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