3. Rotation Tank Geometry – with Sketchup but users of other CAD apps should be able to follow what’s happening.

Now to give an example of drawing the tank. With the constraint that having done steps 1 & 2, lined up the available volume of flow with the available head in the spreadsheet (most recent version 5 posted in reply #14 above), giving us the orifice diameter from the tank diameter. This is the Compact version mentioned in the last post, which we will use in this example. Now a summary of the dimensions we are designing for:

0.75m Height of vortex with turbine (Hvt)

5.88m average diameter of tank

0.88m diameter orifice

inlet having: 0.75m height of water, 0.7m wide, 0.0062 slope, Manning’s 0.015 unfinished concrete, resulting in the velocity 0.91m/s at Q of 480 l/s

Coarse inlet screen of vertical rods having 0.15m spaces

Sliding stop gate on inlet

Fail-safe counter-weighted bottom-hinged weir (flat with river-bed in flood events)

Drawing in the correct order helps give an accurate sketch. The extra radiating lines in this picture are only lines showing the axis of symmetry, not lines we are using, as we are only drawing a 2-dimensional plan for now.

Draw a circle for the orifice 0.44m radius

Calculating tank inner wall radius plus half of the inlet width 2.94 + 0.35 = 3.29

Draw the inlet width: a straight line from 3.29m towards the centre for 0.7m on the left side of the orifice if you are in the southern hemisphere or on the right if you are in the north, or you can just draw for the opposite hemisphere then flip the whole object along the axis. Optionally you can draw a 2.94m radius circle to compare with the area but this tends to get in the way later.

Using the protractor draw guide lines radiating from the centre in 15 or 7.5 degree increments depending on the accuracy you wish for (24 or 48 pie slices). The first guide line should be on top of your inlet width line. With the protractor tool in Sketchup clicking on the marks inside the protractor gives 15 degrees so it was easier to do this completely, then go back and do it again using the mid-way point between to insert extra guidelines.

Divide the width of the inlet by the number of slices you are using: 0.7 / 48 = 0.01458333

Hit m+ on the calculator to store this in the memory. Working in the direction of the flow, the next guideline after the inlet width line use the tape measure from the centre to mark a guide point at 3.29 – mR (memory recall) = 3.2754 leaving this on the screen, so for the next guide point you can minus the mR again. Keep rounding to 2 decimal places, but leave the accurate number on the calculator so that by the time you get around the tank, you should be exactly at the other end-point of the inlet width line.

Join the guide points together with the line tool.

With the tape measure a guide point 0.05m extending outwards from the inlet width line.

From this new guide-point realign the wall with the draw-line tool, to the second guide-point in the direction of the flow.

Extend this new line straight at least one metre from the original inlet width line.

Draw a parallel line to this at the distance of the inlet width.

Draw the gate section and wall thickness offset from the water’s edge.

Including the stop-gate, a distance of at least 1m, at the prescribed width & slope is needed to control the flow of volume per second, at the prescribed velocity per second. You can now delete guides and unwanted lines from the design.

Rgds, Nat