FM-1849-11 Free And Forced Vortex Apparatus


  • Experiment to plot the shape of a free vortex by measurement of surface profile co-ordinates, and thus verify that vr = constant where v is the speed and r is the radius of the vortex.
  • Experiment to plot the surface profiles of various forced vortices formed under different speed conditions.
  • Verification of the formula for forced vortices where h is the height of the surface of the water above the datum point, w is the vortex angular velocity and r is vortex radius


The Free and Forced Vortex apparatus has been designed for students  to experiments to produce and measure free and forced vortex. It  consists of a clear acrylic cylinder where the free vortex is generated by  water discharging through an interchangeable orifice in the base of the  cylinder. The resulting profile is then measured using a combined sliding  and depth scale. The forced vortex is induced by a paddle rotated by jets  of water at the cylinder base.

The profile of the forced vortex is then determined using profile  measuring gauge. Velocity at any point in the free or forced vortex may  be measured using the pitot tube supplied.  The apparatus is designed to be positioned on the hydraulics bench. The  apparatus consists of a cylindrical vessel having two pairs of  diametrically opposed inlet tubes. Overflow cut-outs ensure a constant  level in the tank during experiments. A smooth outlet is centrally  positioned in the base of the vessel, and a set of push-in orifices of various diameters are supplied.  The 12mm diameter inlet tube, which are angled at 15 degree, impart a swirling motion to the liquid entering the vessel,  and are used as entry tubes for the free vortex experiment.

The forced vortex is created by using the 6mm inlet tubes which are angled at tangent to the diameter. The input from  these tubes impinges on a paddle which acts as a stirrer/flow straightener. The paddle rotates on a stud mounted on a  bushed plug inserted in the central orifice.

A bridge piece incorporating profile measuring gauge is used to determine the profile of the forced vortex. The needle can  be travel along X and Y Axis on a pre calibrated scale so a virtually infinite position of the vortex can be drawn rather than  the fixed point measurements.

Velocity heads may be visualized by the insertion of pitot tube in the vortex at any point.