Ekman Pumping & Suction: How to

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We take our clear 16”x16” square container with its circular insert and place it dead center on a white board on the turntable. To create cyclonic and anticyclonic circulation, we will gently blow air over the surface of the water using fans directed tangentially to the circular insert, as shown in Fig.1. The fans (cooling fans used in PC’s work well) are driven by the 12V DC power supply on the turntable.  More information on the fans shown in Fig.1 is available here.

Before attaching the fans, decide whether you want to create a cyclone (fans circulate air in direction of rotation) or an anticyclone (fans circulate air opposite to the sense of rotation). The fans should then be positioned perpendicular to the wall of the tank at the point where the circular insert touches the square container, as shown in Fig.1. Clamp the long screw protruding from the back of the fan to the outer wall (the green clamps in Fig.1 and also clearly in evidence in Fig.4). The fan must be held tightly and arranged to blow horizontally. Next, place the other fan on the opposite wall and direct it to blow in the reverse direction of the first fan. A view from the top is given in Fig.2.

It is also useful to clamp a white sheet vertically (see red clamp in Fig.1) to aid visualization from the side.

Fig.1 Two fans in position (attached via green clamps) to create (in this configuration) an anticyclone for cyclonic turntable rotation. The red clamp secures the vertical white sheet to aid visualization from the side.

Fig.2. A view from the top showing the arrangement of fans used to create an anticyclone.

Now fill the container until the level is approximately 1⁄2 cm below the fans. Plug both fans in to the DC power supply of the turntable making sure that the sense of polarity is correct. The spin rate of the fans should be turned down (using the potentiometer knob) toward the ‘low end’ so as not to stir up too many waves and ripples on the surface. Ascertain that all the wires and connections are completely on the turntable and not dangling down! then spin up the table to a rotation rate of order 10rpm. Wait for 15 to 20 minutes until the circulation has been established.

The surface flow can be visualized using black paper dots sprinkled on the surface. The flow in the interior and at the bottom can be visualized using potassium permanganate crystals, as illustrated in Fig.3. The pattern in the figure corresponds to a cyclonic circulation and was obtained by dropping in crystals in two positions equidistant from the axis of rotation.

Fig.3. Circulation in a cyclonic vortex. Bottom currents are revealed by potassium permanganate crystals: surface currents by paper dots. Note that the bottom flow is convergent and spirals in to the center. The surface flow, by contrast, is divergent and spirals out to the periphery as revealed by the black paper dots evident floating at the surface on the outer edge.

A side view is shown in Fig.4 and vividly demonstrates Ekman suction.

Fig.4. A view from the side showing the vertical structure of a cyclonic circulation created by the action of winds supplied by the two fans acting at the surface. Dyed fluid can be seen collecting in the center and rising through the water column, clear evidence of Ekman suction.

Note that it is also instructive to turn the fans on after solid body rotation has been established. The order does not matter since the primary focus of attention is the quasi-steady state. However, for demonstration purposes it is fun to illustrate solid body rotation first and then ask the audience what they expect to see happen when the fans are turned on.