Fluid Dynamics: Ocean Circulation, Glacier Ice and Lava Flow

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1 GOCE Gravity field & steady state Ocean Circulation Explorer Fluid Dynamics: Ocean Circulation, Glacier Ice and Lava Flow Background Information: Many oceanographers are interested in how much the height of the sea surface changes over time. To do this, they measure the sea surface height relative to an imaginary surface called the geoid. By improving our knowledge of the geoid, the GOCE satellite will help predict ocean circulation. However, there are many other factors affecting ocean circulation, such as the Coriolis Force and convection. The effect of pressure on the melting point of ice and the Magnus, Coanda and Bernoulli Effects also play an important part in governing the flow of water and air. By improving our knowledge of gravity, GOCE will also help predict the movement of lava, glaciers, mud slides and snow avalanches. All of these are said to be non-newtonian in that their viscosity very much depends on the forces applied to them. Learning Objectives: Improved practical skills. Appreciate that it is difficult to make definitive predictions about the oceans and atmosphere, since there are many interrelated factors governing their behaviour. Curriculum Links: Edexcel GCSE in Physics (2109) P : Explain common electrostatic phenomena in terms of the movement of electrons, for example, shocks from car doors, charges on synthetic fibres, dust on television screens and lightning. P : Demonstrate understanding that like charges repel and unlike charges attract P : Explain how insulating and insulated materials can be charged by contact by the transfer of electrons P : Describe some of the uses of electrostatic charges. The Twenty First Century Science suite GCSE Physics A (J635) Explain simple electrostatic effects in terms of attraction and repulsion between charges. AQA Physics 2009 (4451) 10.3 Observation as a stimulus to investigation - If the theories and models we have available to us do not completely match our data or observations, we need to check the validity of our observations or data, or amend the theories or models What is static electricity, how can it be used and what is the connection between static electricity and electric currents. When certain insulating materials are rubbed against each other they become electrically charged. Negatively charged electrons are rubbed off one material onto the other. The material that gains electrons becomes negatively charged. The material that loses electrons is left with an equal positive charge. When two electrically charged bodies are brought together they exert a force on each other. Two bodies that carry the same type of charge repel. Two bodies that carry different types of charge attract.

2 Suggested activities: Explain how GOCE will map the Earth s gravity field, using the balloon experiment to revise electrostatics. Pupils could then carry out some of the other experiments, which look at fluid dynamics. The effects could then be discussed and summarised by watching the video links in the references section. How GOCE s Accelerometers Operate: Cut a strip out of a plastic bag about 1cm wide and 5cm long. Make a series of cuts in the bag as shown, forming a tassel. Rub the balloon and the plastic with your hair. Both the balloon and the plastic bag gain the same type of charge, so they repel one another. This force balances the tassle s allowing it to levitate. Imagine having 6 metal cubes in a box in orbit round the earth. Since they re all in slightly different places, they ll all be in slightly different bits of the earth s gravitational field. To find out how the gravity field changes from one place to another, you would ideally track their movement and look at their relative motion. However, you can t do this as they would rattle around and bump into things. So you hold them still using electrostatic forces to float them in the centre of a small cage. Since they are to remain stationary relative to the satellite, the electrostatic force acting on them must balance their weight. Therefore the size of the voltage which has to be applied between the cage and the cubes to keep them still indicates the strength of the gravitational field.

3 The Bernoulli Effect: 1. Have you ever noticed a toy in the bathtub was moves towards the stream of water and stays in the stream, instead of being pushed away? What you are observing is the Bernoulli Principle. Daniel Bernoulli lived from 1700 to He found that the faster a fluid moves, the less pressure it exerts. Blow hard across the sheet of paper. Describe and explain you observations. 2. Place an egg into the pan in the sink. Turn on the tap. Describe and explain you observations. T 3. Put one of the empty cups inside the other. Hold them quite close to your mouth and blow between the rims of the cups. Describe and explain your observation Gentle blow: Hard blow: 4. Blow between the two ping pong balls. Describe what happens. 5. Turn on the blower. Place the beach ball above it. What happens when you gently change the angle of the blower?

4 Ocean Circulation- A Combination of Convection and The Coriolis Effect There is a clockwise rotation around hills in the northern hemisphere and valleys in the Southern Hemisphere. Conversely, there is an anti-clockwise rotation around valleys in the northern hemisphere and hills in the southern hemisphere. This is known as the Coriolis Effect and it is due to the earth s rotation. Hot air/water is less dense than cold air/water and so it rises. Cold air/water has a higher density than hot air/water and so it sinks. This process is called convection. The circulation of water in the oceans is mainly a combination of convection and the Coriolis Effect. Convection currents look very different in a rotating system Place one of the beakers of water on a rotating turntable. Place a potassium permanganate ice cube on top of the water column in each beaker. Compare the patterns of purple dye. Skating on Thin Ice The Effect of Pressure on Melting Point of Ice This experiment shows that a wire can cut through an ice cube without breaking it into two pieces. Place an ice cube in a retort stand and place a loop of nichrome wire on top. Attach weights to the wire. The pressure of the wire reduces the melting point of ice and allows it to melt. The wire then sinks easily through the melted ice, while the ice above the wire refreezes as it is no longer subjected to pressure. This explains the purpose of ice skates. The pressure they exert on ice causes a layer of water to form under the blades, creating a slippery surface for sliding. It is also the reason why the snow on heavily travelled roads quickly turns to slush. For most substances, increasing the pressure when a system is in equilibrium between liquid and solid phases will increase the phase transition temperature. Water is one of a few special substances for which the pressure lowers the temperature of transition. The basic reason is that water actually expands when it goes from the liquid to solid phase. The melting point of ice falls by C for each additional atmosphere of pressure applied. For example, a pressure of 500 atmospheres causes ice to melt at 4 C. A glacier can exert a sufficient amount of pressure on its lower surface to lower the melting point of its ice. This means liquid water can flow out from the base of a glacier.

5 Non-Newtonian Fluids: You have already seen that applying pressure lowers the freezing/melting point of water and so it can lead to water turning from its solid state into a liquid. Some fluids have a very different behaviour from water. You may have seen the episode of Brainiac where Jon Tickle walks on custard. Custard is a non-newtonian fluid, which means it becomes a solid upon impact and it turns back to a liquid when the pressure is removed. Corn starch is another example of this type of fluid. Place the corn starch on the loudspeakers and increase the amplitude. Observe the effect on the cornstarch every time the cone moves forward and back. A Newtonian fluid s behaviour can be described exclusively by temperature and pressure, not the forces acting on it from second to second. Whereas, a non-newtonian fluid s viscosity (thickness) depends on the force applied to it. Aside from air and water, very few fluids are actually Newtonian. The flow of lava, glacier ice, mud slides and snow avalanches all exhibit non- Newtonian behaviour. The Coanda Effect: Turn the tap on and gently blow through the straw towards the teaspoon. Describe what happens. Light a candle and place it on a table. Place a salt container in front of the candle. Blow against the other side of the container (the side opposite from the lit candle). Be sure to keep your mouth even with the flame of the candle. Describe what happens. Both of these experiments are examples of the Coanda Effect. This describes the tendency of a fluid to follow the curved surface of a wall. The water followed the contour of the spoon. In the candle experiment, the air streams followed the contour of the container, met on the other side and combined to blow out the candle. The upper surface of an airplane wing deflects air downwards because the airflow 'sticks' to the wing s curved surface due to the Coanda Effect. As the air is pushed down, it pushes the wing up, generating lift.

6 The Magnus Effect- Bend It Like Beckham With A Flying Toilet Roll Use cellotape to stick one end of a 2 metre length of elastic on to the middle of the lab bench. Stick the other end of the elastic on to a toilet roll tube. Wind two turns around the tube while the elastic is quite loose. Place the tube down and pull it towards the edge of the bench so that the elastic becomes taut. Wind another three turns round the tube. Let go of the tube. The tube should lift up as it moves through the air. From the Coanda Effect, you know that the air going past a moving tube will tend to stick to it. If the tube did not spin, the air would stick to it symmetrically. However, because the tube is spinning, the air sticks to the only the top of the tube since that part is moving with the air. The bottom of the tube is moving against the air, so the air does not stick to it. This means that the air passing over the tube is pushed down. Newton s third law states that. This means the tube is pushed. When David Beckham causes a football to bend by giving the ball spin. The Magnus Effect causes it to experience lift. The direction of this lift depends on which way he spins the ball. Tennis players like Andy Murray also use spin to make sure the ball goes over the net. The Magnus Effect With Polystyrene Cups Use cellotape to join together the bottom of two polystyrene cups. Knot two rubber bands together. Wrap the rubber bands around the join in the cups about twice. Hold the cups in one hand and the end of the bottom of the elastic in your other hand. Pull back the cups and let go. The cups should follow a loop as they fly through the air.

7 References/Resources: Coriolis Effect: Video clips showing the Non Newtonian fluid moving: How to set up the loudspeakers: A recipe for cornstarch is available from: The Bernoulli Effect: The Aeorodynamics of a Ping Pong Ball Coanda Effect: A description of how to demonstrate the Coanda Effect with a spoon and water: / Answers The Bernoulli Effect The sheet flies up because as you blow across the top of the sheet, you lower the air pressure. This means the pressure on the underside of the sheet is higher than on the top side, pushing the sheet up. This is the same reason airplanes can fly. The egg moves towards the stream. The moving stream of water is pushing less on the side of the egg. The still or slow water on the other side is pushing more, so the egg is continually pushed into the stream of water. If you blow softly the inner cup rises up slowly. If you blow hard the top cup launches itself across the room. Moving air has a lower pressure than still air, so, as you blow, you lower the air pressure between the rims. As the still air in the bottom of the cup is at a higher pressure, it forces the top cup up and out. The Coanda Effect The water follows the contours of the spoon; The candle flame goes out. The Magnus Effect- Bend It Like Beckham With A Flying Toilet Roll For every action there is an equal and opposite reaction; Up How GOCE s Electrostatic Gradiometer Operates electrostatic/repulsive; weight