Fluid Mechanics
MEIT2422 2024 RMIT
36 flashcards · Physics · by Rachaya Youngrod
Define Internal, External, and Open-Channel Flow
Internal: Any fluid flowing through a defined space External: Any fluid flowing through a surface of an object Open Channel: A waterway, canal, or conduit in which a liquid flows over a free surface
Define incompressible flow and incompressible fluid. Must the flow of a compressible fluid necessary be treated as compressible?
Incompressible flow: Fluid flow where density remains relatively similar throughout despite changes in pressure or temperature. Incompressible fluid: Fluid where density remains relatively similar throughout despite changes in pressure or temperature. Flow of compressible fluids can be incompressible when changes resulting from temperature and pressure changes are negligible. The simplification is valid if atm is under 5% or Mach number is below 0.3
Consider the flow of air over the wings of an aircraft. Is this flow internal or external? How about the flow of gases through a jet engine?
External flow, as fluid is flowing over the surface of the wings without restrictions. Depending on the amount of gas released, the flow can range from open channel to internal flow as the jet engine is a defined space
What is forced flow? How does it differ from natural flow? Is flow caused by winds forced or natural flow?
Forced flow: fluid forced to flow by external means Natural flows: flows occuring through natural means (E.g. buoyancy effect) Flows caused by wind flowed as the fluid has force exerted externally, and not naturally occurring force within the fluid
What is a no slip condition? What causes it?
Due to viscosity, fluid in direct contact with a surface "sticks" to the surface and doesn't slip
What is a boundary layer and what causes it to develop?
Boundary layer is the flow region where the viscous effects (and thus, velocity gradient) is significant.
What is Vapour Pressure (Pu) and how is it related to Saturation Pressure (Psat)
Vapour Pressure (Pu): is pressure exerted by the vapour at phase equilibrium at a given temperature. Saturation Pressure (Psat): is the pressure at which the fluid changes phase at a given temperature.
What is Water Hammer?
Phenomenon where there is a sharp rise and fall in pressure as a flow abruptly stops. This results in a hammering on the pipe, which causes damage.
Define Laminar, Turbulent and Transitional flow
Laminar: highly ordered fluid motion characterised by smooth layers of fluid Turbulent: highly disordered fluid motion typically occurring at high velocities and characterised by velocity fluctuations Transitional: A flow that alternated between laminar and turbulent
Does water boil at higher temperatures? Explain
High pressure means that there is higher mass exerted on the surface of the boiling water. The particles at the surface of the water would require more kinetic energy (therefore higher temperatures) to lift off the surface and vapourise in the boiling process
If the pressure of a substance is increased during a boiling process, will the temperature also increase, or will it remain constant? Why?
Increased pressure makes it harder for molecules at the surface to rise away from the surface, therefore requiring more kinetic energy, causing increase in temperature
What is Cavitation? What causes it?
Cavitation is when liquid pressure drops below vapour pressure, causing rapid acceleration and deceleration of the particles which creates steam bubbles that collapse in on itself. The collapse creates highly destructive pressure shockwaves.
What is a Newtonian fluid? Is water a Newtonian fluid?
Fluids in which shear stress is linearly related to shear strain rate. Water abides by the definition and is a Newtonian fluid.
What are dynamic (absolute) and kinetic viscosity?
Absolute viscosity - coefficient of absolute viscosity - is a measure of internal resistance. Dynamic (absolute) viscosity is the tangential force per unit area required to move one horizontal plane with respect to an other plane - at an unit velocity - when maintaining an unit distance apart in the fluid. Kinematic viscosity is the ratio of - absolute (or dynamic) viscosity to density - a quantity in which no force is involved. Kinematic viscosity can be obtained by dividing the absolute visco…
What is viscosity? What is the cause of it in liquids and in gases? Do liquids or gases have higher dynamic (absolute) viscosities?
Kinematic viscosity is the dynamic viscosity of a fluid divided by its density. It describes the fluid's resistance to flow in relation to its mass. For liquids, kinematic viscosity typically decreases with increasing temperature. As a liquid is heated, the molecular motion increases, and the fluid becomes less viscous. For gases, kinematic viscosity increases with temperature. As gas molecules gain kinetic energy with higher temperatures, they move more rapidly, increasing their collision freq…
How does the kinematic viscosity of (a) liquids and (b) gases vary with temperature?
Kinematic viscosity is the dynamic viscosity of a fluid divided by its density. It describes the fluid's resistance to flow in relation to its mass. For liquids, kinematic viscosity typically decreases with increasing temperature. As a liquid is heated, the molecular motion increases, and the fluid becomes less viscous. For gases, kinematic viscosity increases with temperature. As gas molecules gain kinetic energy with higher temperatures, they move more rapidly, increasing their collision freq…
Consider two identical small glass balls dropped into two identical containers, one filled with water and the other with oil. Which ball will reach the bottom of the container first? Why?
The glass ball in oil will reach the bottom of the container first. This is because the ball in oil experiences less viscous resistance as it descends. Oil has a higher dynamic viscosity than water, and as a result, it exerts more resistance on the ball's movement. Therefore, the ball in water will experience greater drag due to the higher viscosity, slowing its descent compared to the ball in oil.
What is surface tension? What is its cause? Why is the surface tension also called surface energy?
Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Due to the cohesive forces a molecule is pulled equally in every direction by neighbouring liquid molecules, resulting in a net force of zero. The molecules at the surface do not have the same molecules on all sides of them and thereforterm-15e are pulled inward. This creates some internal pressure and forces liquid surfaces to contract to the minimum area. Surface energy is the work don…
What is the capillary effect? What is its cause? How is it affected by the contact angle?
A phenomenon where a liquid flows against gravity in narrow spaces due to adhesive and cohesive forces. It is affected by the contact angle, which represents the angle between a liquid/vapor interface and a solid surface. The smaller the contact angle, the stronger the capillary action
A small-diameter tube is inserted into a liquid whose contact angle is 110°. Will the level of liquid in the tube be higher or lower than the level of the rest of the liquid? Explain.
Lower than the level outside. This is because a contact angle greater than 90° triggers a decrease in the liquid level inside the tube.
Consider a soap bubble. Is the pressure inside the bubble higher or lower than the pressure outside?
Higher pressures inside due to the net force inwards, which cause the molecules inside the bubbles to be closer and collide more frequently.
Which statement is most correct? a) Flow across an aircraft wing is one-dimensional but only far away from the wing tips b) Flow across an aircraft wing is two-dimensional but only far away from the wing tips c) Flow across an aircraft wing is three-dimensional but only far away from the wing tips d) Laminar pipe flow is three-dimensional e) None of the above
b) Flow across an aircraft wing is two-dimensional but only far away from the wing tips The flow pattern changes in (x,y) axis as we move forward, with changes in the z axis as well on the wing tip (wingtip vortex)
Fluid at the surface of a flat plate placed parallel to a moving flow does not move, yet the plate experiences drag: a) This is incorrect - there can be no drag, since fluid must move at the surface to cause the drag b) The viscosity of the fluid slipping at the surface causes the drag c) The effect of viscosity in the flow far removed from the surface causes the drag d) Pressure exerted normal to the surface causes the drag e) None of the above is correct
e) None of the above is correct
All fluids are: a) Compressible hence we need to take this into account in a fluid mechanic calculations b) Compressible but only above the speed of sound c) Incompressible but only below the speed of sound d) Compressible but for engineering calculations we can generally ignore it for speeds less than 100 m/s in water e) None of the above
e) None of the above While fluids can be compressible, not all fluids behave the same way, and some, like water under typical conditions, are effectively considered incompressible for many engineering calculations.
Define the resultant hydrostatic force acting on a submerged surface, and the center of pressure.
Resultant hydrostatic force acting on a submerged surface: Total pressure force exerted by fluid in which the surface is submerged. When a body is submerged in a fluid and due to variation of pressure with depth and weight of the fluid, body experiences normal pressure which results into resultant hydrostatic force. Center of pressure: Point of application of resultant hydrostatic force
You may have noticed that dams are much thicker at the bottom. Explain why dams are built that way.
Someone claims that she can determine the magnitude of the hydrostatic force acting on a plane surface submerged in water regardless of its shape and orientation if she knows the vertical distance of the centroid of the surface from the free surface and the area of the surface. Is this a valid claim? Explain.
Yes it's a valid claim. The magnitude of hydrostatic force can be determined using equations F=Pavg∗AF =P_avg *AF=Pavg∗A Pavg=ρghP_avg = ρghPavg=ρgh h = vertical distance of the centroid of the surface from the free surface A = area of surface g = 9.8ms^-2 As these values are known, maagnitude of hydrostatic force can be found.
A submerged horizontal flat plate is suspended in water by a string attached at the centroid of its upper surface. Now the plate is rotated 45° about an axis that passes through its centroid. Discuss the change in the hydrostatic force acting on the top surface of this plate as a result of this rotation. Assume the plate remains submerged at all times.
When the plate is rotated by 45° about an axis passing through its centroid, the depth of the centroid (h) and the area of the submerged surface (A) in contact with the water do not change. The plate remains submerged at all times, so the fluid density (ρ) and the acceleration due to gravity (g) also remain constant. Therefore, the hydrostatic force acting on the top surface of the plate does not change as a result of this rotation. It remains the same before and after the rotation.
Consider a submerged curved surface. Explain how you would determine the horizontal and vertical component of the hydrostatic force acting on this surface.
Identify Geometry: Understand the shape and dimensions of the submerged curved surface. Determine Pressure Distribution: Hydrostatic pressure varies with depth. Integrate the pressure distribution over the curved surface to find the total force. Resolve Forces: Break down the hydrostatic force into horizontal and vertical components. The horizontal component is found by considering the angle of the curved surface relative to the horizontal. Vice versa Apply Formulas: Use principles from fluid m…
What is buoyant force? What causes it? What is the magnitude of the buoyant force acting on a submerged body whose volume is V? What are the direction and the line of action of the buoyant force?
The buoyant force is caused by the increase of pressure in a fluid with depth. The magnitude of the buoyant force acting on a submerged body whose volume is V is expressed as FB=gρVF_B=gρVFB=gρV . The direction of the buoyant force is upwards, and its line of action passes through the centroid of the displaced volume.
Discuss the stability of (a) a submerged and (b) a floating body whose center of gravity is above the center of buoyancy.
(a) A submerged body whose center of gravity is above the center of buoyancy is unstable since it does not return to its original position after a disturbance. (b) While a floating body whose center of gravity is above the center of buoyancy is stable since the body produces a restoring moment to return the body to its original stable position.
Consider two 5-cm-diameter spherical balls—one made of aluminum, the other of iron—submerged in water. Will the buoyant forces acting on these two balls be the same or different? Explain.