Fluid Mechanics Short Flashcards
Cavitation -
Cavitation - when the pressure in liquid-flow systems drops below the vapor pressure at some locations (tip of impeller or suction sides of pumps) it results in vaporization and formation of bubbles, which are swept away from the low-pressure regions and on collapsing generates highly destructive high-pressure waves. It is a common cause for drop-in performance and even the erosion of impeller blades.
Water hammer -
Water hammer - when fluid flow encounters abrupt flow restriction (such as sudden closure of valve) it is locally compressed producing acoustic waves which resemble the sound produced when a pipe is hammered. It is very dangerous & can even damage the pipeline.
why liquid droplets take spherical shape.
surface tension - the surface of the liquid acts like a stretched elastic membrane under tension due to cohesive forces between the molecules.
Net attractive force acting on the molecule at the surface of the liquid tends to pull them towards the interior of the liquid thereby compressing them which causes the liquid to minimize its surface area. For a given volume min. surface area is that of a sphere so liquid droplets take spherical shape.
Pascal’s Law -
Pascal’s Law - the pressure applied to a confined fluid increases the pressure throughout by the same amount
Hydraulic brakes & lifts are based on this principle
Archimedes principle -
Archimedes principle - The buoyant force acting on a body immersed in a fluid is equal to the weight of the fluid displaced by the body, and it acts upward through the centroid of the displaced volume.
Stability of Immersed or Floating Body
stability of immersed body
For an immersed or floating body in static equilibrium, the weight and the buoyant force acting on the body balance each other, and such bodies are inherently stable in the vertical direction
rotational stability of an immersed body depends on the relative locations of the center of gravity G of the body and the center of buoyancy B (i.e centroid of displaced volume)
stability for floating body
metacentric height
distance between the center of gravity G and the metacenter M
metacentre - the intersection point of the lines of action of the buoyant force through the body before and after rotation
Coandă effect? what & why? Applications?
the tendency of a fluid jet to stay attached to an adjacent convex surface.
a region of low pressure around the jet and presence of adjacent curved surface results in unbalanced lateral pressure that causes the jet to deviate towards the nearby surface and to adhere to it.
like use of flaps and slats when fluid is entering through openings.
Venturi effect
The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section (or choke) of a pipe.
Marangoni effect
definitive explanation is still lacking
particles travel from an area of high surface tension to one of low surface tension
its strong enough to drive the particles against gravity and flow of falling water
experimental and computational evidence indicates that the contamination is chiefly driven by surface tension gradients, however the phenomenon is also affected by the dynamics of swirling flows that remain to be fully investigated
Magnus Effect
The phenomenon of producing lift by the rotation
of a solid body is called the Magnus effect.
Results in change in trajectory.
Drag
Types
force acting opposite to the relative motion of any object moving with respect to a surrounding fluid
Parasitic drag, Lift Induced Drag, Wave Drag
Parasitic drag -
pressure drag - Form drag or pressure drag arises because of the shape of the object
skin friction drag - Skin friction arises from the interaction between the fluid and the skin of the body, and is directly related to the wetted surface, the area of the surface of the body that is in contact with the fluid.
Interference drag - mixing of two airflow streams area like at root of wing and fuselage
Lift Induced Drag - directing of fluid for lift
Wave Drag - because of shocks
