Midterm 1 Study Terms Flashcards
what is the definition of a continuum when talking about fluids?
a simplification of disregarding the atomic nature of a substance (gas) and viewed as continuous (homogenous, matter with no holes)
how does cavitation form?
when the pressure of a system drops below vapor pressure, bubbles form then then suddenly collapse
What is the definition of microscopic energy?
energies related to molecular structure system (sum = internal energy)
What is the definition of macroscopic energy?
energies a system possess as a whole with respect to some outside reference frame (kinetic, potential)
How is a water hammer caused and what happens?
occurs when a valve/pump are closed/shut down suddenly
causes the water pressure to rise and fall rapidly (can damage pipes)
What is the speed of sound and how does it work/happen?
FOR COMPRESSIBLE FLOW!!
happens due small disturbances propagating and causing a change in thermodynamic property of the fluid. The small disturbance also causes a slight change in local P
what is the definition of viscosity and how is it caused?
a property that represents the internal resistance to a fluid to motion or the “fluidity”
caused by the cohesive forces between molecules in liquids and by the molecular collisions in gases (varies greatly with temp)
What is the definition of drag force?
The force a flowing fluid exerts on a body in the flow direction
magnitude of this force depends. in part, on viscosity
what is the definition of dynamic viscosity?
a measure of the force (internal resistance, force needed to make a fluid flow at a certain rate)
what is the definition of kinematic viscosity?
a measure of the velocity (how fast a fluid is moving when a force is applied)
what are some examples of non-newtonian fluids
bingham: behaves like solid at small shear stress, then deforms as shear stress increases
pseudoplastic: more shear applied = less viscosity
dilatant: viscosity increases with increased deformation
what are the differences between the viscosity of a liquid vs gas
liquid: molecules have more energy at higher temps, can oppose the large cohesive intermolecular forces more strongly, as result, can move more freely
gas: intermolecular forces are negligible, gas molecules at high temps move randomly at higher velocities, results in more collisions per unit volume per time and therefore in greater resistance to flow
how does surface tension work?
usually, liquids at material interface exert a force per unit length, at interface between liquid and gas or 2 immiscible fluids, forces develop at the interface which cause the surface to behave like a skin or “membrane”
what are the conditions that Bernoulli’s equation is valid for?
inviscid fluid (viscosity = 0)
incompressible fluid
steady flow (flow along streamline)
no shaft work
no heat transfer
irrotational
small vorticity (twisting motion, curling)
How do we describe the motion of a fluid particle
STREAMLINES:
traces the path of the particle
is everywhere tangent to the velocity vector
a mathematical tool to describe a velocity field
What does HGL stand for and what does it represent?
Hydraulic grade line: the line that represents the sum of the static pressure and the elevation heads
what does EGL stand for and what does it represent?
Energy grade line: the line that represents the total head of the fluid
what is dynamic head?
the differences between the heights of EGL and HGL (v2/2g)
when does EGL equal HGL?
for stationary bodies (lakes or reservoirs)
what is the constant distance between EGL and HGL?
dynamic HEAD
In an idealized bernoulli flow, how should the EGL and HGL look?
the EGL should be horizontal with its height remaining constant, HGL should vary depending on flow velocity varying along the flow
for open-channel flow, how should the EGL and HGL look?
HGL is at the liquid free surface and EGL is a dynamic head distance above the free surface
what is the pressure head at a pipe exit?
zero and thus the HGL coincides with the pipe outlet
what happens to EGL and HGL due to frictional effects (mechanical energy loss)
This causes the EGL and HGL to slope downward in the direction of flow. the slope is a measure of the head loss in the pipe. A component such as a valve, that generates significant frictional effects causes a sudden drop in both EGL and HGL in that location
what happens to EGL and HGL whenever mechanical energy is added for removed to or from the fluid (pump, turbine)?
A steep jump or drop occurs
A steep jump occurs when mechanical energy is added (pump)
A steep drop occurs when mechanical energy is removed (turbine)
What happens to the gauge pressure of a fluid at locations where the HGL intersects the fluid?
it is ZERO
the pressure in a flow section that lies above the HGL is NEGATIVE
the pressure in a section that lies below is POSITIVE
what is the definition of static pressure?
it represents the actual thermodynamic pressure of the fluid. This is the same as the pressure used in thermodynamics and property tables
what is the definition of dynamic pressure?
it represents the pressure rise when the fluid in motion is brought to a stop isentropically
what is the definition of hydrostatic pressure?
technically not pressure in a real sense since its value depends on the reference level selected; it accounts for the elevation effects ie fluid weight on pressure
what is the definition of fluid kinematics
the study of how fluids flow and how to describe fluid motion
what are the two distinct ways to describe fluid motion
lagrangian and eulerian
how does lagrangian track a particle
This method follows the path of the individual objects (particles)
requires us to track the position and velocity of each individual fluid parcel and take the parcel to be of fixed identity
how does eulerian track a particle
MORE COMMON METHOD
- a finite volume called a flow domain or control volume is defined through which fluid flows in and out
- instead of tracking individual fluid particles, we define field variables, functions of space and time, within the control volume
what is an application of fluid kinematics
FACKIN CFDDDD (computational fluid dynamic) model to study scour around bridge piers
what is the definition of local acceleration in relation to material acceleration
acceleration due to variation in velocity through time at a given location in space
describes the unsteadiness of flow
what is the definition of convective acceleration in relation to material acceleration
acceleration due to change in velocity as the particle flows from one point in space to another (changes in pipe diameter)
what is flow visualization
visual observations and examination of flow field features
useful not only in physical experiments but in numerical solutions as well (CFD)
how can streamlines help with flow visualization
useful as indicators of the instantaneous direction of fluid motion throughout the flow field
obtained analytically by integrating equations that define lines tangent to the velocity field
