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Fluid Mechanics > Applied Fluid Mechanics > Flashcards

Applied Fluid Mechanics Flashcards

0
Q

Types of measurement quantities

A

Local, global

Direct, indirect

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1
Q

Purpose of measurements

A

Validation: were the correct equations solved ?
Verification: were the equations solved correctly?

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2
Q

3 steps of DoE

A

Isolate the phenomenon
Choose the right tool for the problem
Design the experiment backwards

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3
Q

St

A

Strouhal number
Oscillating velocity/mean velocity
For oscillating flows

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4
Q

Eu

A

Euler number
Pressure forces/inertial forces
For pumps, cavitation

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5
Q

Re

A

Reynolds number
Inertial forces/viscous forces
Almost always used

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6
Q

Fr

A

Froude number
Kinetic energy/potential energy
For free surface flows, where gravity is important.

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7
Q

Ma

A

Mach number
Flow velocity/sonic velocity
For compressible flows (Ma>0.2)

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8
Q

Similar solution

A

When two flows with same BC also have same Re, St, Ma and Fr.

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9
Q

Buckingham Pi-Theory

A

Pi=n-r
Pi: nb of dimensionless quantities
n: nb of influencing quantities
r: nb of basic quantities (MLT)

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10
Q

Requirements on wind tunnels

A

Reproduction of the problem’s flow
Defined conditions (perfect, worst case)
Transferability

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11
Q

Eiffel type: class, parts

A

Open-circuit

Inlet, settling chamber, test section, fan, diffusor

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12
Q

Blower tunnel

A

Fan at entrance-> no diffuser/test section necessary

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13
Q

Göttinger type

A

Closed loop
Corner vanes
Heat exchanger
Wide angle diffuser

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14
Q

Ma dependences in supersonic tunnels

A

Only on ratio of cross-section at nozzle exit and throat. Not on power input as long as it is enough to produce sonic speed at the throat.

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15
Q

Supersonic tunnels: characteristics, types

A

Convergent-divergent nozzle upstream of test section, diffuser with second throat (breakdown shock)
Blow-down, suck-down

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16
Q

Pressure tunnels: types

A

Low density

High-speed

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17
Q

Turbulence dissipates

A

turbulent kinetic energy into heat

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18
Q

Turbulence energy transfer cascade process

A

Energy transfer from large to small scales through deformation work on vortices, induced by strain rates.
Rapid increase of vorticity component in stretching direction, slow decrease in compression direction.

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19
Q

Diffusivity in turbulence

A

Increased rates of momentum, heat and mass transfer. Increase of exchange surface.

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20
Q

Turbulent Reynolds number

A

uL/nu

Molecular diffusion time scale/ turbulence time scale

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21
Q

Meaning of RMS

A

Standard deviation

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22
Q

Turbulence level

A

Tu,i = u_i,rms/u_1,av

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23
Q

Nyquist criteria

A

f_sampling>2f_max

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24
Q

Rough estimate of L_t

A

Time for one large eddy to pass. Half the channel height is a good estimate for the maximum size if large eddies, over velocity.

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25
Q

L_t

A

Integral time scale

Rough measure of the interval over which the velocity u(t) is correlated with itself.

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26
Q

Skewness

A

Describes symmetry of PDF
=0 for Gauss
av(a^3)/arms^3

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27
Q

Flatness/Kurtosis

A

Describes width of PDF
=3 for Gauss
av(a^4)/a_rms^4

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28
Q

Method for calculating autocorrelation R

A

Align graphically the series u’(t) with u’(t+tau), multiply them vertically and compute the average product.

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29
Q

Normalized function condition

A

<=1

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30
Q

Autocorrelation of stationary process: characteristic

A

Symmetry around tau=0 where r=1

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31
Q

Energy spectrum: operation

A

Fourier transform of autocorrelation function.

Energy spectrum and autocorrelation function are a Fourier transformation pair.

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32
Q

Integral of all energy spectrum is

A

Turbulent kinetic energy and R(tau=0)

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33
Q

Energy spectrum of spatial autocorrelation is function of

A

Wavenumber vector Kj

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34
Q

Total turbulent shear stress

A

Molecular momentum transport (viscosity and grad) + turbulent momentum transport (fluctuations)

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35
Q

Velocity profile close to the wall

A

Depends only on local relevant parameters, not on free stream velocity of channel height/boundary layer thickness.

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36
Q

Wall shear velocity

A

u_tau=sqrt(tau_W/rho)

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37
Q

Viscous units

A

u+=avg(u_1)/u_tau

y+=y.u_tau/nu

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38
Q

Universal law of the wall

A

Log region

u+=1/K ln y+ + C

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39
Q

Layers of turbulent BL

A

Viscous sublayer: y+>nu_t u+=y+

Buffer region: 550 nu«nu_t u+=1/K.ln(y+) + C

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40
Q

Definition static pressure, use in fluid mechanics

A

Force per unit area imposed by the flow onto a boundary parallel to the streamline.
Distribution around an object defines its flow resistance or buoyancy. Determines the velocity or volume flow based on Bernouilli.

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41
Q

Variation of pressure in wall-normal direction

A

None (boundary layer theory according to Prandtl)

p at wall = p in free stream

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42
Q

Reference form of wall tappings

A

Perfect straight drill

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43
Q

Dependencies of p error in wall tappings

A

Diameter, length, burr height, velocity utau

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44
Q

Method for calculating Delta p

A

Calculate all dimensionless parameter in Pi equation

Read Pi on diagram of relevant tapping

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45
Q

Static pressure measurements access

A

Wall tappings
Static pressure tube
PSP

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46
Q

Problem with static pressure tubes

A

Blockage effect from shaft and head

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47
Q

Assumption with pitot tubes

A

Hydrostatic pressure can be neglected

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48
Q

Dependency of ptot in pitot tubes

A 
Flow angularity
Diameter ratio (.6)
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49
Q

Conrad sensor

A

2 slanted pitot tubes
+: no angle dependency
-: linear angle dependency

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50
Q

Condition for velocity measurement with pitot tube, solutions

A

Static pressure is known

Wall taping, prandtl tube.

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51
Q

Pressure transducers

A

Liquid manometers
Spring manometers
Multi-point pressure measurements
Wall microphones

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52
Q

Types of liquid manometers

A

Cistern (diff. areas)
Bets
Inclined tube

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53
Q

Types of spring manometers

A

Plate, membrane, spring
Bourdon (circular tube)
Electromechanical (strain gauge, induction)

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54
Q

Multi-point pressure measurement

A

Scani valve, electronic scanning system

Stationary: tapping connected with tube. Instationary: wall microphone.

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55
Q

Wall microphones: type, parameter

A 
Capacitor microphone
Dead volume (volume between measurement point and membrane, needs to be reduced for better frequency response, equal for arrays
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56
Q

Max frequency for pressure sensors

A

1000Hz

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57
Q

Tool for resistance measurement for hot wires

A

Wheatstone bridge

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58
Q

Wheatstone bridge in balance when

A

U_B=0

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59
Q

Types of anemometer for hot wires

A

Constant Current Anemometer -> small quick fluctuations, limited in frequency by thermal inertia.
Constant Temperature Anemometer -> standard, no thermal inertia influence

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60
Q

Overheat ratio

A

Rw/R3=1+alpha(T-Ta)

Higher ratio improves response but decreases lifetime

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61
Q

Influences on quality of hot wire measurements

A

Overheat ratio
Geometry, sleeves
Anemometer quality
Calibration: velocity, dynamic calibration

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62
Q

Hot wire in turbulence

A

Reynolds decomposition of voltage, calculation of turbulence level without calibration.

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63
Q

Hot wire measurement of very low turbulent intensities

A

Parallel hot wire probe. Cross correlation technique. Tu= .1-.2%

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64
Q

Hot film

A

Metallization of thin nickel layer on quartz substrate

65
Q

Advantages/disadvantages of HWA

A

Unbeatable frequency, excellent signal to noise ratio, analog output
Intrusive, not sense-specific, calibration, sensitive to T, pointwise, sensitive to particles

66
Q

% skin friction on commercial aircraft

A

50% of the flow resistance

67
Q

Skin friction due to

A

wall shear stress

68
Q

Viscous sublayer

A

y+<=5

69
Q

Property of turbulent flows

A

Asymmetrical PDF of wall shear stress

70
Q

Wall shear stress measurement devices

A

Wall shear stress balances
Hot films, hot wires (surface, pulse, wall)
Pressure sensor (Preston, surface fence, Stanton, surface)
Optical (oil film, LDA, micro pillar)

71
Q

Measurement of direction of tau_W

A

V-shaped hot film (2 sensors)

Hot film oriented in stream wise direction

72
Q

Preston sensor

A

Pitot tube on wall

73
Q

Diameter ratio for pitot tubes

A

d/D= .6

74
Q

Advantage of surface fences

A

Relatively insensitive to varying boundary conditions -> can be used in more complex flows

75
Q

Oil film laser interferometry

A

Tau_W from gradient dy/dx

76
Q

Flow rate measurements

A

Turbine-/impeller-type meters

Differential pressure measurement, Venturi nozzle and tube

77
Q

Pressure losses in differential pressure measurements

A

prop to square of volume flux

78
Q

Viscosity effects in differential pressure measurements

A

Higher power requirements
Recirculation bubbles
Narrower cross section (mu=A2/A0, m=A1/A0)

79
Q

Flow coefficient

A

Combines all correction factor in estimation of volume flux in differential pressure measurement.

80
Q

Venturi nozzle

A

Separation prevented with curved entrance section

81
Q

Most significant deviation of flow coefficient

A

Swirl components -> use of flow straighteners

82
Q

Design parameter of flow straightener

A

L>2D

83
Q

Purpose of visualization techniques

A

Qualitative analysis
Preliminary information
Quantitative information in some cases

84
Q

Types of visualization techniques

A

Surface
Density based
Tracers

85
Q

Surface visualization techniques

A

Wool threads
Oil film
PSP, TSP

86
Q

PSP principle

A

Oxygen quenching. p/,pO/,I\

87
Q

TSP principle

A

Thermal quenching, T/, I\

88
Q

Identification of flow structures with oil film visualization

A

Distribution on surface or drying speed

89
Q

Important by oil film visualization technique

A

Oil viscosity, must remain on surface but be affected by local flow structures

90
Q

Density based visualization techniques

A

Shadowgraphy
Schlieren
Mach-Zehnder Interferometer
Moiré or Ronchi interferometer

91
Q

Density changes caused by…

A

Compressibility effects, temperature variations, concentration variations

92
Q

Principle of Schlieren

A

Density gradients distorts collimated light which focuses imperfectly. Resulting pattern is a planar distribution of grey levels.

93
Q

Moiré or Ronchi interferometer principle

A

Pattern gets deformed when visualized through a field of variable density

94
Q

Mach Zehnder interferometer in practice

A

Slight tilt of one mirror by angle alpha induces a phase shift and interference pattern with fringe distance Delta x = lambda/alpha
Density gradients induce deviation from reference line proportional to local density.

95
Q

Pathline

A

Trajectory of a single particle as a function of time

96
Q

Streakline

A

All particles that have passed through one point. Sum of pathlines of single particles from that point.

97
Q

Timeline

A

Set of oarticles set at an instant in time and displaced in time

98
Q

Streamline

A

Tangent is parallel to local velocity at that point

99
Q

Flow lines in stationary flows

A

Pathlines, streamlines, streaklines coincide

100
Q

Introduction of particles

A

Far enough upstream, at average velocity, upstream of contraction ratio in wind tunnels

101
Q

Flow lines in turbulent flows

A

Too strong mixing -> anisotropic particles such as aluminum flakes

102
Q

Acronym laser

A

Light Amplification by Stimulated Emission of Radiation

103
Q

Specific qualities of lasers

A 
Coherence
Monochromatic
Small divergence
Extreme short pulse duration
High intensities
104
Q

Condition for interference

A

Spatial coherence

Polarization

105
Q

Proportionality of emission of LIF

A

To concentration and temperature

106
Q

Neutrally buoyant particles

A

Have no slip velocity up-uf

107
Q

Particles response time for turbulent flows

A

Should be in the same order of magnitude as the characteristic flow time tau_f. Stoke number=tau/tau_f< 1%

108
Q

Maximum acceptable particle diameter for turbulent flow tracing

A

Depends on the smallest eddies at which significant turbulent kinetic energy exists

109
Q

PTV

A

Particle tracking velocimetry

Follows one particle in a known time

110
Q

Tracer selection

A

Follow the flow: small, density close to fluid

Scatter light: big, large differences in refractive indices

111
Q

Light scattered by small particles is a function of

A 
Ratio of refractive indices
size
shape
orientation
Polarization
Observation angle
Laser power
112
Q

Tracer stokes number

A

Tau/tau_F= change in flow speed with time/how good can the particle follow<.1

113
Q

Origin of wave diffraction

A

Bending around small obstacles

Spreading past small openings

114
Q

Particle image scales with

A

Particle diameter
Light intensity
Distance particle camera

115
Q

Improper focusing

A

Particle image becomes gradually darker, blurry.

116
Q

Particle image density for different techniques

A

Low Nsource<=10 PIV

High Ns>1 laser speckle velocimetry

117
Q

Important properties of light source for tracer illumination

A

Short pulse
High power
Uniformity in measurement plane
Narrow light sheet

118
Q

Optimal pulse delay by PIV

A

Compromise between
Reasonable particle displacement
Loss of particles
Realtive error decreases with delay increasing
Approximation with finite difference: error / with delay \

119
Q

Imperfect images in PIV

A

Lens aberrations, limited optical access

120
Q

Displacement-correlation peak detectability

A

Ratio of the highest correlation peak to the second highest

121
Q

Loss of correlation due to

A

Out of plane motion, in plane motion.

122
Q

PIV multipass

A

Window shifting in 2nd exposure -> weak influence of inplane losses
With shifting: velocity=window shift+highest correlation

123
Q

Multigrid PIV

A

Sub domain can only host one particle

124
Q

Dynamic range by PIV

A

(Delta Xmax - Delta Xmin)/Delta Xmin

125
Q

Spurious vector in PIV occur with…

A

random acceleration peak exceeds amplitude of displacement correlation peak because of insufficient particle images, large displacements, high spatial velocity gradient, strong background, light sheet inhomogeneity. About 5% in good PIV

126
Q

Limitations of PIV

A

Velocity gradients (BL)
Pixel blur at wall
Inhomogenous seeding density
Peak locking

127
Q

Assumptions in PIV

A

Uniform displacement within interrogation region
Tracers follow flow
Homogeneous distribution

128
Q

LDA response to particle velocity

A

Linear -> no calibration

129
Q

Fringe spacing depends on

A

Laser wavelength, half beam intersection angle

130
Q

Doppler signal in fringe pattern

A

Particles going through the fringe pattern in the measurement volume will scatter light with a frequency proportional to its velocity perpendicular to the fringe planes.

131
Q

Doppler fringe signal split up

A

Pedestal, modulation.

132
Q

Measuring volume is defined by….

A

Amplitude of the modulation (e-2 of max)

133
Q

From of measuring volume

A

Elipsoid

134
Q

Detection volume

A

Elipsoid exceeding signal threshold, depends on particle.

135
Q

LDA MV optimization

A

Beam waist

Beam expansion before focus

136
Q

LDA amplitude depends on

A

Particle size

137
Q

Directional sensitivity in LDA

A

Slight frequency shift with Bragg cell -> fringes are moving

138
Q

Difference between signal and data processing

A

Signal : estimates characteristic parameters from signal such as velocity of particle
Data: use these parameters from many signals to derive flow related quantities such as statistical properties

139
Q

Unique for LDA data processing

A

Irregular sample times

Correlation of particles arrival rate correlated with velocity

140
Q

Doppler burst signal is used to…

A

Determinate the particle’s velocity, arrival time, residence time, evtly acceleration

141
Q

Refractive index matching

A

Mix 2 Diesel oils, exact adjustment to Duran glass n with temperature

142
Q

Advantages of LDA

A 
Non-intrusive
No calibration
High spatial and temporal accuracy
Simultaneous multi-component 
Reverse flow possible
143
Q

Disadvantages LDA

A 
Noisier than hot wire
Careful seeding
Velocity gradients (integration over MV, leads to overestimation of mean and variance)
Optical access
Expensive and complex
144
Q

Turbulence arise from

A

High Re

145
Q

Taylor’s frozen field hypothesis

A

Move a probe rapidly through the flow if rapid enough

146
Q

Reynolds shear stress

A

av(u1’u2’) = R.u1,rms.u2,rms

147
Q

Correlation in Reynolds shear stress

A

Negative correlation (ejection, sweep)

148
Q

Assumptions for Bernouilli equation

A

stationary flow
Non viscous fluid
Constant density
Along streamline

149
Q

Hydrostatic pressure

A

Pressure inside the fluid

150
Q

Error in liquid manometers

A

Due to meniscus

151
Q

Image calibration in PIV

A

Coordinate transformation with grid

152
Q

PIV is considered non-intrusive because…

A

Concentrations <10^-6 particles per m3

153
Q

Amplitude distribution in beam waist

A

Gaussian beam: strong increase

154
Q

LDA in practice

A

Not a point measurement
Limited temporal resolution because of sampling frequency
Noise from detector, multiple particles, electronics, ambient light
Particles may not follow the flow!

155
Q

Bernouilli equation derivation

A

From Euler equation with hypotheses of stationarity, 1D.

Integration along a streamline. g.s=g.h.

156
Q

Mean Square error

A

av((av(uM)-av(u))^2)=2av(u’^2)Lt/T

157
Q

Prevent flow separation in wind tunnel

A

Wide angle diffusor: screens, turbulence generators, slits

Fan: long nacelle

158
Q

Statistically independent samples

A

N=T/2Lt

Sampling once every two Lt is adequate

159
Q

Normalized autocorrelation with

A

Variance^2 (urms^2)

Fluid Mechanics (2 decks)

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