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CFD > Short Answer > Flashcards

Short Answer Flashcards

1
Q

What is the oldest turbulence model still in use, and when was it developed?

A

The mixing length model by Prandtl, in 1925

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2
Q

Who developed the first LES with an eddy viscocity model, and when?

A

Smagorinsky, in 1963

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3
Q

What did Smagorinsky develop in 1963?

A

The first LES with an eddy viscocity model

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4
Q

What did Jones and Launder develop in 1972?

A

The k-epsilon model, the first two-equation turbulence model for RANS

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5
Q

What was the first two-equation turbulence model for RANS called?

A

The k-epsilon model

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6
Q

Who developed the first two-equation turbulence model for RANS, and when?

A

Jones and Launder, in 1972

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7
Q

What did Rhie and Chow develop in 1983?

A

Methods for solving the pressure Poisson equation

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8
Q

Who developed the methods for solving the Poisson equation, and when?

A

Rhie and Chow, in 1983

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9
Q

Who developend the k-omega model, and when?

A

Wilcox, in 1988

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10
Q

What did Wilcox develop in 1988?

A

The k-omega model

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11
Q

Who developed the dynamic eddy viscocity LES model, and when?

A

Germano, in 1990

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12
Q

What did Germano develop in 1990?

A

A dynamic eddy viscocity LES model

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13
Q

Who developed the SST model, and when?

A

Menter in 1994

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14
Q

What did Menter develop in 1994?

A

The SST model

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15
Q

What does SST stand for?

A

Shear Stress Transport

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16
Q

Why do we use CFD (3)?

A

Cheap, hazard-free and field quantities simultaneously accessible without measurement errors

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17
Q

What are (in three words) the three steps of pre-processing?

A

Generation, conditions, modelling

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18
Q

What do we generate in the first step of pre-processing?

A

The computational grid and geometry

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19
Q

What do we model in the final step of pre-processing?

A

The fluid and turbulence

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20
Q

What are (in three words) the three steps of post-processing?

A

Visualization, quantification, interpretation

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21
Q

What do we quantify in the second step of post-processing?

A

The uncertainties and errors

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22
Q

What is the equation for the Knudsen number?

A

Kn = {\displaystyle \lambda}/L

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23
Q

What is the approximate value for Kn in continuum mechanics?

A

Kn &laquo_space;1

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24
Q

What does the Knudsen number represent?

A

The ratio between the mean free path and representative physical length scale of a molecule

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25
Q

What is typical of the Lagrangian frame of reference?

A

It moves with the fluid element

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26
Q

What is typical of the Euler frame of reference?

A

It is a fixed frame, forming a stationary observer with relation to movement of the fluid element

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27
Q

When do we use Euler equations?

A

Analysis of steady, inviscid, compressible flows without jump discontinuities

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28
Q

When do we ignore viscous forces?

A

Re&raquo_space; 1

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29
Q

What are potential flows?

A

Steady, incompressible, inviscid, irrotational flows

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30
Q

What dictates barotropic fluids?

A

The pressure is dependent only on density, removing the use of the energy equation

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31
Q

What are the three most common CFD methods?

A

Finite volume, finite difference, finite element

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32
Q

What characterizes the finite volume method?

A

Uses cell averages, is fast and robust

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33
Q

What characterizes the finite difference method?

A

Uses differences between point values, difficult for complex geometries

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34
Q

What characterizes the finite element method?

A

Uses coefficients of basis functions, arbitrary high order, but slow

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35
Q

What CFD-method is Ansys-CFX based on?

A

The finite volume method

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36
Q

What equation relates the inertial forces to the viscous forces?

A

The Reynolds equation

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37
Q

What number represents advection velocity to the speed of sound?

A

The Mach number

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38
Q

What number relates unsteady to the steady forces?

A

The Strouhal number

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39
Q

What is the equation for the Strouhal number?

A

St = fL/V

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40
Q

What equation relates the inertial forces to the gravity force?

A

The Froude number

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41
Q

What is the equation for the Froude number?

A

Fr = V/sqrt(Lg)

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42
Q

What equation relates the inertial forces to surfaces forces?

A

The Weber equation

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43
Q

What is the equation for the Weber number

A

(rhoLV^2)/sigma

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44
Q

What does it mean if the Weber number is large?

A

The inertial forces dominate the surface forces of a fluid

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45
Q

When do we use the Strouhal number?

A

The Strouhal number relates the unsteady to the steady forces, so oscillating flow mechanisms such as eddy vortices

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46
Q

Which number describes hydraulic jumps?

A

The Froude number

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47
Q

What is the Reynolds number for ‘creeping flow’?

A

Re &laquo_space;1

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48
Q

What does a Reynolds number much smaller than 1 represent?

A

It represent a creeping flow, where the viscous forces in a flow dominate the inertial forces

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49
Q

What order does the Reynolds need to be for the flow to be laminar?

A

Of order 1

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50
Q

What Reynolds number depicts a turbulent flow?

A

Re&raquo_space; 1

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51
Q

What are some characteristics of a turbulent flow?

A

Inertial forces dominate, randomness

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52
Q

What are advantages of having turbulent flow?

A

Enhanced mixing, reduces pressure drag (while skin friction drag increases)

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53
Q

Why would we want laminar flow?

A

It is stable, which means pertubations decay and flow remains determinate

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54
Q

What is the topology of a cell in a grid?

A

The relation to the neighbouring cells

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55
Q

What defines the geometry of a cell in a grid?

A

The shape and size of the cell itself

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56
Q

What are the three types of mesh grids we define?

A

Structured, unstructured and hybrid

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57
Q

What defines a hybrid grid?

A

A mix of structured and unstructured cells, usually structured around surface of interest

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58
Q

What are the (2) advantages of having a structured mesh grid?

A

Generally faster convergence and smaller numerical errors

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59
Q

What is the disadvantage of having a structured grid?

A

It is difficult for complex geometry

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60
Q

Why does having a structured mesh lead to smaller numerical errors?

A

Due to the numerical diffusion being reduced by aligned grid lines

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61
Q

What is the advantage of having an unstructured grid?

A

A more straightforward application to complex structures

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62
Q

Name a disadvantage of having an unstructured grid.

A

The computation time per element is larger, due to more complex relation to neighbouring cell

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63
Q

What are the three different types of structured grids we define?

A

H, O and C

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64
Q

What is the name of the method of triangulation defined by the fact that no point lies inside a defined triangle?

A

Delauney triangulation

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65
Q

What is the name of the method of triangulation defined by a layer being formed along the boundary of the domain in an iterative manner?

A

Advanced front triangulation

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66
Q

When does numerical diffusion occur?

A

When the truncation error of the discrete approximation of continuous operators affects the numerical solution

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67
Q

Why does an unstructured grid have an increased computational time compared to a structured grid?

A

An unstructured grid generally has more elements, but also numerical diffusion increases

68
Q

How do we determine the grid resolution at the wall boundaries?

A

By computing the y-plus value

69
Q

What do we use the y-plus value for?

A

Determining the grid resolution at wall boundaries

70
Q

What can we use turbulence for (2)?

A

Enhanced mixing and postponing flow separation

71
Q

When is the scale separation in the turbulence energy cascade most pronounced?

A

For high Reynolds numbers

72
Q

Name 4 characteristics of turbulent flows

A

Unsteady, rotational, chaotic, breaking symmetry

73
Q

DNS is not used very often, why?

A

Expensive

74
Q

When do we use DNS, despite its expensive nature?

A

Low Reynolds flows, small devices

75
Q

What is the core field for DNS?

A

Fundamental turbulence research

76
Q

How do we define the Reynolds Stress Tensor?

A

Tensor in RANS to account for turbulence fluctuation

77
Q

What does EVM stand for?

A

Eddy Viscocity Model

78
Q

Name two EVM’s

A

k-epsilon, k-omega

79
Q

What dows RSM stand for?

A

Reynolds Stress Model

80
Q

Name two RSM’s

A

LLR RSM, SSG RSM

81
Q

Which names does LLR stand for in the LRR RSM, and in what year was it developed?

A

Launder, Reece, Rody, in 1975)

82
Q

Which names does SSG stand for in the SSG RSM, and in what year was it developed?

A

Speziale, Sarkar, Gatski, in 1991

83
Q

What characterizes the LRR RSM?

A

Solves transport equations for each component of the Reynolds stress tensor, hence more accurate for complex turbulent flows.

84
Q

What characterizes the SSG RSM?

A

Improved turbulence modeling by including pressure-strain correlations

85
Q

Name a model which combines EVM and RSM?

A

EARSM of Wallin and Johansson in 2000

86
Q

What is the pro of Prandtl one-equation model?

A

Modelling turbulence in external flows and attached boundary layers

87
Q

What is the con of Prandtl one-equation model?

A

Bad at internal flows and flow separation, due to assumption constant mixing length

88
Q

What is the pro of k-epsilon model?

A

External flows

89
Q

What is the flaw of k-epsilon model?

A

Prediction of anisotropic influences, such as curvature and directional volume forces

90
Q

What is the k-omega model good at?

A

Boundary layer flows and flows with gradients and separation

91
Q

What is the flaw of the k-omega model?

A

It overestimates the production of turbulence at stagnation points

92
Q

What is the pro of the Spalart-Allmaras model?

A

Good results for simple attached flows and slow-separation locations

93
Q

What is the con for Spalart-Allmaras?

A

It is less suitable for prediction of flow reattachement and free shear layers

94
Q

What do RSM directly solve for?

A

For all components of the unknown Reynolds Stress Tensor

95
Q

What does the slow term of the pressure strain correlation stand for?

A

For the relaxation to the isotropic equilibrium state

96
Q

What does the rapid term of the pressure strain correlation represent?

A

The immediate effects of the mean flow gradients and external forces

97
Q

Which turbulence scales can be computed directly?

A

The resolved scales

98
Q

Wich turbulence scales need to be modelled in order to be solved in LES?

A

The unresolved scales

99
Q

Among which turbulence scales do we expect the most dissipation?

A

The unresolved scales

100
Q

What is the Convolution theorem?

A

A convolution in real space corresponds to a (relatively simple) multiplication in the Fourrier space

101
Q

What does too much dissipation mean for the turbulence energy cascade?

A

Cascade dies out early

102
Q

What does too little dissipation mean for the turbulence energy cascade?

A

The cascade is disrupted

103
Q

What is the approximate value for the Smagorinsky constant if a flow is isotropic and turbulent?

A

Around 0.20

104
Q

What can we do to correct a Smagorinsky model near a wall?

A

Add van-Driest damping

105
Q

What makes a dynamic Smagorinsky model dynamic?

A

The Smagorinsky constant is determined in a dynamic way (Cs = Cs(x,t))

106
Q

Name two methods that combine LES and RANS

A

Zonal coupling, detached eddy simulation

107
Q

In the finite volume method, what does the solution represent?

A

A cell average value

108
Q

What do we cal a control volume for which the evolution of the mean values is computed?

A

A finite volume

109
Q

What is quadrature?

A

The process of determining area

110
Q

Which word do we use for the process of determining area?

A

Quadrature

111
Q

What do we call the type of estimation which construct new data points within the range of a discrete set of known data points?

A

Interpolation

112
Q

What do we call the method where an integral is approximated by the product of the integrand with the area of cell phase Ae?

A

Midpoint rule

113
Q

What determines the rate of grid convergence of a sufficiently smooth solution?

A

The order of the method

114
Q

What order is the trapezoidal rule?

A

Second order

115
Q

What is the trapezoidal rule?

A

A quadrature rule based on approximating the area under a graph as a trapezoid

116
Q

What order is Simpsons rule?

A

Fourth order

117
Q

What does UDS stand for?

A

Upwind Difference Scheme

118
Q

What does CDS stand for?

A

Central difference Scheme

119
Q

What is the numerical effect of a truncation error?

A

Higher order methods converge faster

120
Q

What is the CFL number?

A

The ratio of the physical distance a wave (or particle) travels in one time step to the size of the spatial grid cell

121
Q

What does CFL stand for?

A

Courant Friedrichs Lewy

122
Q

How do we calculate the CFL number?

A

(dt*V)/dx

123
Q

What is the advantage of using a higher order method?

A

Faster convergence after suffciently refined grid

124
Q

When should one not use a higher order method?

A

If grid not sufficiently refined

125
Q

What are three types of interpolation schems in Ansys-CFX?

A

1st order upwind, 2nd order central, 2nd order upwind

126
Q

What does the solution of an unsteady problem rely on?

A

Initial and boundary conditions

127
Q

What is the most simple solution for an unsteady problem?

A

The explicit Forward Euler method

128
Q

What are the two types of time-marching methods?

A

Explicit and implicit

129
Q

Name three implicit time-marching methods

A

Backwards Euler, the mid-point rule, the trapezoidal rule

130
Q

How do we determine a sufficiently small timestep?

A

Dividing the influenced domain by the signal speed in order to find the maximum timestep

131
Q

What are three benefits to explicit time-marching?

A

No iteration needed, straight-forward implementation, low memory requirements

132
Q

What are two detriments to explicit time-marching?

A

Unstable for large timesteps, time step needs to be adjusted for velocity and grid size

133
Q

What are two benefits of implicit time-marching methods?

A

Stable for larger timesteps, costs per timestep can be countered by fewer, larger steps

134
Q

What are two detriments of implicit time-marching methods?

A

High memory requirements, more complex implementation required

135
Q

When should you not use an implicit time-marching method?

A

If timestep is sufficiently small for explicit

136
Q

Why would one choose a local timestep?

A

Accelerate convergence to steady state solution

137
Q

What is the physical timestep in a domain limited by?

A

The smallest cell

138
Q

Name the four types of boundary conditions?

A

Dirichlet, Neumann, Robbin, periodic

139
Q

What is a Dirichlet boundary condition?

A

Apply a value on a boundary

140
Q

What is a Neumann boundary condition?

A

Apply a gradient on a boundary

141
Q

What is a Robbin boundary condition?

A

A combination of a Dirichlet and a Neumann boundary condition

142
Q

What is an example of a periodic boundary condition?

A

Symmetry

143
Q

How many boundary conditions do we exactly need to specify a supersonic inflow?

A

Five Dirichlet

144
Q

How many boundary conditions do we exactly need to specify a subsonic/incompressible inflow?

A

Four Dirichlet

145
Q

In a direct solution with N grid points, how many entries are there?

A

The system matrix will have (NxN) is order N^2 entries

146
Q

Why do programs generally not use direct solution methods?

A

Large amounts of memory, high operation count, unnecessary

147
Q

Name two methods for direct solution?

A

Gauss-elimination, LU-factorisation

148
Q

How many operations does a Gauss elimination of dense matrices require?

A

Of order N^3

149
Q

Why does it generally not matter to solve matrices exactly?

A

Modelling and discretisation errors are larger than computer round-off errors

150
Q

What is the definition of an error?

A

A deviation from the exact solution, where the solution is generally unknown

151
Q

What is the definition of residuum

A

A deviation from the equation that is solved by the iterative solution to the exact solution

152
Q

Between the error and the residuum, which one converges faster?

A

The residuum

153
Q

What is the condition for convergence?

A

That the spectral radius is smaller than 1

154
Q

Why should the spectral radius G for convergence be smaller than one?

A

Otherwise the solution will grow and diverge

155
Q

What does ILU-factorisation stand for?

A

Incomplete Lower Upper

156
Q

What type of factorisation does Ansys-CFX use?

A

Coupled ILU for u,v,w and p

157
Q

Name two multigrid methods

A

Geometric and algebraic

158
Q

What defines the geometric multigrid method?

A

The coarsening is based on an user-defined or automatically generated grid

159
Q

What defines the algebraic multigrid method?

A

The coarsening is based on a coefficient matrix, no actual grid is required

160
Q

What is the definition of verification?

A

Assesses if the model is build correctly

161
Q

What is the definition of validation?

A

Assesses if the model is fit to model the reality of interest

162
Q

What is a bias error?

A

A systematic offset, which can be calibrated

163
Q

What is a precision error?

A

A random error that can be cancelled when statistic averages are performed

164
Q

How does one check for a discretisation error?

A

A mesh convergence study

165
Q

What does a mesh convergence study look like?

A

Run simulations for different mesh resolutions and compare

166
Q

How small should the order of the residual be to be acceptable?

A

In the order of 10^-3

167
Q

How to check for turbulence modelling errors in RANS?

A

Perform simulations with different models and compare

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