20554686
Descripción
Test por Albert Font, actualizado hace más de 1 año
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Creado por Albert Font
hace más de 4 años
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Pregunta 1
Pregunta
Under which circumstances may the volume rate-of-change entail viscous shear stresses?
Respuesta
-
Never for incompressible flow.
-
Always under the Stokes hypothesis.
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Always.
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Never
-
None of the other options.
Pregunta 2
Pregunta
What is true about viscous blockage?
Respuesta
-
All options are correct.
-
It corresponds to a massflow reduction in the near-wall region due to the effects of viscosity
-
It corresponds to a momentum flux reduction in the near-wall region due to the effects of viscosity.
-
It follows from the no-slip condition at the wall due to viscous effects.
-
It may be quantified by the displacement and momen- tum thicknesses of the boundary layer.
Pregunta 3
Pregunta
Which of the following factors may be held responsible for both advancing turbulent transition and inducing early separation of a boundary layer?
Respuesta
-
An adverse pressure gradient.
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Wall roughness.
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High preturbulence levels in the outer flow.
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High Reynolds number.
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None of the other options.
Pregunta 4
Pregunta
Laminar boundary layers, as opposed to turbulent,...
Respuesta
-
have lower friction but separate earlier.
-
have lower friction and separate later.
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have higher friction and separate earlier
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have higher friction but separate later.
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None of the other options.
Pregunta 5
Pregunta
Which of the following properties of the Navier-Stokes equations do the Euler equations retain?
Respuesta
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None of the other options.
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The no-slip condition can be applied on all solid walls.
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They allow computation of friction drag.
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They allow computation of form drag of closed bodies.
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They can anticipate the occurrence of turbulence and wakes.
Pregunta 6
Pregunta
Whichofthefollowingtermsofthestreamwisemomentum equation drops under the boundary layer hypothesis?
Respuesta
-
Streamwise diffusion.
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Wall-normal diffusion.
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Streamwise advection.
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Wall-normal advection.
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Streamwise pressure gradient.
Pregunta 7
Pregunta
The boundary layer (BL) integral equations...
Respuesta
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result from integrating the BL local equations in the wall-normal coordinate over the BL thickness.
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result from integrating the BL local equations along the streamwise coordinate.
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differ for laminar and turbulent BLs.
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result in as many unknowns as there are equations.
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None of the other options.
Pregunta 8
Pregunta
How is the D’Alembert’s paradox removed and the form drag obtained in an inviscid flow – boundary layer coupling calculation?
Respuesta
-
By solving the inviscid problem for a second time over a body enlarged by the displacement thickness and extended with the wake.
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By directly solving the boundary layer equations.
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The paradox remains no matter what.
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By solving the inviscid problem over the original body.
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The paradox was never there in the first place.
Pregunta 9
Pregunta
Which of the following inviscid potential flows admit self- similar solutions for the two-dimensional incompressible laminar boundary layer equations?
Respuesta
-
Wedge/corner flows.
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Source and sink flows.
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Irrotational vortex flow.
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Doublet flow.
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None of the other options.
Pregunta 10
Pregunta
For the laminar boundary layer developing in the vicinity of a stagnation point...
Respuesta
-
momentum thickness is locally finite and constant.
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displacement thickness grows linearly.
-
wall shear stress is locally finite and constant.
-
the form factor is larger than for the Blasius solution.
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None of the other options.
Pregunta 11
Pregunta
Which of the following statements does NOT describe tur- bulent flows?
Respuesta
-
Low energy dissipation.
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Intrinsic three-dimensionality.
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Intrinsically time-dependence.
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Deterministic chaos.
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Enhanced mixing capabilities.
Pregunta 12
Pregunta
Which of the following methods for solving turbulent flows simulates the large turbulent structures but filters out (and then models) the smaller scales?
Respuesta
-
Large Eddy Simulation (LES)
-
Reynolds Averaged Navier-Sokes (RANS).
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Direct Navier-Stokes (DNS).
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Direct Numerical Simulation (DNS).
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None of the others.
Pregunta 13
Pregunta
What is the actual origin of the Reynolds stresses in the RANS equations?
Respuesta
-
Advective momentum transport due to turbulent fluc- tuations.
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Diffusive momentum transport due to enhanced fluid viscosity.
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Turbulent pressure fluctuations.
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Mass conservation in the presence of turbulent fluctu- ations.
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None of the other options
Pregunta 14
Pregunta
How do viscous (foto)
compare within an incompressible, statistically twodimensional, turbulent boundary layer (TBL)?
Respuesta
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mu*(du/dy) prevails in the inmediate vicinity of the wall.
-
-densidad·u'·v' (tot adimensional) prevails over the full TBL thickness
-
mu*(du/dy) prevails over the full TBL thickness
-
-densidad·u'·v' (tot adimensional) prevails in the inmediate vicinity of the wall.
-
They compete in size over the full TBL thickness.
Pregunta 15
Pregunta
What is the sign of the streamwise-wall-normal velocity
fluctuation correlations (u'v') (adimensional todo) within a turbulent boundary
layer?
Respuesta
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(u'v') (adimensional todo) < 0
-
(u'v') (adimensional todo) > 0
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(u'v') (adimensional todo) = 0
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(u'v') (adimensional todo) <0 close to the wall and (u'v') (adimensional todo) >0 far from it.
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(u'v') (adimensional todo) >0 close to the wall and (u'v') (adimensional todo) <0 far from it.
Pregunta 16
Pregunta
How can lift be estimated for an airfoil immersed in inviscid incompressible flow?
Respuesta
-
By introducing a circulation to enforce the Kutta condition.
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In no way because potential flow is irrotational.
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Enforcing wall impermeability, lift naturally arises
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By enforcing the no-slip condition on the walls to the potential flow equations.
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None of the other options.
Pregunta 17
Pregunta
Which statement is true about the lifting problem within
the frame of linear potential theory?
Respuesta
-
The perturbation potential is anti-symmetric in the vertical coordinate.
-
The vertical perturbation velocity is anti-symmetric in the vertical coordinate.
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Both the vertical and horizontal perturbation velocities are symmetric in the vertical coordinate
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The horizontal perturbation velocity is symmetric in the vertical coordinate.
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None of the other options are correct.
Pregunta 18
Pregunta
A problem that aims at computing the pressure distribution given the airfoil shape is known as...
Respuesta
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a direct problem
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an inverse problem
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a symmetric problem.
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an anti-symmetric problem
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None of the others options are correct.
Pregunta 19
Pregunta
On a finite wing producing lift...
Respuesta
-
None of the other options are correct.
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Wingtip vortices roll from suction to pressure side
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The lift distribution is homogeneous along the span
-
There may still be no lift-induced drag
-
Streamlines are deflected towards the fuselage on the pressure side and towards wingtip on the suction side.
Pregunta 20
Pregunta
Prandtl’s lifting-line theory for finite wings...
Respuesta
-
All options are correct.
-
leads to an integro-differential equation for the circulation distribution along the span.
-
shows that minimum lift-induced drag is obtained for an elliptical distribution of circulation along the span.
-
is based on assessing the span-dependent effective angle of attack that results from wingtip vortices.
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provides both wing global lift and lift-induced drag coefficients.
Pregunta 21
Pregunta
Into which terms may the total rate-of-change of a small
fluid volume be split, provided nonlinear effects are neglected?
Respuesta
-
Translation, rotation, volume change, shear deformation.
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Translation, rotation, volume change.
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Translation, rotation, shear deformation.
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Rotation, volume change, shear deformation.
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Volume change, shear deformation.
Pregunta 22
Pregunta
Why is the drag on a sphere smaller when the separating
boundary layer is turbulent rather than laminar?
Respuesta
-
Because wake drag dominates and is smaller
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Because friction drag dominates and is smaller.
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Because both wake and friction drag are smaller.
-
Because wake drag is zero.
-
The assertion is plain wrong. The opposite is true.
Pregunta 23
Pregunta
23. What is true about the streamwise evolution of a boundary
layer (possibly involving a turbulent transition) developing
on a flat plate?
Respuesta
-
Its momentum thickness grows monotonically
-
Its friction coecient decreases monotonically
-
Its momentum thickness decreases monotonically.
-
Its friction coecient increases monotonically.
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None of the other options.
Pregunta 24
Pregunta
Which statement is correct under the boundary layer hypothesis?
Respuesta
-
None of the other options.
-
Streamwise diffusion outweighs streamwise advection
-
Streamwise diffusion outweighs wall-normal diffusion.
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Wall-normal pressure gradients are large.
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Fluid particles slip at the wall.
Pregunta 25
Pregunta
Which of the following interpretations of the displacement
thickness is plain FALSE?
Respuesta
-
It quantifies the friction drag coefficient to be expected
-
It quantifies the deviation of streamlines outside of the boundary layer
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It quantifies the massflow blockage due to viscous effects.
-
It quantifies the amount of wall-normal momentum that must appear for mass preservation within the boundary layer.
-
All statements are TRUE
Pregunta 26
Pregunta
26. Which of the following boundary layers is at a higher risk
of separation, all other parameters unaltered?
Respuesta
-
One that has a larger thickness
-
One that has a larger outer velocity
-
One that is subject to a stronger negative (favorable) pressure gradient.
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One that is turbulent.
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One that is subject to a milder deceleration of the outer flow
Pregunta 27
Pregunta
27. What are the usual assumptions in order to devise an integral method for both laminar and turbulent boundary
layers?
Respuesta
-
H and Cf/2 Re o taken from flat plate values
-
H and Cf/2 Re o taken from stagnation point values.
-
H and Cf/2 Re o taken from separation profile values.
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H taken from stagnation point and Cf/2 Re o from separation profile values
-
H taken from flat plate and Cf/2 Re o from stagnation point values.
Pregunta 28
Pregunta
28. Which term of the Navier-Stokes equations is responsible
for the appearance of the Reynolds stresses in the RANS
equations?
Respuesta
-
The advection term
-
The pressure term.
-
The time-derivative term.
-
The viscous diffusion term.
-
The volume forces term.
Pregunta 29
Pregunta
29. Which of the following statements is true about the
Reynolds stresses inside a turbulent, statistically twodimensional, incompressible boundary layer?
Respuesta
-
u'v' (adim) < 0
-
v'^2 adim > u'^2 adimd
-
w'^2 adim = 0
-
u'v' (adim) =/ 0 at the wall
-
u'^2 v'^2 w'^2 (adim) =/ 0
Pregunta 30
Pregunta
Pregunta 30 To which of the following regions in a turbulent boundary layer does the near wall hypothesis (constant shear
@⌧/@y ' 0, linear mixing length l ' y+) apply?
Respuesta
-
All options are correct.
-
To the inner region.
-
To the viscous sublayer.
-
To the buffer/overlap/blending region
-
To the log-law layer.
Pregunta 31
Pregunta
31. What does viscosity depend on for Newtonian fluids such
as water or air?
Respuesta
-
Temperature
-
Shear deformation rate.
-
Both temperature and shear deformation rate.
-
Neither temperature nor shear deformation rate.
-
Reynolds number
Pregunta 32
Pregunta
32. Which of the following conditions entails separation of a
two-dimensional flow?
Respuesta
-
Zero wall friction
-
Zero wall velocity
-
Infinite wall-normal gradient of streamwise velocity
-
Zero boundary layer momentum thickness
-
Zero boundary layer form factor.
Pregunta 33
Pregunta
Which momentum transport mechanisms are mainly competing within a laminar boundary layer?
Respuesta
-
Streamwise advection and wall-normal diffusion
-
Streamwise diffusion and wall-normal advection
-
Streamwise diffusion and streamwise advection.
-
Wall-normal advection and wall-normal diffusion.
-
Wall-normal and streamwise diffusion.
Pregunta 34
Pregunta
34. How are the integral boundary layer equations obtained?
Respuesta
-
By integrating the local equations over the boundary layer thickness.
-
By integrating the local equations along the streamwise coordinate.
-
By searching for self-similar solutions of the local equations
-
By applying the boundary layer hypothesis to the Navier-Stokes equations in their integral form.
-
By adding whole wheat to the fluid.
Pregunta 35
Pregunta
35. How does the momentum thickness evolve for a laminar
boundary layer in the close vicinity of a stagnation point?
Respuesta
-
It is finite and constant.
-
It remains of negligible size.
-
It debuts with a finite value and grows linearly.
-
It debuts at zero and grows linearly
-
It debuts at infinity and decreases hyperbolically
Pregunta 36
Pregunta
What can be said about the friction coecient Cf when
comparing growing laminar and turbulent boundary layers (BLs) developing under the same outer velocity conditions?
Respuesta
-
It is higher and decreases slowlier for turbulent BL.
-
It is higher and decreases slowlier for laminar BL.
-
It is higher but decreases faster for turbulent BL.
-
It is higher but decreases faster for laminar BL.
-
None of the others.
Pregunta 37
Pregunta
37. Theoretical-empirical turbulent friction laws result from
the matching of the velocity profile in what regions/layers
of the turbulent boundary layer universal profile?
Respuesta
-
Inner and outer regions
-
Viscous sublayer and buffer region
-
Buffer region and log-law layer
-
Viscous sublayer and log-law layer
-
Buffer region and inner region.
Pregunta 38
Pregunta
38. Which of the following phenomena can be ultimately ascribed to the action of viscosity?
Respuesta
-
All of them
-
Lift
-
Friction drag
-
Wake drag
-
Flow separation
Pregunta 39
Pregunta
39. Which of the following actions might be beneficial in terms
of drag coefficient reduction of a streamlined body (e.g.
flat plate at alpha = 0) with a laminar boundary layer naturally developing on its surface?
Respuesta
-
Preserving laminarity of the boundary layer all the way down to the trailing edge.
-
Artificially triggering turbulent transition in the boundary layer before it separates.
-
Reducing the Reynolds number
-
Favouring surface rugosity.
-
None of the others.
Pregunta 40
Pregunta
40. Which of the following is NOT a consequence of the boundary layer hypothesis?
Respuesta
-
Negligible streamwise advection
-
Wall-normal pressure gradient must cancel.
-
The local BL equations are parabolic
-
Negligible streamwise diffusion
-
Streamwise velocity outweighs wall-normal velocity.
Pregunta 41
Pregunta
41. When do boundary layers grow faster?
Respuesta
-
In adverse pressure gradient conditions
-
In favourable pressure gradient conditions.
-
In conditions of accelerated outer flow.
-
In conditions of accelerated (decelerated) outer flow if the boundary layer is laminar (turbulent).
-
In adverse (favourable) pressure gradient conditions if the boundary layer is laminar (turbulent).
Pregunta 42
Pregunta
Upon averaging which term of the Navier-Stokes equations
does the Reynolds stress tensor arise?
Respuesta
-
Advection term
-
Diffusion term.
-
Time-derivative term.
-
Pressure gradient term.
-
None of the others.
Pregunta 43
Pregunta
43. What is true when comparing solution methods for laminar and turbulent boundary layers (BLs)?
Respuesta
-
The BL integral equations are formally the same.
-
The BL local equations are formally the same.
-
The Falkner-Skan self-similar solutions apply all the same, provided that the outer flow is a wedge flow.
-
Experimental input is needed for solving both laminar and turbulent BLs.
-
All of the others.
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