Under which circumstances may the volume rate-of-change entail viscous shear stresses?
Never for incompressible flow.
Always under the Stokes hypothesis.
Always.
Never
None of the other options.
What is true about viscous blockage?
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.
Which of the following factors may be held responsible for both advancing turbulent transition and inducing early separation of a boundary layer?
An adverse pressure gradient.
Wall roughness.
High preturbulence levels in the outer flow.
High Reynolds number.
Laminar boundary layers, as opposed to turbulent,...
have lower friction but separate earlier.
have lower friction and separate later.
have higher friction and separate earlier
have higher friction but separate later.
Which of the following properties of the Navier-Stokes equations do the Euler equations retain?
The no-slip condition can be applied on all solid walls.
They allow computation of friction drag.
They allow computation of form drag of closed bodies.
They can anticipate the occurrence of turbulence and wakes.
Whichofthefollowingtermsofthestreamwisemomentum equation drops under the boundary layer hypothesis?
Streamwise diffusion.
Wall-normal diffusion.
Streamwise advection.
Wall-normal advection.
Streamwise pressure gradient.
The boundary layer (BL) integral equations...
result from integrating the BL local equations in the wall-normal coordinate over the BL thickness.
result from integrating the BL local equations along the streamwise coordinate.
differ for laminar and turbulent BLs.
result in as many unknowns as there are equations.
How is the D’Alembert’s paradox removed and the form drag obtained in an inviscid flow – boundary layer coupling calculation?
By solving the inviscid problem for a second time over a body enlarged by the displacement thickness and extended with the wake.
By directly solving the boundary layer equations.
The paradox remains no matter what.
By solving the inviscid problem over the original body.
The paradox was never there in the first place.
Which of the following inviscid potential flows admit self- similar solutions for the two-dimensional incompressible laminar boundary layer equations?
Wedge/corner flows.
Source and sink flows.
Irrotational vortex flow.
Doublet flow.
For the laminar boundary layer developing in the vicinity of a stagnation point...
momentum thickness is locally finite and constant.
displacement thickness grows linearly.
wall shear stress is locally finite and constant.
the form factor is larger than for the Blasius solution.
Which of the following statements does NOT describe tur- bulent flows?
Low energy dissipation.
Intrinsic three-dimensionality.
Intrinsically time-dependence.
Deterministic chaos.
Enhanced mixing capabilities.
Which of the following methods for solving turbulent flows simulates the large turbulent structures but filters out (and then models) the smaller scales?
Large Eddy Simulation (LES)
Reynolds Averaged Navier-Sokes (RANS).
Direct Navier-Stokes (DNS).
Direct Numerical Simulation (DNS).
None of the others.
What is the actual origin of the Reynolds stresses in the RANS equations?
Advective momentum transport due to turbulent fluc- tuations.
Diffusive momentum transport due to enhanced fluid viscosity.
Turbulent pressure fluctuations.
Mass conservation in the presence of turbulent fluctu- ations.
None of the other options
How do viscous (foto) compare within an incompressible, statistically twodimensional, turbulent boundary layer (TBL)?
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.
What is the sign of the streamwise-wall-normal velocity fluctuation correlations (u'v') (adimensional todo) within a turbulent boundary layer?
(u'v') (adimensional todo) < 0
(u'v') (adimensional todo) > 0
(u'v') (adimensional todo) = 0
(u'v') (adimensional todo) <0 close to the wall and (u'v') (adimensional todo) >0 far from it.
(u'v') (adimensional todo) >0 close to the wall and (u'v') (adimensional todo) <0 far from it.
How can lift be estimated for an airfoil immersed in inviscid incompressible flow?
By introducing a circulation to enforce the Kutta condition.
In no way because potential flow is irrotational.
Enforcing wall impermeability, lift naturally arises
By enforcing the no-slip condition on the walls to the potential flow equations.
Which statement is true about the lifting problem within the frame of linear potential theory?
The perturbation potential is anti-symmetric in the vertical coordinate.
The vertical perturbation velocity is anti-symmetric in the vertical coordinate.
Both the vertical and horizontal perturbation velocities are symmetric in the vertical coordinate
The horizontal perturbation velocity is symmetric in the vertical coordinate.
None of the other options are correct.
A problem that aims at computing the pressure distribution given the airfoil shape is known as...
a direct problem
an inverse problem
a symmetric problem.
an anti-symmetric problem
None of the others options are correct.
On a finite wing producing lift...
Wingtip vortices roll from suction to pressure side
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.
Prandtl’s lifting-line theory for finite wings...
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.
provides both wing global lift and lift-induced drag coefficients.
Into which terms may the total rate-of-change of a small fluid volume be split, provided nonlinear effects are neglected?
Translation, rotation, volume change, shear deformation.
Translation, rotation, volume change.
Translation, rotation, shear deformation.
Rotation, volume change, shear deformation.
Volume change, shear deformation.
Why is the drag on a sphere smaller when the separating boundary layer is turbulent rather than laminar?
Because wake drag dominates and is smaller
Because friction drag dominates and is smaller.
Because both wake and friction drag are smaller.
Because wake drag is zero.
The assertion is plain wrong. The opposite is true.
23. What is true about the streamwise evolution of a boundary layer (possibly involving a turbulent transition) developing on a flat plate?
Its momentum thickness grows monotonically
Its friction coecient decreases monotonically
Its momentum thickness decreases monotonically.
Its friction coecient increases monotonically.
Which statement is correct under the boundary layer hypothesis?
Streamwise diffusion outweighs streamwise advection
Streamwise diffusion outweighs wall-normal diffusion.
Wall-normal pressure gradients are large.
Fluid particles slip at the wall.
Which of the following interpretations of the displacement thickness is plain FALSE?
It quantifies the friction drag coefficient to be expected
It quantifies the deviation of streamlines outside of the boundary layer
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
26. Which of the following boundary layers is at a higher risk of separation, all other parameters unaltered?
One that has a larger thickness
One that has a larger outer velocity
One that is subject to a stronger negative (favorable) pressure gradient.
One that is turbulent.
One that is subject to a milder deceleration of the outer flow
27. What are the usual assumptions in order to devise an integral method for both laminar and turbulent boundary layers?
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.
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.
28. Which term of the Navier-Stokes equations is responsible for the appearance of the Reynolds stresses in the RANS equations?
The advection term
The pressure term.
The time-derivative term.
The viscous diffusion term.
The volume forces term.
29. Which of the following statements is true about the Reynolds stresses inside a turbulent, statistically twodimensional, incompressible boundary layer?
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 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?
To the inner region.
To the viscous sublayer.
To the buffer/overlap/blending region
To the log-law layer.
31. What does viscosity depend on for Newtonian fluids such as water or air?
Temperature
Shear deformation rate.
Both temperature and shear deformation rate.
Neither temperature nor shear deformation rate.
Reynolds number
32. Which of the following conditions entails separation of a two-dimensional flow?
Zero wall friction
Zero wall velocity
Infinite wall-normal gradient of streamwise velocity
Zero boundary layer momentum thickness
Zero boundary layer form factor.
Which momentum transport mechanisms are mainly competing within a laminar boundary layer?
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.
34. How are the integral boundary layer equations obtained?
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.
35. How does the momentum thickness evolve for a laminar boundary layer in the close vicinity of a stagnation point?
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
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?
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.
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?
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.
38. Which of the following phenomena can be ultimately ascribed to the action of viscosity?
All of them
Lift
Friction drag
Wake drag
Flow separation
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?
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.
40. Which of the following is NOT a consequence of the boundary layer hypothesis?
Negligible streamwise advection
Wall-normal pressure gradient must cancel.
The local BL equations are parabolic
Negligible streamwise diffusion
Streamwise velocity outweighs wall-normal velocity.
41. When do boundary layers grow faster?
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).
Upon averaging which term of the Navier-Stokes equations does the Reynolds stress tensor arise?
Advection term
Diffusion term.
Time-derivative term.
Pressure gradient term.
43. What is true when comparing solution methods for laminar and turbulent boundary layers (BLs)?
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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