Aerodynamics EETAC Final Exams Compilation Perm: 0 Exam: 01 Aerodynamics: Boundary Layer Theory { Multiple Choice Test 2018/19 Q2 7. The boundary layer (BL) integral equations... 1. Under which circumstances may the volume rate-of-change (a) result from integrating the BL local equations in the entail viscous shear stresses? wall-normal coordinate over the BL thickness. (b) result from integrating the BL local equations along (a) Never for incompressible flow. the streamwise coordinate. (b) Always under the Stokes hypothesis. (c) result in as many unknowns as there are equations. (c) Always. (d) differ for laminar and turbulent BLs. (d) Never. (e) None of the other options. (e) None of the other options. 8. How is the D'Alembert's paradox removed and the form 2. What is true about the viscous blockage? drag obtained in an inviscid flow { boundary layer coupling (a) All options are correct. calculation? (b) It corresponds to a massflow reduction in the near-wall (a) By solving the inviscid problem for a second time over region due to the effects of viscosity. a body enlarged by the displacement thickness and (c) It corresponds to a momentum flux reduction in the extended with the wake. near-wall region due to the effects of viscosity. (b) By directly solving the boundary layer equations. (d) It follows from the no-slip condition at the wall due to (c) The paradox remains no matter what. viscous effects. (d) By solving the inviscid problem over the original body. (e) It may be quantified by the displacement and momen- (e) The paradox was never there in the first place. tum thicknesses of the boundary layer. 9. Which of the following inviscid potential flows admit self- 3. Which of the following factors may be held responsible similar solutions for the two-dimensional incompressible for both advancing turbulent transition and inducing early laminar boundary layer equations? separation of a boundary layer? (a) Wedge/corner flows. (a) An adverse pressure gradient. (b) Source and sink flows. (b) Wall roughness. (c) Irrotational vortex flow. (c) High preturbulence levels in the outer flow. (d) Doublet flow. (d) High Reynolds number. (e) None of the other options. (e) None of the other options. 10. For the laminar boundary layer developing in the vicinity 4. Laminar boundary layers, as opposed to turbulent,... of a stagnation point... (a) have lower friction but separate earlier. (a) momentum thickness is locally finite and constant. (b) have lower friction and separate later. (b) displacement thickness grows linearly. (c) have higher friction and separate earlier. (c) wall shear stress is locally finite and constant. (d) have higher friction but separate later. (d) the form factor is larger than for the Blasius solution. (e) None of the other options. (e) None of the other options. 5. Which of the following properties of the Navier-Stokes 11. Which of the following statements does NOT describe tur- equations do the Euler equations retain? bulent flows? (a) None of the other options. (a) Low energy dissipation. (b) The no-slip condition can be applied on all solid walls. (b) Intrinsic three-dimensionality. (c) They allow computation of friction drag. (c) Intrinsically time-dependence. (d) They allow computation of form drag of closed bodies. (d) Deterministic chaos. (e) They can anticipate the occurrence of turbulence and (e) Enhanced mixing capabilities. wakes. 12. Which of the following methods for solving turbulent flows 6. Which of the following terms of the streamwise momentum simulates the large turbulent structures but filters out (and equation drops under the boundary layer hypothesis? then models) the smaller scales? (a) Streamwise diffusion. (a) Large Eddy Simulation (LES) (b) Wall-normal diffusion. (b) Reynolds Averaged Navier-Sokes (RANS). (c) Streamwise advection. (c) Direct Navier-Stokes (DNS). (d) Wall-normal advection. (d) Direct Numerical Simulation (DNS). (e) Streamwise pressure gradient. (e) None of the others. 0 0 13. What is the actual origin of the Reynolds stresses (−ρuiuj ) 19. On a finite wing producing lift... in the RANS equations? (a) None of the other options are correct. (a) Advective momentum transport due to turbulent fluc- (b) Wingtip vortices roll from suction to pressure side. tuations. (c) The lift distribution is homogeneous along the span. (b) Diffusive momentum transport due to enhanced fluid (d) There may still be no lift-induced drag. viscosity. (e) Streamlines are deflected towards the fuselage on the (c) Turbulent pressure fluctuations. pressure side and towards wingtip on the suction side. (d) Mass conservation in the presence of turbulent fluctu- 20. Prandtl's lifting-line theory for finite wings... ations. (e) None of the other options. (a) All options are correct. (b) leads to an integro-differential equation for the circu- 14. How do viscous µ @u and Reynolds −ρu0v0 shear stresses @y lation distribution along the span. compare within an incompressible, statistically two- dimensional, turbulent boundary layer (TBL)? (c) shows that minimum lift-induced drag is obtained for an elliptical distribution of circulation along the span. (a) µ @u prevails in the immediate vicinity of the wall. @y (d) is based on assessing the span-dependent effective an- (b) −ρu0v0 prevails over the full TBL thickness. gle of attack that results from wingtip vortices. @u (c) µ @y prevails over the full TBL thickness. (e) provides both wing global lift and lift-induced drag (d) −ρu0v0 prevails in the immediate vicinity of the wall. coefficients. (e) They compete in size over the full TBL thickness. 2018/19 Q1 15. What is the sign of the streamwise-wall-normal velocity 21. Into which terms may the total rate-of-change of a small fluctuation correlations (u0v0) within a turbulent boundary fluid volume be split, provided nonlinear effects are ne- layer? glected? (a) u0v0 < 0. (a) Translation, rotation, volume change, shear deforma- 0 0 (b) u v > 0. tion. 0 0 (c) u v = 0. (b) Translation, rotation, volume change. 0 0 0 0 (d) u v < 0 close to the wall and u v > 0 far from it. (c) Translation, rotation, shear deformation. 0 0 0 0 (e) u v > 0 close to the wall and u v < 0 far from it. (d) Rotation, volume change, shear deformation. 16. How can lift be estimated for an airfoil immersed in invis- (e) Volume change, shear deformation. cid incompressible flow? 22. Why is the drag on a sphere smaller when the separating (a) By introducing a circulation to enforce the Kutta con- boundary layer is turbulent rather than laminar? dition. (a) Because wake drag dominates and is smaller. (b) In no way because potential flow is irrotational. (b) Because friction drag dominates and is smaller. (c) Enforcing wall impermeability, lift naturally arises. (c) Because both wake and friction drag are smaller. (d) By enforcing the no-slip condition on the walls to the (d) Because wake drag is zero. potential flow equations. (e) The assertion is plain wrong. The opposite is true. (e) None of the other options. 23. What is true about the streamwise evolution of a boundary 17. Which statement is true about the lifting problem within layer (possibly involving a turbulent transition) developing the frame of linear potential theory? on a flat plate? (a) The perturbation potential is anti-symmetric in the (a) Its momentum thickness grows monotonically. vertical coordinate. (b) Its friction coefficient decreases monotonically. (b) The vertical perturbation velocity is anti-symmetric in the vertical coordinate. (c) Its momentum thickness decreases monotonically. (c) The horizontal perturbation velocity is symmetric in (d) Its friction coefficient increases monotonically. the vertical coordinate. (e) None of the other options. (d) Both the vertical and horizontal perturbation veloci- 24. Which statement is correct under the boundary layer hy- ties are symmetric in the vertical coordinate. pothesis? (e) None of the other options are correct. (a) None of the other options. 18. A problem that aims at computing the pressure distribu- (b) Streamwise diffusion outweighs streamwise advection. tion given the airfoil shape is known as... (c) Streamwise diffusion outweighs wall-normal diffusion. (a) a direct problem. (d) Wall-normal pressure gradients are large. (b) an inverse problem. (e) Fluid particles slip at the wall. (c) a symmetric problem. (d) an anti-symmetric problem. (e) None of the other options are correct. 25. Which of the following interpretations of the displacement 31. What does viscosity depend on for Newtonian fluids such thickness is plain FALSE? as water or air? (a) It quantifies the friction drag coefficient to be ex- (a) Temperature. pected. (b) Shear deformation rate. (b) It quantifies the deviation of streamlines outside of the (c) Both temperature and shear deformation rate. boundary layer. (d) Neither temperature nor shear deformation rate. (c) It quantifies the massflow blockage due to viscous ef- (e) Reynolds number. fects. 32. Which of the following conditions entails separation of a (d) It quantifies the amount of wall-normal momentum two-dimensional flow? that must appear for mass preservation within the boundary layer. (a) Zero wall friction. (e) All statements are TRUE. (b) Zero wall velocity. 26. Which of the following boundary layers is at a higher risk (c) Infinite wall-normal gradient of streamwise velocity. of separation, all other parameters unaltered? (d) Zero boundary layer momentum thickness. (a) One that has a larger thickness. (e) Zero boundary layer form factor. (b) One that has a larger outer velocity. 33. Which momentum transport mechanisms are mainly com- (c) One that is subject to a stronger negative (favorable) peting within a laminar boundary layer? pressure gradient. (a) Streamwise advection and wall-normal diffusion. (d) One that is turbulent. (b) Streamwise diffusion and wall-normal advection. (e) One that is subject to a milder deceleration of the (c) Streamwise diffusion and streamwise advection.