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The effect of an oscillatory freestream-flow on a NACA-4412 profile at large relative amplitudes and low Reynolds-numbers

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Abstract

Results of an experimental investigation of the flow around a NACA-4412 profile in an oscillating freestream are presented. The experiment took place in an Eiffel-type windtunnel at a chord Reynolds-number of Re = 2 · 105. Measurements of unsteady pressure distributions and boundary-layer profiles as well as flow photographs reveal that even at moderate reduced frequencies \(k = \pi f/\overline u _\infty\) significant changes of the flow field may occur, provided that the relative amplitude of the freestream \(A = \Delta u_\infty /\overline u _\infty\) is sufficiently large. So a periodical separation and reattachment of the flow could be observed while in another case the periodical relaminarization of the boundary-layer could be found.

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Abbreviations

A :

relative amplitude of freestream velocity

A I :

relative amplitude of first harmonic of the freestream velocity

b :

span of the airfoil profile

C A :

lift-coefficient

C A st :

lift-coefficient in steady freestream

C p :

pressure-coefficient

d :

profile thickness

f :

frequency

H 12 :

shape factor

k :

reduced frequency

l :

chord length

p :

phase-average of pressure

p 0 :

total head

p ∞ :

static freestream pressure

p a :

ambient pressure

q ∞ :

dynamic head

Re :

mean Reynolds number

Re δ 2 :

Reynolds number

t :

current time

ΔT :

phase time

u :

velocity in x-direction

u ∞ :

freestream-velocity

Δu ∞ :

amplitude of freestream-velocity

u a :

velocity at boundary-layer edge

u c :

cooling-velocity

u′ :

fluctuation of velocity in x-direction

u′ rms :

mean square of fluctuation

û :

nondimensional velocity, Fig. 3

ν′ :

fluctuation of velocity in y-direction

w′ :

fluctuation of velocity in z-direction

x,y,z :

cartesian coordinates

X A :

distance of separation line from leading edge

α :

angle of attack

Γ :

nondimensional pressure gradient

δ :

boundary-layer thickness

δ 1 :

displacement-thickness

δ 2 :

momentum-thickness

ν :

kinematic viscosity

ω :

angular-velocity

(∼):

periodical component

(-):

time-average

(′):

stochastic component

References

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Authors and Affiliations

  1. Aerodynamisches Institut der RWTH Aachen, Wüllnerstraße zw. 5-7, D-5100, Aachen, Germany

    E. Krause & W. -B. Schweitzer

Authors
  1. E. Krause
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  2. W. -B. Schweitzer
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Krause, E., Schweitzer, W.B. The effect of an oscillatory freestream-flow on a NACA-4412 profile at large relative amplitudes and low Reynolds-numbers. Experiments in Fluids 9, 159–166 (1990). https://doi.org/10.1007/BF00187416

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  • DOI: https://doi.org/10.1007/BF00187416

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