Abstract
Four riblet bends were tested to investigate the effects of riblets on pipe flows including the secondary flow on the Reynolds numbers; Re D =6×103−4×104. The pressure gradients on the smooth pipe downstream from the riblet bends were measured, and also the pressure losses of the bends only were measured. All riblet bends reduced the pressure gradient on the smooth pipe downstream from them, which means a drag reduction. Two of the riblet bends showed the maximum drag reduction of about 4 percent at Re D = 6500; this reduction rate was significant considering the uncertainty of the present experiments. Since the pressure losses of these two riblet bends were almost identical to that of the smooth bend at Re D = 6500, they could cause a net drag reduction of about 4 percent on the piping system including these bends at that Reynolds number. Furthermore, the velocity profiles measured by LDV indicated that the secondary flow becomes weaker downstream from the riblet bends when a drag reduction is recognized there.
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Abbreviations
- D :
-
pipe diameter
- D 0 :
-
the distance from the valley to the valley passing through the pipe center
- H :
-
height of groove
- P :
-
nondimensional static pressure (p/it/(ρU 20 ):p is gauge pressure)
- dP/dX :
-
nondimensional pressure gradient
- Rc :
-
curvature of bend
- Re D :
-
Reynolds number based on bulk velocity and pipe diameter
- s:
-
spacing of groove
- U :
-
mean streamwise velocity along the horizontal diameter
- U 0 :
-
bulk velocity
- V :
-
mean vertical velocity along the horizontal diameter
- x :
-
streamwise direction along the pipe axis (see Fig. 1)
- X :
-
nondimensionalx (=x/D)
- y :
-
radial direction in the horizontal plane which is perpendicular to the plane including the bend (see Fig. 1)
- yUV :
-
swirl intensity (nondimensional swirl intensity:yUV/(DU 20 ))
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Nakao, SI. Effects of riblet bends on pipe flows. Appl. Sci. Res. 54, 237–247 (1995). https://doi.org/10.1007/BF00863511
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DOI: https://doi.org/10.1007/BF00863511