Control of circular cylinder wakes at relatively high Reynolds numbers

A narrow strip has bee introduced as an element to controlcircular cylinder wakes at Reynolds numbers Re=3.0×10 3~2.0× 10 4. The strip is 0.18D (D is diameter of the cylinder) wide, 0.015D~0.025D thick and as long as the cylinder. The long margins of the strip are parallel to and coplanar with the cylinderaxis. The strip is fixed by a row of small rods distributed along a meridianand perpendicularly connected to the surface of the cylinder. The rods havebeen proven to have negligible influence on the flow. The control parameteris strip position, which can be determined byλ/(0.5D)-(unified) distance between the cylinder axis and the length-wise centerline of the strip, and β-angle of attack of the strip.The experiment was conducted in a low turbulence wind tunnel of test section2m long, 60cm wide and 60cm high. Two circular cylinders ofdiameter D=3cm and 5cm were tested respectively in the range ofβ=0°~ 180° and λ/(0.5D)=1.1~3.8. The overall information of the cylinder wake was visualizedby smoke-wire technique and the detailed local information offluctuating velocity was detected by a hot-wire anemometry at various pointsfrom Y/(0.5D)= -6.0 to 6.0 at a stream-wisestation X/D=12.5.Visualization pictures and power spectra of fluctuating velocities show thatvortex shedding on both sides of the cylinder can be effectively suppressedif the strip is located in a certain zone in the near wake. The effectivezones at Re=6.3×103, 1.0×104 and 1.64×104 have been found out with accuracy 0.1 of λ/0.5D and 2.5° of β. The zone enlarges slightly with theincrease of Re in the range tested.Distributions of turbulence intensity for variousβfrom0° to 180° at λ/(0.5D)=2.9 and Re=1.64×104 have been measured. Compare with the case without control, the turbulence intensity on both sides of the wake isreduced when βis in the range of 0°~ 50° or in the vicinity of 180°. However, in the range β=60°~ 120°, an increase appears on the side where the stripresides, while no increase presents on the opposite side.Total resistance on cylinder and control element for variousβfrom 0°to 180°at λ/(0.5D)=2.9 and Re=1.64×104 has been evaluated by momentum integrationusing measured velocity profiles. It shows that the total resistance in therange ofβ=0°~ 40° and in the vicinity of β=180° is obviously smaller than the resistance on bare cylinder. The maximum reduction of resistance is about 32%, which happens atβ=25°~ 30°.All the results mean, local passive interference of the strip can induceglobal changes of the wake at high Reynolds numbers.