Drag reduction of circular cylinders by porous coating on the leeward side

Abstract

The present paper describes the effect of drag reduction of circular cylinders due to a porous coating on their leeward sides. To investigate the coating effect, experiments were conducted in a wind tunnel of Goettingen type. Systematic drag measurements were carried out for different cylinder configurations and flow velocities. The drag measurements were complemented by pressure and particle image velocimetry (PIV) flow field measurements around selected cylinders. The Reynolds numbers were varied in the subcritical range of $3\times 10^{4}<Re<1.4\times 10^{5}$. The results show that a thin porous layer on the leeward side, either incorporated in the cylinder shape or applied on the cylinder surface, leads to an increase of base pressure on the leeward side of the cylinder. It causes a reduction of drag and dampens oscillation amplitudes when compared to a cylinder without coating. Results obtained for different configurations with varying key parameters (coating angles, layer thicknesses and pore sizes of the porous material) clearly indicate the drag-reducing and amplitude-damping potential of leeward coating. The amount of drag reduction and amplitude damping depends on the combination of key parameters. It was demonstrated that the lowered drag coefficients $c_{d}$ were almost constant in the tested range of Reynolds numbers. A maximum reduction of drag of 13.2 % was measured. In addition, the results revealed a strong reduction of the pressure fluctuations around cylinders with a leeward coating due to the shift of the vortex region further downstream.

The internal and external flow fields of a structured porous coated cylinder and implications on flow-induced noise

“…An interesting observation is that Sueki et al [1] experimentally showed that the material type (i.e., comparing a Metal Foam Porous Coated Cylinder (MFPCC) to a Polyurethane Porous Coated Cylinder (PPCC)) had little or no impact on the measured noise reduction, thus showing that the porous material thickness, porosity and spacings of the pores were of greater importance in tonal and broadband noise reduction. Furthermore, suitable application of a porous coating to the cylinder leeward surface can lead to a significant reduction of drag as compared to a bare cylinder [6].…”

Analysis of triangular sharkskin profiles according to second law

“…in which c f is a friction coefficient. The above analysis of friction effects allows formulating a more detailed expression of equation (4 (9) This preliminary hypothesis has allowed producing an effective analysis at macroscopic scale of the specific aerodynamic phenomena and determining in detail the gain produced by sharkskin in different configurations.   (11) We can assume the system is isothermal by neglecting the friction phenomenon that generates a small temperature difference.…”

“…Among several drag reduction methods that could be used are hydrophobic surfaces [6][7][8], compliant coatings [9][10][11], plasma actuators and dielectric barrier discharge [62][63][64], micro-bubbles injection [15][16][17] and riblets [18][19][20]. Riblets, wall grooves and sawtooth surfaces are highly attractive [21][22][23][24][25] because of their simplicity, low cost of manufacturing and easiness of maintenance.…”

Microfiber Coating for Flow Control over a Blunt Surface

“…Passive devices, which do not require external energy, will be a practical option because of broader possible applications using engineered surfaces, materials, and coatings. Examples of passive devices are a trip rod [7][8][9][10], dimple [11,12], vortex generator [13,14], permeable surface [15][16][17], and riblets [18][19][20].…”