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

2017 ◽  
Vol 813 ◽  
pp. 382-411 ◽  
Author(s):  
Katharina Klausmann ◽  
Bodo Ruck
Keyword(s):  
Drag Reduction ◽  
Pressure Fluctuations ◽  
Field Measurements ◽  
Reynolds Numbers ◽  
Circular Cylinders ◽  
Porous Coating ◽  
Leeward Side ◽  
Cylinder Surface ◽  
Amplitude Damping ◽  
Image Velocimetry

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.

An experimental investigation of the oscillating lift and drag of a circular cylinder shedding turbulent vortices

1961 ◽  
Vol 11 (2) ◽  
pp. 244-256 ◽  
Author(s):  
J. H. Gerrard
Keyword(s):  
Experimental Investigation ◽  
Reynolds Number ◽  
Circular Cylinder ◽  
Lift Coefficient ◽  
Reynolds Numbers ◽  
Circular Cylinders ◽  
Cylinder Surface ◽  
Fluctuating Pressure ◽  
Order Of Magnitude ◽  
Lift And Drag

The oscillating lift and drag on circular cylinders are determined from measurements of the fluctuating pressure on the cylinder surface in the range of Reynolds number from 4 × 103 to just above 105.The magnitude of the r.m.s. lift coefficient has a maximum of about 0.8 at a Reynolds number of 7 × 104 and falls to about 0.01 at a Reynolds number of 4 × 103. The fluctuating component of the drag was determined for Reynolds numbers greater than 2 × 104 and was found to be an order of magnitude smaller than the lift.

scholarly journals Passive Flow Control for Drag Reduction on a Cylinder in Cross-Flow Using Leeward Partial Porous Coatings

Fluids ◽  
2021 ◽  
Vol 6 (8) ◽  
pp. 289
Author(s):  
Imogen Guinness ◽  
Tim Persoons
Keyword(s):  
Flow Control ◽  
Numerical Study ◽  
Cross Flow ◽  
Circular Cylinders ◽  
Porous Coating ◽  
Leeward Side ◽  
Porous Coatings ◽  
Passive Flow Control ◽  
Passive Flow ◽  
The Impact

This paper presents a numerical study on the impact of partial leeward porous coatings on the drag of circular cylinders in cross-flow. Porous coatings are receiving increasing attention for their potential in passive flow control. An unsteady Reynolds-averaged Navier–Stokes model was developed that agreed well with the numerical and experimental literature. Using the two-equation shear stress transport k−ω turbulence model, 2D flow around a circular cylinder was simulated at Re = 4.2×104 with five different angles of partial leeward porous coatings and a full porous coating. For coating angles below 130∘, the coating resulted in an increase in pressure on the leeward side of the cylinder. There was a significant reduction in the fluctuation of the pressure and aerodynamic forces and a damping effect on vortex shedding. Flow separation occurred earlier; the wake was widened; and there was a decrease in turbulence intensity at the outlet. A reduction of drag between 5 and 16% was measured, with the maximum at a 70∘ coating angle. The results differed greatly for a full porous coating and a 160∘ coating, which were found to cause an increase in drag of 42% and 43%, respectively. The results showed that leeward porous coatings have a clear drag-reducing potential, with possibilities for further research into the optimum configuration.

Second Law Characterization of Confined Turbulent Flows

2012 ◽  
Vol 134 (11) ◽  
Author(s):  
G. F. Naterer ◽  
O. B. Adeyinka
Keyword(s):  
Entropy Production ◽  
Turbulent Flows ◽  
Particle Image ◽  
Field Measurements ◽  
Reynolds Numbers ◽  
Channel Flows ◽  
Image Velocimetry ◽  
Scale Models ◽  
Confined Channel

In this paper, a new measurement technique for turbulent entropy production is developed and applied to confined channel flows. Past methods of dimensional analysis, Clark gradient, and Smagorinsky models for subgrid turbulent stresses are examined to determine the flow irreversibilities throughout the flow field. The new experimental method obtains the turbulent irreversibilities up to a certain particle image velocimetry (PIV) cut-off wavelength, very close to the wall of the channel. Measured results of turbulence dissipation and entropy production at varying Reynolds numbers are presented and compared successfully against results from direct numerical simulations. The subgrid scale models of turbulent flow irreversibilities are shown to provide an effective alternative to direct PIV averaging of turbulent stresses, particularly close to the wall, where PIV resolution makes it difficult to precisely determine the averaged turbulence fluctuations. This paper develops a new PIV based method that enables the whole-field measurements of turbulent entropy production, and it presents new experimental data for entropy production in channel flows.

Numerical Study of Two-Dimensional Circular Cylinders in Tandem at Moderate Reynolds Numbers

2013 ◽  
Vol 135 (7) ◽  
Author(s):  
Ali Vakil ◽  
Sheldon I. Green
Keyword(s):  
Numerical Study ◽  
Reynolds Numbers ◽  
Separation Distance ◽  
Monotonic Function ◽  
Circular Cylinders ◽  
Cylinder Surface ◽  
Two Dimensional ◽  
Tandem Cylinders ◽  
Laminar Wake ◽  
Cylinder Drag

Computer simulations of the flow around a pair of two-dimensional, tandem circular cylinders in a flow, for Reynolds numbers in the range 1–40, are described. Cylinder surface-to-surface separations in the range 0.1<s/D<400 (D = cylinder diameter) were considered. The computed wake of a single cylinder at these low to moderate Reynolds numbers was in surprisingly good agreement with the laminar wake approximation, and a simple theory is presented to explain this agreement. With tandem cylinders, the drag on the downstream cylinder is a monotonic function of the cylinder separation. The laminar wake approximation can be used to explain reasonably well the variation in drag. The drag on the upstream cylinder is also a monotonic function of separation distance provided that the Reynolds number is less than about 10. For Reynolds numbers between 10 and 40, the upstream cylinder drag first decreases as separation increases up to a few diameters and then increases monotonically with separation distance.

NUMERICAL AND EXPERIMENTAL ANALYSES OF THE FLOW AROUND A HORIZONTAL WALL-MOUNTED CIRCULAR CYLINDER

2009 ◽  
Vol 33 (2) ◽  
pp. 189-215 ◽  
Author(s):  
M. Sami Akoz ◽  
M. Salih Kirkgoz
Keyword(s):  
Numerical Modeling ◽  
Circular Cylinder ◽  
Numerical Solutions ◽  
Turbulence Models ◽  
Reynolds Numbers ◽  
Sensitivity Study ◽  
Cylinder Surface ◽  
Image Velocimetry ◽  
Horizontal Wall ◽  
Drag And Lift

The numerical modeling of two-dimensional turbulent flow around a horizontal wall-mounted circular cylinder at Reynolds numbers in the range of 1000≤ReD≤7000 is investigated. Ansys® 10.0-FLOTRAN program package is used to solve the governing equations by finite element method, and the performance of the standard k-ε, standard k-ω and SST turbulence models are examined. A sensitivity study for the three turbulence models is carried out on eight computational meshes with different densities and structures. The computational velocity fields from the present simulations are compared with the experimental results obtained from particle image velocimetry (PIV) measurements for validation purposes. The point of the boundary layer detachment from the cylinder surface and the lengths of primary and secondary separation regions occurring around the cylinder are determined numerically and compared with those obtained experimentally. From these comparisons it is found that the numerical modeling using either of k-ω and SST turbulence models is reasonably successful. Using the results of numerical solutions, the drag and lift coefficients, Cd and Cl, are also calculated and compared with the measured values.

scholarly journals Active Flow Control on a Square-Back Road Vehicle

Fluids ◽  
2020 ◽  
Vol 5 (2) ◽  
pp. 55 ◽  
Author(s):  
Juan José Cerutti ◽  
Costantino Sardu ◽  
Gioacchino Cafiero ◽  
Gaetano Iuso
Keyword(s):  
Drag Reduction ◽  
Active Flow Control ◽  
Pressure Fluctuations ◽  
Three Dimensional ◽  
Wake Structure ◽  
Rectangular Slot ◽  
Natural Case ◽  
Image Velocimetry ◽  
Dimensional Reconstruction ◽  
Maximum Drag Reduction

An experimental investigation focused on the manipulation of the wake generated by a square back car model is presented. Four continuously-blowing rectangular slot jets were mounted on the rear face of a 1:10 commercial van model. Load cell measurements evidence drag reduction for different forcing configurations, reaching a maximum of 12% for lateral and bottom jets blowing. The spectral analysis of the pressure fluctuations evidence, for all forced cases, an energy attenuation with respect to the natural case, especially close to the shedding frequency. An energy budget highlighted the most efficient forcing configurations accounting for both the drag reduction and the power required to feed the blowing system. Two main configurations are considered: the maximum drag reduction and the best compromise, yielding 5% drag reduction and a convenient energy balance. Particle Image Velocimetry (pPIV) and stereoscopic PIV (sPIV) experiments were performed allowing the three-dimensional reconstruction of the wake in the three considered configurations. Consistently with static and fluctuating pressure measurements, sPIV results reveal a dramatic change in the wake structure when the jets blow in the maximum drag reduction configuration. Conversely, the best compromise configuration reveals a wake structure similar to the natural one.

scholarly journals Surface Roughness Effects on Flows Past Two Circular Cylinders in Tandem Arrangement at Co-Shedding Regime

Energies ◽  
2021 ◽  
Vol 14 (24) ◽  
pp. 8237
Author(s):  
Paulo Guimarães de Moraes ◽  
Luiz Antonio Alcântara Pereira
Keyword(s):  
Surface Roughness ◽  
Drag Reduction ◽  
Vortex Shedding ◽  
Circular Cylinders ◽  
Cylinder Surface ◽  
Outer Diameter ◽  
Grid Turbulence ◽  
Roughness Effects ◽  
Tandem Arrangement ◽  
Relative Roughness

This paper contributes by investigating surface roughness effects on temporal history of aerodynamic loads and vortex shedding frequency of two circular cylinders in tandem arrangement. The pair of cylinders is immovable; of equal outer diameter, D; and its geometry is defined by the dimensionless center-to-center pitch ratio, L/D. Thus, a distance of L/D = 4.5 is chosen to characterize the co-shedding regime, where the two shear layers of opposite signals, originated from each cylinder surface, interact generating counter-rotating vortical structures. A subcritical Reynolds number of Re = 6.5 × 104 is chosen for the test cases, which allows some comparisons with experimental results without roughness effects available in the literature. Two relative roughness heights are adopted, nominally ε/D = 0.001 and 0.007, aiming to capture the sensitivity of the applied numerical approach. Recent numerical results published in the literature have reported that the present two-dimensional model of surface roughness effects is able to capture both drag reduction and full cessation of vortex shedding for an immovable cylinder near a moving ground. That roughness model was successfully blended with a Lagrangian vortex method using sub-grid turbulence modeling. Overall, the effects of relative roughness heights on flows past two cylinders reveal changing of behavior of the vorticity dynamics, in which drag reduction, intermittence of vortex shedding, and wake destruction are identified under certain roughness effects. This kind of study is very useful for engineering conservative designs. The work is also motivated by scarcity of results previous discussing flows past cylinders in cross flow with surface roughness effects.

Pressure fluctuations produced by forward steps immersed in a turbulent boundary layer

2014 ◽  
Vol 756 ◽  
pp. 384-421 ◽  
Author(s):  
Manuj Awasthi ◽  
William J. Devenport ◽  
Stewart A. L. Glegg ◽  
Jonathan B. Forest
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Pressure Fluctuations ◽  
Field Measurements ◽  
Reynolds Numbers ◽  
Step Height ◽  
Stream Velocity ◽  
Step Size ◽  
High Reynolds ◽  
The Impact

AbstractExperiments have been performed on the disturbance of a high-Reynolds-number turbulent boundary layer by three forward steps with sizes close to 3.8, 15 and 60 % of the boundary layer thickness. Particular attention is focused on the impact of the steps on the fluctuating surface pressure field. Measurements were made from 5 boundary layer thicknesses upstream to 22 boundary layer thicknesses downstream of the step, a distance equivalent to over 600 step heights for the smallest step size. Flow speeds of 30 and $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}60\ \mathrm{m}\ {\mathrm{s}}^{-1}$ were studied, corresponding to boundary layer momentum thickness Reynolds numbers of 15 500 and 26 600 and step size Reynolds numbers from 6640 to 213 000. The steps produce a disturbance to the boundary layer pressure spectrum that scales on step size and decays remarkably slowly with distance downstream. When normalized on step height and free-stream velocity, the disturbance is self-similar and appears to develop almost independently of the enveloping boundary layer. The disturbance is still clearly visible at 150 step heights downstream of the mid-size step. Pressure correlations show the disturbance to be characterized by organized quasiperiodic motions that become visible well downstream of reattachment. The coherence and scale of these motions, as seen in the wall pressure correlations, scales on the step height and thus their visibility relative to the boundary layer grows rapidly as the step size is increased.

FLOW THROUGH A ROW OF CLOSELY-SPACED CIRCULAR CYLINDERS NORMAL TO A COLD WATER STREAM

1988 ◽  
Vol 12 (3) ◽  
pp. 139-147
Author(s):  
G.S.H. LOCK ◽  
W.J. O’CALLAGHAN
Keyword(s):  
Static Pressure ◽  
Cold Water ◽  
Freezing Point ◽  
Pressure Change ◽  
Reynolds Numbers ◽  
Bulk Water ◽  
Diameter Ratio ◽  
Circular Cylinders ◽  
Cylinder Surface ◽  
Water Stream

Flow near a row of parallel circular cylinders standing normal to a cross flow has been investigated experimentally. The cylinders spanned the height of the test section in a water tunnel in which the temperature was maintained close to the freezing point. Reynolds numbers, based on the mean bulk water velocity and the cylinder diameter, ranged between 3 x 103 and 3 x 104. The gap-diameter ratio g/D was set at five nominal values: 0.49, 0.20, 0.15, 0.10 and 0.06. For this range of conditions, static pressure readings were taken around the cylinder surfaces and at points well upstream and downstream of the cylinders. The overall static pressure change was converted into a pressure loss coefficient whose dependency on Reynolds number and gap-diameter ratio has been studied. The cylinder surface pressure profiles were used to determine flow behaviour in the gap region and in the wake. A critical transition at free stream Reynolds numbers much lower than that corresponding to a single cylinder was evident; a gap-diameter ratio near to 0.15 appeared to separate two different flow regimes. A cellular wake was often observed and found to be either metastable or bistable.

Investigation of Flow Around and in Wake of a Heated Circular Cylinder at Moderate Reynolds Numbers

2020 ◽  
Vol 142 (11) ◽  
Author(s):  
P. Michálek ◽  
S. Pospíšil ◽  
M. Macháček ◽  
V. Michalcová
Keyword(s):  
Circular Cylinder ◽  
Particle Image ◽  
Reynolds Numbers ◽  
Special Technique ◽  
Cylinder Surface ◽  
Cylinder Wall ◽  
Low Reynolds Numbers ◽  
Contour Maps ◽  
Image Velocimetry ◽  
Wake Patterns

Abstract The flow around a heated circular cylinder and the wake behind it were studied in wind tunnel flow using two methods of anemometry, i.e., particle image velocimetry (PIV) and constant temperature anemometry (CTA) with a special technique using a rotating slanted hot-wire probe. The Reynolds number ranged from 2000 to 20,000 and the cylinder wall temperatures varied between 27 °C and 177 °C. The wake is characterized by mean wind and fluctuation contour maps. Significant changes in wake patterns were observed while the cylinder was being heated, thus increasing its wall temperatures at low Reynolds numbers. At higher Reynolds numbers, the effects of cylinder heating on wake properties were negligible. The research fills a gap observed in the literature for a certain combination of velocity, cylinder aspect ratio, and cylinder surface temperature.

Sign in / Sign up

Export Citation Format

Share Document