Due to its unique pore structure, porous materials have the potential to be used in the fields of acoustic noise reduction and flow drag reduction control. In order to study their effects and mechanism of drag reduction on the flow around a circular cylinder, experiments are conducted in a low-speed wind tunnel with low turbulence intensity. The drag forces acting on a circular cylinder model are measured using wind tunnel balance when porous materials with different permeability are applied within different intersection angles on the trailing-edge and leading edge, and the flow fields are visualized with a particle image velocimetry system with high time resolution. The method of dynamic mode decomposition (DMD) is also used for reduced-order analysis of the vorticity field in the wake of the cylinder. The measured drag forces and wake flow fields are then compared with those of a smooth cylinder, and the results show that porous materials laid on the trailing-edge can reduce drag, when a porous material with 20 pores per inch is laid within 270° on the leeward side, the best effect of the drag reduction ratio of 10.21% is reached. The results of flow visualization indicate that after the porous material is applied, the vortex region in the wake of the cylinder is expanded; both the frequency of vortex shedding and the magnitude of vorticity fluctuation decrease; the Reynolds-shear-stress decreases significantly, and both indicate that vorticity is dissipated earlier. The results of DMD analysis show that porous materials can effectively relax the energy of vortices in different modes.

Drag reduction in cylindrical wake flow using porous material

A series of reduced-order numerical simulations on a specific bluff body type (v-gutters) in a subsonic duct flow is done to assess the unsteady wake dynamics. Two of the v-gutter's geometrical parameters are varied: the v-gutter's base angle (θ) and the size of a slit (ξ) at the leading edge of the v-gutter. Five v-gutter angles are considered (θ,{degree sign} = π/6, π/4, π/3, 5π/12, π/2) and three slit sizes (ξ, mm =0,0.25,0.5) are considered only for a particular θ = [π/6]. High fluctuations in velocity and pressure are seen for the bluffest body in consideration (θ = π/2) with higher drag (cd) and total pressure loss (Δp0). On the other hand, the presence of a slit on a streamlined body (θ = π/6) tends to efficiently stabilize the wake and thus, producing almost a periodic shedding structure with lower cd and Δp0. For θ = [π/6], broadened spectra in vortex shedding are seen with a peak at [fL/u∞] ∼ 0.08. For θ {greater than or equal to} [π/4], a dominant discrete shedding frequency is seen with a gradual spectral decay. Similarly, the effects of ξ on the θ = [π/6] case produce a discrete shedding frequency instead of a broadened one, as told before. The shedding frequency increases to a maximum of [fL/u∞] ∼ 0.26 for the maximum slit size of ξ = 0.5. More insights on the shedding vortices, momentum deficit in the wake, varying energy contents in the flow field, and the dominant spatiotemporal structures are also provided.

https://aip.scitation.org/doi/pdf/10.1063/5.0097235

Unsteady dynamics in a subsonic duct flow with a bluff body

Two separate experimental campaigns of a spatially developing turbulent boundary layer under approximately zero-pressure-gradient at moderate Reynolds numbers ($1700<Re_{\theta} < 3400$) are conducted with stereoscopic Particle Image Velocimetry and one component Hot Wire Anemometry. This range of Reynolds numbers is found to be of particular interest for turbulent boundary layer control investigations.The motivations behind this work rely on the lack of recent studies which provide a rigorous experimental database on a flat plate turbulent boundary layer, openly available online. This is critical as, in most of the cases, the modification of the statistics resulting from turbulent boundary layer control strategies are compared with a smooth baseline reference.The statistics of the velocity fields, obtained with the two techniques, show a good match with the Direct Numerical Simulation in literature results. We focused on the skin friction evaluation by means of Clauser's chart technique.The near wall turbulence activity and the associated coherent structures are investigated by means of the Variable Interval Time Averaging technique using the hot wire signal. The influence of the acquisition and algorithm parameters as well as the effect of the Reynolds number are reported. The logarithmic and outer structures are investigated by applying the Uniform Momentum Zones technique to the PIV data-set. The hierarchical distribution of the uniform momentum zones as a function of the the wall distance as well as their variation with the Reynolds number confirm the validity of the Attached Eddy Model even at the moderate Reynolds numbers of the current investigation.

On the turbulent boundary layer over a flat plate at moderate Reynolds numbers

This study investigates the potential of finite-length porous surfaces with a subsurface chamber for the control of the turbulent boundary layer. The effect of the subsurface chamber on the boundary layer is investigated by hot-wire anemometry measurements of the boundary layer response to different chamber configurations. Three different chamber configurations were investigated: a common cavity that connected the array of surface perforations, a locally reacting chamber with individual cavities underneath each perforation, and chambers that connected the perforations in streamwise or spanwise flow directions. It was found that a common backing cavity and individual cavities reduced the peak turbulence intensity, whereas the test case with streamwise or spanwise channels increased the turbulence intensity and strengthened large-scale turbulent structures within the boundary layer. While both common and individual cavities were effective in reducing turbulence, the individual cavities created a larger reduction in the pre-multiplied spectrum with an average of 80% at large scales compared to between 40% and 60% reduction at large scales for common cavities with different volumes. Hence, a short porous surface with individual cavities underneath each perforation was found to be the most effective turbulence-reducing configuration among the investigated cases.

Finite-length porous surfaces for control of a turbulent boundary layer

The compressible flows past a wavy-axis square cylinder are numerically carried out by means of large-eddy simulation technique for two different free-stream Mach numbers (M∞=0.71 and 0.85) , which are less than the critical Mach number Mcr (≈ 0.9). The Reynolds number based on side-length of wavy-axis square cylinder is chosen as 4×105. For comparison, the compressible flows around the corresponding normal square cylinder are also calculated. The control effects and mechanisms are systematically analyzed. Comparing wavy-axis square cylinder with normal square cylinder for M∞=0.71 and 0.85, about 23.5% and 8.1% drag reductions are acquired, respectively, and the fluctuating forces are suppressed significantly. Based on the analysis of drag decomposition, when M∞<Mcr, the drag reduction related to vortex force prevails over that relevant to compressible effect. Moreover, wavy-axis square cylinder also can provide the effective control for type C moving shock. The effective drag reduction and suppression of fluctuating force obtained by wavy-axis square cylinder are closely associated with the higher base-pressure and lower turbulent fluctuations in the near wake, which can be achieved by the strengthened compressibility and waviness effect of shear-layer. However, when M∞ approaches Mcr, the effective flow control from wavy-axis square cylinder is attenuated due to the competition between strengthened compressibility and waviness effect of shear-layer.

Large-eddy simulation of the compressible flows around a wavy-axis square cylinder

The complex surface of an aircraft generates a nonzero pressure gradient flow. In this study, the boundary conditions of favorable and adverse pressure gradients are constructed in a small low-turbulence wind tunnel test section. Hot-wire anemometers and time-resolved image velocimetry are used to analyze the flow structure in a fully developed turbulent boundary layer with porous media. The effects of the porous surface on the statistical characteristics of the turbulent flow field and turbulent flow structure are analyzed and discussed. The results show that porous media reduce the velocity gradient in the linear layer, and the friction drag reduction effect is higher downstream of the porous wall. The drag reduction effect decreases along the flow direction. A wall with a 10 pores per inch produces a slightly better drag reduction effect than smooth wall. The maximum local drag reduction effect of a 10-pores-per-inch porous wall is 43.7% under a favorable pressure gradient and 42.3% under an adverse pressure gradient. The velocity streaks in the inner layer show that the porous wall widens the low-velocity streaks, making them more stable, while the high-speed streaks decrease in size under the pressure gradient. In the case of the adverse pressure gradient, the structure of the streaks becomes blurred, and their strength weakens. Under both favorable and adverse pressure gradients, the porous media lift up the coherent structures near the wall, thus weakening the large-scale coherent wall structures. 

Experimental study on drag reduction of the turbulent boundary layer via porous media under nonzero pressure gradient

Skin friction is a primary source of total aircraft drag, and it is important, therefore, in science and engineering to achieve drag reduction control in a boundary layer. In this paper, under the experimental conditions of Re q=5909 (x/c=0.55), and with a zero-pressure gradient, the drag reduction control of a plate boundary layer in porous media is studied. The global skin friction of the plate is measured using fluorescent oil film test technology. The results show that, in contrast with the downstream frictional resistance coefficient of a flat plate that possesses a smooth surface, the coefficient for porous media reduced significantly. And the lower the pores-per-inch (PPI) of the porous media, the greater the drag reduction effect. Among the three porous media with different PPI, porous media with 10 PPI has the best drag reduction effect. With increasing distance from the porous media, the drag reduction effect decreases gradually. Porous media significantly increases the slope of the logarithmic region of the velocity profile of the downstream turbulent boundary layer, the dimensionless wall-velocity u+ moves upward, and the velocity pulsation in the logarithmic region increases, so as to reduce skin friction.

Experimental study on flat plate skin friction control by porous media based on global fluorescent oil film measurement technology

Under low Reynolds number conditions, the airfoil is prone to a laminar separation bubble (LSB), which causes problems such as lift drop and drag increase. In this work, the fluorescent oil film was applied to study the leading edge LSB from the perspective of skin friction. The experimental study is carried out on a flat plate with the leading edge LSB under the Reynolds number of [Formula: see text] (the characteristic length of the plate chord). The camera captures the leading edge LSB that is formed by airflow separation and reattachment, and the flow in the bubble is opposite to the free flow. The position of separation and reattachment is determined by the distribution of skin friction, which is consistent with the visual image of the oil film. Applying the Poincare–Bendixson (P-B) index formula to test the topology of the separation bubble skin friction, the quantitative relations between the critical points and the boundary switching points in the closed region of several permeable boundary all confirm this topology law. The results show that the fluorescent oil film can quantitatively map the flow structure of the LSB, and further verify the advantage of this method in the visual measurement of the complex flow structure.

Fluorescent oil-film applied to measure skin friction of flat plate with leading edge separation bubble in low Reynolds number

To improve the low aerodynamic efficiency and reduce the high energy consumption of a single-stage circulation control wing, a multistage circulation control wing was designed. By combining force measurement and particle image velocimetry (PIV), the aerodynamic and flow-field characteristics of an aerofoil were investigated with respect to the increase in the number of blowing slots, changes in the blowing coefficient, and different blowing ratios for three slots. The force measurement results revealed that the maximum lift-to-drag ratio resulting from simultaneous blowing into the three slots increased by 95.3% compared with that in the absence of circulation control. With an increase in the blowing coefficient, two stages were observed: separation control and supercirculation control. In the separation control stage, the lift and drag coefficients significantly increased and decreased, respectively. In the supercirculation control stage, the lift coefficient gradually increased with the blowing coefficient, whereas the drag coefficient remained unchanged. When the blowing ratio (blowing flow ratio of three slots) in the three slots was 3:1:2, the maximum lift-to-drag ratio of the wing could reach 143.48%. The effects of different slot positions on the aerodynamic control were found to vary. The effects of Slot.1 and Slot.3 in terms of the drag reduction and lift, respectively, were evident, and the influence of Slot.2 on blowing between these two slots played a role in jet relay. The PIV results revealed that multistage blowing circulation increased the curvature of the trailing-edge streamline, thus increasing the equivalent aerofoil camber and improving the wing lift. At a high angle of attack, this circulation demonstrated a flow separation control effect.

Hai Du

Papers

Experimental study on drag reduction of the turbulent boundary layer via porous media under nonzero pressure gradient

Drag reduction in cylindrical wake flow using porous material

Experimental study on flat plate skin friction control by porous media based on global fluorescent oil film measurement technology

Fluorescent oil-film applied to measure skin friction of flat plate with leading edge separation bubble in low Reynolds number

Effect of Multistage Circulation Control on Blade Aerodynamic Performance