Session U17: Flow Control: Drag Reduction II

Shark-inspired high-low alternating riblets by inkjet printing of UV-curable ink

Riblets reducing fluid friction drag were originally inspired by ridges of dermal denticles of sharks. In some shark species including a great white shark (Carcharodon Carcharias), ridge height in a denticle is not uniform: the middle ridge is high and the side ridges are low. We hypothesized that the high-low alternating ridges adapt to wide range of swim speed in the sharks. In this study, we realized the riblets with high-low alternating ribs by inkjet printing of UV-curable ink on flat plates, followed by drag measurement in a water tunnel. The printing method enabled us to fabricate and compare different riblet designs with low cost. The height and spacing of the riblets were determined by our micro X-ray CT observation of the denticles of a great white shark for different locations in the body. Friction speed of the each location was estimated by modeling the body as a flat plate. The size scale of the riblets and range of flow speed of the water tunnel were decided to simulate the wide range of swim speed of the shark. The results of the measurement support our hypothesis and suggest that the high ribs are effective for low speed and the low ribs work for high speed.   

Hydrodynamic performance enhancements of shark skin-inspired surfaces on foils and robotic fish fins

Enhancements to improve the hydrodynamic performance of unpiloted underwater vehicles (UUVs) are crucial for next-generation designs, with applications in nearshore and ocean monitoring. One approach is to mimic advantageous biological features, such as shark skins for turbulent drag reduction and fish fins for maneuverability. In particular, sharks have dermal denticles, or microstructures that decrease skin friction. Bioinspired designs using rigid denticles on rigid or compliant surfaces have demonstrated increased lift-to-drag ratios at low angles of attack in laminar flow, but have not yet been tested experimentally at higher Reynolds numbers, or used compliant denticles. To address these open questions, we manufactured denticle panels using stereolithography, collected hydrofoil force measurements in a recirculating water tunnel with flow speeds up to 3.5 m/s, and validated preliminary results of a 3D unsteady Navier-Stokes solver. Denticles were then applied to robotic fish fins to test the combined effect of multiple bioinspired features on swimming performance. Future work will focus on improving the speed, energy efficiency, and antifouling properties of UUVs to enhance persistence and minimize vehicular noise signatures.   

Modeling effective transpiration over riblets to improve drag predictions

Riblets are flow-aligned micro-grooves that reduce turbulent skin friction below that of a smooth wall. The drag reduction has been quantified by viscous protrusion-heights theory (Luchini et al., J. Fluid Mech., vol 228, 1991, pp. 87-109). The theory uses viscous (Stokes) flow to characterize the offsets of the mean flow and turbulence above small riblets, where the flow is similar to that above a smooth wall (Luchini, Comp. Meth. Appl. Sci., vol. 3, 1996, pp. 466-470). In this talk, I will assess direct numerical simulation data against the theory and propose an extension to the theory. The assessment points to the importance of crest-transpiration effects (Ibrahim et al., J. Fluid Mech., vol. 915, 2021, pp. A56), which is not accounted for in the protrusion-heights theory, and I will discuss a reduced-order calculation of turbulence structures to incorporate this effect.   

Influence of Incidence Angle on the Flow Past Tandem Cylinders with Rounded Corners

The present study focuses on the variation of wake structures and aerodynamic forces with changes in the corner radius, spacing, and orientation of Cylinders. Numerical simulations were performed for flow past Tandem square cylinder with different corner radii placed at an angle to the incoming flow. The fixed Reynolds number (Re.) 100 is selected for the present study. However, corner radius to diameter ratio, R/D = 0 to 0.5 and L/D = 1.5 to 5 spacing between two cylinders and incidence angle α = 0°–90° is used as a varying parameter. The flow visualization parameters, the drag and lift coefficients are comprehensively presented and compared for different cases in order to reveal the effect of corner radius, gap spacing, and incidence angle on the behavior of the flow. The presented results shows the highly importance of the incidence angle and rounded corners of the tandem cylinders for reduction of aerodynamic forces.   

Shape optimisation for a stochastic two-dimensional cylinder wake using ensemble variation

This talk concerns the optimisation of a two-dimensional cylinder shape to minimise the mean drag in the presence of random noise at Reynolds number Re=100. The noise is provided by the Ornstein-Uhlenbeck process added to the free stream inlet velocity. Because of the added mass effect, the noise introduces large fluctuations to the instantaneous drag. The optimisation problem is solved using an ensemble-variation method (EnVar). To keep the same internal cylinder space, we constrain the cylinder cross-section area to be a constant value. The cylinder shape is built with Fourier coefficients. We also added a penalty term to avoid non-smooth cylinder surfaces. The optimal shape that minimises the mean drag is found to be close to an oval, the rear side of which is a little blunter than the front. Moreover, it is shown that this oval shape is robust to the noise level because the stochastic oscillations do not significantly alter the cylinder wake mean flow. We observed that the optimisation process significantly reduces the pressure drag component related to the vortex shedding. However, the viscous drag originating from the cylinder surface is marginally increased by 3.5%. Finally, the total drag is decreased by 23.5%. 

    

Effect of Surface Roughness and Trips on the Drag of a Circular Cylinder at Subcritical Flow

Aerodynamic drag is the major source of resistive force on elite and amateur athletes in speed sports such as cycling. Over a circular cylinder, typically used as a low-order model of an athlete's leg or arm, passive flow control methods have shown to be effective towards reducing aerodynamic drag. In the range of Reynolds numbers experienced by cyclists, surface roughness and boundary layer trips can reduce the aerodynamic drag by transitioning the local boundary layer at the surfaces to turbulence resulting in a drag crisis at a lower range of Reynolds numbers. Experiments were conducted to study the impacts of surface roughness and trip on the drag and vortex shedding of a circular cylinder. The cylinder model was covered with textile sleeves made from seven different fabrics designed to provide a variety of surface roughness. Moreover, each type of fabric possessed one, two, three, and four spanwise seam configurations to trip the flow. The drag was measured in a closed-loop wind tunnel in the range of 40000 < Re < 120000 using two parallel mounted load cells and the Constant Temperature Anemometry technique was employed to measure the vortex shedding frequency. Results indicate that the drag reduction of the cylinder for the Reynolds number range of cyclists can be achieved when the surface roughness is combined with the seams. The number of seams and their location with respect to the freestream are shown to be influential parameters. For most of the fabrics, a drag reduction of up to 25% and a decrease in the vortex shedding frequency are observed compared to the smooth cylinder in the subcritical regime. However, for some cases with specific seam configurations, a significant decrease in the critical Reynolds number is detected, which leads to around 40% drag reduction and an increase in the vortex shedding frequency in the range of Reynolds numbers considered.   

Control of Reverse Flow using Trailing Edge Morphing

At high speeds, a portion of the retreating blade of a rotorcraft can enter a regime of reverse flow. The reverse flow starts to separate at very low angles of attack, leading to an increase in drag, production of negative lift, and a pitching moment impulse. In addition, periodic formation and shedding of the separation bubble during dynamic pitching leads to a large increase in hysteresis. The proposed method for mitigating these negative effects is a trailing edge reflex camber over the final quarter chord of the blade. Experiments were performed on a 20° forward swept blade and 20° backward swept blade with and without trailing edge morphing. Load cell measurements were taken at a chord-based Reynolds number of 2.38 × 10⁵ across a wide range of angles of attack. Stereoscopic Particle Image Velocimetry (SPIV) was also performed at the same Reynolds number at a single angle of attack. The load cell measurements showed a decrease in adverse effects when using the cambered geometry for both sweep angles. The SPIV results showed a significant reduction in the size of the separation bubble over the cambered blade, which was the cause of the reduction in the adverse effects.    

Control of Shockwave Boundary Layer Interaction using Plasma-Assisted Shock Control Bump

The shockwave boundary layer interaction (SWBLI) is a key phenomenon in high-speed flows. Many attempts, active and passive, have been made to manipulate SWBLI. Shock control bumps (SCBs) are passive flow control devices that have shown control authority in transonic and supersonic applications. At on-design conditions, SCBs reduce wave drag significantly, while increasing viscous drag slightly. However, at off-design conditions, the SCB promotes boundary layer thickening which drastically increases viscous drag and causes boundary layer separation. A combination of two-dimensional SCBs with plasma actuators is proposed to address the off-design limitations of SCBs when used alone. Dielectric barrier discharge (DBD) actuators driven by nanosecond repetitively pulsed (NRP) plasmas will be imbedded in the surface upstream of the SCBs to perturb the impinging shock. The control authority of the combined plasma actuators and SCBs will be assessed in Mach 2 flow using characterization of the base flow and controlled flow by streamwise pressure measurements and schlieren visualization.   

Author not Attending

This study reports on the experimental application of Machine Learning (ML) methods for the optimization of the open-loop Active Flow Control (AFC) via the Synthetic Jet (SJ) technology. Specifically, the AFC of the turbulent wake past a cylinder is addressed with the purposes of drag reduction and vortex shedding suppression. SJ actuators have been proven to be a promising tool in this direction, due to both their effectiveness in separation control and advantageous features, like compactness and small power requirements. In this work, two ML techniques, the Deep Reinforcement Learning and the Genetic Programming Control, are applied to seek for optimal control strategies in terms of the temporal law of the SJ actuator driving signal, thus overcoming the limitations inherent to conventional optimization methods. Experimental tests are carried out in a subsonic wind tunnel on a circular cylinder equipped with a SJ located at the rear stagnation point. An a-posteriori flow field analysis is also performed via particle image velocimetry. Therefore, the present investigation aims at both assessing feasibility of ML approaches for flow control on the experimental side and improving fundamental knowledge on the behavior of SJs and their interaction with the wake past bluff bodies.

    

Large-scale control in turbulent channels over surface riblets

As a passive drag control method, surface riblets have been studied for a long time. However, its drag reduction rate is no more than 10%, affected by many riblets issues such as shape, size and arrangement. Recently, a large-scale control scheme has been developed via spanwise opposed wall-jet forcing, generating a pair of counterrotating streamwise swirls that combine near wall streaks together and hence reduce drag. Thus, it is interesting to check whether a pair of large-scale swirls over surface riblets would lead to drag reduction higher than each of the individual control method. Here, we report direct numerical simulations of turbulent channel at Re_τ=180, with equicrural-triangle riblets (height h⁺=10, width s⁺=20) and large-scale swirls (controlling-height y_c⁺=30, spanwise size λ⁺=1200). It is found that the new method leads to at least 16% drag reduction (could be higher by different placements of riblets in the spanwise direction), in contrast to the mere surface riblets control (which is around 3%). As a conceptual validation, our results support the perspective of drag control for higher Reynolds number by generating large-scale swirls over various surface modifications.   

Application of Flow Control in the Slat Cut-Out Region for Lift Enhancement

The introduction of ultra-high bypass ratio (UHBR) engines and associated increased in nacelle size necessitates a cut-out in the leading edge slat, which induces separated flow during take-off and landing resulting in a loss of lift and increased drag. Active flow control (AFC) is a promising technology with several potential applications for improving the performance of an aerodynamic body. Pulsed jet AFC has been shown capable of suppressing turbulent boundary layer separation by blowing air from the wing surface to create streamwise vortices which energise the boundary layer. AFC has been proposed as a potential solution to recover the loss in lift coefficient due to a slat cut-out. The purpose of this study – part of the Clean Sky 2 WINGPULSE project (https://cordis.europa.eu/project/id/887092), is to computationally and experimentally investigate AFC strategies in the slat cut-out region. The goal is to recover lost maximum lift coefficient. This will be demonstrated by conducting numerical (CFD) and and experimental (wind tunnel) tests. Force balance measurements to measure lift and drag coefficients, and pressure taps to measure chord-wise pressure coefficients are used in high-lift configuration of a representative wing (swept wing model with a high-speed DLR wing section and Fowler flap), which has been used in prior AFC studies on slat cut-outs. To-date, 3D numerical simulations and force balance tests have been conducted for the no flow control case and baseline flow control case. Further flow control cases will be explored, aimed at maintaining the aerodynamic benefit of AFC while reducing the net mass flow and hence energy input.   

Effective composite control for drag reduction at high Reynolds numbers

We explore further the composite drag control (CDC) [Yao, Chen & Hussain, Phys. Rev. Fluids 2, 062601(R) (2017)] that combines opposition control (OC) and spanwise opposed wall-jet forcing (SOJF) methods together via direct numerical simulation of incompressible turbulent channel flow. For three Re_τ's (180, 395 & 550), we achieve drag reductions above 30% -- much higher than obtained by either individual method (e.g. 14% for SOJF and 22% for OC at Re_τ=550). The net power saving is >30% as less power is required. Flow analysis shows that CDC is more effective in suppressing the random turbulence than OC, and the coherent motion is also suppressed more compared to SOJF due to the reduction of near-wall vortical structures by OC. The distribution of wall shear stress shows structures periodic in the streamwise direction, and corresponding spectrum show a sharp peak at k_x^+_~7*10-4 . Vorticity field analysis shows that the turbulent transport of spanwise vorticity in the normal direction also displays periodic structures. Moreover, the triple decomposition (mean, coherent and random) supports our previous results that coherent vorticity transport and random transport of wall normal vorticity in spanwise direction reduce drag, while random normal transport of spanwise vorticity increases drag. In summary, our results strongly promote prospects for employing CDC for effective skin friction drag reduction at high Reynolds numbers.