Low-speed Flow Research Articles

Quantifying the Tip Leakage Vortex Wandering in a Low-Speed Axial Flow Fan via URANS Simulation

The tip leakage vortex is a dominant noise and aerodynamic loss source of low-speed axial flow fans. The tip leakage vortex often has an unsteady behavior, like the periodic motion of the vortex core: the so-called vortex wandering. This periodic vortex wandering results in additional noise, aerodynamic loss, and an oscillation in the moment acting on the blading, which causes an unfavorable vibration of the rotor. In the research campaigns focusing on vortex wandering, complicated measurement techniques (PIV) or high-fidelity but computationally costly simulations are commonly used. The present paper aims to introduce a relatively cost-effective unsteady Reynolds-averaged Navier-Stokes simulation-based investigation method of vortex wandering. The capabilities of the proposed technique are presented through a low-speed axial flow fan case study.

Fabrication and characterization of austenite-ferrite stainless steel bimetals by twin wire arc additive manufacturing

In this work, gas metal arc welding (GMAW) based twin wire arc additive manufacturing (TWAAM) system has been employed to deposit to wire materials simultaneously to fabricate austenite-ferrite stainless steel (AFSS) bimetal of austenite stainless steel (SS304L) and ferrite stainless steel (SS430L). A combination of high voltage and wire feed with low torch speed and gas flow resulted in optimal deposition as determined through response surface analysis based on experimental data of single-track bimetal bead on plate depositions. Mechanical and microstructural characterization of the AFSS bimetal was carried out using tensile test, hardness tests, fractography, SEM with EDS, and phase analysis. Tensile properties of the AFSS bimetal were seen to improved with the yield strength of 343 MPa, ultimate strength of 613 MPa and elongation of 67%, and a hardness value of 225 HV. The interface of the AFSS bimetal was found defects free and microstructures contained both austenite and ferrite phases with higher amounts of Ni-content. The fracture morphology of the fabricated AFSS bimetal was seen to contain dimple sizes lying between that of individual steels and resulted in higher elongation than that of the pure ferrite steel. This study demonstrated defects free fabrication of bimetallic structures with unique microstructural features and enhanced mechanical properties.

The mechanical environment regulates the extent of aortic valve calcification and thickness: an ex vivo whole mouse heart study

Abstract Background Aortic stenosis (AS) is a narrowing of the aortic valve opening due to calcification and thickening of the leaflets. Calcifications are mostly located at regions with high mechanical stress and disturbed flow suggesting that mechanical environment is important in the regulation of calcification. However, direct evidence linking valve calcification and mechanical stress is still lacking. Recently, we established an ex vivo calcification model for whole mouse hearts (Miniature Tissue Culture System MTCS) which allows the study of the mechanical environment underlying aortic valve calcification. Purpose Aim of this study was to investigate the role of mechanical stress on aortic valve calcification and thickening using the MTCS. Methods Whole mouse hearts were cultured in the MTCS with 2 distinct configurations: 1) medium flowing from the aorta towards the aortic valve to create a continuous closed aortic valve (n=28) and 2) medium flowing from the left ventricle through the aortic valve towards the aorta to create a continuous open aortic valve (n=17) (Figure1). The hearts were subjected to low (300 ul/min) or high (3000 ul/min) flow speed in the presence of calcifying medium (3mM phosphate buffer). After culture for 1 week, the hearts were isolated and fixed overnight with 4% PFA/PBS. Alizarin red staining was performed to determine calcification presence. Quantifications were performed using Caseviewer software. Results Aortic valves in closed configuration exposed to a high flow speed, exhibited significant increase in calcification compared to aortic valves in the closed configuration exposed to low flow speed or aortic valve cultured in the open position (p<0.01, Figure2A). Of interest, the extent of calcification in the aortic valve in closed position exposed to high flow speed strongly correlated with the diameter of the aortic annulus (R²=0.86, p<0.0001, Figure2B). Conversely, leaflet thickness was significantly lower in the closed aortic valve configuration as compared with the open aortic valves configuration (p<0.05, p<0.001, Figure2C). Conclusion Our study demonstrates the influence of mechanical environment on aortic valve calcification and thickening. The presence of calcification in the closed but not the open aortic valve configuration exposed to high flow suggests that increased pressure promotes calcification. The correlation of the extent of aortic valve calcification with the diameter of the aortic annulus implies that configuration of the valve is an important determinator of calcification, with potential implications for the results of aortic valve repair. The presence of a minimal amount of mechanical stress as experienced by aortic valves in the open position, however, resulted in thicker aortic valves, indicating that the right balance of mechanical stresses is required to maintain normal valve morphology. These findings provide valuable insights into AS pathogenesis, guiding future therapeutic approaches.ex vivo cultured aortic valvesCalcification in ex vivo aortic valves

Optimization of hydraulic structure and well flushing parameters of a truncated cone drill bit for drilling shaft sinking

ABSTRACT Based on a research method combining numerical simulations and model testing, a numerical model and test platform for gas lift reverse circulation multiphase coupling in well washing were developed. The hydraulic structure and well washing parameters of a φ5 m truncated cone bit used in the Beifeng well at Taohutu Coal Mine were optimized, and the distribution law of the well washing flow field was determined. The findings revealed that: (1) with a bit taper of 40 and a length-diameter ratio of 0.32 for the truncated cone, the hydraulic structure was optimal. Compared with the original bit structure, the axial speed of mud at the slag suction port increased by 15.9%, and the average sliding speed of mud at the inclined rock surface increased by 63.2%. (2) The multiphase fluid in the slag discharge pipe flowed successively in the form of “low-speed stable two-phase flow” and “high-speed disordered three-phase flow,” while the multiphase fluid in the horizontal and inclined bottom holes flowed predominantly in radial and tangential patterns, respectively. (3) The optimal parameter combination for efficient well washing with the truncated cone bit included a bit rotation speed of 8 r/min, a gas injection rate of 5600 m3/h, an air duct sinking ratio of 0.79, and a mud viscosity of 176 mPa·s, with bit rotation speed being the most significant factor affecting effective washing efficiency. These results could provide a valuable reference for improving the efficiency of drilling and well flushing in Jurassic strata of western China.

Variance-reduction kinetic simulation for characterization of surface and corner effects in low-speed rarefied gas flows through long micro-ducts

Variance-reduction kinetic simulation for characterization of surface and corner effects in low-speed rarefied gas flows through long micro-ducts

A vibration-based method to estimate the low-wavenumber wall pressure field in a turbulent boundary layer

When an elastic structure is excited by a low-speed turbulent flow, it generates flow-induced vibration which is primarily due to the low-wavenumber components of the wall pressure field (WPF) beneath the turbulent boundary layer (TBL). Therefore, to accurately predict the vibration response of the structure, an accurate estimation of the low-wavenumber WPF is needed. While existing TBL models for WPF well predict the convective region, they exhibit significant discrepancies in predicting the low-wavenumber levels. This numerical study aims to explore the feasibility of estimation of the low-wavenumber WPF by analysing vibration data obtained from a structure excited by a TBL. An inverse method is proposed based on the relationship between the TBL forcing function and structural vibrations in the wavenumber domain. The random TBL force is simulated with deterministic loading using the uncorrelated wall plane wave technique. A model of a simply supported plate under a TBL excitation is developed to demonstrate the proposed method. The plate's acceleration data is then used to estimate the WPF in the low-wavenumber range. It is shown how using multiple discrete frequencies in the analysis can reduce the required snapshots for accurate estimation of the WPF.

Enhancing LSPIV accuracy in low-speed flows and heterogeneous seeding conditions using image gradient

Flow measurement in rivers and channels is crucial for water resource management and infrastructure planning, especially under the context of climate change. However, traditional methods like mechanical current meters and hydroacoustic instruments face limitations in terms of cost, intrusiveness, and accessibility. In recent years, image-based velocimetry techniques have emerged as promising alternatives due to their non-contact nature and cost-effectiveness. Nevertheless, persistent challenges remain, particularly concerning the uniform distribution of surface tracers necessary for precise measurements. These challenges are particularly pronounced in cases involving artificial seeding, where ensuring uniform distribution poses a significant obstacle. To address this issue, this study presents a novel methodology for filtering Large Scale Particle Image Velocimetry (LSPIV) data based on indicators of pixel intensity gradients. The methodology was evaluated across various field measurements under low flow conditions, encompassing a wide range of seeding characteristics. The evaluations demonstrated improvements in mean surface velocity profile estimation, showing reductions of up to 70 % in normalized root mean square error compared to not using filters. Additionally, the results were compared with filters typically employed by experienced LSPIV users, such as background subtraction and cross-correlation coefficient thresholds, showing improvements with the proposed filter. Implementation of the proposed strategy reduces the subjectivity in LSPIV implementation, particularly for users with limited knowledge of the technique, but also require minimal post-processing efforts. The methodology is anticipated to be integrated into existing software tools, thereby enhancing the accessibility of LSPIV for individuals with limited expertise in image velocimetry. Overall, this methodology facilitates cost-effective expansion of hydrological information availability, particularly in resource-constrained regions.

A DSMC-CFD coupling method using surrogate modelling for low-speed rarefied gas flows

A new Micro-Macro-Surrogate (MMS) hybrid method is presented that couples the Direct Simulation Monte Carlo (DSMC) method with Computational Fluid Dynamics (CFD) to simulate low-speed rarefied gas flows. The proposed MMS method incorporates surrogate modelling instead of direct coupling of DSMC data with the CFD, addressing the limitations CFD has in accurately modelling rarefied gas flows, the computational cost of DSMC for low-speed and multiscale flows, as well as the pitfalls of noise in conventional direct coupling approaches. The surrogate models, trained on the DSMC data using Bayesian inference, provide noise-free and accurate corrections to the CFD simulation enabling it to capture the non-continuum physics. The MMS hybrid approach is validated by simulating low-speed, steady-state, force-driven rarefied gas flows in a canonical 1D parallel-plate system, where corrections to the boundary conditions and stress tensor are considered and shows excellent agreement with DSMC benchmark results. A comparison with the typical domain decomposition DSMC-CFD hybrid method is also presented, to demonstrate the advantages of noise-avoidance in the proposed approach. The method also inherently captures the uncertainty arising from micro-model fluctuations, allowing for the quantification of noise-related uncertainty in the predictions. The proposed MMS method demonstrates the potential to enable multiscale simulations where CFD is inaccurate and DSMC is prohibitively expensive.

Investigation of varying tip clearance gap and operating conditions on the fulfilment of low-speed axial flow fan

Abstract The present study aims to identify the suitable tip clearance and volumetric flow rates for low-speed axial flow fans. The performance of an axial fan depends on various parameters like fan speed, available tip clearance gap, volumetric flow rate, power consumption, and working fluid. Leakage flow occurring at a rotating component such as a blade and solid casing of a ducted axial fan is typically linked to losses and the potential emergence of a rotating stall. The numerical analysis for this study uses the Reynolds Averaged Navier–Stokes equations with the k-omega SST turbulence model to perform the steady-state simulations by varying tip gap and volumetric flow rate. The results proved that the optimum performance was obtained with a tip clearance gap of 1 mm, and maximum fan efficiency was achieved at a volumetric flow rate of 3.9–4.5 m3/s. The novelty of this proposed work is to enhance the efficiency of axial flow fans with circular arc-cambered aerofoils using optimum tip clearance and volumetric flow rate through steady-state simulations. This method can be used in turbomachinery to improve fan performance.

Measurement of unsteady force on rotor blade surfaces in axial flow compressor

To assess the aerodynamic performance and vibration characteristics of rotor blades during rotation, a study of unsteady blade surface forces is conducted in a low-speed axial flow compressor under a rotating coordinate system. The capture, modulation, and acquisition of unsteady blade surface forces are achieved by using pressure sensors and strain gauges attached to the rotor blades, in conjunction with a wireless telemetry system. Based on the measurement reliability verification, this approach allows for the determination of the static pressure distribution on rotor blade surfaces, enabling the quantitative description of loadability at different spanwise positions along the blade chord. Effects caused by the factors such as Tip Leakage Flow (TLF) and flow separation can be perceived and reflected in the trends of static pressure on the blade surfaces. Simultaneously, the dynamic characteristics of unsteady pressure and stress on the blade surfaces are analyzed. The results indicate that only the pressure signals measured at the mid-chord of the blade tip can distinctly detect the unsteady frequency of TLF due to the oscillation of the low-pressure spot on the pressure surface. Subsequently, with the help of one-dimensional continuous wavelet analysis method, it can be inferred that as the compressor enters stall, the sensors are capable of capturing stall cell frequency under a rotating coordinate system. Furthermore, the stress at the blade root is higher than that at the blade tip, and the frequency band of the vibration can also be measured by the pressure sensors fixed on the casing wall in a stationary frame. While the compressor stalls, the stress at the blade root can be higher, which can provide valuable guidance for monitoring the lifecycle of compressor blades.

Influence of tip air injection on the unsteady blade force under circumferential pressure distortion

In order to further understand the impact of tip air injection on compressor stability under circumferential pressure distortion, a series of experimental studies on tip air injection under uniform and circumferential distortion were conducted on a low-speed single rotor axial flow compressor. The unsteady measurements were carried out by use of microsensors and strain gauges bonded on the rotor blade surfaces, and a collection of dynamic pressure sensors installed on the casing wall. Results indicate that with the increase in the injected momentum ratio, the load at the leading edge near the rotor blade tip obviously decreases, thereby delaying the stall. By comparing the pressure distribution on the rotor blade surface under different injected momentum ratios, tip air injection can effectively suppress flow separation at the blade tip to enhance flow capability. In addition, it can also weaken the fluctuation of tip leakage flow and reduce the dynamic stress similar to natural vibration near the blade root both in stationary and rotating coordinate systems. Under the condition of inlet circumferential distortion, tip air injection can significantly reduce the load at the distorted region and suppress the low-frequency disturbances measured on the rotor blade surface caused by inlet distortion. In addition, the dynamic stress near the blade root can also be effectively reduced by tip air injection. It implies that tip air injection can both extend the stall margin and reduce blade vibration when suffering from circumferential distortion; thus, the compressor can safely operate.

Efficiency high-order discontinuous Galerkin method for low speeds flows with time-derivative preconditioning

A local preconditioning high-order discontinuous Galerkin (DG) method is introduced to enhance the efficiency and accuracy of the density-based approach for low-speed flows. The method is based on the time-derivative preconditioning technique typically employed in finite volume methods. Traditional approximation methods that only precondition low-order derivatives lead to incompatibility of the equations. To extend the preconditioning technique to the DG method, we employ an analytic preconditioning mass matrix derived from the DG formulation of the preconditioned Navier–Stokes equations. Modified numerical flux functions and a self-adaptive local velocity truncation parameter are employed for DG systems with preconditioning. We demonstrate the efficiency and accuracy of the preconditioned DG method using explicit and implicit schemes for steady flows and a dual-time scheme for unsteady flows. The method has been implemented and used to compute a variety of flow problems, including inviscid and laminar flows, utilizing various degrees of polynomial approximations. Computations with and without preconditioning are performed to analyze the influence of spatial discretization on the accuracy and convergence of the DG solutions at low Mach numbers. Numerical results underscore the enhanced accuracy and convergence properties of the proposed method for low-speed flows, indicating significant performance improvements over traditional methods.

Experimental study on enhancing the performance of hydrokinetic integrated Darrieus-Savonius rotor in open water-channel

The kinetic energy stored in water streams could be used to generate electricity via hydrokinetic rotors, due to the availability of kinetic energy from flowing water. Darrieus and Savonius are vertical axis kinetic rotors, they are promising for generating power in low speed flows. The integrated Darrieus- Savonius hydrokinetic rotor works on the principles of integrated forces including lift and drag forces. An experimental investigation has been conducted to get the optimum configuration for Darrieus-Savonius integrated rotor in order to attain improved self-starting and high-power-factor in open channel water flow, under different inflow conditions. This study investigated the effects of the radius ratio, add-on angle, water heights, and flow velocity on performance of hydrokinetic integrated Darrieus-Savonius rotor with three standard NACA 020 airfoil shape as Darrieus rotor and single/double stage semi-circular blades as Savonius rotor on the power factor. Three radius ratios (R.R) namely (0.8, 0.6 and 0.4) and add-on angles of (0o, 30o, 45o, 60o and 90o) were studied. The experimental channel was (26 m) long water flume with a (1m wide ×1.2 m depth). The horizontal walls of the flume were made of glass with a strengthened frame, to permit visual examination. The flume entrance consisted of a receiving tank (3 m long, 1.5 m wide, and 0.75 m depth), a honeycomb pattern pipe box, and a screen box field with a large stone to dissipate the water turbulence and energy at the entrance. Two centrifugal pumps were used to supply water to the channel. Darrieus-Savonius integrated rotor was placed at distance equal to ten times width of the experimental channel, to avoid any turbulence and to make sure that the flow was steady.Based on the experimental results, it was observed that the optimum Cp values at λ (1.84), add-on angle of (45o) and R.R (0.4, 0.6 and 0.8) were (0.335, 0.29 and 0.299) respectively. The maximum hybrid performance was attained at R.R (0.4), add-on angle (45o) and tip-speed ratio (1.8) and Re (240x106) with value of (0.339). Decrease in the Re values by 25 % enhanced the performance by 1.19 %. Accordingly, it is not necessary to rely on Re values in calculating the power-factor values compared to the influence of the rest of the variables.

Study on secondary motions in supersonic boundary layers of a bent pipe

The present study employed direct numerical simulation to investigate the supersonic flow of Mach 3 in a bent pipe with a curvature of 0.0825, elucidating the dynamic mechanism of secondary motions within the turbulent boundary layer. The findings indicate that the compressible flow, affected by the wall curvature, is differentiated into several motion patterns as the bending angle increases: a portion of the outer fluid close to the wall, driven by the circumferential pressure gradient, moves inward through the lateral wall, causing an increase in the mass rate toward the lateral boundary layer and promoting the circumferential transport of energy and vorticity; other outer fluids at the start of the bent section, due to the centrifugal force, approach the wall to form a thinner boundary layer downstream; meanwhile, the fluid near the inner wall experiences the expansion, followed by the flow separation and reattachment at a bending angle of 14.6° and 22.0°, respectively, which induce a shear layer that develops from the inner end point toward the mainstream center, gradually reshaping the high-speed flow area within the pipe cross section into a U-shape, and enhancing the vorticity and temperature field of the inner region. Additionally, this study reveals a remarkable phenomenon that the separated flow in a localized inner region forms a rotating field, inducing vortices distinct from the mainstream Dean vortices in the low-speed flow region enclosed by the shear layer.

Experimental study on pulsed suction under different momentum coefficients in variable compressor cascade with annular gaps

Variable Stator Vanes (VSVs) with penny cavities are crucial in the compressors of variable cycle engines, where complex interactions between annular and radial gaps can adversely affect aerodynamic performance. Unsteady active flow control techniques have significant potential for reducing secondary flow losses in compressors. This research investigates the internal flow characteristics of a variable compressor cascade with penny cavities using low-speed wind tunnel experiments and flow visualization techniques. This study is the first to experimentally explore the relationship between loss reduction and momentum coefficient using pulsed suction (PS) compared to steady continuous suction (SCS), evaluating the effects of various excitation frequencies. The findings show that at a momentum coefficient of 0.11 and an excitation frequency of 100 Hz, PS achieves optimal aerodynamic performance, reducing total pressure losses by 11.59 %, marking a 92.59 % improvement over SCS. As the momentum coefficient increases, the effectiveness of PS control significantly improves, whereas the changes in SCS effectiveness are less pronounced. The control effects of PS under varying excitation frequencies are complex and influenced by multiple factors. At higher momentum coefficients, the loss reduction effectiveness of PS is strongly correlated with excitation frequency, while at lower coefficients, performance changes due to frequency variations are minimal. This study provides valuable guidance for the design of modern multistage variable compressors.

High-order compact gas-kinetic scheme in arbitrary Lagrangian-Eulerian formulation

This study proposes an extension of the high-order compact gas-kinetic scheme (CGKS) to compressible flow simulation in an arbitrary Lagrangian-Eulerian (ALE) formulation in unstructured mesh. The ALE method is achieved by subdividing arbitrary mesh into tetrahedrons and integrating flux function in a local coordinate system at the cell interface to ensure geometric conservation law. The scheme incorporates a compact reconstruction with third-order accuracy for updating both cell-averaged conservative flow variables and their gradients. HWENO-type nonlinear reconstruction and gradient compression factors are adopted to improve the accuracy and robustness of the scheme. A multi-stage multi-derivative (MSMD) time-stepping method is also implemented to achieve high-order time accuracy with fewer middle stages. The scheme is used to study problems involving moving boundaries. The numerical experiments demonstrate the effectiveness of the scheme in capturing the accurate solutions of both low-speed smooth flow and highly compressible ones with strong shock waves.

Feasibility Study For Large Scale Thermal Convective Flow Measurements In Building Physics

The Department of Building Physics / Energy-efficient Buildings at the Rheinland-Pfälzische Technische Universität Kaiserslautern-Landau is about to put a novel research-building into commission, the “Small House III”. Among other measurement equipment, a system for volumetric, large-scale, high-resolution flow velocity measurement should be utilized. In this paper, requirements for such a system are detailed, a suitable measurement system is proposed and a feasibility study for the verification of the system requirements is conducted. To this end, a tailored system for large-scale Lagrangian particle tracking (“Shake-the-Box”) is applied to thermal convective flow measurements above an electric radiator close to a wall. As a result, it is found that the designed volumetric velocity measurement system is capable of successfully operating in the design environment without minimal disturbance of the low-speed convection flow. . After a relatively short setup time (about 5 hours, including camera setup, calibration and self-calibration) up to 10000 images at 50 Hz (200 seconds) have been recorded continuously. Shake-the-Box results show a decline of the number of tracked particles from initially 48000 to about 8000 (with a half-life span of about 100 seconds for the helium filled soap bubbles used for seeding). Applying fine scale reconstruction (data assimilation), a vector spacing of 4 mm could be achieved for instantaneous volumetric velocity snapshots. Averaging resulted in a 2 mm vector spacing with an average of 3300 particles per bin and 410 particles per bin for 1 mm vector spacing (with 75% bin overlap). Weaker particles could only be detected in front of a black background. Using a white background (default background for later applications), weak particles could not be detected as well, so the number of detected particles and the measurement volume decreased slightly compared to the black background. Overall, the feasibility study successfully confirmed the readiness of the measurement system for the intended use for “Small House III” measurements.

Effect of low blockage ratio obstacle on explosion characteristic in methane/air mixture

Methane/air explosion is one of the common hazards in process and mining industries. In this study, the methane explosion propagation is studied, and two-dimensional configuration is considered. The gas phase equations are solved using an OpenFOAM code for compressible reacting flow, EXiFOAM. The effects of the low blockage ratio (LBR) obstacles are investigated. The results show that the flame propagation speed, flame structure, shock wave propagation, and gas flow are considerably affected by LBR obstacles. Specifically, the flame propagation speed first increases and then decreases. As the initial pressure increases, the flame propagation speed gradually increases. The maximum speed is up to 500 m/s, when the initial pressure is 1.5 MPa. If the flame propagation is prevented, resulting in the flame deformation. Moreover, the flame disruptive phenomenon is captured due to the reflected waves. In front of the obstacle, the cellular structures of the high-pressure distribution are formed. Note that the tiny cellular structures are produced in the wake of the leading shock. Furthermore, a high-speed flow region and two low-speed flow regions are produced around the LBR obstacles. It is found that as the initial pressure increases, the explosion pressure is larger, while the temperature change is not obvious. These research findings have implications for enhancing safety production in coal mines.

Direct numerical simulation of supersonic boundary layer transition induced by gap-type roughness

The transition of the supersonic boundary layer induced by roughness is a highly intricate process. Gaining a profound understanding of the transition phenomena and mechanisms is crucial for accurate prediction and control. In this study, to delve into the flow mechanisms of a transition in a supersonic boundary layer induced by the medium gap-type roughness, direct numerical simulation is employed to capture and analyze the transition process. Research indicates that as the flow over the flat plate passes the gap, the spanwise convergence effect leads to the formation of both upper and lower counter-rotating vortex pairs. As the flow progresses, these counter-rotating vortex pairs in the central region exhibit attenuation, with streamwise vortices developing on both sides. At a certain downstream distance, the boundary layer becomes unstable, triggering the formation of streamwise vortex legs. These streamwise vortex legs undergo further evolution, transforming into hairpin vortices and leg-buffer vortices. The formation of the central low-speed zone downstream of the roughness element is mainly attributed to the lift-up effect of the low-speed flow propelled by the central counter-rotating vortex pairs. The low-speed streaks on both sides are primarily influenced by the streamwise vortices. Through a meticulous analysis of the turbulent kinetic energy distribution and its generation mechanisms during the transition phase, this study infers that the primary sources of turbulent kinetic energy are the hairpin vortices, leg-buffer vortices, and their consequent secondary vortices. Combined with modal analysis, the study further elucidates the generation and breakdown of hairpin and leg-buffer vortices.

Experimental Studies of Bioinspired Shark Denticles for Drag Reduction.

Shark skin is composed of denticles, or complex scale-like features, which have been shown to reduce drag in turbulent and laminar flow. The denticle crown features undulating structures called riblets that interact with the turbulent boundary layer to reduce drag. Two mechanisms thought to contribute to the drag-reducing properties of riblets include the lifting of streamwise vortices and the hampering of spanwise vortex interactions to reduce crossflow, which could translate to similar flow mechanisms for denticles. Because of the varied morphologies of dermal denticles on different shark species, which also depend on body location, the impact of these denticle geometries on flow is of interest to the biology community, including related fields such as fluid mechanics and oceanography. This review highlights the past 15 years of manufacturing techniques and experimental measurements of drag over denticle-inspired surface structures, including real shark skin samples and engineered denticles and riblets. State-of-the-art additive manufacturing and other techniques are primarily limited to mm-length denticle scales, which have demonstrated drag reduction in lower flow speeds, under 1 m s-1. New manufacturing approaches can create sub-mm length denticles and nanotextured surface structures, which have achieved reported drag reductions of up to 31%. We synthesize results from the literature to illustrate the drag reduction properties of bioinspired denticles and riblets according to their geometry and flow conditions. Using these trends, we suggest design features and focus areas for future research, such as increasing studies of different denticle morphologies, hydrophobicity, antifouling properties, and acoustic noise reduction. Continued work on bioinspired denticles for drag reduction has wider implications in comparative biology and applications to design more energy-efficient, persistent vehicles for environmental monitoring.