Low-speed Flow Research Articles

Dynamics and scaling of particle streaks in high-Reynolds-number turbulent boundary layers

Inertial particles in wall-bounded turbulence are known to form streaks, but experimental evidence and predictive understanding of this phenomenon is lacking, especially in regimes relevant to atmospheric flows. We carry out wind tunnel measurements to investigate this process, characterizing the transport of microscopic particles suspended in turbulent boundary layers. The friction Reynolds number$Re_\tau = {O}(10^4)$allows for significant scale separation and the emergence of large-scale motions, while the range of viscous Stokes number$St^+=18$–870 is relevant to the transport of dust and fine sand in the atmospheric surface layer. We perform simultaneous imaging of both carrier and dispersed phases along wall-parallel planes in the logarithmic layer, demonstrating that streamwise particle streaks largely overlap with large-scale low-speed flow regions. The fluid–particle slip velocity indicates that with increasing inertia, the particle streaks outlive the low-speed fluid streaks. Moreover, two-point statistics show that the width of the particle streaks increases linearly with Stokes number, bounded by the size of the coherent flow structures. Finally, the particle-sampled flow topology suggests that particle streaks reside between the legs of hairpin packets. From these observations, we infer a conceptual view of the formation of particle streaks in the frame of the attached eddy model. A scaling for the particle streaks’ width is derived as a function of$Re_\tau$and$St^+$, which reproduces the measured trends and predicts widths${O}(0.1)$m in the atmospheric surface layer, comparable to aeolian streamers observed in the field.

Effect of wire mesh casing treatment on axial compressor performance and stability

In this paper, a kind of Wire Mesh Casing Treatment (WMCT) is proposed to improve the stable operating range of the compressor. In contrast to the traditional circumferential groove, as for WMCT, a layer of wire mesh is laid on the surface of the circumferential groove. Parametric studies were conducted on the low-speed axial flow compressor, including the groove width, axial location, and mesh count. The optimum axial location for WMCT is related to its groove width. A higher wire mesh count results in a smaller compressor stall margin improvement. Steady simulations were carried out to study the effect of WMCT on the flow structure of the compressor. The wire mesh in the WMCT has a certain flow resistance, which restricts the flow into and out of the groove. Due to the WMCT, the flow parameter in the tip region of the rotor is less sensitive to changes in the operating conditions of the compressor. The WMCT causes the rotor tip blade loading to shift backward, inhibiting the formation of spill forward of the leakage flow, and thus improving the stability of the compressor. The flow resistance on the groove surface is a new degree-of-freedom for the casing treatment designer.

Experimental and numerical investigation of the blade tip-related aeroacoustic sound source mechanisms of a ducted low-speed axial flow fan

A combined aerodynamic and aeroacoustic study of a ducted low-speed axial flow fan at various operating points is presented herein, focusing on the blade tip-related noise sources. The investigation includes upstream and downstream phased array microphone measurements and computational fluid dynamic simulations. The downstream acoustic measurements are carried out by using an acoustically transparent duct. The resulting flow fields and noise source maps are evaluated simultaneously. The results show that at higher flow rates, the dominant noise source is the turbulent boundary layer – trailing edge noise. At lower flow rates, the dominating noise sources can be found at the blade tip’s leading edge. These noise sources are found to be tip leakage flow related. Also, downstream microphone array measurements indicate strong noise sources at the blade tip’s trailing edge. These are determined to be a combination of the turbulent boundary layer - trailing edge noise, tip leakage vortex impingement noise, and double-leakage noise.

Investigation on the compressibility characteristics of laminar flow in rotating channel

In high-speed rotating channels, significant compressive effects are observed, resulting in distinct flow characteristics compared to incompressible flows. This study employs a finite volume method, based on the implicit formulation, to solve for low-speed compressible laminar flow in rotating channels using an orthogonal uniform grid. The governing equations include the full Navier–Stokes equations and the energy equation. The alterations in flow within rotating channel are primarily influenced by the compressive effects of centrifugal force and the compressibility of fluid within the flow's normal section. The first effect involves a reduction in the velocity due to centrifugal force, leading to an increasing influence of the Coriolis force compared to inertial forces. This trend change aligns closely with the increase in rotation speed. The second effect arises from the increase in the rotating Mach number and the Coriolis compression, resulting in slight density differences within the cross section. Strong centrifugal forces generate significant centrifugal additional force (buoyancy force). Consequently, under the same local rotation number, the velocity profiles of the mainstream experience considerable changes. Additionally, a higher rotating Mach number significantly impacts wall shear stress, with the leading side being notably affected. For instance, at cross-sectional Ro = 0.6 and MaΩ = 2.1, the dimensionless shear stress on the leading side decreased by 13%. Furthermore, while an increase in the rotating Mach number has minimal impact on the cross-sectional secondary flow structure, changes in mainstream velocity profiles influence secondary flow intensity, resulting in an enhanced velocity peak and a shift toward the trailing side.

Measurement of interfacial shear stress in gas–liquid two-phase stratified flow

Gas–liquid two-phase stratified flow exists in many industrial processes. Although the flow pattern is simple, the interfacial shear prediction of stratified flow is still the focus of the study. The calculation of the shear stress at the gas–liquid interface is closely related to the measurement of the void fraction and pressure drop of the stratified flow. In this study, a new method for the calculation of interfacial shear stress of gas–liquid two-phase stratified flow is proposed. Differential pressure measurement and planar laser-induced fluorescence technology are combined to obtain important parameters for stratified flow under low-speed flow conditions (Ql = 0.10–0.25 m3/h, Qg = 0.35–1.00 m3/h). The interfacial shear stress is successfully calculated using macroparameters. The uncertainty associated with the calculated parameters using the proposed method is 2.67%, and this study verifies the accuracy of the linear relationship. The method provides a new way to obtain the interfacial shear stress of gas–liquid stratified flow.

Investigation of water separation capabilities of demisters integrated with microstructured surfaces

Demisters with high water separation efficiencies play an important role in performance enhancement of thermal desalination. Bio-inspired, microstructured surfaces have been highlighted due to the potential to considerably increase water separation by enhancing water capturing and transporting features. However, experimental investigation on the performance enhancement of the demisters with those functional surfaces is still lacking. Herein, we demonstrate water collection and separation characteristics of the large-scale demisters integrated with hybrid microstructured surfaces. Particularly, the hybrid microstructured polypropylene surfaces to possess the vertically-aligned spiky cone arrays along with multiple sets of open-type microchannels are successfully fabricated in uniform manner by using an injection molding process. The fabricated hybrid microstructured surfaces perform twice and up to 3.2 times higher water collection than the control planar surfaces in low-speed and high-speed air flows, respectively, owing to their enhanced water capturing and transporting capabilities by the capillary pressure generated by the microstructures. As a model study, the inner surfaces of the axial cyclone-type demister ducts are modified with the hybrid microstructured surfaces, leading to up to 12 % enhancement of the water separation efficiencies of the demisters, compared to the control planar surface demister in high-speed air flow.

Design and performance investigation of a triple blade dual stage Savonius-alike hydrokinetic turbine from low flow stream reserves

ABSTRACT A trending technology that is being employed to generate hydro energy from low-flow stream reserves is the Savonius-alike hydrokinetic turbine (SAHT). Clearance between the stages of a dual-stage two-bladed SAHT was found to improve its performance at low flow speeds; however, the impact of clearance on the triple-bladed configuration of SAHT was not studied earlier. In this paper, a triple blade dual stage configuration of SAHT is designed, and its performance is investigated in a water flume under various stage clearances (0,5,10,15 and 20 mm), low flow speeds (0.45,0.55 and 0.65 m/s) and different brake loads. Detailed turbine performance under different design and off-design conditions are investigated to obtain meaningful performance insights. The findings show that torque production by the turbine increases with the increase of brake load, with maximum hydrodynamic torque generated at the highest brake load. The highest coefficient of performance and torque of 0.071 and 0.261 are obtained at a stage clearance of 5 mm, tip speed ratio of 0.273, and free-stream flow speed of 0.55 m/s. The present SAHT under design condition has improved performance compared to a dual blade dual stage SAHT exhibiting a wider tip speed ratio range for its application in low flow stream reserves. Further, this turbine may also be recommended for torque generation to work as a motor in a flow speed condition less than 0.5 m/s. The novelty of this work is the application of an additional flow control measure in the form of flow-through clearance to negotiate vertical water thrust through the clearance and exert additional pressure on the advancing blades of the SAHT in the upper stage.

Modified advection upstream splitting method: Revolutionizing accuracy and convergence speed in low-Mach flows

The vital role of the numerical scheme is becoming increasingly critical as the use of computational fluid dynamics grows. To address the unfavorable effects experienced in low-speed flows when using the AUSM+M scheme (Improved Advection Upstream Splitting Method), the present paper presents an improved approach known as Modified-AUSM+M (M-AUSM+M). This novel method offers enhanced reliability in simulating low-Mach number flows, effectively mitigating the challenges associated with low-speed symptoms encountered in the original AUSM+M scheme. The novel scheme is facilitated by the parameter-free form of the pressure diffusion term in the mass flux and the low-dissipative form of the velocity diffusion term in the pressure flux. The impacts of these critical ingredients are then thoroughly evaluated, and the different characteristics are explored in terms of robustness and accuracy using a wide range of low-Mach test cases. The proposed scheme maintains a consistent correlation between accuracy and convergence speed. In addition, the recently devised technique demonstrates superior accuracy compared to AUSM+M and AUSM+UP schemes when dealing with low-Mach flows. Furthermore, the findings indicate an incredible reduction in iteration numbers, ranging from 30% to 80%, by employing the enhanced scheme in low-Mach domains. In the investigation of high-Mach test cases, the newly developed method preserves the accuracy achieved by AUSM+M in high-Mach flows.

Application of fast-response pressure sensitive paint to low-speed vortical flow at high angles of attack

Application of fast-response pressure sensitive paint to low-speed vortical flow at high angles of attack

Estimation of the low-wavenumber component of turbulent boundary layer pressure field using vibration data

In structures subjected to a high-speed flow, the convective region of the wall pressure field (WPF) beneath a turbulent boundary layer (TBL) plays a crucial role in their vibration behaviour. However, when it comes to underwater structures experiencing low-speed flow, they effectively filter out the convective domain of WPF and the bending wavenumber of structures align with the low-wavenumber domain of the WPF. As a result, the primary cause of vibration in this case is the low-wavenumber components of the WPF. Thus, accurate estimation of the WPF at low-wavenumber domain is crucial for predicting vibration responses of these structures. Existing models for WPF accurately predict the convective region but differ significantly in predicting the low-wavenumber levels. This numerical study aims to investigate the feasibility of estimating the low-wavenumber WPF by analysing measured vibration data from a flat plate excited by a TBL. The WPF's cross spectrum in the wavenumber domain can be linked to the cross spectrum of the plate's acceleration. By employing regularization techniques and solving an inverse problem, the low-wavenumber components of the WPF can be then estimated. Virtual experiments are performed to evaluate the accuracy of the studied process by comparing its prediction to the input WPF model.

Phased array microphone measurement of a ducted low-speed axial flow fan at various operating points with the involvement of the acoustically transparent duct technique

The aeroacoustic investigation of ducted turbomachines is not evident. Wall-mounted and strut-mounted microphones placed in the flow field are both sensitive to installation, placement, and flow-related effects. Therefore, it is advantageous to place the microphone sensors outside of the ducting of the turbomachine while also accounting for the acoustic characteristic of the ducting. In this paper, a ducted low-speed axial flow fan is investigated with the acoustically transparent duct (ATD) and the phased array microphone (PAM) Rotating Source Identification (ROSI) beamforming techniques at design and off-design conditions. The combination of these methods is capable of identifying the dominant sound sources of the fan in a non-intrusive approach at stall condition, pre-stall operation, design condition, and part-load operating condition as well.

Triple Decomposition of Unsteady Flow in a Low-Speed Mixed-Flow Fan

Numerous studies (including theoretical modeling, numerical simulation, and experimental measurement) have confirmed that wake interaction is a primary factor affecting tonal and broadband noise emitted by aeroengine fans. The knowledge and techniques derived from these studies have often been applied to analyze and control the aerodynamic noise of low-speed axial/mixed-flow fans used in ventilation and cooling applications. Despite the structural similarity between aeroengine fans and axial-flow fans for ventilation and cooling, the differences in their rotor–stator spacing and operating parameters raise the question of whether or not they share completely identical or approximately similar unsteady flow and noise generation mechanisms. In this paper, we employ a large-eddy simulation to investigate the low-speed unsteady flow inside a mixed-flow fan. The triple decomposition method is used to analyze the spatiotemporal characteristics of the time-averaged, ensemble-averaged, and stochastic fluctuation velocities and pressures in both stationary and rotating coordinate systems. The results show that the unsteady flow and noise generation mechanisms inside the fan investigated in this paper are significantly different from those of aeroengine fans, mainly due to two aspects. First, the wake effect on the velocity field of the fan studied in this paper is almost negligible, and the potential interaction is the primary source of the nonuniform spatial distribution of the time-averaged velocity field and the periodic velocity disturbance. Second, the turbulent energy spectrum of the stochastic fluctuation velocity is greatly distinct from the classic turbulence spectra in the low-frequency bands.

Eco-speed optimization model for active rear-end collision avoidance of connected and automated vehicles on freeways

This paper proposed an eco-speed optimization model for active rear-end collision avoidance of connected and automated vehicles on freeways in the scene of dangerous sources downstream, and the optimal speed curve was obtained to guide connected and automated vehicles to travel or stop safely and smoothly to avoid rear-end collision accidents. To implement the eco-speed optimization model, a traffic state prediction model based on confidence interval was proposed for forecasting the state of the front low-speed traffic flow. To improve the computational efficiency, the original optimal control problem was decomposed into several sub-problems based on the hyperbolic tangent function and only the leading vehicle was optimized. To eliminate the deviation caused by traffic variation during the period of solving the eco-speed optimization model, a prediction-based optimization strategy was adopted. To verify the performances of the eco-speed optimization model, the control strategy of in-vehicle driving alert systems was taken as the study case for comparison. The results show that the approximate model can basically ensure the computational accuracy and significantly improve the computational efficiency, that the proposed traffic state prediction model can ensure traffic safety, and that the speed control strategy obtained in this paper can make the vehicle decelerate in advance to avoid rear-end collision, smooth the vehicle’s speed curve, save the fuel consumption by 24.08%, restrain traffic oscillation and improve the safety of the traffic flow.

Multiscale finite element method combined with local preconditioning for solving Euler equations

In this work we present a nonlinear multiscale finite element method combined with local preconditioning for solving compressible Euler equations in conservative variables. The formulations are based on the strategy of separating scales, in which it is the core of the variational multiscale (finite element) methodology. The subgrid scale space is defined using bubble functions that vanish on the boundary of the elements, allowing to use a local Schur complement to define the resolved scale problem. The resulting numerical procedure allows the fine scales to depend on time. The formulation proposed added artificial viscosity isotropically in all scales of the discretization. Due to the fact that, density-based schemes suffer with undesirable effects of low speed flow including low convergence rate and loss of accuracy, local preconditioning is applied to the set of equations in the continuous case. We evaluate the multiscale formulation with local preconditioning in the low Mach number comparing with the non-preconditioned case. The experiments show that density-based schemes combined with local preconditioning yields good results.

Application of nonlinear stiffness mechanism on energy harvesting from vortex-induced vibrations

This study investigates the potential of vortex-induced vibration (VIV) as a renewable energy source, achieved when fluid flow interacts with a bluff body, inducing self-sustained oscillations through vortex shedding in the wake. While VIV research has traditionally focused on understanding its mechanisms and mitigating detrimental effects, interest in VIV energy harvesting has surged as a means to convert marine hydrokinetic (MHK) energy into usable electrical power. The nonlinear effects of two linear oblique springs on VIV energy harvesting are explored using the wake oscillator model, encompassing bistable and Duffing hardening stiffness. The study examines the response and energy harvesting performance while considering the impact of undeformed spring length, structural damping, and initial conditions on VIV energy conversion. Findings show that nonlinear stiffness application in the VIV system can broaden the synchronization bandwidth or reduce the VIV initiation flow speed. Bistable stiffness may broaden the synchronization velocity range, while Duffing hardening stiffness efficiently reduces the VIV initiation speed with small energy harvesting loss. Combining both stiffness types with appropriate control strategies presents a promising approach for achieving a broad synchronization VIV bandwidth and low initiation flow speed. Key parameters, such as the nondimensional parameter defining spring system obliquity and the ratio between undeformed spring length and cylinder diameter, significantly influence VIV response and energy harvesting. Moreover, optimal structural damping is vital to maximize energy harvesting efficiency, and understanding and controlling initial conditions are crucial for optimizing VIV synchronization bandwidth and energy harvesting efficiency for both bistable and Duffing hardening stiffness. This study provides valuable insights into VIV system dynamics and energy conversion potential with nonlinear springs, offering promising avenues for enhancing energy harvesting efficiency and inspiring further applications of nonlinear effects in VIV energy converters.

Investigation of the Influence of Wake Field Characteristic Structures on Downstream Targets Using the POD Method

This research investigated the impact of complex low-speed wake flow structures on the aerodynamic characteristics of objects downstream. It employed the proper orthogonal decomposition (POD) method and the domain precursor simulation method to compare traditional methods and validate this approach. The study generated several flow structures of parallel dual-cylinder wakes with different scales and spacing. The variations in the aerodynamic coefficient of three downstream objects at various times passing through wakes of varying scales were appropriately compared and analyzed. The study established that the wake with a cylinder spacing of G = 1.5 has a more compact and concentrated modal structure than that with a cylinder spacing of G = 0.35. Smaller objects were more responsive to the wake flow structure with a spacing of G = 1.5, whereas larger objects responded more to the flow structure with a spacing of G = 0.35. The achieved results also revealed that the aerodynamic force coefficients of objects passing through the wakefield at different times were closely related to the temporal characteristics of the wake flow structure with different scales.

High-subsonic boundary-layer flows of an organic vapour

Boundary layers of Novec649, a low-global-warming potential fluid of interest for low-grade heat recovery, are investigated numerically by means of linear stability theory, direct numerical simulation (DNS) and large-eddy simulations (LES). This organic vapour is of interest in organic Rankine cycle (ORC) turbines and realistic thermodynamic conditions are selected. Under these conditions, the vapour behaves as a dense gas and, due to its high molecular complexity, real-gas effects occur. In addition, the fluid exhibits large and highly variable heat capacities and density- as well as temperature-dependent transport properties. More specifically we report the first direct and LES of transitional and turbulent boundary layers of Novec649 at high-subsonic conditions$M=0.9$. A controlled transition is performed by using oblique modes determined by linear stability theory extended to dense gases. An oblique-type transition is obtained as in low-speed air flows, where sinuous streaks develop by the lift-up mechanism and break down into turbulence. In the turbulent state, the profiles of dynamic flow properties (velocities, turbulent intensities, turbulent kinetic energy budgets) are little affected by the gas properties and remain very close to incompressible DNS, despite the high-subsonic flow speed. The fluctuations levels for thermodynamic properties have been quantified with respect to air flows. Notwithstanding a drastic reduction, genuine compressibility effects are present. For example, the fluctuating Mach number and the acoustic mode are characteristic of high-speed flows. The influence of forcing frequency and amplitude on the established turbulent state has been investigated using LES. An analysis of integral quantities shows a slow relaxation towards a canonical equilibrium turbulent state for all cases due to the high Reynolds numbers typical of dense gas flows. Overall the present DNS constitutes a valuable reference not only for forthcoming experiments but also for future studies of free-stream transition and loss mechanisms in ORC turbines.

Effect of variations in blade width on vertical shaft kinetic water turbine power

Various studies and observations have been carried out to produce a turbine design that suits the needs and the relatively low flow speed is a challenge that currently still requires further research. ThereforeThe aim of this research is to determine the effect of variations in the width of the bowl blade on the vertical shaft kinetic water turbine power.The research method used is an experimental method with an experimental design on a laboratory scale. Tests were carried out on three variations of blade width, from the results of the study it can be concluded that increasing the water flow rate, turbine rotation, and the ratio of tangential speed to the turbine blade width can increase the kinetic water turbine power. In this study, variations in blade width were 10 cm, 11 cm, and 12 cm. Increasing turbine blade sheets increased turbine power due to the additional mass of the flow hitting the turbine blades thereby affecting the kinetic water turbine power. Analysis of the water flow rate shows that the maximum turbine power occurs at a blade width of 12 cm and a discharge of 0.017 m3/second with a turbine power value of 25.455 Watt. Analysis of turbine rotation, Maximum turbine power occurs at a blade width of 12 cm and rotation of 80 rpm with a turbine power value of 23,864 Watts. Meanwhile, analysis of the tangential speed ratio shows that the maximum turbine power occurs at a blade width of 12 cm and at a tangential speed ratio of 0.5 rad/s2 with a turbine power value of 23,864 Watts.
 Keywords: Blade width, power, kinetic turbine, vertical shaft.

GPU implementation of the discrete unified gas kinetic scheme for low-speed isothermal flows

In this paper, two GPU parallel algorithms are proposed for the discrete unified gas kinetic scheme (DUGKS) for simulating low-speed isothermal flows. Algorithm-I uses a two-level fine-grain technique for the parallelization of physical spatial space, while Algorithm-II adopts this technique for both physical spatial and particle velocity spaces. To evaluate the performance of the proposed algorithms, several typical benchmark problems are simulated, including the two-dimensional (2D) and three-dimensional (3D) lid-driven cavity flows, the micro channel and cavity flows. Numerical results show that our GPU algorithms can achieve satisfactory computational efficiency. For Algorithm-I, the speedup can reach 250 and 338 on a Tesla V100 GPU card for the 2D and 3D continuum cavity flows, respectively, and a hundredfold acceleration can be obtained for the rarefied cases. While for Algorithm-II, a speedup of about 70 can be attained for rarefied cases. However, it is not applied to continuum problems that only require a small number of velocity points. Moreover, comparisons between the two GPU algorithms are also conducted for the rarefied flows with various grid meshes and velocity directions. The results show that Algorithm-I performs better when physical mesh size is large, while Algorithm-II can provide higher efficiency for a coarser mesh with medium number of discrete velocities. Special attention is also paid to comparisons between Algorithm-I and MPI parallelization with 128 CPU cores based on physical space discretization approach, and it is found that Algorithm-I has a clear advantage on V100 GPU when dealing with sparse physical grids in both continuum and rarefied cases.

Numerical Simulation of the Internal Flow Field of Test Chamber at High Altitude Environment

In order to further promote the digitalization extent of environmental test, ensure that the test chamber can more realistically simulate the actual environment, and improve the environmental adaptability of vehicle equipment in extreme environments, the three-dimensional CFD simulation method was used to calculate simulation test chamber of an armored vehicle in high-altitude environment in the form of low-speed return flow wind tunnel. There are movable suspension and flap in the entrance to the contraction section of the test chamber. The velocity field and temperature field were analyzed, the temperature distribution and heat transfer in the characteristic section of the test section under different flap deflection angles were compared and analyzed by selecting working conditions, and make the relationship among flow speed and flow rate and the temperature field of the test section clear.