Streamwise Position Research Articles

Resonator-like behavior of a wall-bounded precessing vortex core in a diffuser with wall asymmetries

This paper reports a detailed investigation of the interaction between a wall-bounded precessing vortex core (PVC) occurring in swirling flows after vortex breakdown and a wall asymmetry. Experiments are carried out in an axisymmetric diffuser downstream of an axial swirl generator inducing a swirling flow with a swirl number of S = 1.1. Wall pressure measurements and two-component particle image velocimetry (PIV) are conducted for Reynolds numbers (Re) ranging from 20 000 to 76 000 in the initial axisymmetric configuration and several asymmetric configurations, with an additional cylindrical protrusion placed on the diffuser wall at different streamwise and circumferential positions. It is first confirmed that synchronous pressure fluctuations at the PVC frequency are only produced in asymmetric configurations. Furthermore, the analysis of the pressure data in several asymmetric configurations revealed for the first time a resonator-like behavior of a wall-bounded PVC. While a change of the protrusion circumferential position in a given cross section of the diffuser only affects the phase of the synchronous pressure fluctuations, the amplitude of the latter features successive minima (pressure node) and maxima (pressure anti-node) as the protrusion is moved along the diffuser in the streamwise direction. In addition, as the protrusion is moved closer to a pressure node, the phase of the synchronous pressure fluctuations exhibits a sudden variation of ±π. Similar results are observed for all tested values of Reynolds number, whereas the PVC frequency linearly increases with Re. A reconstruction of the PVC helical structure based on PIV measurements showed that these consecutive pressure nodes are spaced by a distance equal to approximately one third of the PVC helical pitch. Finally, it also revealed that two different states are observed, depending on the position of the protrusion along the diffuser: the synchronous pressure component reaches its maximum value as the PVC center is approaching either its closest or farthest angular position with respect to the protrusion. The transition from one state to another one depends on the streamwise position of the protrusion with respect to the pressure nodes. These unprecedented experimental observations pave the way to novel theoretical developments for a better understanding and modeling of synchronous pressure fluctuations induced by wall-bounded PVC in asymmetric geometries.

A Comprehensive Analytical Model for Vortex Shedding From Low-Speed Axial Fan Blades

Abstract The paper presents a comprehensive analytical model for the characterization of von Karman vortex shedding in the wake of models of low-speed axial fan blades. The elaborated minimal model is based on the Reynolds-averaged Navier–Stokes and continuity equations. For validation purposes, hot-wire measurements have been carried out in a wind tunnel on representative blade profiles. The measurement data obtained for various streamwise positions downstream of the blade trailing edge, i.e., transversal profiles of mean velocity as well as root-mean-square of fluctuating velocity, are evaluated. As the experimental validation demonstrates, the minimal model fairly localizes the transversal position of the vortex centers and represents the motion of the vortices along the wake. The validated minimal model serves the following benefits: (a) an extensive understanding of the underlying physics related to the flow field featuring vortex shedding in the near-wake region (easy-to-use quantitative correlation among the characteristics of wake flow affected by the shed vortices); (b) extension of the literature-based methodology for determination of the transversal distance between the shed vortex rows, being used as a scaling parameter for the Strouhal number utilized in calculation of vortex shedding frequency; and (c) modeling the behavior of rows of shed vortices farther away from the trailing edge. Such behavior may influence the acoustic signature of VS, and, as such, it is to be considered in fan noise modeling.

Near wall temperature measurements and turbulent features in a water flow at transition Reynolds numbers in a square heated asymmetric cavity channel

• Temperature profile and fluctuations in water flow in a heated cavity channel are measured at medium Reynolds numbers. • The conductive sublayer is clearly detected with a micro-thermocouple. • Temperature turbulent intensity shows peaks within the conductive sublayer with increasing magnitude along the channel. • Autocorrelation analysis reveals two trends in and outside the boundary layer attributed respectively to random turbulence and recirculation. • A slight peak in the Nusselt number distribution is observed in the centre of the channel as the Reynolds number increases. Passive cooling configurations are attracting an increasing interest in different thermal management applications. Nevertheless, a more efficient design requires deeper understanding of heat transfer and flow mechanisms. Among these, turbulence and convection play an important role. In particular, the so-called conductive sublayer close to the wall, where significant temperature gradients are present, is critical. Despite its importance, to the best knowledge of the authors only few experimental works have measured the temperature profile in the laminar sublayer of forced-convective turbulent flows. In the present work, measurements of the temperature profiles and temperature fluctuations in the boundary layer for water forced-convective flow in a heated cavity channel (for Reynolds number in the low turbulent regime [Re ≅ 3640-4970] and for different imposed heat fluxes [q w ≅ 9500, 13700, 19350 W/m 2 ]) were carried out using a micro-thermocouple at different streamwise positions. A new method is proposed for validating the temperature measurements. A minimum point in the temperature profile towards the inlet of the channel is observed which attributed to flow recirculation in the cavity. Increasing the fluid velocity resulted in a shift of the position of the minimum point towards the heated wall. An increase in heat flux led to a narrowing of the amplitude of the region around this minimum point. Peaks of temperature turbulence intensity with increasing magnitude along the streamwise direction were observed within the laminar sublayer. A slight peak in the Nusselt number distribution is observed at the centre of the channel as the Reynolds number was increased. Among the geometrical passive cooling configurations, the one presented in this work showed superior heat transfer performance.

Numerical Study on a Cylinder Vibrator in the Hydrodynamics of a Wind–Wave Combined Power Generation System under Different Mass Ratios

A hydrodynamic wind–wave combined power generation system is a new type of energy device that uses wind and ocean current energy to generate electricity. In this paper, the hydrodynamics of a wind–wave combined power generation system was simulated in Fluent. The fluid–structure coupling simulation of the vortex vibration of the cylindrical oscillator was realized using UDF and dynamic mesh technology. The Vortex-Induced Vibration (VIV) characteristics of the cylindrical oscillator were analyzed, and the reliability of the numerical simulation method was verified by comparing the amplitude and trajectory of the eddy-excited vibration with the classic experiments of Jauvtis and Williamson. The VIV characteristics of cylindrical oscillators with different mass ratios were studied in terms of vibration response, motion trajectory, and the streamwise equilibrium position. The effect of the mass ratio on the hydrodynamics of a wind–wave combined power generation system was simulated using spring damping, achieving the goal of carrying out preliminary research work simulating the wind–wave combined power generation device. Some useful conclusions were obtained through calculation, which provided data support for the corresponding platform device. This study shows that in cylindrical oscillators with different mass ratios, the overall trend at the same reduced velocity is that the larger the mass ratio, the smaller the crossflow amplitude. The cylindrical oscillators with mass ratios of one and two appear in the upper branch, while cylindrical oscillators with mass ratios of three and four do not appear, and with the increase in the mass ratio, the frequency ratio in the lower branch tends toward one. At the same reduced velocity, the lower the mass ratio, the larger the corresponding downstream equilibrium position, and the higher the energy acquisition efficiency.

Study on the Sensitivity of the Streamwise Location of MVG on SWBLI in MVG-Based Supersonic Flow Control

Micro vortex generator (MVG) is a currently facile, robust, and feasible device for supersonic and hypersonic flow control. The purpose of this study is to investigate the impact on SWBLI from the streamwise location of MVG. Large eddy simulation (LES) was conducted on MVG controlled supersonic ramp flow to reveal the sensitivity of MVG streamwise position on shock-wave boundary-layer interaction (SWBLI) control. Numerical cases with minor different distances between MVG and ramp corner are carried out. The results are analyzed in time-averaged and instantaneous view, respectively. The results show that streamwise position has a significant effect on SWBLI in some aspects. With minor changes on the streamwise position, the ring-like vortices generated by MVG were very similar, with only small changes in height and intensity. However, the small changes made on the ring-like vortices produced relatively significant changes to the separation region in front of the ramp. In terms of the time-averaged solution, the farther the MVG is from the ramp, the higher the ring-like vortices are lifted, and the shock wave is also disturbed/reduced more strongly. Further, the flow separation zone on the wall also appears smaller. The results of this study play a guiding role for further optimal configuration of MVG in flow control.

Calibration and performance characterization of a Mach 5 Ludwieg tube.

Calibration, commissioning, and design features of a new Mach 5 Ludwieg Tube wind tunnel at the University of Arizona are discussed. Mach number uniformity and free-stream noise levels are measured using a Pitot rake at a range of unit Reynolds numbers and at multiple spanwise and streamwise positions. The wind tunnel is shown to have a free-stream Mach number of 4.82 with maximum variance less than 0.8% (and less than 0.5% at most streamwise positions). The average free-stream acoustic noise level in the core (based on Pitot pressure) is shown to be less than 1.2% at an intermediate Reynolds number with some regions dropping locally below 1.0%. The core flow region is measured to be 282.4mm (11.1 in.) in diameter at the nozzle exit.

Interscale transfer of turbulent energy in grid-generated turbulence with low Reynolds numbers

Direct numerical simulations are performed for grid-generated turbulence with low Reynolds numbers to study the interscale transfer of turbulent energy. The Reynolds numbers based on the uniform velocity and grid mesh size, M, are set to ReM=5000, 9000, and 15000. The results show that, for ReM=9000 and ReM=15000, the turbulent dissipation constant increases in the upstream region but it becomes nearly constant in the region of x/M>15. In contrast, for ReM=5000, it continues to increase and do not level off even in the downstream region. Scale-by-scale analysis using the Karman–Howarth–Monin–Hill equation indicates that, the non-linear transfer term balances with the dissipation term for all the cases. Instead, it is found that, in the downstream region of the ReM=5000 case, where the turbulent Reynolds number becomes Reλ<25, the contribution of the advection term varies depending on the streamwise positions. Further analysis for the one-point statistics indicates that, in such conditions, the coherences among the advection term, dissipation term, and diffusion term become small at small scales, while they are generally large in the other cases including the upstream region for ReM=5000. These indicate that energy cascade mechanism in the downstream region for ReM=5000 is different from the cases of 30<Reλ<60, even though both of them are generally considered as small Reλ, and it is caused by the increase of the dissipation scale range appearing in the transition period to the final decay.

Effects of streamwise oriented riblets on spot nucleation in free-stream turbulence induced transition

The effects due to streamwise oriented riblets on the free-stream turbulence (FST) induced transition of a flat plate boundary layer (BL) are studied by means of Particle Image Velocimetry (PIV) and Laser Doppler Velocimetry (LDV). The experiments are performed on a flat plate installed between adjustable endwalls providing an adverse pressure gradient typical of turbine blades for aeronautical applications. Four different ribbed surfaces are compared with a smooth reference case to test the effects of riblet dimension and location on turbulent spot nucleation. Riblets are installed in the laminar, the transitional, and the fully turbulent BL. Their effects are studied for three Reynolds numbers (Re=70000, 150000, 220000) under fixed pressure gradient and free-stream turbulence intensity (Tu=5%). Varying the flow Reynolds number leads to different non-dimensional riblet height and spacing values, providing an overall test matrix of 13 different conditions.LDV measurements allowed the computation of the wall shear stress and the BL statistical moments at selected spatial positions. On the other hand, PIV data are used to characterize the riblet effects on turbulent spot nucleation and transition: the higher the non-dimensional riblet height, the more the breakup events occur upstream with respect to the smooth case. Momentum losses significantly increase when the non-dimensional riblet spacing becomes larger than 30 wall-units. Otherwise, beneficial effects are obtained for smaller riblet spacing, regardless of their streamwise extension and position with respect to the BL transition region.

Applicability of Taylor's frozen hypothesis and elliptic model in the atmospheric surface layer

Based on the synchronous multi-point temperature data measured at different streamwise positions with the application of distributed temperature sensing, a field investigation on the applicability of Taylor's frozen hypothesis and elliptic model was performed in the atmospheric surface layer (ASL). In this work, several important spatial statistical functions of temperature field, such as longitudinal space–time correlation [CTT(r, t)], space correlation [RTT(r)], normalized second-order structure function [⟨ΔT+2(r)⟩], and wavenumber spectrum [ΦTT(k)] of temperature fluctuations, were directly measured in the ASL. By comparing the directly measured spatial statistical functions with the predicted results, our study indicates that both Taylor's frozen hypothesis and elliptic model are applicable in the near-neutral and stable ASLs when the turbulence level is low. However, only the elliptic model is substantially accurate in the unstable ASL when the turbulence level is high. The elliptic model can relate CTT(r, t) to RTT(rE), where rE = [(r−Ueτ)2+(Veτ)2]1/2, Ue is the convection velocity, and Ve is the sweeping velocity. With the application of Ue and Ve, RTT(r) and ⟨ΔT+2(r)⟩ can be estimated by the elliptic model in the near-neutral, unstable, and stable ASLs.

Numerical study on a novel device for hydrogen mixing enhancement in a scramjet engine: Coaxial injector

The mixing process between the fuel and air is very important for the efficient operation of the scramjet engine, and it has attracted an increasing attention all over the world. In this work, the coaxial injector was developed and investigated by means of the three-dimensional Reynolds-averaged Navier-Stokes equations coupled with the SST k-ω turbulence model, and the numerical approaches were validated against the non-dimensional experimental streamwise velocities in four different streamwise positions on the symmetry plane. Further, the influence of the jet-to-crossflow pressure ratio for the hydrogen and air is evaluated numerically as well. The obtained results show that the mixing process induced by the coaxial injector is conducted more quickly, and it is a promising mixing enhancement device for the scramjet engine. The smallest value of the mixing length for the cases studied in the current study is 1.69. It brings nearly no additional total pressure loss when compared with the single injector, and the jet-to-crossflow pressure ratio has only a small impact on the total pressure recovery coefficient.

Unsteady flow behavior and two-phase region prediction in the SCO2 centrifugal compressor

In this study, unsteady numerical calculations of centrifugal compressors with volute were carried out. The unsteady evolution of two-phase regions and flow behavior in impeller passages were determined. The results show that the two-phase regions in impeller passages are governed mainly by the low static pressure strip LP1 on the casing wall. Meanwhile, the unsteady trajectory of a two-phase region is consistent with the trajectory of LP1. The unsteady evolution of a two-phase region undergoes two stages: In the first stage, it appears near the middle streamwise position and simultaneously expands to the leading edge (LE) and trailing edge (TE) of the blade. In the second stage, it shifts from the middle streamwise position to the LE of the blade and finally disappears. Moreover, based on the rotor enthalpy conservation, the prediction model for the formation of two-phase region in the impeller passage under different inlet conditions was determined.

An experimental investigation of a roof-mounted horizontal-axis wind turbine in an idealized urban environment

Wind-tunnel experiments are performed to investigate the effect of minor modifications to the roof edge shape on the power performance and wake characteristics of a horizontal-axis wind turbine sited on a cube-shaped building. Three roof edge shapes are considered: a sharp edge, a rounded edge, and a solid fence. The power performance of the turbine is dependent on its streamwise position, and on the roof edge shape due to the difference in the flow shear induced by different roof edges. The sharp edge and solid fence cases show high variation in power with the turbine position, which is reduced significantly in the rounded-edge case. The turbine shows the worst power performance regardless of its position in the fence case. The roof edge shape is also found to affect the wind turbine wake. Specifically, the wake recovery and expansion rates are found to be greatest for the fence case, and they decrease with the increasing smoothness of the roof edges. This is related to the difference in the base flow turbulence induced by different roof edge shapes. Compared to the base flow, turbulence intensity in the wake is reduced on the building, except at the rotor top tip level.

Proper Orthogonal Decomposition for a Turbulent Boundary Layer with Aero-Optic Distortion

The velocity field associated with the deflection of a laser beam passing through a heated incompressible boundary layer in the wall-normal direction was studied using variants of the classical proper orthogonal decomposition (POD) technique. Due to the heating of the boundary layer, temperature, density, and index of refraction fields were variable in the boundary layer and were instantaneously affected by turbulent coherent structures. The time-varying index of refraction field led to a time-varying beam path of a laser traversing the boundary layer; the final angle of the laser beam was measured simultaneously with particle image velocimetry. The relationship between coherent structures in the velocity field and the final beam angle was studied using conditional POD and space-time POD. Conditional POD was performed on the wall-normal velocity field using conditions of large upstream and downstream laser deflections. The modes from conditional POD were compared to modes from POD without conditions. Physical features within some conditional POD modes were found to be shifted in the streamwise direction compared to unconditioned modes, suggesting that the streamwise position of velocity features was correlated to laser deflection. Space-time POD resulted in the same modes as the conditional POD and showed the modes convecting in the streamwise direction without a significant change in the size of the structures. This suggested a hypothesis that the laser deflects when coherent structures reach particular positions relative to the laser position as the structures convect, aligning the largest local velocity and density gradients with the laser beam path.

Heat transfer characteristics of a periodically transient flow for turbulent wall-bounded dual jet

The thermal characteristics and aerodynamic behaviours of an unsteady flow for a turbulent wall-bounded dual jet are computationally studied in the present paper. It is quite important to analyse thermal characteristics of such flows, as a function of independent parameters like Reynolds number. These jets are used in different industrial applications, such as, cooling of electronic devices and heat exchange systems. The capabilities of different Reynolds-Averaged Navier-Stokes (RANS) turbulence modeling approaches, such as, renormalization groupk-ɛ(RNG), realizablek-ɛ, standardk-ω,and shear-stress transportk-ω(SST) models in predicting thermal performance of a turbulent dual jet are investigated. The comparison shows that among four turbulence models, the SSTk-ωmodel provides a good agreement with the reported experimental results. The transient RANS equations have been used to simulate the complex behaviour of a turbulent dual jet. The Reynolds number (Re) is varied from 10,000 to 40,000 for a fixed offset ratio of 2 under isothermal boundary wall conditions. The results reveal that the axial position of the merge point is independent of Re and remains almost constant as Re is further increased. It is also noticed that the velocity becomes self-similar and independent of Re after a streamwise position of X = 30. The time series of U and V velocity signals unveil sinusoidal oscillations and display peak to peak amplitude difference at the far axial location. The FFT assessment of the U velocity signal displays a solo peak dominant frequency which is the same as shown by V velocity in the FFT plot corresponding to the Strouhal number of the vortex shedding phenomenon. The average Nusselt number and the average heat flux increase with Re. A correlation has been developed for the average Nusselt number and the average heat flux. These outcomes will improve the current knowledge of various engineering and industrial applications to enhance the heat transfer in different heat exchange systems, such as, electronics cooling systems.

A new three-dimensional analytical model for wind turbine wake turbulence intensity predictions

In wind farm design, accurately predicting the wake turbulence level is crucial for turbine power and load evaluation. However, a knowledge gap still exists on the characteristics of wake turbulence, meanwhile there has been little work on the development of related engineering models. In view of this, firstly, one-dimensional analytical models that can estimate the wake width and maximum wake turbulence level at any streamwise positions are proposed and validated. Based on these, a highly simple three-dimensional cosine shape (3D-COTI) model is proposed for estimating the wake turbulence intensity in an effective way. Moreover, by taking into account the wind shear and ground effects, this proposed model is capable of describing the anisotropic property of the 3-D wake field. Afterwards, model evaluations are performed through several test cases consisting various types of turbines operating under a wide range of inflow conditions. Overall, the proposed model shows a good agreement with the measurements about the spatial distribution of turbulence enhancement within the wake flow. Because of its good accuracy, simplicity and universality, the present model has potential for large-scale wind farm applications.

Flow Dynamics of a Contra-Rotating Axial Fan Under Windmilling and Locked Conditions

The present work aims to understand the flow dynamics of a low-speed contra-rotating axial fan under windmilling and locked state. To gain a deep understanding of flow behavior, four configurations are evaluated, both experimentally and numerically, in which one of the rotors will be in windmilling or locked mode while the other is in powered mode. The spanwise distribution of the loading coefficient and flow angles at different streamwise positions reveals the details of the flow development across the rotors. The windmilling rotor depicts the existence of a “neutral radius,” where compressor and turbine behaviors coexist. When rotor-2 is windmilling, the neutral radius is identified closer to the tip of the blade. In contrast, when rotor-1 is windmilling, the neutral radius is identified closer to the hub of the blade. The flow coefficient has a significant effect on the position of the neutral radius. Results from numerical simulations imply that the flow separates from the pressure surface in the span regions where the rotor behaves as a turbine and from the suction surface in the span regions where the rotor behaves as a fan.

Flow field and performance of a vertical-axis wind turbine on model buildings

The placement of a scaled-down Savonius (drag) vertical-axis wind turbine on model buildings is analysed experimentally by the use of turbine performance and flow field measurements in a wind tunnel. The set-up consists of two surface mounted cubes aligned in the flow direction. The turbine is tested at six different streamwise positions – three on each cube. Velocity field measurements are performed with particle image velocimetry along the centreline of the cubes with and without the turbine. The performance at each position is evaluated based on measurements of the produced torque and the rotational speed of the turbine. It is demonstrated that the common practice of estimating wind resources based on the urban flow field without the turbine present is insufficient. The turbine has a substantial influence on the flow field and thus also on the available power. The performance is found to be optimal in the front and centre of the first building with a significant drop-off to the back. This trend is reversed for the downstream building. Holistically, for more generic geometries and varying wind directions, the results suggest the central position on a building is a good compromise.

Enhancing turbulence: a potential strategy for drag reduction based on ground transportation systems (GTS) model

PurposeThe purpose of this paper is to investigate the mechanism and performance of a potential strategy, which is to enhance turbulence to carry out drag reduction for heavy trucks.Design/methodology/approachEnhancing turbulence deflector (ETD) was placed on the roof surface of an ground transportation system (GTS) to investigate the drag reduction mechanism of enhancing turbulence. Transition shear-stress transport improved delay detach eddy simulation model was adopted to simulate the unsteady small-scale flow around the ETD.FindingsBy optimizing the three influencing factors, diameter, streamwise length and streamwise position, the optimized ETD has achieved a maximum drag reduction of 7.04%. The analysis of flow field results shows that enhancing turbulence can effectively suppress flow separation and reduce the negative pressure intensity in the wake region of GTS.Originality/valueThe present work provides another potential possibility for the improvement of the aerodynamic performance of heavy trucks.

Forced and mixed convection experiments in a confined vertical backward facing step at low-Prandtl number

In this paper, we present experimental results for a non-isothermal vertical confined backward facing step conducted with a low-Prandtl number fluid. The eutectic alloy gallium–indium–tin is used as the working fluid. We conducted experiments for different Reynolds and Richardson numbers covering both forced and mixed convection regimes. Time-averaged velocity profiles were measured at six streamwise positions along the test section center-plane with so-called permanent magnet probes. The local Nusselt number was measured in streamwise and spanwise directions along the heating plate mounted right after the step. We further ran RANS simulations of the experiment to study the qualitative influence of assuming a constant specific heat flux thermal boundary condition for the experiment heating plate. The measured velocity profiles show the expected behavior for both studied convection regimes, while the measured streamwise local Nusselt number profiles do not. This is explained by how the heating plate thermal boundary condition is defined. We performed an order of magnitude estimate to estimate the forced- to mixed convection transition onset. The estimate shows good agreement with the experimental data, although further measurements are needed to further validate the estimated transition threshold. The measurement of fluctuating quantities remains an open task to be addressed in future experiments, since the permanent magnet probe measurement equation needs further adjustments.Graphical

Experimental hydrodynamic investigations on the effectiveness of inverted flag-based piezoelectric energy harvester in the wake of bluff body

The flapping motion and energy harvesting performance of an inverted flag in the wake of a bluff body are experimentally studied. The experimental measurements are carried out by changing the bending rigidity, streamwise gap (which represents the distance from the bluff body to the tip of the flag), and Reynolds number to explore their impact on the flapping dynamics and power generated by the piezoelectric flag. The optimal values of the Reynolds number, bending rigidity, and streamwise gap are determined based on the power generated. Variation in the flapping modes ranging from continuous to deflected mode is observed. The results show that the inverted flag having a high peak-to-peak amplitude is preferred for piezoelectric energy harvesting as it produces high strain energy. It is also demonstrated that the vortices shed by the upstream bluff body have a strong effect on the flapping amplitude of the downstream inverted flag. This research would help in determining the most effective streamwise position of the inverted flag behind the bluff body and bending rigidity for generating voltage at low Reynolds numbers.