Wake Velocity Measurements Research Articles

The influence of crosswinds and leg positions on cycling aerodynamics

This work investigated the influence of crosswinds and leg positions on the aerodynamics of an articulated cycling mannequin on a track bicycle. Force, wake total pressure and wake velocity measurements were made in a low-speed sports wind tunnel of closed-circuit type. The freestream velocity and wheel speeds were kept at 15m/s\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$15\\, \\hbox {m/s}$$\\end{document}. The crank angle of the mannequin was varied across a pedal cycle. Yaw angles from 0∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$0^{\\circ }$$\\end{document} to 20∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$20^{\\circ }$$\\end{document} were examined. The experimental results reveal that the leg position significantly affects the aerodynamic performance of a cyclist. At high yaw angles, the aerodynamic drag on the cyclist showed noticeable deviations between most leg positions and their 180∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$180^{\\circ }$$\\end{document}-apart pairs. A wake analysis technique effectively captured the influence of leg positions on drag. The total pressure deficit contributes dominantly to the overall drag. The wake pressure contours demonstrate how leg-wheel interaction affects the total pressure distribution and drag under crosswinds. This study offers valuable insights into the flow behavior and drag generation around a cyclist with varying leg positions under crosswinds.

Flow reconstruction by multiresolution optimization of a discrete loss with automatic differentiation.

We present a potent computational method for the solution of inverse problems in fluid mechanics. We consider inverse problems formulated in terms of a deterministic loss function that can accommodate data and regularization terms. We introduce a multigrid decomposition technique that accelerates the convergence of gradient-based methods for optimization problems with parameters on a grid. We incorporate this multigrid technique to the Optimizing a DIscrete Loss (ODIL) framework. The multiresolution ODIL (mODIL) accelerates by an order of magnitude the original formalism and improves the avoidance of local minima. Moreover, mODIL accommodates the use of automatic differentiation for calculating the gradients of the loss function, thus facilitating the implementation of the framework. We demonstrate the capabilities of mODIL on a variety of inverse and flow reconstruction problems: solution reconstruction for the Burgers equation, inferring conductivity from temperature measurements, and inferring the body shape from wake velocity measurements in three dimensions. We also provide a comparative study with the related, popular Physics-Informed Neural Networks (PINNs) method. We demonstrate that mODIL has three to five orders of magnitude lower computational cost than PINNs in benchmark problems including simple PDEs and lid-driven cavity problems. Our results suggest that mODIL is a very potent, fast and consistent method for solving inverse problems in fluid mechanics.

Quiet Spacecraft Cabin Ventilation Fan: Wake Measurements Results

A spacecraft cabin ventilation fan suitable for aerodynamic and acoustic ground tests was designed and two copies of the fan assembly were fabricated - designated as Quiet Space Fan. Both fans were tested for aerodynamic performance and acoustic levels in the NASA Glenn Research Center Acoustical Testing Laboratory. A new test rig for small axial flow fans was designed to accommodate the instrumentation and back-pressure adjustments. Measurements acquired were - static pressures for measuring performance, a 72-channel in-duct microphone array, external microphone measurements for acoustics, and inter-stage hot wire measurements of the fan wake. This report documents the wake velocity and turbulence measurements as part of a series of reports.

Performance of the porous disk wind turbine model at a high Reynolds number: Solidity distribution and length scales effects

A new design methodology for porous disk wind turbine modeling is proposed, where a disk is matched to a horizontal axis wind turbine (HAWT) on (i) thrust coefficient, (ii) radial solidity distribution, and (iii) length scale criteria. Three disk designs are tested, allowing for isolation of the effects of each criterion, with performance evaluated through experimental wake comparisons with a model HAWT at a diameter-based Reynolds number of 4 × 106 and free-stream turbulence intensity of 1.2%. Wake velocity measurements reveal excellent agreement on mean profiles in the near wake (as early as 112 diameters downstream) when the rotor’s radial solidity distribution is incorporated into the disk design. Higher order velocity statistics can also be matched farther downstream (312 diameters). To match the higher order moments, the disk must generate near wake turbulence of similar characteristics to the rotor, since this turbulence dominates the development of the wake in a high Reynolds number, low free-stream turbulence environment. This is achieved by the third design criterion, where physical features that match the rotor length scales are incorporated. Thus, including all three criteria in a single porous disk yields a model that performs well at field-relevant Reynolds numbers, is not performance dependent on the free-stream turbulence intensity, and does not require iterative tuning.

Performance assessment of vertical axis wind turbines (VAWT) through control volume theory

Renewable technologies are the focus of today’s energetic scenario. In this context, vertical axis wind turbines (VAWT) have been attracting interest in the last decade as they emerge as good candidates for urban applications and deep-water, multi-megawatt, offshore projects. Nevertheless, their current development state is still limited. Wind tunnel testing is one of the most important tools for their study, but involves high infrastructure costs, which lead to an important downscaling of the prototypes and the consequent problems associated. This work presents the formal development and use of an alternative methodology for characterization of turbine thrust, torque and performance without contact, using data from flow measurements. This is achieved by applying Control Volume (CV) theory which overcomes the typical mechanical issues derived from small-scale prototypes. Complete inlet pressure and wake velocity measurements have been performed and are presented here. From these measurements, turbine thrust, torque and performance have been obtained through CV theory, revealing good agreement with previous experimental data.

Experimental investigation and analytical modelling of active yaw control for wind farm power optimization

In this study, the physics and effectiveness of active yaw control under various wind conditions are investigated systematically, based on wind tunnel experiments and a new analytical wind farm model. The power and wake velocity measurements of a three-row miniature wind farm reveal that the peak power gain (18%) is reached in partial-wake conditions, when the wind direction is misaligned with the turbine column by 2–4°. In contrast, the power gain in the full-wake condition (5.4%) is a local minimum. For a single-column wind farm, the optimal yaw angle distribution always exhibits a decreasing trend from upstream to downstream, which can be associated with the secondary wake steering effect. Analytical model predicts that with increasing number of rows, both the peak power gain and the leading-turbine yaw angle increase asymptotically. The maximum value of the yaw angle is mainly determined by the cosine exponent of the thrust coefficient (p). With a typical value of p=1.8, the maximum yaw angle value is approximately 30°. Turbulence intensity and streamwise spacing have similar effects on active yaw control. When these two parameters increase, the relative power gain decreases monotonically.

The effect of the incoming boundary layer thickness on the aeroacoustics of finite wall-mounted square cylinders.

This paper is concerned with the influence of the incoming wall boundary layer thickness on the noise produced by a square finite wall-mounted cylinder in cross-flow. Acoustic and near wake velocity measurements have been taken in an anechoic wind tunnel for a cylinder in two different near-zero-pressure gradient turbulent boundary layers with thicknesses of 130% and 370% of the cylinder width, W. The cylinders have an aspect ratio of 0.29≤L/W≤22.9 (where L is the cylinder span) and were examined at a Reynolds number, based on width, of ReW = 1.4 × 104. The results presented in this paper demonstrate that increasing the height of the boundary layer delays the production of acoustic tones to higher aspect ratios. The height of the boundary layer changes the balance between upwash and downwash across the cylinder span, resulting in a delayed onset of the shedding regimes and correspondingly, the production of acoustic tones.

Spanwise wake development of a pivoted cylinder undergoing vortex-induced vibrations with elliptic trajectories

The wake development of a pivoted circular cylinder undergoing vortex-induced vibrations with elliptical trajectories is examined experimentally at a fixed Reynolds number of 3027 and mass ratio of 10.8. Simultaneous cylinder displacement measurements and time-resolved, two-component particle image velocimetry in multiple horizontal and vertical planes are used to quantify the structural response and wake development. The selected test cases pertain to $$U^*=U_0/f_\mathrm{n} D=5.48$$ and 7.08, and exhibit different orientations of elliptical cylinder trajectory, both with a clockwise direction of orbiting. Three-dimensional reconstructions of the phase-averaged wake velocity measurements reveal 2S shedding along the span of a stationary cylinder and hybrid shedding for the two vibrating cylinder cases, with planar wake topology transitioning from 2S to P+S to 2S for $$U^*=5.48$$ , and 2S to P+S for 7.08. The observed wake topologies show significant deviation from predictions based on the Morse and Williamson (J Fluids Struct 25(4):697–712, 2009) shedding map. Vortex identification and strength quantification are used to provide insight into vortex dynamics and to propose a model of the dislocations. Examination of the time averaged wake characteristics shows the formation length, wake half-width, and maximum velocity deficit exhibit distinct spanwise trends aligning with the regions associated with specific shedding regimes.

Fluid force and symmetry breaking modes of a 3D bluff body with a base cavity

A cavity at the base of the squareback Ahmed model at Re≃4×105 is able to reduce the base suction by 18% and the drag coefficient by 9%, while the flow at the separation remains unaffected. Instantaneous pressure measurements at the body base, fluid force measurements and wake velocity measurements are investigated varying the cavity depth from 0% to 35% of the base height. Due to the reflectional symmetry of the rectangular base, there are two Reflectional Symmetry Breaking (RSB) mirror modes present in the natural wake that switch from one to the other randomly in accordance with the recent findings of Grandemange et al. (2013b). It is shown that these modes exhibit an energetic 3D static vortex system close to the base of the body. A sufficiently deep cavity is able to stabilize the wake toward a symmetry preserved wake, thus suppressing the RSB modes and leading to a weaker elliptical toric recirculation. The stabilization can be modelled with a Langevin equation. The plausible mechanism for drag reduction with the base cavity is based on the interaction of the static 3D vortex system of the RSB modes with the base and their suppression by stabilization. There are some strong evidences that this mechanism may be generalized to axisymmetric bodies with base cavity.

Unsteady Modes in Flowfield About Airfoil with Horn-Ice Shape

An experimental investigation was conducted on an NACA 0012 airfoil with a leading-edge horn-ice shape to identify and characterize flowfield unsteadiness related to ice-induced flow separation. This type of iced-airfoil flowfield was dominated by a leading-edge separation bubble, which is associated with several inherent modes of unsteadiness. Using unsteady surface pressure and wake velocity measurements, three distinct modes of unsteadiness were identified at various locations throughout the flowfield. These unsteady modes included a regular mode of vortical motion and a shear-layer flapping mode, which are both associated with separation bubbles. A low-frequency mode that was associated with the thin-airfoil stall type of the iced airfoil was also observed and was represented by a global oscillation of the airfoil circulation. The characteristic frequencies and flowfield locations corresponding to these unsteady modes were compared with those reported in the literature for iced-airfoil flowfields and flows about canonical geometries.

Momentum Coefficient as a Parameter for Aerodynamic Flow Control with Synthetic Jets

The influence of periodic excitation from synthetic jet actuators on boundary-layer separation and reattachment over a NACA 0025 airfoil at a low Reynolds number is studied. Flow-visualization results showed a vertical jet pulse accompanied by two counter-rotating vortices being produced at the exit of the simulated slot, with the vortices shed at the excitation frequency. Hot-wire measurements determined the maximum jet velocity for a range of excitation frequencies and voltages, and were used to characterize the excitation amplitude in terms of the momentum coefficient . With the synthetic jet actuator installed in the airfoil, flow-visualization results showed that excitation produces boundary-layer reattachment, with the associated significant reduction in wake width. Wake-velocity measurements were performed to characterize the effect of flow-control excitation amplitude and frequency on airfoil drag and wake topology. The results demonstrate that is the primary governing flow-control parameter. Applying excitation above a specific threshold produced a 50% reduction in drag, significantly affecting wake topology. However, power consumption of a piezoelectric synthetic jet actuator depends substantially on the excitation frequency. Hence, by varying excitation frequency, significant gains in efficiency are possible.

Three-dimensional vortex flow near the endwall of a short cylinder in crossflow: Stepped-diameter circular cylinder

The effect of geometry on the flow around a cylinder in crossflow is investigated in this study. Three different stepped-diameter circular cylinders (SDCC s) with varying step heights are used. Extensive flow visualization using the oil-lampblack and smoke-wire techniques and near wake velocity measurements using a hotwire anemometer reveal complex secondary flows on and around the SDCC. Six vortices are observed in the horseshoe vortex system near the cylinder–endwall junction and six additional vortices are found in the step-induced vortex system on the step surface. Based on these experimental results, new secondary flow models are proposed. The step-induced vortices separate from the step surface at both sides and move toward the endwall, washing down the sides of the top/bottom larger diameter cylinders and interact with the separated shear layer and horseshoe vortices. In this process, they modify the near wake flow significantly: they produce an increase in velocity near the endwall region (below the step) and a decrease in velocity near the mid-span region, even altering the oscillatory behavior of the wake.

The flow over a thin airfoil subjected to elevated levels of freestream turbulence at low Reynolds numbers

Micro Air Vehicles (MAVs) can be difficult to control in the outdoor environment as they fly at relatively low speeds and are of low mass, yet exposed to high levels of freestream turbulence present within the Atmospheric Boundary Layer. In order to examine transient flow phenomena, two turbulence conditions of nominally the same longitudinal integral length scale (Lxx/c = 1) but with significantly different intensities (Ti = 7.2 % and 12.3 %) were generated within a wind tunnel; time-varying surface pressure measurements, smoke flow visualization, and wake velocity measurements were made on a thin flat plate airfoil. Rapid changes in oncoming flow pitch angle resulted in the shear layer to separate from the leading edge of the airfoil even at lower geometric angles of attack. At higher geometric angles of attack, massive flow separation occurred at the leading edge followed by enhanced roll up of the shear layer. This lead to the formation of large Leading Edge Vortices (LEVs) that advected at a rate much lower than the mean flow speed while imparting high pressure fluctuations over the airfoil. The rate of LEV formation was dependent on the angle of attack until 10° and it was independent of the turbulence properties tested. The fluctuations in surface pressures and consequently aerodynamic loads were considerably limited on the airfoil bottom surface due to the favorable pressure gradient.

Vortex modes and vortex-induced vibration of a long, flexible riser

Investigations of the velocity and vorticity fields in the wake of a flexible riser with a length to diameter ratio of 181 were conducted in a towing tank at moderate Reynolds numbers in the range of 9400–47,000. Wake velocity measurements were made with the riser freely vibrating in both in-line and cross-flow directions. The motion and wake field of the riser, undergoing free vibration, were simultaneously measured by accelerometers installed inside the riser and by using a digital particle image velocimetry (DPIV) system. The vortex-induced vibration (VIV) results show that the riser freely oscillated at multiple vibration frequencies and amplitudes at each Reynolds number. Mixed vortex modes, ‘2S’, ‘2P’ and ‘P+S’, were observed in the near wake of the riser at different instants of time. The occurrence of these vortex modes depended on the Reynolds number, dominant frequency and mean amplitude. At lower Reynolds number, the single stable mode ‘2S’ dominated the wake. With the increase of Reynolds number, the percentage of the ‘2S’ modes decreased while the percentage of ‘2P’ modes increased steadily except at Reynolds numbers of 14,100 and 47,000. The ‘P+S’ modes occurred mostly at a Reynolds number of 14,100 accompanied by more ‘2P’ modes and less ‘2S’ modes. At this Reynolds number, the frequency of the VIV was very close to the natural frequency of 0.72 Hz, which was obtained from a riser decay test in steady water and the average amplitude to diameter ratio reached 0.95, the highest found in these tests.

Flow Regimes and Drag Characteristics of Yawed Elliptical Cavities With Varying Depth

The effect of yaw angle and cavity depth on the resulting flow field of cavities with elliptical planform areas embedded in a low velocity turbulent boundary layer was investigated experimentally. A 2:1 elliptical cavity with depth to minor axis ratios ranging from 0.1 to 1.0 was tested in a wind tunnel facility. Surface pressure measurements and wake velocity measurements, using hot-wire anemometry, were conducted to examine the resulting flow regimes. The results indicated several different flow regimes for the different yaw angle and cavity depth configurations. Cellular structures were observed when the minor axis was aligned with the streamwise direction. Yawing the cavity with respect to the streamwise direction resulted in a highly asymmetric flow regime. This flow regime was also associated with high drag for certain cavity depth configurations. A nominally two-dimensional flow regime was observed for large yaw angles, when the major axis of the cavity was aligned with the streamwise direction. The yaw angle had only a minor effect on the flow regimes associated with the shallowest and deepest cavities examined. A strong resemblance was found between the flow regimes associated with elliptical and rectangular cavities for similar yaw and depth configurations. This similarity was also observed in the lift and drag coefficients for the different yaw angles and cavity depths. This indicated that the wall radius of curvature of elliptical cavities has a negligible effect on the resulting flow regimes when compared to rectangular cavities.

주기적으로 회전진동하는 원주 후류의 공진특성에 관한 연구

Lock-on characteristics of flow around a circular cylinder oscillating rotationally with a relatively high forcing frequency have been investigated experimentally. Dominant governing parameters are Reynolds number (Re), angular amplitude of oscillation (θ_A), and frequency ratio F_R=f_f/f_n, where f/_f is a forcing frequency and f_n is a natural frequency of vortex shedding. Experiments were carried out under the conditions of Re=4.14 x 10³, π/90≤θ_A≤π/3, and F_R=1.0. The effect of this active flow control technique on the lock-on flow characteristics of the cylinder wake was evaluated with wake velocity measurements and spectral analysis of hot-wire signals. The rotational oscillation modifies the flow structure of near wake significantly. The lock-on phenomenon always occurs at F_R=1.0, regardless of the angular amplitude θ_A. In addition, when the angular amplitude is less than a certain value, the lock-on characteristics appear only at F_R=1.0. The range of lock-on phenomena expands and vortex formation length is decreased, as the angular amplitude increases. The rotational oscillation creates a small-scale vortex structure in the region just near the cylinder surface. At θ_A=60°, the drag coefficient was reduced about 43.7% at maximum.

Flow over yawed circular cylinders: Wall pressure spectra and flow regimes

The fluctuating wall pressure on a circular cylinder in cross flow has been investigated experimentally in a water tunnel to examine the effect of yaw angle on the spectral characteristics of the wall pressure field and to provide insight into the periodic and turbulent structures of the flow. Wall pressure was measured at five azimuthal positions around the periphery of the cylinder for seven yaw angles from α=90° (normal flow) to α=0° (axial flow) at three subcritical Reynolds numbers (ReD=7200, 13 500, 27 600). As the yaw angle is varied from α=90° to 0°, large systematic and nonmonotonic variations in both the narrowband (periodic vortex shedding) and broadband (turbulent) spectral levels occur. The results provide additional insight into the structural characteristics of the yawed cylinder flow regimes over that which has been identified previously through measurements of mean wall forces and flow field characteristics. Significant Reynolds number effects were also observed over the range of the measurements, particularly at small yaw angles, that may indicate fundamental shifts in the structure of the wake or underlying regime transition mechanisms. Possible effects due to vortex-induced cylinder vibrations are not entirely clear but predominantly confined to the larger yaw angles. Simultaneous wall pressure, cylinder vibration, radiated acoustic pressure, and wake velocity measurements are required to appropriately isolate the various potential contributions to the wall pressure field and thus provide a clearer understanding of the underlying boundary layer separation, transitional wake, and fluid-structural coupling mechanisms.

Interference effect of an upstream larger cylinder on the lock-in vibration of a flexibly mounted circular cylinder

Experiments have been carried out to investigate the flow-induced vibration response of a flexibly mounted circular cylinder located in the vicinity of a larger cylinder and subjected to cross-flow. The interfering larger cylinder was placed upstream and had a diameter twice that of the vibrating cylinder. Complex interaction was observed between the flow over the two cylinders. The vibration responses of the flexible cylinder were classified into different regimes according to the relative positions of the two cylinders. In the-side-by-side arrangement and the tandem or near-tandem arrangement, flow-induced vibrations of the flexible cylinder were greatly suppressed. In the staggered arrangement which covered a large portion of the relative cylinder positions being investigated, vibrations of the smaller cylinder were greatly amplified. The vibration response curves were also largely modified with a broadening of the lock-in resonance range. A shift of the peak reduced velocity for maximum vibration response was also found. Flow visualizations and wake velocity measurements suggested that the modifications of the vibration responses were related to the presence or absence of constant or intermittent flow through the gap region between the two cylinders. The proposed mechanisms of flow interactions and the resulting vibration response characteristics could explain previous observations on flow-induced vibrations of two equal-sized circular cylinders reported in the literature.

2次元煙風洞における複合翼型まわりの流れの可視化

The lift and drag characteristics of specially designed complex airfoil are investigated by surface pressure and wake velocity measurements in a smoke tunnel. To understand the characteristics obtained, flow mechanism induced by this complex airfoil was investigated in the same tunnel by flow visualization technique using the smoke-line, tuft and oil flow methods. Both airfoil characteristic study and flow visualization were carried out under the same following conditions; Reynolds Number Re=3.3 × 105, angle of attack α=-615 degrees.

Free-stream turbulence: A limitation on fractal descriptions of open-flow systems

Cylinder wake velocity measurements were made at Reynolds number 100 with varying levels of free-stream turbulence. The Grassberger–Procaccia (G–P) correlation function, which is often used for estimating fractal dimension, was computed for these data, and also for data obtained by adding random noise to the iterations of the Henon map. A comparison of the two plots shows that the free-stream turbulence acts as a highly random external noise source. Since free-stream turbulence determines the details of the smaller flow scales, no low-order dynamical system will be able to model all the scales in this open flow.