Constant Free-stream Velocity Research Articles

Drag reduction of a D-shaped bluff-body using linear parameter varying control

In this work, we report on a closed-loop flow control strategy that consistently reduces the drag of a D-shaped bluff body under variable freestream velocity conditions. The control strategy is guided by open-loop tests with pulsed Coanda blowing at two freestream velocities that yield optimal frequencies (Strouhal number of 0.33 and 1.3), which reduce the drag by up to 40%. The strong correlation between drag coefficient (Cd) and the wake fluctuations is exploited for the feedback signal, where a microphone signal is used to measure the pressure fluctuations at the model base. The results demonstrate the ability to perform accurate and robust H∞-based control for drag reduction using solely the wake pressure fluctuations at the model base as feedback signal. The robust control strategy at constant freestream velocity is shown to improve output stability and enhance performance in terms of settling time, even when employing simple models of the flow response with large uncertainty. Building on that success, an H∞-based linear parameter varying controller is designed and implemented to reduce drag under free stream variations and/or fluctuations. Similarly, the results demonstrate improved robustness and performance enhancements.

Flow Field Measurement of Laboratory-Scaled Cross-Flow Hydrokinetic Turbines: Part I—The Near-Wake of a Single Turbine

Recent developments in marine hydrokinetic (MHK) technology have put the cross-flow (often vertical-axis) turbines at the forefront. MHK devices offer alternative solutions for clean marine energy generation as a replacement for traditional hydraulic turbines such as the Francis, Kaplan, and Pelton. Following previous power measurements of laboratory-scaled cross-flow hydrokinetic turbines in different configurations, this article presents studies of the water flow field immediately behind the turbines. Two independent turbines, which operated at an average diameter-based Reynolds number of approximately 0.2×105, were driven by a stepper motor at various speeds in a closed circuit water tunnel with a constant freestream velocity of 0.316 m/s. The wakes produced by the three NACA0012 blades of each turbine were recorded with a monoscopic particle image velocimetry technique and analyzed. The flow structures with velocity, vorticity, and kinetic energy fields were correlated with the turbine power production and are discussed herein. Each flow field was decomposed into the time averaged, periodic, and random components for all the cases. The results indicate the key to refining the existed turbine design for enhancement of its power production and serve as a baseline for future comparison with twin turbines in counter-rotating configurations.

Twin Marine Hydrokinetic Cross-Flow Turbines in Counter Rotating Configurations: A Laboratory-Scaled Apparatus for Power Measurement

This article proposes an experimental apparatus design to measure the power of a cross-flow marine hydrokinetic turbine system operating in a laboratory water tunnel. Data, from one Hall sensor output signal, was processed to capture the three types of torque exerted on the turbines: mechanical loss, brake, and hydrodynamic torque. The method was then applied to compare the power of a twin turbine system in different counter-rotating configurations. Controlled by a hysteresis brake, the tip-speed-ratio was varied in a constant freestream velocity of 0.316 m/s. While the braking torque was independent of the speed, the mechanical loss was found to depend on the system rotational speed and the amount of mass mounted on the mechanical support. In a counter-rotating configuration, the turbines were synchronized through a pair of spur gears and timing pulleys. Operating at the average chord based Reynolds number of 8000, each turbine had three NACA0012 blades mounted at 15∘ pitch angle. The power coefficient results of 8 turbine configurations showed the tendency of power enhancement of counter-rotating configurations due to blade interaction and increase in blockage ratio. Comparison of the results suggested direct application in a river flow scenario and manipulation of the blade interaction for optimal power production.

An Introductory CFD Analysis Study of Novel Cavity Vane Driven Wind Turbine Blade Design

Climate in Malaysia is heavily influenced by monsoon seasons, where the wind speed pattern is inconsistent and highly unsteady. Existing commercial wind turbines are only designed to accommodate high wind speed regions. In order to harvest energy in Malaysia, design modification is required on conventional wind turbine blade. This paper presents an introductory computational investigation of proposed drag driven vertical axis wind turbine. The objective of this research is to investigate the factors that governs the aerodynamics attributes of the proposed design. The design is simulated in 2D using sliding mesh technique via FLUENT based on URANS model. Previous mesh dependency study shows that fine and medium grid topology indicated a good agreement. In this numerical study, the proposed wind turbine is simulated based on SST k-ω turbulent transport model under fine grid spatial discretization. The design is analyzed under the influence of constant freestream velocity of 25 m/s in order to study the behavior of the turbine at high speed and low RPM. Result shows that high static pressure of 64.8 kPa exerts on the returning blade 1 which overwhelms the motion of advancing blade. Blade 1-3 indicated stable numerical values after 216°. Hence, the governing factors in terms of flow properties that affected the wind turbine is studied for future improvement. It is found that the cavity vane of the proposed design requires modification in order to improve moment.

Numerical Study of Bach-bladed Savonius Wind Turbine with Varying Blade Shape Factor

Savonius wind turbine with Bach-profile blades is considered in this study. Previous studies have shown that a rotor with the Bach-profile blade produces better performance than a standard Savonius turbine. This study focuses on the blade shape factor variations of the Bach-profile blade to give the best performance. Two-dimensional unsteady simulations are performed with moving mesh. The configuration being tested is the Savonius rotor with Bach-profile blades with an arc angle of 135◦ . The blade shape factor is varied 0.2, 0.3, 0.4 at a constant freestream velocity of 4 m/s, with a corresponding Reynolds number of 20,000. The k-ω Shear Stress Transport turbulence model was used, with secondorder discretization schemes for the pressure and momentum equations. The boundary conditions were set as velocity inlet for the inlet, outflow for the outlet, and walls for the blade surfaces. The top and bottom sides were set as symmetric. Results showed that the configuration with a shape factor of 0.4 gave the best performance among the others. This configuration gave a higher moment coefficient and power coefficient of about 6.8% and 7.3%, respectively. Results extracted from the simulation includes the flow structure, and the distribution of the pressure coefficients along the blade surface.

Mixed Convection on MHD Flow with Thermal Radiation, Chemical Reaction and Viscous Dissipation Embedded in a Porous Medium

In this paper, a theoretical analysis has been made to study the effect of mixed convection MHD oscillatory Couette flow in a vertical parallel channel walls embedded in a porous medium in the presence of thermal radiation, chemical reaction and viscous dissipation. The channel walls are subjected to a constant suction velocity and free stream velocity is oscillating with time. The channel walls are embedded vertically in a porous medium. A magnetic field of uniform strength is applied normal to the vertical channel walls. The nonlinear and coupled partial differential equations are solved using multi parameter perturbation techniques. The effects of physical parameters, viz., the radiation absorption parameter, Prandtl number, Eckert number, dynamic viscosity, kinematic viscosity, permeability of porous medium, suction velocity, Schmidt number and chemical reaction parameter on flow variables viz., temperature, concentration and velocity profile have been studied. MATLAB code is used to analyze theoretical facts. The important results show that an increment in the radiation absorption parameter and permeability of porous medium results in an increment of the temperature profile. Moreover, an increment in the Prandtl number, Eckert number and dynamic viscosity results in a decrement of the temperature profile. An increment in suction velocity results in a decrement of the velocity profile. An increment in the Schmidt number, chemical reaction parameter and kinematic viscosity results in a decrement of the concentration profile.

A moment coefficient computational study of parametric drag-driven wind turbine at moderate tip speed ratios

ABSTRACT This paper presents the CFD numerical investigation of novel drag-driven wind turbine blade inspired by spiral optimisation algorithm (SPO) and cycloid curve. In this study, six hybrid spiral geometries were analysed via CFD simulation using FLUENT based on URANS and SST numerical model. The simulated results of six shapes were compared against simulated conventional Savonius wind turbine. In terms of moment coefficient, all the turbines were simulated under similar computational configuration. The turbines were studied at two moderate tip speed ratios, which are λ = 0.59 and λ = 0.94, under constant freestream velocity of 8 m/s. The result shows that design shape S-4 displayed higher moment coefficient than Savonius wind turbine with an improvement of 7.2% in moment coefficient at λ = 0.59. However, at λ = 0.94, the percentage of improvement in moment coefficient is only 4%. It is observed that reduction in blade height and modification of blade curve configuration improve the moment of the rotor. The sharp edge presented by the hybrid spiral shape induces higher pressure gradient than Savonius wind turbine on the convex side of the blade.

The Role of Free-Stream Turbulence in Attenuating the Wind Updraft above the Collector of Precipitation Gauges

AbstractIn operational conditions, wind is the main environmental source of measurement biases for catching-type precipitation gauges. The gauge geometry induces a deformation of the surrounding airflow pattern, which is generally characterized by relevant updraft zones in front of the collector and above it. This effect deviates the trajectories of the lighter hydrometeors away from the collector and thus is responsible for a significant reduction of the collection performance. Previous approaches to this problem, using computational fluid dynamics simulations and wind-tunnel tests, mostly assumed steady and uniform free-stream conditions. Wind is turbulent in nature, though. The role of natural free-stream turbulence on collection performance is investigated in this work for the case study of a calyx-shaped precipitation gauge and wind velocity between 10 and 18 m s−1. The unsteady Reynolds-averaged Navier–Stokes model was adopted. Turbulent conditions were simulated by imposing constant free-stream velocity and introducing a fixed solid fence upstream of the gauge to generate the desired turbulence intensity. Wind-tunnel measurements allowed validating numerical results by comparing measured and simulated velocity profiles in representative portions of the investigated domain. Results revealed that in the case of turbulent free-stream conditions both the normalized magnitude of the flow velocity and the updraft above the collector are reduced by approximately 20% and 12%, respectively. The dissipative effect of the turbulent fluctuations in the free stream has a damping role on the acceleration of the flow and on the updraft. This would result in a reduced undercatch with respect to literature simulations that employed the traditional uniform free-stream conditions.

Numerical analysis of wind velocity effects on fire-wind enhancement

Variation in flow characteristics triggered through the fire-wind interface can potentially damage the buildings during bushfires. Fire-wind enhancement which is referred to as the increase of wind velocity, caused by the fire-wind interaction, is one of the destructive phenomena in this regard. In spite of the significance, the underlying mechanism contributing to this phenomenon is still not well understood. This study employs computational fluid dynamic (CFD) simulation to fundamentally investigate the effects of free-stream wind velocity on fire-wind enhancement through analyzing the momentum and buoyancy of fluid. Fire-wind interaction is shown to cause the generation of fire-induced longitudinal negative pressure gradient which results in fire-induced pressure and viscous forces in longitudinal direction. These forces are further found as the prime reason for the distortion of the wind velocity profile. A module is implemented to the FireFOAM solver to calculate and extract these forces quantitatively. The results reveal that under a constant fire intensity, the level of distortion and/or enhancement in the wind velocity profile comparatively reduces with the increase of free-stream wind velocity. A new non-dimensional group (modified Euler number) is introduced to take into account dominant fire-induced forces causing fire-wind enhancement. Richardson number and the modified Euler number are employed to determine the influence of free-stream wind velocity and longitudinal distance from the fire source on wind velocity enhancement. Large-eddy simulation (LES) results indicate that while the level of enhancement generally depends on both Richardson and the modified Euler number, the location of the maximum level of enhancement along the plume centreline coincides with the maximum value of modified Euler number under a constant free-stream wind velocity scenario.

Numerical Investigation of Transitional Flow over a Flat Plate under Constant Heat Fluxes

In this study, a boundary layer flow over a flat plate is investigated numerically at constant inlet freestream velocity and turbulence intensity. After intensive mesh refinements, an adequate computational domain is determined. Four turbulence models (k-epsilon, k-omega, k-omega SST, Transition SST) are used to analyze the boundary layer flow. Local surface friction coefficient distribution is obtained and compared to each other to assess the most convenient turbulence model. The Computational Fluid Dynamics (CFD) results show that the Transition SST turbulence model demonstrates the most realistic surface friction coefficient (Cf) distribution in agreement with the experimental data. Additionally; the effects of constant heat fluxes on Cf values are investigated and it is found that the heating process moves transition backward compared to isothermal case. Moreover, it is fount that Cf values in the turbulent region decrease compared to isothermal case.

Internal and external cooling of a full coverage effusion cooling plate: Effects of double wall configuration and conditions

The present study provides new heat transfer data for both the surfaces of the full coverage effusion cooling plate within a double wall cooling test facility. To produce the cooling stream, a cold-side cross-flow supply for the effusion hole array is employed. Also utilized is a unique mainstream mesh heater, which provides transient thermal boundary conditions, after mainstream flow conditions are established. For the effusion cooled surface, presented are spatially-resolved distributions of surface adiabatic film cooling effectiveness, and surface heat transfer coefficients (measured using infrared thermography). For the coolant side, presented are spatially-resolved distributions of surface Nusselt numbers (measured using liquid crystal thermography). Of interest are the effects of streamwise development, blowing ratio, and Reynolds number. Streamwise hole spacing and spanwise hole spacing (normalized by effusion hole diameter) on the effusion plate are 15 and 4, respectively. Effusion hole diameter is 6.35 mm, effusion hole angle is 25°, and effusion plate thickness is 3 hole diameters. Considered are overall effusion blowing ratios from 2.9 to 7.5, with subsonic, incompressible flow, and constant freestream velocity with streamwise development, for two different mainstream Reynolds numbers. For the hot side (mainstream) of the effusion film cooling test plate, results for two mainflow Reynolds numbers of about 145000 and 96000 show that the adiabatic cooling effectiveness is generally higher for the lower Reynolds number for a particular streamwise location and blowing ratio. The heat transfer coefficient is generally higher for the low Reynolds number flow. This is due to altered supply passage flow behavior, which causes a reduction in coolant lift-off of the film from the surface as coolant momentum, relative to mainstream momentum, decreases. For the coolant side of the effusion test plate, Nusselt numbers generally increase with blowing ratio, when compared at a particular streamwise location and mainflow Reynolds number.

On the acoustics of a circulation control airfoil

A two-dimensional elliptical circulation control airfoil model is studied in the Florida State Aeroacoustic Tunnel. Far-field acoustics are obtained via a 55 microphone phased array. Single microphone spectra are also obtained, and it is shown that background noise is significant. In order to circumvent this problem, beamforming is employed. The primary sources of background noise are from the tunnel collector and jet/sidewall interaction. The deconvolution approach to mapping acoustic sources (DAMAS) is employed to remove the effects of the array point spread function. Spectra are acquired by integrating the DAMAS result over the source region. The resulting DAMAS spectral levels are significantly below single microphone levels. Although the DAMAS levels are reduced from those of a single microphone or delay and sum beamforming (DAS), they are still above those of a NACA 0012, estimated using NAFNoise, at the same geometric and free-stream conditions. A scaling analysis is performed on the processed array data. With a constant free-stream velocity and a varying jet velocity the data scale as jet Mach number to the 6th power. If the momentum coefficient is held constant and the free-stream velocity is varied the data scale as free-stream Mach number to the 7th power.

Stability of Boundary Layers over Porous Walls with Suction

Although there is considerable interest in using wall suction to increase boundary-layer stability, stability analyses suggest that porous walls are inherently destabilizing. We explore this contradiction by performing a spatial linear stability analysis of the asymptotic suction boundary layer using a realistic model of wall suction. The porous wall is modelled as a layer of rigid, homogeneous, isotropic, porous material of small permeability, in which inertial effects may be neglected. The porous layer is bounded above by a semi-infinite region in which a boundary layer is driven by a constant freestream velocity. The wall suction is created by applying a suction pressure to a semi-infinite region below the porous layer. Our stability analysis takes account of the full coupling between the flowfields in the boundary-layer and suction regions, governed by the Navier–Stokes equations, and the flow in the porous layer, governed by the volume-averaged Navier–Stokes equations. We find that small amounts of wall permeability destabilize the Tollmien–Schlichting wave and cause a substantial broadening of the unstable region. As a result, the stabilization of boundary layers by wall suction is substantially less effective and more expensive than what is predicted by classical boundary-layer theory.

Micro-PIV Measurement of Flow Upstream of Papermaking Forming Fabrics

In the flow through porous media such as screens the structure of the screen affects the upstream flow field. This is relevant for filtration and deposition processes where the local filtrate deposition rate depends on the upstream flow field, while deposition also alters this flow field over time. Here, we investigate the flow through forming fabrics that retain pulp fibers during the papermaking process, while the liquid phase passes through. The associated drainage velocity through the fabric has a significant impact on the quality of the finished product. Microparticle image velocimetry (micro-PIV) was used to measure the drainage velocity distribution upstream of two different fabrics. To make measurements at the scale of the forming fabric filaments, an experimental micro-PIV procedure was developed to permit velocity measurements several millimeters into the fluid stream with a depth of correlation of \(28\,\upmu \hbox {m}\) and an in-plane resolution of \(40 \,\upmu \hbox {m} \times 40 \,\upmu \hbox {m}\). For the single phase flow of water, the experiments show the existence of a highly variable drainage velocity field upstream of both fabrics. The normalized standard deviation (NSD) of the velocity fields decays exponentially with distance upstream of the fabric. The decay constant is on the order of one filament diameter at moderate Reynolds numbers \((Re=52-140)\). At a constant freestream velocity, the deposition of a small number of wood fibers onto a fabric results in a higher NSD than that of the bare fabric. This is believed to be due to partial obstruction of some holes in the fabric, which leads to higher velocities through unobstructed holes. Once about 5 g per square meter of fiber has been deposited onto the fabric, the velocity field above the fabric with adhering fiber mat becomes uncorrelated with the velocity field through the bare fabric.

Numerical study on the fluid flow pass a square cylinder: The temperature-viscosity dependence

In this paper, the 2D fluid flow pass a heated/cooled square cylinder exposed to a constant free-stream upward velocity is simulated via a multiple relaxation time (MRT) lattice-Boltzmann (LB) method. The buoyancy effect on the drag and lift coefficients as well as Nusselt number related is compared with the results in the existing literatures to validate the code used. The effect of temperature-viscosity dependence is then investigated to test whether the effect can be neglected or not for the mixed convection case. It is shown that the effect cannot be ignored when |Ri| > 0.15. Fortunately, the effect can be captured by using an effective temperature formula [J. M. Shi, D. Ferlach, M. Breuer, G. Biswas and F. Durst, Phys. Fluids16, 4331 (2004)] in a rather large range of Ri. All the numerical results, from another angle, also demonstrate that the MRT method is an efficient tool in simulating the problems such as the present one.

Aeroacoustic measurements on a NACA 0012 applying the Coherent Particle Velocity method

Aeroacoustic measurements on two NACA 0012 airfoil sections with different chord length and sharp trailing edge were conducted at the Laminar Wind Tunnel (LWT) of the University of Stuttgart. The LWT is a closed test section wind tunnel with a very low turbulence level and an acoustically optimized diffusor section allowing for high-quality aerodynamic as well as aeroacoustic measurements. Trailing edge noise measurements were performed using the Coherent Particle Velocity (CPV) method, which is based on a cross-spectral analysis of two hot-wire sensor signals placed on the suction and the pressure side of the airfoil trailing edge, respectively. At high angles of attack, the cross-spectral analysis of the two sensor signals used for the measurement of the trailing edge noise can be prone to a disturbing influence of hydrodynamic fluctuations. Hence, continuous shifts in the phasing of the cross-correlation are observed mainly for low sensor distances to the trailing edge. The quantitative evaluation of the trailing edge noise predominately in the low frequency range is, therefore, considerably disturbed. A new approach is proposed, which allows for the correction of the cross-correlation function based on the averaged single wire auto-spectrum. The results are compared to measurements with increased sensor distance and show good agreement. In the following, trailing edge noise measurements were performed on a NACA 0012 airfoil in a wide range of angles of attack (α = 0°–8°) and free-stream velocities ( $$u_{\infty} = 30{-}70\,{\hbox{m/s}}$$ ). The tripped flow cases exhibit a very good consistency for the scaling of the 1/3 octave spectra based on outer variables. Moreover, a common intersection point of the sound pressure level was observed for trailing edge noise spectra measured at constant free-stream velocity and different angles of attack. In cases without boundary layer tripping, the presence of an acoustic feedback loop was observed and linked to the presence of a laminar separation bubble on the pressure side in the vicinity of the trailing edge. Finally, a comparison of the aeroacoustic measurements based on the CPV method showed reasonably good agreement with published data obtained with both a microphone array and the Coherent Output Power method in open-test section facilities.

Inhalability of micron particles through the nose and mouth

Aspiration efficiencies from nose and mouth inhalations are investigated at low and high inhalation rates by using the commercial Computational Fluid Dynamics (CFD) software CFX 11. A realistic human head with detailed facial features was constructed. Facial features were matched to represent the 50th percentile of a human male, aged between 20 and 65 years old, based on anthropometric data. The constant freestream velocity was 0.2 ms−1, normal to the face, and inhalation rates through the mouth and nose were 15 liters per minute (LPM) for light breathing and 40 LPM for heavy breathing. It was found that the flow field in the near breathing region exhibited vertical direction caused by the presence of the torso where the airstream diverges as it flows around and over the body. The critical area concept was used as a tool to determine the aspiration efficiency of particles. Comparisons between critical areas for the nose and mouth inhalations show similar geometric properties such as the area’s shape, and its vertical distance location on the x-z plane located at y = 80 cm upstream. The critical area sizes were found to be slightly larger for the mouth inhalation mainly due to the larger mouth area and also the aligned orientation of the mouth to the upstream flow, whereas the nose is perpendicular to the upstream flow. This study was undertaken to establish the flow field in the near breathing region that will help to characterize the flow and particle field for initial boundary conditions leading to a more holistic modeling approach of respiration through the internal nasal cavity and mouth.

Modified amplitude and strouhal number scaling for correction of turbulent wall pressure fluctuations

Pressure fluctuations were measured in an external turbulent boundary layer over a buoyantly propelled axisymmetric body of revolution. Data were measured for three cases, resulting in axial length Reynolds numbers of 6.88x10 , 4.27x10 , and 3.21x10 at the measurement locations. The fresh water measurements were made in a fully developed turbulent boundary layer, following natural transition, with a near zero (very mildly adverse) pressure gradient. The salt water measurements were made in a favorable pressure gradient following a flow trip to force transition. The momentum thickness Reynolds number was greater than 4400 for all measurements, and the data were scaled with outer variables. The turbulence data were corrected using Corcos correction factors and then further scaled in both amplitude and frequency by the square root of (transducer radius/displacement thickness), in an attempt to resolve spatial resolution effects. This resulted in excellent agreement among the spectra to a modified Strouhal number of 1, which was the range of validity for the data. A second data set (constant freestream velocity and increasing displacement thickness with downstream measurement location), scaled somewhat better with the ratio of transducer radius to displacement thickness.

Unsteady boundary layer flow due to an impulsively started moving plate

The problem dealing with the unsteady two-dimensional boundary layer flow of a viscous fluid past an impulsively started infinite plate moving parallel to itself with a constant velocity has been investigated. The fluid at infinity is assumed to be flowing with a constant free-stream velocity. By using the appropriate transformations for the stream function, the basic governing equations are reduced to a single equation. This equation is solved analytically subject to the relevant boundary conditions by homotopy analysis method. The present analytic solution is uniformly valid for all time. Finally, the results are discussed through graphs.

Boundary layers in a dilute particle suspension

The general problem of a boundary-layer flow carrying a dilute, mono-disperse suspension of small particles (together with gravitational effects) is considered. The problem is modelled using the ‘dusty-gas’ equations, which are a coupled equation set linking the fluid motion to that of the particle motion (both of which are modelled as continua). A number of qualitatively distinct potential scenarios are predicted. These include a variety of boundary-layer breakdowns, and the formation of shock transitions in the distribution of the particulate phase (together with the development of particle-free zones). Numerical results predicting these differing behaviours are confirmed through local asymptotic analyses of the governing equations. Although we consider a general class of boundary layer, our results are compared and contrasted with previous studies of specific cases, most notably the constant freestream fluid velocity case (akin to the ‘clean’ Blasius boundary layer). In the case of a boundary-layer flow driven by a linearly retarding free stream (the analogue of the classical ‘Howarth’ boundary-layer problem), the effects of the particle phase are surprisingly seen to (slightly) delay the separation of the boundary layer.