Constant Freestream Research Articles

Revisiting RANS prediction of transitional flow on T3A flat plate subject to various freestream turbulences

A laminar–turbulent transition subject to a freestream turbulence involves highly complex flow mechanisms, which have received considerable interest in the past decades. Accurately predicting such complex physical phenomena is significantly challenging using computational fluid dynamics solvers, particularly in industrial engineering applications. In this study, the capability of two transitional turbulence models—k−kl−ω and k−ω−γ−Re˜θt—to predict a T3A plate transition scenario was revisited. First, the predicted boundary layer data was compared to ERCOFTAC experimental measurements. Subsequently, a transitional flow subject to constant and sinusoidal freestream turbulences was systematically investigated. Under a constant freestream turbulence, the two models exhibited opposite tendencies of the transition starting and ending positions with the increase in Refst. However, there were still some similarities between the results of the two models. When the length scale was fixed, the transition area moved upstream with increasing turbulence intensity. The transition zone was more sensitive to a small length scale than to a considerable length scale. The dynamic boundary layer under the sinusoidal combination condition was less disturbed than the sinusoidal turbulence intensity and the sinusoidal specific turbulence dissipation rate. Regardless of the type of sinusoidal freestream turbulence the boundary layer was exposed to, increasing the oscillation frequency significantly disturbed the transitional flow. It was observed that the inertia effect of the dynamic onset point was enhanced by increasing the oscillation frequency. Concurrently, a significant phase delay occurred, and the delay time was approximately proportional to the oscillation frequency. The extreme position of the transition onset close to the leading edge of the plate was not affected by the oscillation frequency, whereas it depended on the sinusoidal variable. However, the extreme downstream transition onset point was significantly affected by the oscillation frequency.

High streamwise airfoil oscillations at constant low and high incidence angles

Ratios of streamwise airfoil oscillations to the freestream velocity above 30% have not been well investigated in the literature for a reduced frequency range relevant to unsteady applications. A known departure from the experimental correlation to analytical theory for lower magnitudes of this ratio, known as surge amplitude, motivates a parameter study for constant freestream, at constant low- and high-incidence angles, to understand the circulatory lift dependence on angle of attack, Reynolds number, surge amplitude, and reduced frequency in comparison with theory and higher-order computations. To better understand the increased deviation between theory and experiment with increasing velocity fluctuation, a detailed study of surge amplitude of 0.5 is investigated. The experiment for comparison was a free-surface water tunnel with a NACA (National Advisory Committee for Aeronautics) 0018 airfoil oscillated in the streamwise direction. Force measurements, normalized by instantaneous dynamic pressure, reveal that unsteady lift is dependent on Reynolds number and reduced frequency in both attached and fully separated conditions. In separated conditions, mean and fluctuating lift show a dependency on reduced frequency for larger velocity fluctuations than a relative surge amplitude of 10%. Two-dimensional computations were found to agree well with experimental data for Reynolds number 75 k, low incidence cases, and for high incidence with reduced frequencies less than 0.15, where a fully separated upper surface boundary layer condition occurred. Agreement between computations and experiments was not favorable for reduced frequencies above 0.15 for high incidence cases, where partial upper surface boundary layer reattachment is predicted.

Pressure And Velocity Fields Decomposition Around A Flapping Wing

In the present work, the velocity and pressure fields around a flapping airfoil in pitching/heaving sinusoidal motion are decomposed in modes in the wing-fixed reference frame. This approach is aimed at assessing the contribution of the fluid structures forming on the wing surface to the aerodynamic forces. The velocity field used for this work have been obtained through Particle Image Velocity experiments at a chord-based Reynolds number of 3600 and a Strouhal number equal to 0.2. The pressure field has been reconstructed from the phase-average velocity fields using a finite-differences Poisson solver. Finally, the velocity and pressure fields have been interpolated on a wing-fixed domain and decomposed using Proper Orthogonal Decomposition and Quadratic Stochastic Estimation to account for the quadratic relation between velocity and pressure. The decomposition identified vortical flow structures over the wing which model the evolution of bound vorticity, as well as leading edge and trailing edge vortices, over the wing. These modes contribute linearly to the chord-normal force by interacting with the constant free stream contained in the time average, as well as quadratically to the chord-wise force.

Turbulent Flow in Forced Convection Heat Transfer - Numerical Validation

Forced convective heat transfer of airflow through circular pipe with constant heat input and different free stream velocities is numerically validated. The significance of the present work is that the suction flow has been employed in the forced convection set up domain kept in the wind tunnel. From first law of thermodynamics and applying the energy balance equation, experimental heat transfer coefficient is determined. Further correlations are used to validate the experimental results. Although correlations provide reasonable estimates from the point of feasibility and accuracy, computational methods are used to estimate the convective heat transfer coefficient. Hence in this paper experimental, theoretical and computational analysis is carried out. The results reveal that the numerical validation is an effective tool from the point of feasibility and accuracy to determine the convective heat transfer coefficient.

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.

HIFiRE-5b Boundary-Layer Transition Length and Turbulent Overshoot

Hypersonic International Flight Research Experimentation 5 (HIFiRE-5) is a hypersonic flight-test experiment designed to investigate the aerothermodynamics of a 3-D geometry. The vehicle is an elliptic cone with a 2:1 aspect ratio and 2.5 mm nose radius. The HIFiRE-5b achieved a Mach number of 7.7–7.9 and freestream unit Reynolds number of to during descent. Heat flux was calculated from thermocouple pairs and boundary-layer transition locations were identified from these heating rates. A multilobed transition front was observed; it is suspected that a different mechanism is responsible for each of these lobes. The vehicle’s attitude oscillated, which has been exploited to assess the sensitivity of heat flux to yaw angle along the leading edges and ascertain the appropriateness of the filtering applied to the thermocouple signals. Spatial heat-flux profiles have been constructed for constant freestream unit Reynolds numbers. The heat-flux rate at the end of transition is examined; turbulent overshoot was present along some azimuths, but not others. This flight-test result indicates the phenomenon is not specific to ground test, freestream noise level, or wall-to-stagnation-temperature ratio. Finally, transition length (its end location normalized by its onset location) is reported and connected to turbulent spot generation rates.

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.

Role of chemical reactions in the stagnation point heat flux of rarefied hypersonic cylinder flows

This work investigates the variations of the stagnation point heat flux (SPHF) in hypersonic cylinder flows using the direct simulation Monte Carlo method, with the consideration of a constant freestream Knudsen number but different cylinder diameters. Four different freestream Mach numbers and the accompanying chemical reactions are considered. The result reveals a high-density effect in chemical reactions inside the thermal boundary layer, which induces an increasingly rising SPHF with a decreased cylinder diameter for all the cases. The cases at Ma∞ = 30 exhibit a characteristic of peculiarity that the value of SPHF increases the fastest, which is strongly correlated with the different high-density effects at different Ma∞. Further analysis demonstrates that the NO dissociation and recombination reactions always play a vitally important role in the high-density effect. A secondary NO dissociation reaction was observed inside the thermal boundary layer when Ma∞ > 30. This observation is the result of the shift of chemical equilibrium induced by violent recombination reaction and sufficiently high flow temperature. Subsequently, the newly emerging secondary dissociation reaction weakens the influence of recombination reaction; thus, the growth of SPHF at a high Mach number is not so strong as that with Ma∞ ≤ 30. Furthermore, in order to provide more reliable results, additional simulations are discussed by employing the widely accepted total collision energy and catalytic surface reaction models.

Numerical investigations on the dynamic behavior of a 2-DOF airfoil in the transitional Re number regime based on fully coupled simulations relying on an eddy-resolving technique

The paper is the numerical counterpart of the experimental investigation on the fluid–structure interaction (FSI) of a wing with two degrees of freedom (DOF), i.e., pitch and heave. Wood et al. (2020) has provided the experimental basis by studying the flutter stability of an elastically mounted straight wing (NACA 0012 airfoil) in a wind tunnel considering the transitional Reynolds number regime. Three different configurations with varying distances between the fixed elastic axis and the variable center of gravity were considered. Additional free-oscillation tests in still air were carried out in order to make the mechanical properties of the setup available for the simulations. The present contribution describes the numerical methodology applied consisting of a partitioned coupled solver combining eddy-resolving large-eddy simulations on the fluid side with a solver for the governing equations of the translation and rotation of the rigid wing. In order to prove the parameters provided by the experiment and to determine the pure material damping coefficients not available from the measurements, simulations of 1-DOF free-oscillation tests in still air are carried out and analyzed. For validation purposes the corresponding 2-DOF free-oscillation tests in still air are assessed and a good agreement with the experimental data is achieved. Finally, the wing exposed to a constant free-stream of varying strength is analyzed leading to the characterization of complex instantaneous FSI phenomena such as limit-cycle oscillations and flutter. Under full utilization of the supplementary measurements the predictions are evaluated in detail. Contrary to the experiments the simulations provide the entire fluid data and unique data for the translatory and rotatory movement allowing to investigate the causes of the observed phenomena. Both limit-cycle oscillations and flutter can be reproduced by the coupled FSI predictions.

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.

Force generation of smooth and corrugated airfoil performing unsteady motions at low Reynolds number

Structurally, corrugated airfoil for flapping wing MAV applications is more desirable due to its increased strength. Therefore, any aerodynamic (penalty) effects due to corrugation on such airfoil have found considerable attention. The objective of the paper is to computationally examine the effects of corrugation on an elliptical airfoil performing translation and pitching motion at Re = 100 by comparing it with the performance of a smooth NACA 0012 airfoil. The forces and the flow structures generated over the airfoils during their unsteady motion are solved using Navier–Stokes equations. These unsteady motions include the airfoil accelerating from the rest position and quickly pitching up in a constant free stream. The two airfoils are considered in the study with 50 and 100 equally spaced perturbations on the upper and the lower surfaces, respectively. The surface of a standard ellipse has been modified with the regular perturbations or ‘corrugation’ of the order of 1% c on the upper and the lower surfaces. The current study reveals that during the airfoil acceleration from the rest to U and, subsequently, in a constant velocity translation, the corrugated airfoils (regular surface perturbations) have similar behavior of the force and the moment coefficients (also similar vorticity plots) in comparison with the smooth airfoil. However, it is also found that for the fast and the slow pitching motion in a free stream, the corrugated airfoil (after certain angle of attack) has slightly lower values of the lift and the moment coefficients in comparison with the smooth airfoil; which is due to the small effect of corrugation on the velocity profile. It is concluded that the unsteady effect dominates the geometric differences (smooth versus corrugated airfoil); therefore, corrugated airfoil can be utilized to provide structural strength for flapping wing MAV applications.

Indoor particle inhalability of a stationary and moving manikin

Understanding particle inhalation caused by human activity is vital for developing estimates of inhalation exposure in indoor environments. This study used Computational Fluid Dynamics to compare nasal aspiration efficiency of a stationary manikin in uniform airflow and a moving manikin in stagnant air. The model was a full-scale body with detailed facial features at the nose. The stationary manikin case adopted a constant freestream velocities of 0.2 m/s and 0.4 m/s representing typical office wind environments; while the moving manikin case considered a walking speed of 0.4 m/s as a comparison case, followed with additional higher walking speeds (0.8 m/s and 1.6 m/s) to investigate realistic walking scenarios. Inhalation rates through the nose included light and medium breathing at 15 and 27 L per minute.The aspiration efficiency (AE) was used to quantify the nasal particle inhalability from an upstream release source. In order to evaluate the inhalability for the moving manikin, a 3-dimensional particle source was created and validated with a 2-dimensional particle source, which was common in studies for stationary manikins. Discrepancies in aspiration efficiencies were quantified between using 2- and 3-dimensional particle sources. The particle inhalability for a moving manikin was estimated for three particle sizes and three walking speeds. The inhalability of a walking manikin was found higher than a stationary manikin facing the wind, less relevant to particle sizes but more related to walking speeds. This study quantified the particle inhalability for moving manikins to characterise a more comprehensive scenario for modelling human respiration and developing estimations of particle exposure.

Angle of Attack Investigations on the Performance of a Diverterless Supersonic Inlet

An extensive experimental investigation to study the effects of angle of attack (AOA) on the performance of a body-integrated supersonic inlet has been carried out. The present inlet, known as Diverterless Supersonic Inlet (DSI), is utilized with a three-dimensional bump to provide both supersonic flow compression and boundary layer diversion. Experiments were conducted at the presence of a typical fore-body including an elliptical nose to further contemplate the effects of fore-body geometry on the approaching flow. All tests were conducted at a constant free stream Mach number, M_∞=1.65 zero degrees angle of sideslip (AOS), and at various angles of attack (AOA) ranging from -2 to 6 degrees. The results showed that the present DSI had acceptable performance characteristics for all ranges of AOA tested. It should be noted that the present DSI does not have any moving, adjustable or auxiliary mechanisms as such systems or mechanism are used to improve the performance of an inlet.

A methodology for simulating 2D shock-induced dynamic stall at flight test-based fluctuating freestream

A comprehensive methodology for simulating 2D dynamic stall at fluctuating freestream is proposed in this paper. 2D CFD simulation of a SC1095 airfoil exposed to a fluctuating freestream of Mach number 0.537 ± 0.205 and Reynolds number 6.1 × 106 (based on the mean Mach number) and undergoing a 10° ± 10° pitch oscillation with a frequency of 4.25 Hz was conducted. These conditions were selected to be representative of the flow experienced by a helicopter rotor airfoil section in a real-life fast forward flight. Both constant freestream dynamic stall as well as fluctuating freestream dynamic stall simulations were conducted and compared. The methodology was carefully validated with experimental data for both transonic flow and dynamic stall under fluctuating freestream. Overall, the results suggest that the fluctuating freestream alters the dynamic stall mechanism documented for constant freestream in a major way, emphasizing that inclusion of this effect in the prediction of dynamic stall related rotor loads is imperative for rotor performance analysis and blades design.

Melting Heat Transfer and Radiation Effects on Jeffrey Fluid Flow over a Continuously Moving Surface with a Parallel Free Stream

This article is proposed to address the melting heat transfer of a Jeffrey fluid in Blasius and Sakiadis flow caused due to a moving surface. Thermal radiation and a constant free stream are considered in this mathematical model. The non-linear coupled dimensionless equations from the governing equations are attained by employing appropriate similarity transformations. The resulting dimensionless equations are solved by implementing RKF method. The impact of sundry emerging parameters on different flow fields are interpreted with the help of figures and tables. For augmented values of Deborah number, the velocity profile diminishes in the case of Blasius flow and the reverse behavior in the Sakiadis flow is observed. Moreover, the velocity of non-Newtonian liquid in case of Blasius flow is superior to that of the Sakiadis flow. The present work is demonstrated by matching with the computational results in the literature and found to be outstanding agreement.

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.