Open-circuit Wind Tunnel Research Articles

Laser velocimetry of a plane mixing layer with a specific initial condition for study of turbulence characteristics of enhanced growth rate

Mixing layers are sensitive to the mixing angle and turbulence in the primary streams. Although there is extensive available research on this rather basic flow, there are still no suggestions for a clearly best configuration. For example, the combination of a laminar initial boundary layer and a large mixing angle has received little attention. In this work we test a new experimental configuration with large mixing angle and laminar/turbulent initial boundary layer that was not examined experimentally by LDA and PIV. This setup is expected to be a representation of the initial conditions that must result in better mixing. A plane mixing layer with a velocity ratio of 0.6 is produced by rebuilding an open circuit wind tunnel. Extensive calibration tests on velocity profiles and Reynolds stresses established the position of the self similar region. Velocity field measurements with laser Doppler anemometer (LDA) and particle image velocimeter (PIV) showed enhanced mixing layer growth. PIV plots showed the presence of stream-wise and cross stream vortices in the self-similar region without any considerable change in turbulence characteristics to that of reported in the literature. The article presents a combination of different experimental results that give a deeper understanding of this very configuration.

Wall pressure sources near an airfoil trailing edge under turbulent boundary layers

In the present work the wall pressure sources under turbulent boundary layers developing on the surface of an airfoil have been investigated in the region near the trailing edge. A trip wire placed on the surface of the airfoil close to the leading edge ensured a turbulent boundary layer for the two Reynolds numbers investigated. When the angle of attack was different than zero, for the lower Reynolds number, a laminar/transitional boundary layer was observed on the airfoil pressure surface due to relaminarisation of the flow. This study aimed at improving the understanding of the relationship between the vortical velocity field and its surface pressure signature which is a matter of relevance for the so-called airfoil self-noise and for the vibration phenomena observed in ship hulls. A NACA0012 airfoil of 30cm chord and aspect ratio of 1 placed at the exit of an open-circuit blower type wind tunnel was used in the investigation. Two different flow conditions corresponding to Reynolds numbers based on the chord of the airfoil of Rec=2×105 and 4×105 and three different angles of attack have been analysed. Simultaneous measurements of the unsteady surface pressure fluctuations and of the velocity field in the boundary layer and wake of the airfoil were performed, allowing the calculation of cross-correlations and cross-spectra between the two components of the velocity and the surface pressure. This has permitted the analysis of the surface pressure generating flow structures and revealed information about the sources of wall pressure fluctuations, which are closely related to those of the radiated noise.

Overspeed Control of a Variable-Pitch Vertical-Axis Wind Turbine by Means of Tail Vanes

This paper describes the overspeed control of a vertical-axis wind turbine (VAWT) without an electric control system or mechanical brakes. To improve its performance with straight blades, a VAWT with a variable-pitch mechanism consisting of a four-bar linkage was designed. This wind turbine exhibits directivity to the wind, which is able to make use of rotational speed control in a VAWT. This wind turbine can achieve passive yaw control by means of tail vanes, which are installed on the eccentric link lever. The yaw angle characteristics of the tail vanes were measured in an open-circuit wind tunnel and in field tests. The yaw angle variations from the numerical analysis are in good agreement with the experimental data from the wind tunnel tests. It was found that overspeed prevention by means of tail vanes is useful in protecting a VAWT from strong winds without using mechanical brakes.

Flow Control over a Conical Forebody by Duty-Cycle Actuations

Duty-cycle modulation alternately blowing from two opposite-facing plasma actuators on the leeward surface near the apex of a cone with a 10° semi-apex angle is adopted to control mean lateral force and moment, and the flow control mechanisms are presented. Pressure distributions over the forebody of the cone are measured by steady pressure tappings. The experiments are performed in a 3.0×1.6 m open-circuit wind tunnel at a wind speed of 20 m/s, a 45° angle of attack and a Reynolds number of 2×105, based on the diameter of the base of the cone. Almost linearly proportional control of the lateral forces and moments over a slender conical forebody at a high angle of attack has been demonstrated by employing a pair of single dielectric barrier discharge plasma actuators near the apex of the cone, combined with a duty-cycle technique. The pressure distribution measurements indicate that the bi-stable vortex pattern appears to be shifted in the opposite direction when the port or starboard actuator is activated, while the other is kept off during the test. It is shown that the reduced pulse-repetition frequency based on the local diameter at the plasma actuator equal to one yields the highest effectiveness among the cases considered.

Experimental Study of Half-Delta Wing Vortex Generator for Flow Separation Control

This paper describes an experimental study of the effects of passive half-delta wing vortex generators on a backward-facingramp flow.Fourmodelsofvortexgeneratorswithaspectratiosof0.4,0.8,and1.6weretestedinan open-circuit wind tunnel. The vortex generators were placed upstream of the backward-facing ramp where their heights were 24 and 48% of the boundary-layer thickness. Flow visualization and velocity measurements were conducted using a particle image velocimetry system. The results showed that an array of vortex generators with an aspect ratio of 1.6 had the highest efficiency in reducing the flow separation region. Nomenclature c = vortex generator chord length c=h = vortex generator chord length to vortex generator height ratio d = vortex generator spacing from trailing edge to trailing edge d=h = vortex generator trailing-edge spacing to vortex generator height ratio h = vortex generator height h=� = vortex generator height to boundary-layer thickness ratio � = vortex generator angle of incidence � = boundary-layer thickness � = displacement thickness � = momentum thickness

DESIGN, CONSTRUCTION AND TESTING OF LOW SPEED WIND TUNNEL WITH ITS MEASUREMENT AND INSPECTION DEVICES

A low speed open circuit wind tunnel has been designed, manufactured and constructed at theMechanical Engineering Department at Baghdad University - College of Engineering. The work is one ofthe pioneer projects adapted by the R & D Office at the Iraqi MOHESR. The present paper describes thefirst part of the work; that is the design calculations, simulation and construction. It will be followed by asecond part that describes testing and calibration of the tunnel. The proposed wind tunnel has a testsection with cross sectional area of (0.7 x 0.7 m2) and length of (1.5 m). The maximum speed is about (70m/s) with empty test section. The contraction ratio is (8.16). Three screens are used to minimize flowdisturbances in the test section. The design philosophy is discussed and methods for wind tunnelcalculation are outlined. Simulation of wind tunnel using ANSYS shows no separation of flow alongwind tunnel. Construction steps are also included in present work.

Drag performance of divergent tubular-truncated cones: a shape optimization study

The use of more efficient energy consuming devices, which are closely associated with reduction of environmental pollution, has gained significant interest in the recent decades. The reduction of drag coefficient also improves safety and durability of environmental structures subjected to high-velocity fluid flow, and causes the noise and vibration to decrease as well. This paper describes the efficiency improvement in energy management by means of reducing drag coefficient in a practical divergent tubular-truncated cone. Extensive numerical simulations with emphasis on the shape optimization study were performed in order to find minimum drag coefficient for both laminar and turbulent flows (9.41 × 102 ≤ Re ≤ 1.882 × 107) around the mentioned cone. The numerical results were validated with experimental data, obtained by performing tests in an open circuit wind tunnel. The results showed that the minimum drag coefficient of optimum model within the shape modification process, comparable to the value for the primary model in turbulent flow decreased by 69.8% (the maximum discrepancy) at the highest considered Reynolds number of 1.882 × 107. Furthermore, the study of streamwise velocity profile led to more useful result. In this regard, the velocity magnitude inside the inlet span of the primary model, close to the entrance was approximately 1.6 times as great as the free upstream velocity. This is a noticeable result for the use of efficiency important structures in industrial applications.

Design of a low Reynolds number airfoil for small horizontal axis wind turbines

A low Reynolds number airfoil was designed for applications in small horizontal axis wind turbines to achieve better startup and low wind speed performances. Experiments were performed on the improved airfoil (AF300) in an open circuit wind tunnel at Reynolds numbers of 38,000, 75,000, 128,000 and 205,000. Pressure distributions were obtained over the surface of the airfoil and the lift and drag forces were measured with a dynamometer at different angles of attack, α. A CFD analysis was also performed to get additional information on the flow characteristics. Particle Image Velocimetry (PIV) together with smoke flow visualization were used to study the flow around the airfoil. At the Reynolds numbers of 75,000, 128,000 and 205,000, maximum lift coefficients of 1.72, 1.81 and 1.86 respectively were obtained at the stall angle of 14°. The lift coefficient increased from 0.41 to 1.05 at Re = 38,000 in the α range of 0–18°, in which no stalling was documented. The results from PIV and smoke flow visualization showed that the flow stayed fully attached to the airfoil surface from Re as low as 56,000 at an angle of attack of 8° and maintained a fully attached flow up to 14° angle of attack for Re as low as 75,000.

Mitigation of flow separation using DBD plasma actuators on airfoils: A tool for more efficient wind turbine operation

In this study, dielectric barrier discharge plasma actuators (DBD-PA) were used to actively control flow separation over a NACA0024 airfoil. Experiments were conducted at a free stream velocity up to U ≈ 10 m/s ( Re ≈ 1.3 × 10 5) in an open-circuit blower type wind tunnel with a test section measuring 200 mm × 200 mm × 600 mm. The airfoil model was designed specifically to incorporate minimum flow disturbances from the components of the DBD-PA and was made using rapid prototyping. A sheet of dielectric polyimide (125 μm) with copper electrodes (35 μm) was attached to the outer surface of the airfoil. A layer of DBD plasma across the airfoil was produced when a peak-to-peak voltage of V p-p = 8.0 kV was applied between top and bottom electrodes at a frequency of f p = 9.0 kHz. This development of plasma produced a tangential air jet across the surface of the airfoil, which reached its maximum value ( u j-max ) in the range of 0.5 m/s < u j-max < 0.7 m/s. Varying degrees of separation flow control was observed under these conditions. Performance comparisons were made between electrodes located at the leading edge (LE) and the quarter chord (QC, 25% of chord length) at angles of attack of α = 8°, 12°, 16°. The plasma-induced jet velocities and flow profiles were measured using particle image velocimetry (PIV). Characteristics such as power consumption, voltage waveform, and current magnitude were quantified through the use of a digital oscilloscope.

Experimental study of laminar separation phenomenon combining with numerical calculations

The separation is much more sensitive for laminar flow than for turbulent flow. These remarks have been attested for both subsonic and supersonic flows. However, they are not applicable to transonic flows when there are interactions between boundary layer and shock wave. Along with the Reynolds number, the Mach number is a necessary dimensionless parameter for the condition and the mechanism of separations. The report presents one part of studies on laminar separation with Mach number of incompressible flow. The laminar regimes correspond to flows through wind turbine blades. Our experimental work for laminar separation phenomenon was carried out in a subsonic open circuit wind tunnel by taking photographs. The accuracy of experimental results basically depends on the accuracy of wind tunnel and the quality of smoke on density and constitutive materials. Experimental results permit to determine the position of separation and the form of turbulent region followed from the separation point. Numerical studies were simultaneously realized. Based on obtained experimental and numerical results, the report presents also the comparison between the laminar separation and the turbulent separation.

Simulation of honeycomb–screen combinations for turbulence management in a subsonic wind tunnel

Honeycomb and screens, mainly used for turbulence reduction, are the key elements of a subsonic wind tunnel. In this paper, design aspects of these elements are addressed for an open-circuit wind tunnel, installed at Indian Institute of Technology Guwahati (IITG), India. The effectiveness of honeycomb and honeycomb–screen combinations, in reducing swirl and turbulence level in the test section, is studied by simulating the flowfield using commercial computational fluid dynamics (CFD) package ANSYS-CFX. RNG k−ε turbulence model, with scalable wall functions, is used for modeling turbulence. Results of these simulations for turbulence management, using honeycombs of different lengths, cell shapes and screens of different open area ratios, are found to be in good agreement with experimental and theoretical results available in open literature. These simulations have confirmed the methodology to be adopted for design of wind tunnel subsections. Thus validated design parameters have been used for fabricating the honeycomb and screens for the IITG wind tunnel.

Experimental study of forced convection from helical coiled tubes with different parameters

The forced convection from outside surfaces of helical coiled tubes is studied experimentally with a constant wall heat flux. Ten helical coiled-tubes of different parameters with a range of diameter ratio ( D/ d o ) ranged from 7.086 to 16.142 and pitch ratio ( P/ d o ) ranged from 1.81 to 3.205 are employed in the present study. The experiments are performed in an open-circuit airflow wind tunnel system operated in suction mode. The experiments covered a range of Reynolds number (Re) of 6.6 × 10 2 to 2.3 × 10 3. The experimental results indicated that these parameters ( D/ d o and P/ d o ) have important effects on the average heat transfer coefficient. The average Nusselt number (Nu m) increases with the increase of D/ d o at constant Re and P/ d o . Also, Nu m increases with the increase of P/ d o at constant Re and D/ d o . A considerable agreement between the present experimental data and previous work is achieved. The average Nusselt numbers is correlated with Re, D/ d o and P/ d o .

An experimental approach to turbulent heat transfer using a symmetric expanded plane channel

The flow in a symmetric expansion plane channel is known to deflect to one side of a channel even at a low Reynolds number due to the Coanda effect. Details of flow structure have been investigated by various authors; however, there have been a few works conducted in the area of on heat transfer. This paper presents experimental results of turbulent heat transfer in separated and reattached flows in a symmetric expansion plane channel. Experiments were conducted using a low-speed open-circuit wind tunnel. The step H was 20 mm high and 200 mm wide, with an expansion ratio of 2.0. The Reynolds number based on the uniform flow velocity at step and H was varied from 5,000 to 35,000, respectively. The mean and turbulent fluctuating velocities were measured using mainly two types of split film probe. A cold single wire probe was used for measuring the mean and turbulent fluctuating temperatures. It was found that the local Nusselt number profile was considerably different on the upper and lower walls due to the Coanda effect, which is was caused by instability between the upper and lower separated shear layers. Empirical formulae for the maximum Nusselt number in the reattachment region are hereby proposed for the upper and lower walls, respectively. The two formulae are well correlated with the previous general formula proposed. The location of maximum Nusselt number is found to be very close to the flow reattachment point. Details of the velocity and temperature fields were clarified and their correlations with the heat transfer characteristics described above investigated. Furthermore, the wavelet transformation methodology was employed to study instantaneous flow and temperature behaviors, which exhibited its usefulness in the study of the present complicated flow and temperature fields.

Two-sided wind catcher performance evaluation using experimental, numerical and analytical modeling

Experimental wind tunnel and smoke visualization testing as well as CFD and analytical modeling were conducted to investigate the performance of a two-sided wind catcher. This type of wind catcher is divided internally into two halves for the purposes of air supply and extract. In this study, the two-sided wind catcher model was constructed of two similar one-sided wind catcher models, which were attached together back to back. These one-sided models are 1:40 scale models of Kharmani's School wind catcher in the city of Yazd. Experimental investigations were carried out using an open-circuit wind tunnel and both the induced volumetric airflow into the building and the pressure coefficients around all surfaces of the wind catcher model were measured at various wind angles. Furthermore, the CFD simulation was also used to evaluate the pressure coefficient distribution and airflow pattern around and through the wind catcher. Additional experimental tests and computational fluid dynamics simulation of the wind catcher in the wind tunnel were also conducted in order to assess the accuracy of measurement procedures and the uncertainty of experimental results. This article also represents a semi-empirical approach in which experimental data were used for a detailed analytic model, in order to provide an accurate estimate of the performance of wind catchers. It was found that for an isolated two-sided wind catcher model, the maximum efficiency is achieved at the angle of 90°. At this air incident angle the wind catcher efficiency increases approximately 20% more than the one at zero angle. The experimental investigations demonstrated the potential of two-sided wind catcher for enhancing the natural ventilation inside buildings. It can be seen that CFD simulation and analytical modeling results have a good agreement with the experimental results. Theoretical modeling can also help to assess the accuracy of measurement procedures and the uncertainty of experimental results.

Performance of a Vertical Axis Wind Turbine with Variable-Pitch Straight Blades utilizing a Linkage Mechanism

This paper describes the performance of a micro vertical-axis wind turbine with variable-pitch straight blades. The proposed variable-pitch angle mechanism has an eccentric point that is different from the main rotational point. One feature of the mechanism is its ability to vary the pitch angle of the blades according to the azimuth angle of the main links, without actuators. The performance of the wind turbine was measured in an open-circuit wind tunnel. The performance of the vertical-axis wind turbine with variable-pitch straight blades was better than one with fixed-pitch blades. A wind turbine with variable-pitch straight blades has wind directivity. It was found that the performance of a wind turbine is dependent upon the blade offset pitch angle, the blade pitch angle amplitude, the size of the turbine, the number of blades, and the airfoil profile.

Computational fluid dynamics analysis of a combined three-bucket Savonius and three-bladed Darrieus rotor at various overlap conditions

In this paper, a computational fluid dynamics analysis using the FLUENT package 6.2 was carried out to predict the performance characteristics such as power coefficient (Cp), torque coefficient (Ct), and tip speed ratio of a combined three-bucket Savonius and three-bladed Darrieus rotor for various overlap conditions, namely, 16.2%, 20%, 25%, 30%, and 35%. In the upper part of the rotor model, there was a three-bucket Savonius rotor of bucket diameter of 8 cm and height of 10 cm, whereas, in the lower part, there was a three-bladed Darrieus rotor of blade diameter of 8 cm and height of 10 cm. A two-dimensional unstructured computational grid was developed for the combined Savonius–Darrieus rotor model. A k-ε turbulence closure model with enhanced wall treatment function was chosen. A first-order upwind discretization scheme was adopted for pressure-velocity coupling of the flow. The values of Cp and Ct obtained computationally were then compared with those of the values of Cp and Ct obtained experimentally for all the overlap conditions. The experimental values of Cp and Ct for different overlap conditions were obtained from the tests conducted previously in an open-circuit subsonic wind tunnel available in the department. The comparison of experimental and computational studies is quite encouraging.

Experimental study on natural ventilation performance of a two-sided wind catcher

Experimental wind tunnel and smoke visualization testing was conducted to investigate the performance of a two-sided wind catcher. This type of wind catcher is divided internally into two halves for the purposes of air supply and extract. In this study, the two-sided wind catcher model was constructed of two similar one-sided wind catcher models, which were attached back to back. These one-sided models are 1: 40 scale models of Kharmani's School wind catcher in the city of Yazd. Experimental investigations were carried out using an open-circuit wind tunnel, and both the induced volumetric airflow into the building and the pressure coefficients around all surfaces of the wind catcher model were measured at various wind angles. Additional experimental tests and computational fluid dynamics simulation of the wind catcher in the wind tunnel were also conducted in order to assess the accuracy of measurement procedures and the uncertainty of experimental results. To evaluate the stack effects on the natural ventilation performance of a two-sided wind catcher system, the results were compared to the corresponding results of a one-sided one. As a result of placing urban full-scale wind catchers in the boundary layer of atmospheric winds, the effect of this phenomenon was also examined experimentally. The experiments were performed when the wind catcher model with an adjoining house was placed in the wake of upstream objects, resembling neighbouring buildings. It was found that for an isolated two-sided wind catcher model, maximum efficiency is achieved at an air incident angle of 90°, and this can only be explained by the domination of a stack effect rather than dynamic pressure. At this air incident angle the wind catcher efficiency is ≈20 per cent more than the one at zero angle. The results show that the approaching air incident angles, the presence of an object upstream of the structure, and the blowing of atmospheric wind, influence phenomena such as pressure coefficients, induced airflow rate, and the airflow pattern of the two-sided wind catcher. The results show the potential of the two-sided wind catcher as a passive device for providing natural ventilation in buildings.

F301 CONTROL OF SEPARATION FLOW FROM AIRFOIL BY FUNCTIONAL JET USING DBD PLASMA ACTUATOR(Components-1)

Using a dielectric barrier discharge plasma actuator (DBD-PA), an active control system for flow separation around the MEL001 airfoil was attempted successfully at Re = 3.2×10^4. DBD-PA consists of top and bottom electrodes and a dielectric layer placed between the electrodes. Tangential jet velocities reach their maximum (V_<j-max>) in the range of 0.12 &le; V_<j-max> &le; 1.05 m/s, when bipolar sinusoidal wave voltages are applied, ±2.0, ±2.25, ±2.5, ±2.75, ±3.0 kV at 50 kHz. A series of experiments was carried out in the open-circuit blower type wind tunnel with 200mm×200mm×500mm test section under free stream velocity of U_∞ = 3.2 m/s. A DBD plasma actuator was installed on the MEL001 airfoil (chord length: L = 50 mm, wingspan: W = 200 mm). The wing was set in the test section with angles of attack (a) at a = 8.2 and 12.0 degrees. The effective jet velocities and power consumptions required to suppress flow separation were estimated by the velocity distributions around the airfoil and the current and voltage data so far accumulated on DBD-PA.

Hot-wire anemometry of transitional boundary layers exposed to different freestream turbulence intensities

Transitional boundary layer flows on a flat plate, subjected to different freestream turbulence intensities, have been investigated experimentally in details. The inlet freestream turbulence levels were varied from 1.5 to 4.4 per cent using different circular bar grids of various sizes installed upstream in a proper distance from the plate leading edge. A single hot-wire probe was used for the measurements of instantaneous flow velocities within the boundary layer. Longitudinal and vertical movements of the hot-wire probe were performed by a two-component traversing device, which was itself controlled by a computerized system. Experiments were conducted in an open circuit wind tunnel of suction type. For each level of freestream turbulence, boundary-layer surveys of the mean longitudinal velocity and root mean square of the velocity fluctuations were obtained at several streamwise locations. Experimental results were used to determine the variations of streamwise skin-friction coefficient and the boundary-layer shape factor versus the local Reynolds number. Evaluation of the final results showed that increasing the freestream turbulence intensity causes the transitional region to shrink and to shift up-stream. Current experimental results together with reliable data presented by other researchers led to propose a new correlation capable of predicting the onset of transition for a wide range of freestream turbulence intensities.

Two-Dimensional Analysis of Tandem/Staggered Airfoils Using Computational Fluid Dynamics

The two-dimensional analysis, using computational fluid dynamics (CFD), of tandem/staggered arranged airfoils of the canard and wing of an Eagle 150 aircraft and also the aerodynamic tests conducted in an open-circuit wind tunnel are presented in the paper. The wind tunnel tests were carried out at a speed of 38m/s in a test section of size 300 mm (width), 300 mm (height) and 600 mm (length), at Reynolds number 2.25 × 105. The tests were carried out with tandem and staggered placement of the airfoils in order to determine the optimum position of the wing with respect to the canard and also to determine the lift coefficient at various angles of attack. The CFD code FLUENT 5 was used to investigate the aerodynamic performance of a two-dimensional model to validate the wind tunnel results. The flow interaction was studied in the tandem and staggered arrangements in the wind tunnel as well as by the computational method. The k-ε turbulence model gave exceptionally good results.