Closed-circuit Wind Tunnel Research Articles

Surface temperature variations of gyrating hailstones and effects of pressure-temperature coupling on growth

Simultaneous measurements of surface temperatures in the equatorial and polar regions of gyrating hailstones, growing in a closed circuit wind tunnel, were performed using two infrared microscopes. Results indicate that a temperature difference as high as 5.9°C existed between these regions, with maximum differences occurring in the liquid water content range of 1.5 to 6.4 g m −3. Under such conditions growth was generally characterized by a dry, cold regime at the poles and a wet, warmer regime at the equator. Thus, contrary to standard assumptions, the surface temperature of growing hailstones cannot be considered homogeneous. A comparison of the hailstone growth results from the present study, performed at laboratory pressure, with those of García-García and List (1992), who varied air pressure with temperature according to the Denver soundings (Beckwith, 1960), indicates that the mass growth rate is enhanced at lower pressures due to the combination of increased terminal velocity and collection efficiency. Smaller ice fractions in deposits were measured at lower air pressures for comparable air temperatures and liquid water contents. This investigation demonstrates the difficulty of extrapolating from laboratory pressure to hail growth regions at lower pressure. An examination of the governing equations shows that such a simulation is physically impossible.

Wall interference correction to drag measurements in automotive wind tunnels

A low-order panel method has been devised for calculating wall interference corrections to the measured drag force in automotive wind tunnels with closed, 34-open or slotted-wall test sections. The method is applied to the Motor Industry Research Association (MIRA) generic car model, tested in three working sections of the German-Dutch Wind Tunnel (DNW), and to three subscale models tested in the DSMA closed-circuit pilot wind tunnel with closed and slotted walls. The calculations show that the drag measurements in closed test sections and 34-open test sections require negative and positive corrections, respectively, whereas inside a test section with 30% open longitudinally slotted walls the measurement is nearly interference-free.

Outline of a new closed circuit wind tunnel of Yokohama National University

A new closed circuit wind tunnel of Yokohama National University was completed in July 1994. This wind tunnel is a 2nd wind tunnel of our laboratory and planed for intensive research in fundamental fluid dynamics, structural wind engineering and environmental engineering. Fundamental idea of this wind tunnel can be summarized as follows.1) Closed circuit wind tunnel, free from small fluctuation of the external weather condition.2) Excellent basic characteristics as the lowspeed wind tunnel for efficient experiments and reliable results3) Comfortable and low noise work space not to disturb intensive research4) Flexibility for future extension of wind tunnel research.The wind tunnel has been satisfactorily operated for more than a year since its opening ceremony on July 26 1994.

Aerodynamic drag reduction of goods trains

Tests on one-tenth scale hopper and gondola rail cars have been undertaken in a closed-circuit wind tunnel with a fixed floor. By varying the number of wagons in the train length, it was concluded that simulation of a typical wagon away from end effects (i.e. the influences of the locomotive(s) and trailing wagon) could be achieved with one and a half “dummy” wagons upstream and one half a “dummy” wagon downstream. An active model coupled to a drag balance was used to measure drag coefficients and drag reductions from geometric modifications on a typical wagon. The largest drag savings arose from fitting an enclosing top. Other effects, including inter-wagon gap and end fairings were also investigated. It was found that a practical wagon design, that was based on an existing wagon and allowed the current methods of loading and unloading, would reduce averaged drag coefficients by 27% for an unladen wagon. Considerably larger savings (up to 50%) could be made with more substantial wagon redesign.

Free-stream turbulence and tube spacing effects on surface pressure fluctuations for two tubes in an in-line arrangement

An experimental investigation of surface pressure distributions (mean and r.m.s.) on two tubes (circular cylinders) mounted in an in-line or tandem arrangement was carried out in a low-speed, closed-circuit wind tunnel. The measurements were mainly performed at a subcritical Reynolds number, Re = 2·0 × 104, at three different turbulence intensities of the approaching cross-flow (0·1, 1·4 and 3·2%). The spacing between the tubes was in the range of 1·25 ≤ S/D ≤ 5·0. The fluctuating pressures were recorded by a microphone in a pinhole arrangement. End plates were used to minimize the influence of the boundary layers on the wind tunnel walls. The results indicate that the effect of an increased free-stream turbulence intensity is strongly dependent on the tube spacing S/D. It is suggested that this is due to the influence of the free-stream turbulence on the separated boundary layer from the upstream tube. The behaviour of this shear layer determines the type of flow occurring around the tubes and therefore also the magnitude of the forces acting on the tubes. The so-called critical spacing between the tubes was found to decrease as the turbulence intensity was increased. An overall increase in the r.m.s. pressure coefficient was also found as the turbulence intensity was increased.

Dynamics of fluid mixing in separated flows

Separated flows at high Re (>103) are highly turbulent. In some situations the turbulence generation and mixing processes associated with flow separation are desirable, e.g., in heat exchangers or in many chemical engineering applications. In others, e.g., stalled airfoils, separation must be avoided as it causes loss in pressure and kinetic energy. To control the phenomenon effectively, physical mechanisms of flow separation and related aspects, such as the growth of flow instabilities in shear layers, the process of vortex formation, and the dynamics of fluid mixing in recirculating flow regions, must be understood. In many cases numerical procedures, e.g., Navier–Stokes calculations including k-ε turbulence modeling, fail to predict real physical mechanisms in separated flows.1,2 Separated flows in the lee of bluff bodies have been studied for many years.3,4 However, accurate measurements of the magnitude and direction of velocities and the magnitude of the terms of the Reynolds stress tensor have been restricted by the unsuitability of the hot-wire anemometer in recirculating flows. The development of the pulsed-wire anemometer, flying hot-wire anemometer, and laser-Doppler anemometry (LDA) allows more reliable measurements also in turbulent separated flows.5–8 The aim of this paper is to investigate the dynamics of undisturbed fluid mixing in separated regions of 2-D, incompressible flows with visualization techniques and LDA. Measurements were performed with a vertical flat plate model, mounted in a closed-circuit wind tunnel at low blockage ratio. Because of the noninvasive character, optical techniques like LDA are more suitable to analyze complex fluid motions than pulsed-wire and flying-wire anemometry. The LDA system used to investigate turbulent flow structures consists of a two-channel version operating in backscatter mode and a specifically developed phase detector to extract phase-averaged information from recorded measurement ensembles.9 Endplates were used to get nearly 2-D behavior. Downstream of the plate the flow field exhibits a distinct periodic component at Strouhal frequency S=0.14. In the investigated Re number range, 103–105, S remains constant. The flow dynamics were visualized by the smoke-wire technique.10 Mixing between laminar external and turbulent separated fluid is evident. The process is initiated by entrainment of external fluid into the turbulent recirculating region. Corresponding quantitative results of LDA measurements are discussed. The plots show the developing of flow structures in a section from x/D=1.0 – 4.5 and y/D=−1.0 – 1.0, where D is the dimension of the plate model. The time difference between consecutive phases is t=0.02 sec. The saddle points are emphasized by short arrows indicating streamlines. Flow visualization and LDA measurements are in good agreement concerning the overall flow motions of the turbulent structures. Dynamic behavior of the Reynolds stress terms (u′2, v′2, and u′v′) are discussed. From these distributions the fields of phase-averaged turbulent energy production 〈P〉 could be derived. The isoline diagrams clarify that the location of extremum values in 〈P〉 are dependent on time and the position of the saddle point. In conclusion: (1) The dynamics of fluid mixing can be investigated with phase-locked LDA technique; (2) qualitative results of flow visualizations and quantitative LDA measurements support each other and shed new light upon the physics of fluid mixing; (3) extreme values of phase-averaged turbulent energy production coincide with the occurrence of saddle points in the flow field; and (4) mixing is initiated by entrainment of laminar external fluid into the turbulent recirculating region.

Velocity vector and turbulence in the symmetry plane of a Darrieus wind generator

This paper reports experimental results for the flow field of a Darrieus type wind generator model of diameter 1.7 m tested in a closed circuit wind tunnel. Velocity vector and turbulence measurements in the symmetry plane of the rotor (horizontal plane) were taken using an X-hotwire. The measurements covered the near field around the model from half a rotor diameter upstream to one diameter downstream. The results indicated that the edges of the wake are easily identifiable by the abrupt change in the wake velocity to that of the undisturbed flow and also by the high turbulence level. At tip speed ratios of up to about 5.4, the upstream flow, even at a location within half a rotor diameter, seems not to be affected by the operation of the wind generator, whilst small effects are present at a tip speed ratio of 7. The flow field downstream of the rotor is asymmetric with a tendency for the flow to turn in the direction of rotor rotation by only 7° at a tip speed ratio of around 4.

A Transient Method for Determining Thermal Diffusivity of Tobacco Stems

Abstract A microwave generator and a closed-circuit wind tunnel were used to measure the thermal diffusivity of tobacco (Nicotianatabacum L.) stems in vivo by the unsteady-state method. A simple mathematical model for heat flow, based on Fourier's heat-conduction equation and Newton's law of cooling, was used in this study. The microwave method was found to be relatively rapid as both heating and cooling of a cylindrical stem in an air stream could be completed in approximately 30 minutes for thermal-diffusivity determinations. Thermal-diffusivity value of the tobacco stems, containing 94 % moisture and a mean stem temperature of 30°C, was found to be (1.38 ± 0.06) × 10-7 m2 s-1. The coefficient of variation for the measurements did not exceed 1.4 % as determined through the analysis of cooling curves for five different air-flow rates over the stems. This study showed that the microwave technique could be effectively used to determine both accurately and reliably the thermal diffusivity of tobacco stems in vivo.

Stability effects in a normal triangular cylinder array

Fluid-elastic vibrations in tube bundles are often responsible for major damage and very high shutdown costs. The onset is manifested by a rapid increase in the tube vibration amplitude at a particular critical flow velocity and is strongly dependent upon various physical parameters. In this paper, experimental results showing a second stability boundary for a flexibly mounted cylinder in an otherwise fixed cylinder array are presented. It is shown that minor variations of the array geometry affect its vibration mode. The stability boundaries, for various configurations of flexibly mounted and fixed cylinders in the array, have been measured. The resulting stability boundaries are plotted as functions of the mass-damping parameter and amplitude-diameter ratio in stability diagrams. The results are critically compared with known results from the literature. A closed-circuit wind tunnel with a 1·5 m diameter, 2·1 m long test section is used for the tests. The eighteen 80 mm × 800 mm aluminium cylinders in the multi-row normal triangular array have a pitch-to-diameter ratio of 1·25 and are, when not fixed, linear iso-viscoelastically mounted. Additionally, the linear damping of nine cylinders (4, 3 and 2 cylinders in the first, second and third cylinder row, respectively) can be continuously varied over the wide mass-damping parameter range of 10 2

Growth and Development of the Main Greek Oriental Tobacco Cultivars

Abstract A microwave generator and a closed-circuit wind tunnel were used to study thermoregulatory responses of tobacco leaf. Heating and cooling curves at various wind velocities showed the maximum reduction of steady-state temperature occurred at 10 cm / s. Wind gusts of high intermittency were very effective in transferring heat from leaf tissue; gust interval and dynamic equilibrium leaf temperature were found to be linearly related. Thermal-time constant and half-cooling time of a tobacco leaf were determined as a function of wind velocity under the conditions of the Newtonian law of cooling. It was determined that transpirational cooling of tobacco leaf in total darkness could occur if leaf temperature was raised above 40°C. This study confirmed that microwaves can be effectively used to study heat exchanges of flue-cured tobacco leaves in vivo under both continuous and fluctuating wind conditions.

Unsteady heat transfer problems related to a high-power laser flow loop

Introduction A PULSED closed-cycle electric discharge laser (EDL) is a very important device for achieving high energy intensity laser beams. Figure 1 shows a simplified sketch of the EDL. A working gas flows through a closed loop as in a closed circuit wind tunnel used in aerodynamic applications. The flow is circulated by the fan. In the cavity section, pulsed electron beam energy is deposited into the flowing gas to achieve population inversion leading to lasing action. Pulse frequencies of 100 Hz are typical, and each pulse typically lasts for 5 //s. As far as the fluid dynamics of the cavity flow is concerned, the energy deposition is instantaneous, and, therefore, the process can be described as an instantaneous constant volume heating process. Thus, the gas pressure and temperature are instantaneously increased by the energy deposition in the cavity. Because of this, shock waves traveling away from the cavity in both upstream and downstream directions are created. Acoustic mufflers for attenuating these shocks are located upstream and downstream of the cavity. The pressure in the cavity region relaxes back to near its original value in a short period of time. However, the temperature discontinuities created by the energy deposition do not relax in a short period of time. A hot region of gas remains long after the pressure waves have traveled away. This hot slug of gas flows from the cavity at the speed of the flow and encounters the heat exchanger located downstream. The heat exchanger not only removes the heat from the hot slug of gas but also causes longitudinal mixing of this hot slug with the previous cold slug which passed through the heat exchanger. The fan also causes mixing and, thus, the working gas enters the cavity at a lower temperature. For the cavity medium to produce a high-quality output laser beam, there are stringent requirements on density homogeneity. Typically (Ap/p) rms ~ 10~. Variations in density are due to several sources. One source is the unsteady temperature profile at the inlet of the heat exchanger, shown in Fig. 2. The pulse width in Fig. 2 is small compared to the interpulse time, and, therefore, the pulse width is considered infinitesimal. A second source of density variation is the temperature balance between the gas and the solid surfaces in the loop the gas temperature changes with time. Estimating the relaxation times and density changes requires a knowledge of unsteady heat transfer. One unique feature of these unsteady heat transfer problems is that the fluid bulk temperature fluctuates as opposed to problems where the body surface temperature fluctuates. Available literature shows that even though a large number of papers are published concerning unsteady heat transfer, little work is available which considers fluid bulk temBOUNDARY LAYER SUCTION

Ruffed Grouse Metabolic Rate and Temperature Cycles

We measured the effects of temperature and wind on ruffed grouse (Bonasa umbellus) metabolic rates in an open-circuit respiration system with a closed-circuit wind tunnel. Metabolic heat production was linearly related to operative temperature (Te) below a lower critical temperature of 1.5 ? 0.99 (SE) C. Standard metabolic rate was 3.2 + 0.11 W (66.1 kcal/day) for a grouse of mean weight (607 g). Metabolic rate was related linearly to wind speed 05. The regression predicting net metabolic heat production (metabolic heat production evaporative cooling [M E]) was 3.1589 0.1176T, + 0.4141(wind speed)s5 (r2 = 0.98, P mean low nocturnal cloacal temperature (38.3 + 0.54 C) (P = 0.006). J. WILDL. MANAGE. 52(3):450-453 Ruffed grouse are distributed throughout north temperate and subarctic North America (Aldrich 1963) where winter thermostatic energy demands are potentially high. Grouse may feed frequently (Thomas et al. 1975) or use fat reserves (Norman and Kirkpatrick 1984) to meet winter metabolic demands. Rasmussen and Brander (1973) reported effects of cold temperatures on ruffed grouse metabolic rates under free convection (no wind). Effect of forced convection on heat loss by birds has been estimated from predictions based on heat transfer theory of cylinders and flat plates (Calder and King 1974, Robinson et al. 1976). Only a few studies have directly measured effects of forced convection on birds (Gessaman 1972, Robinson et al. 1976, Bakken et al. 1981), and none have been on galliforms. Most birds exhibit daily cyclical changes of 1-3 C body temperature between active and inactive periods (Calder and King 1974). This variation may increase with food deprivation or cold exposure (Calder and King 1974, Chaplin 1976, Bucher and Worthington 1982). Body temperature of ruffed grouse has been reported (Rasmussen and Brander 1973), but cyclic fluctuations or response to cold temperatures have not been measured. We report on effects of forced convection and perature on the metabolic heat production of ruffed grouse. We also measured daily fluctuations in body temperature and changes due to cold exposure. H. C. Gerhardt, J. Maruniak, and the Division of Biological Sciences, University of MissouriColumbia provided equipment necessary for this study. We thank D. E. Figert, D. E. Hoffman, B. J. Wilson, and C. S. James for assistance with various aspects of this study. M. R. Ryan and T. V. Dailey reviewed this manuscript. Financial support for this study was provided by the Ruffed Grouse Society and the Missouri Department of Conservation. This is Contribution 10382 of Missouri Agricultural Experiment Station ProjThis content downloaded from 157.55.39.45 on Fri, 02 Sep 2016 05:31:36 UTC All use subject to http://about.jstor.org/terms J. Wildl. Manage. 52(3):1988 GROUSE METABOLIC RATES * Thompson and Fritzell 451 ect 189 and the Missouri Cooperative Fish and Wildlife Research Unit (U.S. Fish and Wildl. Serv., Mo. Dep. Conserv., Univ. Missouri-Columbia, and Wildl. Manage. Inst., cooperating).

Thermal Diffusivity Measurements of Apples by Microwave Radiation

A microwave generator and a closed-circuit wind tunnel were used to measure the thermal diffusivity of four varieties of apples (Malus pumila (Mill.)) by the non-steadystate method. The coefficient of variation for the measurements of thermal diffusivity of apples did not exceed 5.8% as determined through the analyses of the cooling curves. The method is relatively rapid as a single measurement of thermal diffusivity was completed within two hours. Thermal diffusivities for the post-harvest apples ranged from 1.22 x 10-7 m2s-1 for Atlas to 1.47x 107m2s-1 for Cortland. The use of a semi-empirical relationship based on fruit moisture content and temperature range was found insufficient to fully account for the variation in the thermal diffusivity values of apples observed in the present study.

Pressurized Icing Tunnel for Graupel, Hail and Secondary Raindrop Production

Abstract The closed-circuit icing wind tunnel with pressure control serves for experiments on the growth and melting of ice particles, such as graupel and hailstones, and the observation of shedding of rain-sized drops by growing and melting hailstones. The pressure variation allows a better simulation of nature by duplicating all state parameters of the atmosphere and, consequently, also the correct relative speeds between test particle and air under all conditions. Thus, the hailstone growth according to different in-cloud trajectories can be simulated. The double-wall construction of the measuring and downstream calming sections provides excellent heat transfer at the outside wall and thus, minimal temperature gradients and radiative imbalances across the tunnel. This is of particular importance for the study of small particles requiring low speeds. The tunnel is 4.3 m high and has a 70 cm vertical measuring section with an inner cross section of 17.8 cm × 17.8 cm. The operational pressure range is bet...

Heat transfer studies of leaves by microwave radiation

Thermal responses of microwave exposed leaves in steady and intermittent winds were studied in a closed-circuit wind tunnel. The transient heat transfer coefficients of water hyacinth leaves were found to be approximately 2 to 4 times higher than those of tobacco. Observed leaf temperatures agreed fairly well with the predictions of a mathematical model. Wind intermittency and equilibrium leaf temperatures were linearly related for both dry and wet leaves. Stomatal response to leaf temperature of tobacco occured at 40°C.

Turbulence and reynolds number effects on the flow and fluid forces on a single cylinder in cross flow

The flow around a smooth and rigid tube (circular cylinder) in cross flow has been investigated experimentally. The experiments were carried out in a closed-circuit low-speed wind tunnel. Two test tubes (diameters 41 and 120 mm, respectively) fitted with end plates were used. The fluctuating pressures on the tube surface were sensed by microphones in pinhole arrangements while the modulus of the surface shear stress was measured using a surface hot film. The flow field was investigated using single hot wires. Effects of a low intensity freestream turbulence was studied by placing a grid at the entrance of the working section of the wind tunnel. The Reynolds number ranged from 2 x 104 to 3 x 105 and the turbulence intensities in the freestream were 0·1% and 1·4%, respectively. The results distinguished different flow regimes depending on the Reynolds number and turbulence intensity. For Reynolds numbers less than about 10 5 , an increase in the turbulence intensity gave an increase in the pressure forces, while the opposite effect was observed at higher Reynolds numbers. A conditional sampling technique using the different flow gauges provided information on the relation between the flow field in the near wake and the forces on the tube. Distributions of mean- and r.m.s.-coefficients, spectra of fluctuating pressure and the alteration of various coefficients (e.g. Strouhal number) when entering the critical regime are presented. The separation from the tube was found to be delayed in the precritical regime. Also, the build-up of a turbulent zone towards reattachment, coupled to a narrowing of the wake, was observed.

Wind-Related Heat Transfer Coefficient for Flat-Plate Solar Collectors

Experiments were performed to evaluate the convective heat transfer coefficient for a flat plate mounted in a wooden model of a roof of a building. The experiments were carried out in a closed-circuit wind tunnel and included parametric adjustments of the roof tilt and Reynolds number, based on the length of the plate. The roof tilt was set at 0, 30, 45, 60, and 90 degrees and the Reynolds number ranged from 58,000 to 250,000. A transient, one lump, thermal approach was used for heat transfer calculations. Due to a separation bubble at the leading edge of the model, i.e., the roof, at angles of attack of less than 40 degrees, the flow became turbulent after reattachment. This resulted in a higher heat transfer than previously reported in the literature. At higher angles of attack, the flow was not separated at the leading edge and remained laminar. The heat transfer coefficient for higher angles of attack, i.e., α > 40 deg, was found to be approximately independent of the angle of attack and in good agreement with the previously published results.

A method for measuring skin friction drag on a flat plate in contaminated gas flows

A technique for measuring friction drag in turbulent gas and gas/particle flows over flat plates is presented, and preliminary results are reported. A 0.25-in.-thick 72 x 6-in. Al plate is suspended by six horizontal support air bearings and four vertical alignment air bearings between fixed dummy plates and leading-edge and trailing-edge fairings in the 32-in.-high 48-in.-wide 11-ft-long test section of a closed-circuit atmospheric wind tunnel operating at 50-150 ft/sec. Particles of Fe and Al oxides of diameter 20-150 microns and density up to 0.3 lb particles per lb air are injected via a 6 x 0.167-in. nozzle; turbulence is induced by a roughened section of the leading-edge fairing; and friction drag is measured using a load-cell pressure transducer. Sample results are shown in a graph, demonstrating good agreement with theoretical drag calculations.

Experimental studies of the separated flow over a NASA GA(W)-1 airfoil

The separated flow over the 17-percent-thick blunt-trailing-edge NASA GA(W)-1 airfoil is investigated experimentally at angles of attack 10.3-18.4 deg and velocities up to 92 m/sec in the 1.09 x 0.76 x 1.22-m working section of a closed-circuit wind tunnel, with typical Reynolds numbers of the order 4 x 10 to the 6th. The results are presented graphically and characterized. Separated-flow velocity profiles and skin-friction distributions are obtained, and the boundary-layer integral properties are shown to be in good agreement with the separation criteria of Sandborn and Kline (1961). The chordwise development of the mean velocity is reduced to a universal curve, and some similarity is found in the outer-region velocity profiles downstream of the separation.

On The Basic Design and Construction of an Industrial Aerodynamics Wind Tunnel

Three types of wind tunnel has been built in the research laboratory of Shimizu Construction Co., Ltd.The first type, Eiffel, was built to study the shape of contraction corn in 1974. The second type, a closed-circuit boundary layer wind tunnel, was built to investigate a testing facility for architectural and industrial aerodynamics in 1975.The third type wind tunnel has been built in 1982.This wind tunnel has a wide angle diffuser in front of the contraction corn. This paper describes about the basic design and characteristics of these three different types of wind tunnels.