Low Speed Flow Field Research Articles

Investigating the spatiotemporal distribution of refractivity structure index in low-speed flow field by moiré deflectometry

Abstract Atmospheric optical turbulence is a significant restraint on free-space optical communication and atmospheric laser transmission, whose intensity is generally quantified by the refractivity structure index. In this paper, moiré deflectometry is applied to study the combined effects of thermal disturbance and flow speed on the spatiotemporal distribution of the refractivity structure index. Firstly, three sets of comparative experiments are conducted, and 3600 frames of moiré fringes are captured within an hour in each set. Then, the spatiotemporal distribution is obtained based on the theoretical relationship between the refractivity structure index and the phase information recorded by moiré fringes. Finally, its regularities under different experimental conditions are comparatively analyzed. The findings reveal that both thermal disturbance and flow speed will change the intensity of optical turbulence. And, although the latter has a much weaker impact than the former, their combined effects will increase the intensity by three orders of magnitude. In this process, the contribution of thermal disturbance accounts for more than 75%. In short, the related results could provide some insights for research on atmospheric optical detection and communication.

Controllable vertical and radial corrosion by step flow fields for fabricating large aspect ratio micro-cone arrays in through-mask electrochemical micromachining

Ionic liquid electrospray thrusters (ILET) have the advantages of high efficiency, small size and low power consumption, and are widely used in micro and nanosatellite propulsion systems. As the core component of the electrospray thruster, the height and tip diameter of the micro-cone emitter array determine the performance of the thrust system. To increase the height of the micro-cone emitter, a through-mask electrochemical micromachining (TMEMM) processing method was proposed in this study. The eddy current generated under the mask in the low-speed flow field was innovatively used to make the gas and solid products gather on the processing side wall to form a product film, which effectively reduced the radial corrosion rate and achieved higher longitudinal processing. In sequence, high-speed flow field was applied to achieve high radial corrosion rate, high contour accuracy and high surface quality. By switching the low-speed flow field and the high-speed flow field, the vertical and radial corrosion rates were controlled. Finally, a microcone array with a height of 256.2 μm and a tip diameter of 20.3 μm was fabricated.

A new accelerating technique for low speed flow: pseudo high speed method

This paper proposes a new accelerating technique for simulating low speed flows, termed as pSeudo High Speed method (SHS), which uses governing equations and numerical methods of compressible flows. SHS method has advantages of simple formula, easy manipulation, and only need to modify flux of Euler equations. It can directly employ the existing well-developed schemes of hyperbolic conservation laws. To verify the technique, several numerical experiments are performed, such as: flow past airfoils and flow past a cylinder. Analysis of SHS method and comparisons with some precondition methods are made numerically. All tests show that SHS method can greatly improve the efficiency of compressible method simulating low speed flow fields, which exhibits in accelerating the convergence rate and increasing the accuracy of the numerical results.

Ruthenium-based fast-responding pressure-sensitive paint for measuring small pressure fluctuation in low-speed flow field

A sprayable fast-responding pressure-sensitive paint (fast-PSP) has been developed to measure time-resolved small pressure fluctuation on model surfaces. To realize PSP with fast response, high robustness, and high luminescence intensity, we have developed a novel ruthenium complex-based fast-PSP. The binder layers of the proposed fast-PSP were prepared by mixing room-temperature-vulcanizing silicone (silicone) and titanium dioxide (TiO2) particles with different sizes and their effects on the properties including the time response, pressure sensitivity, and luminescence intensity were investigated. The effects of the hydrophilic and hydrophobic surface treatments of TiO2 particle on the structure and mechanical robustness of the binder layer were also studied. The developed fast-PSP was applied to the flow field measurements behind a square cylinder and was evaluated. We have succeeded in obtaining time-series data of pressure fluctuation with high accuracy even at a mean velocity of 20 m s−1.

Flow measurement for the pre-stage of jet pipe servo valve using 2D -PIV technique

In order to investigate the internal flow field characteristic for the pre-stage of jet pipe servo valve, a large-scale low-speed flow field visualisation experimental platform was developed. The i...

Experimental detection of bubble-wall interactions in a vertical gas–liquid flow

Abstract Bubble motions and bubble-wall interactions in stagnant liquid were experimentally investigated by high-speed CCD and PIV technique with the main feature parameters such as Eotvos numbers Eo = 0.98–1.10, Morton number Mo = 3.21 × 10 − 9 and Reynolds numbers Re = 180 ~ 190. The effect of bubble injecting frequency and the distance S between the gas injection nozzle and the wall on the statistical trajectory of bubbles, average velocity distribution of flow field and Reynolds shear stress were studied in detail. It was shown that the combination of bubble injecting frequency and the distance S caused different bubble motion forms and hydrodynamic characteristics. When the normalized initial distance was very little, like S ⁎ ≈ 1.2 (here S ⁎ = 2 S / d e , and d e is the bubble equivalent diameter), bubbles ascended in a zigzag trajectory with alternant structure of high and low speed flow field around the bubbles, and the distribution of positive and negative Reynolds shear stress looked like a blob. With the increase of distance S ⁎ , bubbles' trajectory would tend to be smooth and straight from the zigzag curve. Meanwhile, with the increase of bubble injecting frequency, the camber of bubble trajectory at 20 y S ⁎ ≈ 3.6, the low-frequency bubbles gradually moved away from the vertical plane wall in a straight trajectory and the high-frequency bubbles gradually moved close to the vertical wall in a similar straight trajectory after an unstable camber motion. Under the circumstances, high-speed fluid was mainly distributed in the region between the wall and the bubbles, while the relative large Reynolds shear stress mainly existed in the region far away from the wall.

Computational Approach for Aeroheating with Thermally Coupled Fields

In this paper, a coupling methodology was proposed to solve simultaneously the external flowfield of a hypersonic reentry vehicle/capsule and the resultant heating on the vehicle’s structure and internal components with buoyancy-driven very low-speed flows resulting from conjugate heat transfer through the thermal protection system of the vehicle/capsule. Two flow solvers were employed to solve the hypersonic flowfield and the very low-speed flowfield, respectively. The coupling was done through the exchange of appropriate interface data between the two solvers. One of them was a density-based solver, and the other was a pressure-based solver. Validations of the individual solvers were done with two benchmark cases. The proposed coupling method was proved through simulating two conjugate heat transfer problems. Finally, an aerothermal–man–machine system exposed in a hypersonic flow was computed using the proposed coupling method.

翼型低<italic>Re</italic>数小攻角非线性非定常层流分离现象研究

Laminar separation bubble (LSB) is the characteristic feature of the low speed flowfields with low Reynolds number, which has the strong non-linear and unsteady effects. There are three key influences of the LSB to the airfoil aerodynamic characteristics: (1) Rapidly decreasing of the lift coefficient and the ratio of the lift to drag, increasing of the drag coefficient; (2) Non-linear effects of the aerodynamic coefficients exist at the small angle of attack; (3) The non-linear static hystereses appear at the middle and large angle of attack. Based on the former authors numerical simulations about airfoil E387 and SD8020, the trailing-edge laminar separation bubble was found by author, which is different from the classical LSB. In this paper, furthermore, the unsteady numerical simulation methods and water tunnel PIV technologies were used to carefully study the alterations of the time-averaged and unsteady flow separation structures, the distributions of the pressure coefficients on the surface, around the SD8020 airfoil. Both the results of the CFD and experiment confirmed very well, which proved our former discovery through numerical simulations correct. The main conclusions of this paper are described briefly following: (1) The PIV experiments validated that the classical Long LSB (LLSB) is the time-averaged flow of a series laminar separation vortex, shedding from the upper surface. A stable long LSB does not exist; (2) the experimental results proved that the time-averaged trailing-edge laminar separation bubble(TLSB) really exists for the symmetrical airfoil at low Reynolds number with low incidence, which is obviously different with the LLSB, not only for the shape and structure but also for the evolvement. The sketch of the two kinds of LSB are painted and analyzed carefully in this paper; (3) the numerical simulations and the water tunnel experiments also showed that the structure and the pressure distribution of the unsteady flow fields of the two kinds of LSB are different with each other. And the non-linear effect of the lift coefficient of the symmetrical airfoil at low incidence are caused by the sudden alteration between the LLSB and TLSB; (4) the unsteady vortex structure about the TLSB almost keeps the dead water near the separation point, and alters to the Karman vortex street near the trailing-edge point. Otherwise, the unsteady flow field about the LLSB appears remarkable unsteady character near the separation point, and a series of independent vortexes generate, develop, move, incorporate and shed along the upper surface of the airfoil. Therefore, maybe the deep mechanism and reason that cause the sudden alteration between the LLSB and the TLSB are the unsteadiness of the flow field near the separation bubble.

GS32 ポリマー型感圧塗料を用いた低速流れ場における非定常圧力計測技術

Pressure sensitive paint (PSP) is a useful noncontact measurement technique to obtain continuous pressure distribution on solid surfaces. The application of PSP to low speed flows in mechanical engineering fields, including unsteady flow measurement related to analysis of aerodynamic noise, is highly demanded. Polymer-based PSP has high pressure sensitivity for low-speed flow field, but the unsteady response of the polymer-based PSP is not examined enough. In this study, we investigate the unsteady response of the polymer-based PSP, to evaluate the feasibility of the polymer-based PSP as a measurement tool for unsteady low-speed flows. We have applied the polymer-based PSP in low-speed flow to measurement of unsteady pressure distribution around a circular cylinder, and compared the amplitude spectra of PSP and that of pressure probe, which are obtained by FFT analysis. Furthermore we have visualized the distribution of the integrated intensity of the PSP amplitude spectrum around the peak, to visualize the area with large pressure fluctuation on the cylinder.

An Investigation of Compressible Turbulent Forced Convection by an Implicit Turbulence Model for Large Eddy Simulation

An investigation of compressible turbulent forced convection in a three-dimensional channel flow is studied numerically by an implicit turbulence model for large eddy simulation (LES). Because of a high temperature difference between two walls and turbulent flow, the compressibility and viscosity of fluid should be taken into consideration simultaneously. Methods of the Roe scheme, preconditioning, and dual time stepping coordinating an implicit turbulence model for LES are used for resolving the effect of the compressibility of fluid on a low speed flow field. The magnitudes of Re τ based on the friction velocity changing from 180 to 940, with the high temperature difference of two walls of 500 k are conducted. The results of the mean velocity profiles and turbulent intensities are in good agreement with the benchmark DNS data obtained by spectral codes from a low Reynolds number (Re τ = 180) to a high Reynolds number (Re τ = 940). Besides, the larger the Re τ is, with the exception of acquirement of larger average Nusselt number, the more drastic variation of local instantaneous Nusselt number is observed.

G403 感圧塗料を用いた低速流れ場における非定常計測技術(GS4-1 計測技術,一般セッション)

Pressure sensitive paint (PSP) is a useful noncontact measurement technique to obtain continuous pressure distribution on solid surfaces. The application of PSP to low speed flows in mechanical engineering fields, including unsteady flow measurement related to analysis of aerodynamic noise, is highly demanded. In this study, we clarify the feasibility of PSP as a measurement tool for unsteady low-speed flow fields. Because the response time depends on the square of the thickness of the PSP layer, we have developed the thinner-layer PSP to improve its time response, and the dependence of the thickness of PSP layer on pressure sensitivity and time response was verified. Furthermore we have applied the PSP to measurement of pressure distribution around the circular cylinder in steady flow, to clarify the problems when PSP is applied to measurement of unsteady pressure distribution in low speed flows.

Numerical Simulation of Loitering Aircraft Aerodynamics in Low Speed Flight

Aerodynamic design rules of unconventional layout loitering aircraft against the issue of stall in low speed flight were proposed in this paper. Using the SST k-ω two equations turbulence model, the stalling characteristics of the loitering aircraft were studied. Based on the low speed flow field of the loitering aircraft without the deflexion of the helm, aerodynamic parameters such as lift, drag, pitching moment and yawing moment in different workings were calculated. Analysis of the results shows that the loitering aircraft designed has good stalling capability whose critical angle of attack is 18 degrees. The loitering aircraft has good longitudinal and lateral stability under different angle of attack.

C15 重ね塗り感圧・感温塗料を用いた低速流れ場の圧力計測の妥当性検証(C1流体機械I)

C15 重ね塗り感圧・感温塗料を用いた低速流れ場の圧力計測の妥当性検証(C1流体機械I)

Simultaneous measurement of particle velocity and size based on gray difference and ai]tocorrelation

The gray of two images of a same particle taken by a digital camera with different exposure times is different too. Based on the gray difference of particle images in a double-exposed photo and ai]tocorrelation processing of digital images, this paper proposes a method for measuring particle velocities and sizes simultaneously. This paper also introduces the theoretical foundation of this method, the process of particle imaging and image processing, and the simultaneous measurement of velocity and size of a low speed flow field with 35 μm and 75 μm standard particles. The graphical measurement results can really reflect the flow characteristics of the flow field. In addition, although the measured velocity and size histograms of these two kinds of standard particles are slightly wider than the theoretical ones, they are all still similar to the normal distribution, and the peak velocities and diameters of the histograms are consistent with the defai]lt values. Therefore, this measurement method is capable of providing moderate measurement accuracy, and it can be further developed for high-speed flow field measurements.

1502 重ね塗り感圧・感温塗料を用いた低速流れ場における圧力・温度計測技術(OS15-1 先端的熱流体計測法,オーガナイズドセッション)

Recently, measurement method using pressure sensitive paint (PSP) and temperature sensitive paint (TSP) is attractive in measurement of pressure and temperature field on surfaces. However, PSP has temperature dependence as well as pressure sensitivity. Temperature dependence will be the factor which brings an error in pressure measurement using PSP, so temperature correction is needed to obtain the highly precise pressure distribution. So, in this research we apply use the combined method using both PSP and TSP by stacking PSP and TSP layer up. Using the combined method, the temperature distribution obtained by TSP can be used for the temperature compensation of PSP. We call this combined method as "Dual-layer PSP/TSP". In this research, we clarified that the sensitivity of the dual-layer PSP/TSP in low-speed flow fields against pressure and temperature is comparable to that of mono-layer PSP or TSP, showing the feasibility of the dual-layer PSP/TSP for the measurement in low-speed flow fields.

0908 感圧塗料を用いた低速流れ場における計測技術の開発(OS9-2 先端的熱流体計測法,オーガナイズドセッション)

0908 感圧塗料を用いた低速流れ場における計測技術の開発(OS9-2 先端的熱流体計測法,オーガナイズドセッション)

Numerical prediction and wind tunnel experiment for a pitching unmanned combat air vehicle

The low-speed flowfield for a generic unmanned combat air vehicle (UCAV) is investigated both experimentally and numerically. A wind tunnel experiment was conducted with the Boeing 1301 UCAV at a variety of angles of attack up to 70 degrees, both statically and with various frequencies of pitch oscillation (0.5, 1.0, and 2.0 Hz). In addition, pitching was performed about three longitudinal locations on the configuration (the nose, 35% MAC, and the tail). Solutions to the unsteady, laminar, compressible Navier–Stokes equations were obtained on an unstructured mesh to match results from the static and dynamic experiments. The computational results are compared with experimental results for both static and pitching cases. Details about the flowfield, including vortex formation and interaction, are shown and discussed, including the non-linear aerodynamic characteristics of the vehicle.

Linearized and non‐linear acoustic/viscous splitting techniques for low Mach number flows

AbstractComputation of the acoustic disturbances generated by unsteady low‐speed flow fields including vortices and shear layers is considered. The equations governing the generation and propagation of acoustic fluctuations are derived from a two‐step acoustic/viscous splitting technique. An optimized high order dispersion–relation–preserving scheme is used for the solution of the acoustic field. The acoustic field generated by a corotating vortex pair is obtained using the above technique. The computed sound field is compared with the existing analytic solution. Results are in good agreement with the analytic solution except near the centre of the vortices where the acoustic pressure becomes singular. The governing equations for acoustic fluctuations are then linearized and solved for the same model problem. The difference between non‐linear and linearized solutions falls below the numerical error of the simulation. However, a considerable saving in CPU time usage is achieved in solving the linearized equations. The results indicate that the linearized acoustic/viscous splitting technique for the simulation of acoustic fluctuations generation and propagation by low Mach number flow fields seems to be very promising for three‐dimensional problems involving complex geometries. Copyright © 2003 John Wiley & Sons, Ltd.

A221 アセトンレーザー誘起蛍光法による不足膨張自由噴流の計測

The use of laser-induced fluorescence (LIF) from acetone is becoming increasingly widespread as a flow diagnostic, however, limited to measurement of low speed flowfields. In this paper, acetone planar laser-induced fluorescence (PLIF) is applied to measure the flowfield of under-expanded supersonic freejets. The field of number density taken by the present method is compared with the theoretical one obtained by CFD method. As a result, it is found that this method may be the powerful tool for measuring the density in supersonic flowfields.

Air movement around a worker in a low-speed flow field.

A knowledge of the air movement around a worker in a low-speed airflow is important in a number of areas: containment testing of fume cupboards; testing of personal dust samplers; testing of LEV effectiveness; and measurement of worker exposure. Measurements of velocity vectors around the upper torsos of manikins and a human in low-speed airflows have been made using a laser Doppler anemometer. Both heated and unheated manikins, as well as a 'breathing' manikin were used. The results show that quite distinctive flow patterns develop with heated and unheated bodies. Comparison of the flows around two- and three-dimensional manikins with that around a human shows that only a three-dimensional heated manikin gives good results. The unheated breathing manikin gave results which were unrepresentative of the real situation. A suitable manikin for use in sampling or testing in low-speed airflows would have a heated, rounded, three-dimensional body of reasonably human dimensions and would be non-breathing and clothed.