Homogeneous, Isotropic Flow in Grid Generated Turbulence

1999 ◽  
Vol 122 (1) ◽  
pp. 51-56 ◽  
Author(s):  
Riccardo Tresso ◽  
David R. Munoz
Keyword(s):  
Wind Tunnel ◽  
Three Dimensional ◽  
Wind Tunnel Test ◽  
Flow Region ◽  
Hot Wire ◽  
Spatial Homogeneity ◽  
One Dimensional ◽  
Streamwise Location ◽  
Dimensional Measurements ◽  
Approach Zero

Detailed grid generated turbulent analysis has been completed using a three-dimensional hot-wire anemometer and traversing mechanism to identify a homogeneous, isotropic flow region downstream of a square mesh. The three-dimensional fluctuating velocity measurements were recorded along the centerline of a wind tunnel test section and spatially over the entire wind tunnel cross section downstream of the square mesh. Turbulent intensities for various grid sizes and Reynolds numbers ranged from a minimum of 0.2 percent to a maximum of 2.2 percent in each of the three principal velocity directions. Spatial homogeneity and isotropy were determined for several turbulent flow conditions and downstream positions using the method of covariances. Covariances, in theory, should approach zero asymptotically; however, in practice, this was not achievable. A subjective judgment is required to determine downstream location where the variance of the three covariances reaches a value close to zero. The average standard deviation provides an estimate for defining the limit or subjective threshold needed to determine the onset of homogeneous, isotropic flow. Implementing this threshold, a quantitative method was developed for predicting the streamwise location for the onset of the homogeneous, isotropic flow region downstream of a 25.4 mm square grid as a function of Reynolds number. A comparison of skewness, determined from one-dimensional hot wire anemometer measurements, and covariances, determined from three dimensional hot wire anemometer measurements, indicates a need for caution when relying solely on one-dimensional measurements for determination of turbulence isotropy. The comprehensive three-dimensional characterization also provides an improved understanding of spatial distribution of fundamental turbulence quantities generated by the grid within a low-speed wind tunnel. [S0098-2202(00)02501-3]

Turbulence structure of a reattaching mixing layer

1981 ◽  
Vol 110 ◽  
pp. 171-194 ◽  
Author(s):  
C. Chandrsuda ◽  
P. Bradshaw
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Reynolds Shear Stress ◽  
Mixing Layer ◽  
Three Dimensional ◽  
Flow Region ◽  
Turbulent Energy ◽  
Turbulence Structure ◽  
Hot Wire ◽  
Recirculating Flow

Hot-wire measurements of second- and third-order mean products of velocity fluctuations have been made in the flow behind a backward-facing step with a thin, laminar boundary layer at the top of the step. Measurements extend to a distance of about 12 step heights downstream of the step, and include parts of the recirculating-flow region: approximate limits of validity of hot-wire results are given. The Reynolds number based on step height is about 105, the mixing layer being fully turbulent (fully three-dimensional eddies) well before reattachment, and fairly close to self-preservation in contrast to the results of some previous workers. Rapid changes in turbulence quantities occur in the reattachment region: Reynolds shear stress and triple products decrease spectacularly, mainly because of the confinement of the large eddies by the solid surface. The terms in the turbulent energy and shear stress balances also change rapidly but are still far from the self-preserving boundary-layer state even at the end of the measurement region.

Wind Tunnel Test on a Slowed Mach-Scaled Hingeless Rotor with Lift Compounding

2021 ◽  
Author(s):  
Shashank Maurya ◽  
Xing Wang ◽  
Inderjit Chopra
Keyword(s):  
Wind Tunnel ◽  
Reverse Flow ◽  
Wind Tunnel Test ◽  
Bending Moment ◽  
Flow Region ◽  
Dynamic Stall ◽  
Rolling Moment ◽  
Slowing Down ◽  
Slowed Rotor ◽  
Compressibility Effects

A single main rotor helicopter's maximum forward speed is limited due to the compressibility effects on the advancing side and reverse flow and dynamic stall on the retreating side. Compound helicopters can address these issues with a slowed rotor and lift compounding. There is a scarcity of test data on compound helicopters, and the present research focuses on a systematic wind tunnel test on lift compounding. Slowing down the rotor increases the advance ratio and, hence, the reverse flow region, which does not produce much lift. The lift is augmented with a wing on the retreating side. A hingeless rotor hub helps to balance the rolling moment with lift offset. Wind tunnel tests were carried out on this configuration up to advance ratios of 0.7 at two different wing incidence angles. Rotor performance, controls, blade structural loads, and hub vibratory loads were measured and compared with in-house comprehensive analysis, UMARC. A comparison between different wing incidences at constant total lift provided many insights into the lift compounding. It increased the vehicle efficiency and reduced peak-to-peak lag bending moment and in-plane 4/rev hub vibratory loads. The only trade-off was steady rotor hub loads and rolling moment at the wing root carried by the fuselage.

A Unified Correction Method for the Acoustic Refraction (UCMAR) Caused by a Three Dimensional Shear Layer

2019 ◽  
Vol 105 (5) ◽  
pp. 732-742
Author(s):  
Lican Wang ◽  
Rongqian Chen ◽  
Yancheng You ◽  
Wenjun Wu ◽  
Ruofan Qiu
Keyword(s):  
Wind Tunnel ◽  
Temperature Gradient ◽  
Shear Layer ◽  
Ray Tracing ◽  
Three Dimensional ◽  
Correction Method ◽  
Three Dimensions ◽  
Acceptable Solution ◽  
One Dimensional ◽  
Planar Shear

The acoustic refraction induced by the shear layer in an open-jet wind tunnel causes a source shift when estimating the source location with beamforming. Traditional correction methods of the shear layer refraction are achieved through a computational eff ort or limited using one-dimensional or planar shear layer. In this paper, the unified correction method for acoustic refraction (UCMAR) is suitable for the three dimensions that covers several traditional forms. Meanwhile, the UCMAR can consider more general configurations, such as the temperature gradient on both sides of the shear layer and the off -axis source in a circular wind tunnel. These configurations are validated through a ray tracing technique and a benchmark example. In addition, the principle of time reverse is integrated with UCMAR. This results in a reverse UCMAR, which can quickly attain an acceptable solution.

The aeroacoustics of finite wall-mounted square cylinders

2017 ◽  
Vol 832 ◽  
pp. 287-328 ◽  
Author(s):  
Ric Porteous ◽  
Danielle J. Moreau ◽  
Con J. Doolan
Keyword(s):  
Boundary Layer ◽  
Experimental Study ◽  
Reynolds Number ◽  
Wind Tunnel ◽  
Three Dimensional ◽  
Flow Structures ◽  
Hot Wire ◽  
Radiated Noise ◽  
Aspect Ratios ◽  
Square Cylinders

This paper presents the results of an experimental study that relates the flow structures in the wake of a square finite wall-mounted cylinder with the radiated noise. Acoustic and hot-wire measurements were taken in an anechoic wind tunnel. The cylinder was immersed in a near-zero-pressure gradient boundary layer whose thickness was 130 % of the cylinder width, $W$. Aspect ratios were in the range $0.29\leqslant L/W\leqslant 22.9$ (where $L$ is the cylinder span), and the Reynolds number, based on width, was $1.4\times 10^{4}$. Four shedding regimes were identified, namely R0 ($L/W<2$), RI ($2<L/W<10$), RII ($10<L/W<18$) and RIII ($L/W>18$), with each shedding regime displaying an additional acoustic tone as the aspect ratio was increased. At low aspect ratios (R0 and RI), downwash dominated the wake, creating a highly three-dimensional shedding environment with maximum downwash at $L/W\approx 7$. Looping vortex structures were visualised using a phase eduction technique. The principal core of the loops generated the most noise perpendicular to the cylinder. For higher aspect ratios in RII and RIII, the main noise producing structures consisted of a series of inclined vortex filaments, where the angle of inclination varied between vortex cells.

The Effect of the Source Location on the Acoustic Excitation on Laminar-Turbulent Transition on Swept Wing Boundary Layer

2002 ◽  
Author(s):  
B. Gu¨lac¸ti ◽  
S. Aubrun ◽  
A. Seraudie ◽  
D. Arnal
Keyword(s):  
Wind Tunnel ◽  
Test Section ◽  
Three Dimensional ◽  
Source Location ◽  
Sound Level ◽  
Acoustic Excitation ◽  
Hot Wire ◽  
Swept Wing ◽  
Turbulent Transition ◽  
Laminar Turbulent Transition

The effect of the source location and the direction of the propagation on the laminar-turbulent transition on swept-wing three-dimensional boundary layers are investigated experimentally. Also the crossflow case is handled in detail. The source for the acoustic excitation is placed in four different locations: in front of the wing, on top of the test section, behind the wing and in front of the wind tunnel. Three different experimental cases (streamwise, crossflow and mixed cases) are examined for each location with two different excitation bands. For the most efficient frequency ranges and the highest sound pressure levels an upstream shift of transition motion between 20%–35% of chord length for streamwise case and between 5%–10% for the crossflow case are observed. While in front of the wing and behind the wing are the most efficient loudspeaker positions in the streamwise case, in the crossflow case the most efficient locations are observed to be in front of the wing and on top of the test section. It is concluded that acoustic sound level plays a more important role in the upstream shift of the transition than the source location and placing the loudspeaker in front of the wind tunnel is not an efficient position. For the crossflow instabilities dominated transition the stationary vortices are clearly seen from the velocity contours obtained by the hot-wire. Secondary instabilities couldn’t be observed in the hot-wire spectra. The surface roughness of the wing that is reduced to 0.25µm does not change the transition location in the crossflow case.

Efficient stall compliance prediction method for trimmed very light aircraft with high-lift devices

2016 ◽  
Vol 231 (6) ◽  
pp. 1124-1137
Author(s):  
Nhu Van Nguyen ◽  
Daeyeon Lee ◽  
Maxim Tyan ◽  
Jae-Woo Lee ◽  
Sangho Kim
Keyword(s):  
Wind Tunnel ◽  
Stokes Equations ◽  
Transport Model ◽  
Three Dimensional ◽  
Wind Tunnel Test ◽  
Prediction Method ◽  
Aircraft Design ◽  
Design Stage ◽  
High Fidelity ◽  
Configuration Generation

An efficient stall compliance prediction method using quick configuration generation, adapted mesh, high fidelity analysis, and wind tunnel test data for trimmed very light aircraft is proposed. The three-dimensional Navier–Stokes equations are used to determine the characteristics of the flow field around the aircraft, and the [Formula: see text] shear stress transport model is used to interpret the turbulent flow as a solver in the high fidelity analysis. The calibrated mesh and model are developed by comparing the results with the wind tunnel test and adjusting the adapted mesh to match the wind tunnel data. The calibrated mesh and model are applied to conduct the full-scale very light aircraft analysis for the clean and full flap extended flight conditions to comply with the CS-VLA stall regulations. It is recommended that the flap area be increased in the trimmed full flap extended condition. The proposed method demonstrates the feasibility and effectiveness of very light aircraft VLA stall compliance prediction in reducing the development cost and time with small configuration changes at the preliminary very light aircraft design stage.

Wind Tunnel Test of a Rotorcraft with Lift Compounding

2021 ◽  
Vol 66 (1) ◽  
pp. 1-16
Author(s):  
Andŕe Bauknecht ◽  
Xing Wang ◽  
Jan-Arun Faust ◽  
Inderjit Chopra
Keyword(s):  
Wind Tunnel ◽  
Flow Field ◽  
Reverse Flow ◽  
Lift Coefficient ◽  
Wind Tunnel Test ◽  
Flow Region ◽  
Field Analysis ◽  
Rolling Moment ◽  
The Impact ◽  
Side Flow

Rotorcraft flight speed is limited by compressibility effects on the advancing blade side and decreasing lift potential on the retreating blade side. It may thus be beneficial to employ a hingeless rotor to generate additional lift with the advancing blade and compensate the resulting rolling moment with a fixed wing on the retreating blade side. This concept is a form of "lift compounding" that appears to show enormous potential. The present paper presents results of a wind tunnel test with a slowed, hingeless rotor and single fixed wing on the retreating blade side. Based on rotor test stand data and flow field measurements, the impact of operational and rotor parameters on system performance and aerodynamics is examined, mutual interaction effects between rotor and fixed wing are analyzed, and dominant flow structures are characterized in the reverse flow region on the retreating blade side. Flow field analysis reveals a reverse flow entrance vortex that freely convects through the reverse flow region and rivals the blade tip vortices in strength. Contrary to previous beliefs, this vortex originates from upstream of the reverse flow region and only its detachment from the rotor blade is related to entering this region. The combination of finite rolling moment trim and aft shaft tilt significantly increases rotor lift coefficient and corresponding peak lift-to-drag ratio of the compound rotorcraft. Results are compared with predictions from a comprehensive rotor analysis that is expanded to cover the main effects of the added fixed wing and is able to reproduce general performance trends of the rotorcraft. The present study highlights that adding a single fixed wing and hingeless rotor to a high-speed rotorcraft could significantly improve its performance.

scholarly journals A study of droplet evaporation coupling model based on Eulerian method

2020 ◽  
Vol 36 (4) ◽  
Author(s):  
Y. Tang ◽  
J. Shang ◽  
Y. Zhan
Keyword(s):  
Flow Field ◽  
Three Dimensional ◽  
Great Influence ◽  
Wind Tunnel Test ◽  
Internal Flow ◽  
Coupling Model ◽  
One Dimensional ◽  
Model Based ◽  
Eulerian Method ◽  
Thermodynamic Coupling

Research on engine icing is a hot topic among the world. Different from the aircraft wing or airframe icing, the evaporation phenomenon in the internal flow field has a great influence on the engine icing. Moreover, the thermodynamic coupling between droplets and flow field is not available in current particle trajectory calculations, or only for one-dimensional situation. Therefore, a three-dimensional droplet trajectory calculation model based on Eulerian method is used to demonstrate the thermodynamic coupling between droplets and flow field. The model was verified by NRC small engine icing wind tunnel test data and the flow field evolution is obtained which cannot be obtained by the one-dimensional coupling model. In the meanwhile, the effects of different initial LWC, relative humidity and MVD on the internal flow evaporation were studied, and the trends of droplets and flow field affected by evaporation were obtained. The numerical method in this paper can provide guidance for the subsequent research on engine icing.

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