Generalized Airloads Prediction for Bluff Bodies Transported as Slung Loads

2015 ◽  
Author(s):  
Nicholas Motahari ◽  
Nandeesh Hiremath ◽  
Dhwanil Shukla ◽  
Brandon Liberi ◽  
Nikolaus Thorell ◽  
...  
Keyword(s):  
Aspect Ratio ◽  
Bluff Body ◽  
Flight Speed ◽  
Navier Stokes ◽  
Bluff Bodies ◽  
Aerodynamic Loads ◽  
Rotation Method ◽  
Continuous Rotation ◽  
Speed Prediction ◽  
Body Shapes

Objects of arbitrary shapes have to be carried as slung loads under aircraft, particularly rotorcraft. The flight speed is limited by the possibility of slung loads going into divergent oscillations. In 2014 we presented a testing-based approach to predict the safe flight speed, applicable to bluff bodies of arbitrary shape. Since then, an extensive variety of bluff-body shapes has been tested, and we venture further towards generalized airload prediction, required for generalized divergence speed prediction. Extending recent work, the Continuous Rotation method is applied to obtain aerodynamic loads on generic shapes: a circular cylinder and a rectangular prism, both with aspect ratio varied systematically. The genesis of the side force on the yawed cylinder, and the differences between rough and smooth cylinders, have been derived from comparisons between experiments and diagnostic computations with an unsteady Navier-Stokes solver. Interpolating Fourier coefficients of the azimuthal load variation appears to be viable to generalize loads on cylinders of varying aspect ratio for both the generic shapes.

Generalizing Prediction of Bluff Body Aerodynamic Load Maps

2015 ◽  
Author(s):  
Sorin Pirau ◽  
Brandon Liberi ◽  
Natasha Barbely ◽  
Narayanan Komerath
Keyword(s):  
Aspect Ratio ◽  
Bluff Body ◽  
Navier Stokes ◽  
Gradual Transition ◽  
Aerodynamic Load ◽  
Rotation Method ◽  
Continuous Rotation ◽  
Rate Effects ◽  
Body Shapes ◽  
Definition Of

The Continuous Rotation method enables efficient definition of all aerodynamic load components on bodies of arbitrary shape for arbitrary attitudes. This is applied to several bluff body shapes including cylinders, a cuboid, a flat plate and a porous box. Rate effects and unsteadiness are shown to be negligible using a cylinder of aspect ratio 1. The genesis of the side force on the yawed cylinder, and the differences between rough and smooth cylinders, are derived from comparisons between experiments and diagnostic computations with an unsteady Navier-Stokes solver. Interpolating Fourier coefficients of the azimuthal load variation appears to be viable to generalize loads on cylinders of varying aspect ratio. A large variation is seen for aspect ratio 0.5 to 1, with a more gradual transition to ‘high aspect ratio’ features beyond aspect ratio 2.

Aerodynamic Load Mapping of Bluff Bodies: An Update and Summary

2016 ◽  
Author(s):  
Nicholas Motahari ◽  
Dhwanil Shukla ◽  
Nandeesh Hiremath ◽  
Narayanan Komerath
Keyword(s):  
Aspect Ratio ◽  
Rectangular Prism ◽  
Circular Cylinders ◽  
Aerodynamic Load ◽  
Bluff Bodies ◽  
Aerodynamic Loads ◽  
Bridge Model ◽  
Rotation Method ◽  
Continuous Rotation ◽  
Large Interaction

Measurements of 6-DOF aerodynamic loads on bluff bodies using the continuous rotation method are summarized. New results are presented on two rectangular prisms and on three practical shapes — an Engine Canister model, a Ribbon Bridge model, and a model of a standardized aerodynamic container to carry automobiles. Efforts are described, to relate the aerodynamic loads on the practical shapes, to weighted combinations of aerodynamic loads on interpolated canonical shapes. The Engine Canister aerodynamic loads are obtained to good approximation from those on an interpolated cylinder of aspect ratio 1.9. Likewise, the Ribbon Bridge aerodynamic loads are approximated from loads on rectangular prisms and circular cylinders. An attempt is made to predict the load variation on a rectangular prism, from those measured on a prism 1/3 as long using topological arguments; this attempt shows the large interaction effects on such a shape.

Pressure Field of a Yawed Aspect Ratio 1 Circular Cylinder

2017 ◽  
Author(s):  
Nandeesh Hiremath ◽  
Dhwanil Shukla ◽  
Anshuman Pandey ◽  
James W. Gregory ◽  
Narayanan Komerath
Keyword(s):  
Aspect Ratio ◽  
Circular Cylinder ◽  
Sensor Data ◽  
Navier Stokes ◽  
Windward Side ◽  
Test Case ◽  
Single Shot ◽  
Aerodynamic Loads ◽  
Streamline Curvature ◽  
Continuous Rotation

The aerodynamic loads and the flow around an Aspect Ratio 1 circular cylinder in the range of 300K to 400K Reynolds number, pose a surprisingly rich fundamental problem. This aspect ratio demonstrates features from the ‘coin’ limit as well as the high aspect ratio limit. Well-resolved measurements of all 6 components of aerodynamic loads became possible with the Continuous Rotation method. The genesis of the side loads, drag and yawing moment on a yawed aspect ratio 1 cylinder, is examined using 3 different methods. The first is direct pressure sensing on the flat side surface using pressure taps and individual sensors, done on similar setups at two different facilities. This is of course sparse but gives direct quantitative measures for validation. The second is pressure-sensitive paint (PSP), which provides a full distribution of pressure across the surface, with high spatial resolution that is limited only by the optics of the setup. The third is to extract the surface pressure using an algorithm that uses measured three-dimensional velocity data, along with the continuity and Navier-Stokes equations with a streamline-curvature model used to obtain the initial estimate. As possible, the measurements are compared with those computed from first principles using a Navier-Stokes solver. The streamline curvature method is validated against a numerical test case for an infinite cylinder, operated in the Foppl regime. It is then applied to the AR1 cylinder test case, and found to yield satisfactory comparison with interpolated surface plots from point measurements. The single-shot/transient PIV technique is applied to obtain the pressure map on the leeside and windward side of the yawed cylinder. The results are compared to point sensor data and PSP data. Comparisons of all these are shown, and linked to prior aerodynamic load measurements and computations. Agreement is good, with reasons for disagreement and uncertainty identified.

Efficient Airload Determination for Bluff Body Aeromechanics

2014 ◽  
Author(s):  
Sorin Pirau ◽  
Vrishank Raghav ◽  
Alex Forbes ◽  
Brandon Liberi ◽  
Narayanan Komerath
Keyword(s):  
Flat Plate ◽  
Bluff Body ◽  
Free Stream ◽  
Periodic Problem ◽  
Ensemble Averaging ◽  
Aerodynamic Loads ◽  
Load Variations ◽  
Continuous Rotation ◽  
Cylinder Model ◽  
Support Interference

A continuous-rotation testing technique is applied to capture the variation of aerodynamic loads with attitude on objects of arbitrary shape. The technique converts the problem of measuring static air loads at various attitudes into a periodic problem. Phase-resolved ensemble-averaging is used to capture load variations with arbitrarily fine azimuthal resolution. The airload variations are obtained in closed form as discrete Fourier series. Experiments on a cylinder model of equal length and diameter were used to study the ability to capture asymmetries, and resolve support interference issues. A closed cuboid is used to correlate with prior work. A flat plate with a central cylindrical load, and a porous box are also studied. Free-swing tests using rigid tethers fixed to a pitch-yaw-roll gimbal mount are used to derive dynamic behavior in a free stream. The cylinder results showed the ability to resolve the effect of minor geometric asymmetries on airloads. The flat plate at 10 degrees pitch shows strong differences in dynamics between cases with a rounded versus squared-off edge facing the freestream. The porous box shows the differences between cases with and without one side blocked.

Divergence Prediction for Practical Helicopter Slung Loads

2016 ◽  
Author(s):  
Nicholas Motahari ◽  
Thomas Kim ◽  
Dhwanil Shukla ◽  
Nandeesh Hiremath ◽  
Narayanan Komerath
Keyword(s):  
Bluff Body ◽  
General Pattern ◽  
Oscillation Amplitude ◽  
Flight Test ◽  
Scale Model ◽  
Aerodynamic Load ◽  
Stable Operation ◽  
Rotation Method ◽  
Angle Data ◽  
Continuous Rotation

Certifying the highest safe speed for an aircraft with a slung load, is life-critical yet daunting. Two flight cases are considered, to test an iterative procedure that predicts the divergence speed from experimental scale model data and simulations. The first is an empty engine canister. The second is a segment of a water-floatable military Ribbon Bridge. In each case, mass, geometry, tether length from a single rotation-bearing attachment, and moments of inertia, come from flight preparations. An initial aerodynamic load map is interpolated and synthesized from a growing library on bluff body aerodynamics. Dynamic simulation with these data predict maximum roll, pitch and yaw angles reached as functions of freestream speed. This yieldw a good initial estimate of critical speeds and dynamics. Model-scale wind tunnel data using our Continuous Rotation method about the required axes, refine simulation. For the engine canister, simulations matched detailed flight test data on maximum trailing and rolling amplitudes over the operational speed range. Trail angle data showed that Reynolds number errors are not significant. In this paper, model-based results explained the correct speed where the ribbobn bridge flight test was stopped. While flight test oscillation amplitude histories depend on initial perturbations of the load, a 15-degree initial amplitude gives conservative results. Ribbon bridge airloads resemble those on a long container but with asymmetries. Dynamic behavior follows the general pattern of an intermediate hump in roll amplitude followed by stable operation at higher speeds until divergence occurs.

Review—Bluff Body Flows Applicable to Vehicle Aerodynamics

1980 ◽  
Vol 102 (3) ◽  
pp. 265-274 ◽  
Author(s):  
P. W. Bearman
Keyword(s):  
Bluff Body ◽  
Main Flow ◽  
Wind Tunnels ◽  
Prediction Methods ◽  
Bluff Bodies ◽  
Turbulence Level ◽  
Vehicle Aerodynamics ◽  
Body Shapes ◽  
Bluff Body Flows ◽  
Relative Wind

This paper attempts to review those aspects of bluff body aerodynamics that are relevant to the understanding of vehicle flows. Vehicles often have complex body shapes and are influenced by the proximity of the ground. The effect of the ground is discussed in some detail and results for bluff bodies mounted in wind tunnels above fixed and moving ground planes are presented. It is concluded that drag is little affected by ground proximity and ground representation whereas lift is often sensitive to both. The effect of slanting the base of a bluff body is discussed and the two main flow regimes that result are described. The influence of the wind on vehicle flows is investigated and it is found that vehicle mean flows are sensitive to the turbulence level in the relative wind. Finally numerical prediction methods are considered.

scholarly journals An Observational Study of Vortex Spacing in Island Wake Vortex Streets

2006 ◽  
Vol 134 (8) ◽  
pp. 2285-2294 ◽  
Author(s):  
George S. Young ◽  
Jonathan Zawislak
Keyword(s):  
Aspect Ratio ◽  
Bluff Body ◽  
Three Dimensional ◽  
Width Ratio ◽  
Bluff Bodies ◽  
Von Karman ◽  
Vortex Streets ◽  
Moderate Resolution Imaging Spectroradiometer ◽  
Island Wake ◽  
Von Kármán

Abstract Vortex streets are a frequent occurrence in stratocumulus-topped flow downwind of mountainous islands. Theoretical studies dating back to von Kármán, supported by laboratory and numerical studies, have yielded similarity theories for the size and spacing of these vortices behind bluff bodies. Despite dynamical differences between such two-dimensional flows and the three-dimensional flow past isolated islands, satellite case studies suggest these geometric similarities may also hold for the atmospheric case. In this study, two of the resulting dimensionless ratios are measured using satellite imagery. One is the aspect ratio between cross-street and along-street spacing of the vortices. The second is the ratio of the cross-street spacing to the crosswind width of the island. A 30-image sample from the Aqua and Terra Moderate Resolution Imaging Spectroradiometer satellites is analyzed to obtain these ratios. The resulting set of values for the two dimensionless ratios is tested against the values found in bluff body studies. The aspect ratio is tested against the value of 0.28 resulting from von Kármán’s inviscid theory, and the dimensionless width ratio is tested against the value of 1.2 from Tyler’s laboratory study of flow around a bluff body. It is found that atmospheric vortex streets do indeed follow the geometric similarity theories, but with different values for the two ratios than those predicted by von Kármán and Tyler. The aspect ratio is larger than predicted as is the dimensionless width ratio. Both differences are consistent with the turbulent diffusion of vorticity in the wake of the island. The vortex streets more closely follow inviscid theory close to the island, with vortex expansion taking place a few vortex diameters downwind of the island.

A proposal concerning laminar wakes behind bluff bodies at large Reynolds number

1956 ◽  
Vol 1 (4) ◽  
pp. 388-398 ◽  
Author(s):  
G. K. Batchelor
Keyword(s):  
Reynolds Number ◽  
Steady Flow ◽  
Bluff Body ◽  
The Body ◽  
Navier Stokes ◽  
Large Reynolds Number ◽  
Bluff Bodies ◽  
Proper Solution ◽  
Navier Stokes Equation ◽  
Limit Solution

This note advocates a model of the steady flow about a bluff body at large Reynolds number which is different from the classical free-streamline model of Helmholtz and Kirchhoff. It is suggested that, although the free-streamline model may be a proper solution of the Navier-Stokes equation with μ = 0, it is unlikely to be the limit, as μ → 0, of the solution describing the steady flow due to the presence of a bluff body in an otherwise uniform stream. The limit solution proposed here is one which gives a closed wake.A closed wake contains a standing eddy, or eddies, whose general features can be inferred from the results of an earlier investigation of steady flow in a closed region at large Reynolds number. In all cases, the drag (coefficient) on the body tends to zero as the Reynolds number tends to infinity. The proccedure for finding the details of the closed wake behind two-dimensional and axisymmetrical bodies is described, although no particular case has yet been worked out.

Vortex Shedding in a Linear Shear Flow From a Vibrating Marine Cable With Attached Bluff Bodies

1985 ◽  
Vol 107 (1) ◽  
pp. 61-66 ◽  
Author(s):  
R. D. Peltzer ◽  
D. M. Rooney
Keyword(s):  
Shear Flow ◽  
Vortex Shedding ◽  
Bluff Body ◽  
Bluff Bodies ◽  
Near Wake ◽  
Cable System ◽  
Sheared Flow ◽  
Linear Shear ◽  
Body Shapes ◽  
Marine Cable

The present study examines the vortex street wake behavior of a flexible, helically wound, high aspect ratio marine cable in a linear shear flow. Particular attention is paid to the lock-on phenomena associated with uniform and sheared flow past the cable when it is forced to vibrate in the first mode, normal to the flow. An analysis is given of the effects on the vortex shedding and synchronization phenomena that are generated by placing distributions of spherical bluff body shapes along the span of the cable in uniform and sheared flow. The latter geometry is representative of a number of cable system deployments and has special consequencies for strumming in a shear flow. The effectiveness of these attached spheres as strumming-suppression devices is evaluated. Synchronized vibration and/or the presence of the bluff bodies significantly affected the spanwise character of the near wake cellular vortex shedding structure. The spanwise extent of the resonant, vortex-excited oscillations was significantly extended by the presence of the spheres along the cable span. This finding was particularly significant because it meant that the undesirable effects that accompanied synchronization would be extended over a longer portion of the cable span.

Effect of Body Aspect Ratio and Tank Size on the Hydrodynamics of a Rotating Bluff Body During the Initial Spin-Up Period

2001 ◽  
Vol 123 (3) ◽  
pp. 649-655
Author(s):  
D. Maynes ◽  
M. Butcher
Keyword(s):  
Aspect Ratio ◽  
Flow Field ◽  
Bluff Body ◽  
Initial Period ◽  
The Body ◽  
Bluff Bodies ◽  
Physical Description ◽  
Time Scaling ◽  
Rectangular Cylinders ◽  
Tank Geometry

Hydrodynamic torque measurements on rotating bluff bodies are presented for 32 different bodies and three different sized tanks for Reynolds numbers in the range 104-105. The present results focus on the initial period, build-up regime, where the torque remains constant before the tank walls have impacted the flow field in the vicinity of the body. The results show that during the build-up regime, the torque coefficient is a function only of the aspect ratio and increases to a maximum at an aspect ratio near unity, followed by a decrease for further increases in the aspect ratio. This behavior is similar to a uniform flow past rectangular cylinders of varied width and a physical description for the observed variation is proposed. A nondimensional time scale describing the time until the tank geometry impacts the flow field near the body is also presented. This time scaling is based on all of the measurements and appears to be quite general, predicting the spin-up time for bodies differing in volume by three orders of magnitude and tanks differing by two orders of magnitude.

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