Dhwanil Shukla

Generalized Airloads Prediction for Bluff Bodies Transported as Slung Loads

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.Copyright

Genesis of the Airload Variations on Cylinders of Small Aspect Ratios

A cylinder of aspect ratio 1 experiences a rich mixture of phenomena when yawed through 360 degrees. Understanding this variation is crucial to aerodynamic load definition for objects of practical shape. This paper uses several diagnostics to explain the detailed airload map of cylinders with length to diameter ratios 0.25 to 4 in steady incompressible flow, at Reynolds numbers from 50,000 to 450,000. A sharp linear variation of side force coefficient with yaw is bounded by apparent stall. Drag and pitching moment depend more on the curved surfaces, while side force is dominated by flow over the end plates. Tuft visualization and stereo Particle Image Velocimetry complement 6-DOF loads. Pressure distributions are obtained from velocimetry as well as from computations. The flow over the suction side shows a curved leading edge vortex followed by reattachment and an aft separation. The lift at low yaw is largely due to suction from the zones near the front corner, while at higher yaw it comes from the pressure difference between the two sides. The presence of helical vortices differentiates the lift generation from models based purely on separation.Copyright

Divergence Prediction for Practical Helicopter Slung Loads

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.Copyright

Vortical Features of a Rotor Blade in Reversed Flow

The nature of the flow around a rotating blade in reverse flow is described, integrating results from fixed and rotary wing experiments. The highly 3-dimensional flow phenomena do not conform to expectations based on 2-D airfoil aerodynamics. Fixed-wing results from load measurements and flow visualization showed that the sharp-edge vortex (SEV) is a primary feature when the blade is yawed either forward or backward. The loads are better modeled using the Polhamus leading edge suction analogy. Vortex-induced pressure gradient induces an inward radial flow overcoming centrifugal effects, but away from the vortex, outward radial flow is evident everywhere. A strongly three-dimensional and attached SEV is evident under the blade at 240 degrees azimuth. This detaches and convects with the flow, remaining close to the blade by 270 degrees. The vortex seen at 300 degrees is clearly detached, but growth of the core diameter corresponding to vortex bursting, causes strong suction under the blade. The flow around the blunt edge is again strongly 3-D. Some evidence of intermittent separation is seen, but the azimuth-resolved, ensemble-averaged flow is mostly attached around the blunt edge. Preliminary static pressure contours derived from the measured 3-component velocity field are presented.Copyright

Control of Centrifugal Instability in Vortex-Surface Interaction

The interaction of a rotating conical flow with a solid surface generates a centrifugal instability. This occurs in the flow over the wings of certain types of aircraft at high angles of attack. Efforts at our laboratory have detected such structures using near-surface flow diagnostics, and shown that they can be effectively alleviated using passive flow control near the surface. Their alleviation removes the narrowband spectral peak at the nominal location of vertical fins on these aircraft. This paper explores the substitution of active flow control techniques that remain conformal to the surface and are only powered during high angle of attack operation. The occurrence of the phenomenon and its 15-dB alleviation with geometric fences are shown on a rounded-edge 42-degree swept, cropped delta wing at 25 degrees angle of attack. The feasibility and power requirements for the plasma actuator are estimated in this paper. The generation of counter-rotating vortices using a double dielectric barrier discharge actuator is demonstrated.Copyright