M. S. Chandrasekhara

Compressibility effects on dynamic stall

Abstract Dynamic stall delay of flow over airfoils rapidly pitching past the static stall angle has been studied by many scientists. However, the effect of compressibility on this dynamic stall behavior has been less comprehensively studied. This review presents a detailed assessment of research performed on this subject, including a historical review of work performed on both aircraft and helicopters, and offers insight into the impact of compressibility on the complex aerodynamic phenomenon known as dynamic stall. It also documents the major effect that compressibility can have on dynamic stall events, and the complete change of physics of the stall process that can occur as free-stream Mach number is increased.

Unsteady Stall Control Using Dynamically Deforming Airfoils

Introduction An experiment documenting the compressible flow over a dynamically deforming airfoil is presented. This airfoil, which has a leading edge radius that can be dynamically changed, was tested at various defor- mation rates for fixed airfoils angle of attack. Selected leading-edge shapes were also tested during airfoil os- cillation. These tests show that for a range of Mach numbers observed on the retreating blades of heli- copter rotors the dynamic stall vortex can be avoided by the judicious variation of leading-edge curvature

Measurements in Vertical Plane Turbulent Plumes

Mean-flow and turbulence measurements have been obtained in two-dimensional vertical turbulent plumes in a nominally still ambient. The plumes were generated by injecting hot water vertically upwards from the bottom of a reservior containing cold water. A two-component Laser Doppler Anemometer (LDA) and a “cold-film” resistance thermometer were used to obtain instantaneous velocity and temperature measurements in the plume. The present mean-flow measurements have confirmed many of the earlier measurements on plane plumes, but have also indicated some important differences. The use of the two-component LDA made it possible to obtain data on turbulent intensities, turbulent fluxes and other details of the structure of turbulence in plane plumes. The turbulence measurements have shown that the eddy viscosity and turbulence are significantly higher in the plume compared to an isothermal jet. Detailed measurements of energy balance suggest that buoyant production contributes substantially to this increase.

Interferometric Investigations of Compressible Dynamic Stall Over a Transiently Pitching Airfoil

The compressible dynamic stall flowfield over a NACA 0012 airfoil transiently pitching from 0 to 60 deg at a constant rate under compressible flow conditions has been studied using real-time interferometry. A quantitative description of the overall flowfield, including the finer details of dynamic stall vortex formation, growth, and the concomitant changes in the airfoil pressure distribution, has been provided by analyzing the interferograms. For Mach numbers above 0.4, small multiple shocks appear near the leading edge and are present through the initial stages of dynamic stall. Dynamic stall was found to occur coincidentally with the bursting of the separation bubble over the airfoil. Compressibility was found to confine the dynamic stall vortical structure closer to the airfoil surface. The measurements show that the peak suction pressure coefficient drops with increasing freestream Mach number, and also it lags the steady flow values at any given angle of attack. As the dynamic stall vortex is shed, an anti-clockwise vortex is induced near the trailing edge, which actively interacts with the post-stall flow.

Quantitative study of unsteady compressible flow on an oscillating airfoil

Detailed interferometric measurements of the flow near the leading edge of an oscillating airfoil offer the first detailed experimental quantification of the locally compressible flow field that surrounds an oscillating airfoil at moderate subsonic Mach numbers. Interferograms obtained by a specially adapted real-time point-diffraction interferometry technique have revealed significant characteristics of this complex, and very rapidly varying, locally supersonic flow. Instantaneous pressure distributions determined from these interferograms document the effect of unsteadiness on the leading-edge flow environment.

Boundary-Layer-Tripping Studies of Compressible Dynamic Stall Flow

The challenging task of properly tripping the boundary layer of a leading-edge-stalling airfoil experiencing compressible dynamic stall at Reynolds numbers between 3.6 X 10 5 and 8.1 X 10 5 has been addressed. Real-time interferometry data of the flow over an oscillating airfoil have been obtained at freestream Mach numbers of 0.3 and 0.45. The airfoil was tripped by separately placing five different trips of varying lengths near the leading edge. The trip heights ranged from 40 to 175 μm. The resulting flow and airfoil performance were evaluated using the criteria of elimination of the laminar separation bubble that otherwise forms, delay of dynamic stall onset to higher angles of attack, and production of consistently higher suction peaks. Quantitative analysis of the interferograms showed that the laminar separation bubble was still present with the smallest trip and premature dynamic stall occurred with the largest trip. The right trip was determined to be a distributed roughness element extending from 0.5 to 3% chord. Its height was found to compare reasonably with the airfoil boundary-layer thickness at the dynamic stall vortex formation angle of attack, at a location slightly upstream of the vortex origin in the adverse pressure gradient region.

Competing Mechanisms of Compressible Dynamic Stall

Earlier experiments have documented the onset of compressible dynamic stall either from the bursting of a leading-edge laminar separation bubble or from a leading-edge shock, depending on the Reynolds number and Mach number. For certain combinations of conditions, the supersonic flow and the bubble dynamics compete with each other. The consequent complex interactions lead to a newly discovered mechanism of dynamic stall onset. Details of these various mechanisms are discussed

Schlieren studies of compressibility effects on dynamic stall of transiently pitching airfoils

Abstract : Compressibility effects on the flowfield of an airfoil executing rapid transient pitching motion from 0-60 deg over a wide range of Mach numbers and pitching rates were studied using a stroboscopic schlieren flow visualization technique. The studies have led to the first direct experimental documentation of multiple shocks on the airfoil upper surface flow for certain conditions. Also, at low Mach numbers, additional coherent vortical structures were found to be present along with the dynamic stall vortex, whereas at higher Mach numbers the flow was dominated by a single vortex. The delineating Mach number for significant compressibility effects was 0.3 and the dynamic stall process was accelerated by increasing the Mach number above that value. Increasing the pitch rate monotonically delayed stall to angles of attack as large as 27 deg.

Compressible dynamic stall control using a shape adaptive airfoil

Compressible dynamic stall control using a dynamically deforming leading edge airfoil is reported. The technique uses real-time leading edge shape adaptation to manipulate the local adverse pressure gradient over an oscillating airfoil. This produces attached leading edge flow at all angles of attack in the cycle. The shape change schedule is tailored to achieve the “right” airfoil shape through the dynamic stall regime by rounding the nose on the upstroke and sharpening it on the downstroke. The method exploits the favorable effects of shape adaptation on the potential flow to modify the pressure field and maintain the peak vorticity level below the critical level at which the dynamic stall vortex develops. The sensitivity of the flow to rate of nose curvature change and the angle of attack at which the initiation occurs has been investigated.

Analysis of compressible light dynamic stall flow at transitional Reynolds numbers

Numerical and experimental results of steady and light dynamic stall flow over an oscillating NACA 0012 airfoil at a freestream Mach number of 0.3 and Reynolds number of 0.54 x 10 6 are compared. The experimental observation that dynamic stall is induced from the bursting of a laminar separation bubble points to the role of transition in influencing the flow development. Its modeling, including the changes in transition onset location and transition length with increase in airfoil angle of attack, is critical for computing the dynamic stall flow properly. In this study, the transition onset point is specified suitably and a simple transition length model is incorporated to determine the extent of the laminar separation bubble. The thin-layer approximations of compressible, Reynolds-averaged, Navier-Stokes equations are used for the numerical solution, with an implicit, upwind-biased, third-order-accurate scheme for the numerical integration.