The control of flow separation by periodic excitation

Nomenclature c = chord CD =drag coefficient, D/qA CL =lift coefficient, L/qa CP = pressure coefficient, P-PW /q CM = pitching-moment coefficient, M/qA D =drag, Ib k = reduced frequency, wc/2U L =lift, Ib M =Mach number; pitching moment, ft-lb P = pressure, lb/in q = dynamic pressure, 1 /2p U R, Re, Re, Rn = Reynolds number t time U,V = freestream velocity, ft/s x = distance a = angle of attack, deg f =nondimensional chord length, x/c $tr transition location co = rotational frequency, rad/s p = density, lb/ft A = sweep angle, deg

Progress in analysis and prediction of dynamic stall

The axial and radial velocity components w and u, and the concentration c of a Rhodamine 6G dye were measured simultaneously in a turbulent buoyant jet, using laser-Doppler anemometry combined with a recently developed laser-induced-fluorescence concentration measurement technique. These non-intrusive techniques enable measurements in a region of plume motion where conventional probe-based techniques have had difficulties. The results of the study show that the asymptotic decay laws for velocity and concentration of a tracer transported by the flow are verified experimentally in both jets and plumes. The momentum and volume fluxes and the mean dilution factor are determined in dimensionless form as a function of the normalized distance from the flow source. Contradictory results from earlier experimental plume investigations concerning the decay laws of w and c and the plume width ratio b_c/b_w are discussed. The turbulence properties and the transition from momentum-driven jets to buoyancy-driven plumes are presented. The turbulence is found to scale with the mean flow as predicted by dimensional analysis and self-similarity. Buoyancy-produced turbulence is found to transport twice as much tracer as jet turbulence. Although velocity statistics in jets and plumes are found to be highly self-similar there is a strong disparity in the distribution of tracer concentration in the two flows. This occurs in the time-average mean flows as well as the r.m.s. turbulent quantities. Instantaneous concentration fluctuations are found to exceed time averages by as much as a factor of 3. The experimental results should provide a reasonable basis for validation of computer models of axisymmetric plumes.

/pdf/investigations-of-round-vertical-turbulent-buoyant-jets-fts3lg6qh5.pdf

Investigations of round vertical turbulent buoyant jets

The characteristics of the unsteady boundary layer and stall events occurring on an oscillating NACA 0012 airfoil were investigated by using closely spaced multiple hot-film sensor arrays at . Aerodynamic forces and pitching moments, integrated from surface pressure measurements, and smoke-flow visualizations were also obtained to supplement the hot-film measurements. Special attention was focused on the behaviour of the spatial-temporal progression of the locations of the boundary-layer transition and separation, and reattachment and relaminarization points, compared to the static values, for a range of oscillation frequency and amplitude both prior to, during and after the stall. The initiation, growth and rearward convection of a leading-edge vortex, and the role of the laminar separation bubble leading to the dynamic stall, as well as the mechanisms responsible for the stall events observed at different test conditions were also characterized. The hot-film measurements were also correlated with the aerodynamic load and pitching moment results to quantify the values of lift increment and stall angle delay as a result of the observed boundary layer and stall events. The results reported here provide an insight into the detailed nature of the unsteady boundary-layer events as well as the stalling mechanisms at work at different stages in the dynamic-stall process.

Investigation of flow over an oscillating airfoil

The increasing need for thermal comfort has led to a rapid increase in the use of cooling systems and, consequently, electricity demand for air-conditioning systems in buildings. Heat-driven ejector refrigeration systems appear to be a promising alternative to the traditional compressor-based refrigeration technologies for energy consumption reduction. This paper presents a comprehensive literature review on ejector refrigeration systems and working fluids. It deeply analyzes ejector technology and behavior, refrigerant properties and their influence over ejector performance and all of the ejector refrigeration technologies, with a focus on past, present and future trends. The review is structured in four parts. In the first part, ejector technology is described. In the second part, a detailed description of the refrigerant properties and their influence over ejector performance is presented. In the third part, a review focused on the main jet refrigeration cycles is proposed, and the ejector refrigeration systems are reported and categorized. Finally, an overview over all ejector technologies, the relationship among the working fluids and the ejector performance, with a focus on past, present and future trends, is presented.

https://re.public.polimi.it/bitstream/11311/984261/1/2016_RSER_BesagniMereuInzoli.pdf

Ejector refrigeration: A comprehensive review

Active Separation Control Measurements and Computations for a NACA 0036 Airfoil

Ensemble-averaged two-component velocity measurements over an airfoil experiencing oscillatory dynamic stall under compressibility conditions were obtained. The measurements show the formation of a separation bubble over the airfoil that persists till angles of attack close to when the dynamic stall vortex forms and convects. The fluid attains mean velocities as large as 1.6 times the free stream velocity with instantaneous values of 1.8 times the free stream velocity. The airfoil motion induces these large velocities in regions that are far removed from the surface.

Laser velocimetry measurements of oscillating airfoil dynamic stall flow field

The approaches of compressible dynamic stall control using real-time airfoil adaptation and slatted airfoils are compared. Each method attempts to solve the unsteady e ow separation and the underlying causes differently. The approaches lead to unexpected results: For the slatted airfoil, dynamic stall alleviation on the main airfoil with a fully stalled slat occurred, and for the shape adapting airfoil, leading-edge attached e ow with trailing-edge separation was obtained. In both cases, no dynamic stall vortex was present. As can be expected, the control effectiveness of each method varies over the full cycle and depends on the Mach number due to the new factors introduced by the use of the methods. These issues are addressed. conditions. At low Reynolds numbers and Mach numbers, it arises from a large leading-edge adverse pressure gradient. At slightly highervaluesofthesequantities,whenthelocale owbecomessuper- sonic, the mechanism changes to that due to an interaction between the local supersonic e ow and the laminar separation bubble, at very low pressure gradients. Tripping the airfoil causes dynamic stall again from the strong leading-edge adverse pressure gradient. At higher Mach numbers, shock-induced separation initiates dynamic stall. A rotor blade encounters a variety of freestream conditions de- pending on the local e ow conditions. Hence, it may see dynamic stall arisingfromany of these causes. In general,athigher Reynolds numbers, two main mechanisms, the adverse leading-edge pressure gradient and the shock-induced separation, can be expected to pre- vail. It is imperative that any dynamic stall e ow control schemes that are developed be robust and effective against all dynamic stall onset mechanisms over the full range of e ow conditions observed in the rotor e ight envelope. Specie c to dynamic stall control is that the unsteady vorticity e uxincreasesdramaticallywhenanairfoilisrapidlypitchedpastthe staticstallangle.Thisisparticularlyinterestingbecauseoneeffectof unsteadinessistoreducetheequivalentangleofattackfortheairfoil, which is responsible for the delay of stallmeasured.Eventually,this unsteady vorticity abruptly coalesces into the dynamic stall vortex.

Compressible Dynamic Stall Control: Comparison of Two Approaches

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 deformation rates for fixed airfoil angle of attack. Selected leading-edge shapes were also tested during airfoil oscillation. These tests show that for a range of Mach numbers observed on the retreating blades of helicopter rotors the dynamic stall vortex can be avoided by a judicious variation of leading-edge curvature

M. S. Chandrasekhara

Papers

Active Separation Control Measurements and Computations for a NACA 0036 Airfoil

Laser velocimetry measurements of oscillating airfoil dynamic stall flow field

Compressible Dynamic Stall Control: Comparison of Two Approaches

Unsteady stall control using dynamically deforming airfoils

The effect of compressibility on suppression of dynamic stall using a slotted airfoil