David Greenblatt

Technion - Israel Institute of Technology | technion · Faculty of Mechanical Engineering

Combined pitching-and-surging of an airfoil at the identical frequency (i.e., synchronously), at four different phase-differences, was investigated theoretically and experimentally. The most general unsteady theoretical formulation was adopted to calculate the lift coefficient, and then extended to explicitly compute the unsteady bound vortex sheet...

An experimental wind tunnel study was performed to investigate the viability of incompressible turbulent boundary layer skin friction drag modification by temporal acceleration and deceleration, motivated by similar pipe and channel flows studies. The acceleration and deceleration phases were achieved by rapidly opening and closing a set of downstr...

A two-element natural laminar flow airfoil-commonly used on medium-altitude, long-endurance, unmanned air vehicles-was conformally decambered for application on a two-bladed, H-rotor, vertical-axis wind turbine. Blade kinematics were used to determine the virtual camber line, which was then used to conformally map the original profile onto the chor...

Dynamic stall on the blades of low-solidity vertical axis wind turbines (VAWTs) is a major problem due to its adverse effects on performance, drive-train components, and generator sizing. However, when the turbine chord-to-radius ratios are relatively large, greater than approximately 0.5, counterintuitively, dynamic stall can be harnessed to produ...

We achieved the first experimental validation of conformally decambered blades for VAWT applications and the first adaptation of slotted natural laminar (NLF) airfoils as VAWT blades. Torque was produced by the dynamic stall phenomenon due to large chord-radius ratios. Decambered blades produced 60% and 27% greater power and torque respectively, th...

The driving torque on vertical axis wind turbines with large chord-to-radius ratio blades, typically greater than 0.5, is generated mainly by the phenomenon of dynamic stall. In order to determine the optimum configuration, an extensive experimental study was conducted on two-bladed, H-rotor test turbine, by systematically varying the blade chord-t...

An experimental wind tunnel study was performed to investigate the viability of incompressible turbulent boundary layer skin friction drag reduction by temporal acceleration and deceleration, motivated by similar pipe and channel flows studies. The acceleration and deceleration phases were achieved by rapidly opening and closing a set of downstream...

The supply of freshwater based on wind-powered desalination is technically feasible, but energetic losses associated with conventional systems necessitate new approaches. This study explores a direct wind-powered desalination (D-WPD) system using a small-scale vertical axis wind turbine (VAWT) for reverse osmosis via a high-pressure pump without el...

Oscillating wind energy conversion systems that rely on vortex-induced vibrations (VIV) suffer from low energetic efficiency, but active flow control can produce significant improvements. This research investigates wind energy generation resulting from alternating slot blowing (ASB) on a circular cylinder, experimentally and theoretically. On the b...

A series of wind tunnel and flight experiments was performed on a mini, or small, unmanned air vehicle (sUAV) model, where pulse-modulated dielectric barrier discharge (DBD) plasma actuators were used to control wing stall. Plasma actuation on the unpowered model produced modest increases in maximum lift coefficient, while post-stall lift coefficie...

The benefits of dielectric barrier discharge (DBD) plasma actuators on battery-powered helicopter hover performance were estimated on the basis of recently-acquired experimental data. Two analyses were performed, constrained by nominally incompressible tip Mach numbers. In the first analysis, experimental data were used together with dimensional an...

A Mach 4 parallel flow supersonic nozzle, intended for arc-plasma wind tunnel applications, was designed for high-pressure and high-enthalpy working point conditions. The objective was to ensure effective cooling and structural integrity under conditions of no shock wave formation and parallel flow streamlines at the nozzle exit. The design was per...

A novel direct wind-powered desalination (D-WPD) system for brackish water utilizes a small-scale vertical axis wind turbine to directly power a reverse-osmosis-based desalination system via a high-pressure pump without electricity generation or a system controller. A comprehensive parametric study examined the effects of feed water salinity, modul...

Noise reduction was studied on a model helicopter rotor using pulsed dielectric barrier discharge (DBD) plasma actuators mounted at the blades' leading edges. The evaluation was performed in a fully anechoic chamber, where thrust and torque were measured concurrently with the acoustic signature using an array of 15 microphones. Acoustic measurement...

Alternating slot-blowing (ASB) on a circular cylinder is used to generate energy from the wind by directly exciting or forcing the vortex shedding instability. Low amplitude excitation produces energetic peaks near the natural vortex shedding frequency, while high amplitude forcing relies on circulation control associated the unsteady Coandă effect...

The effect of high-amplitude harmonic surging on airfoil laminar separation bubbles, at small angles of attack, was investigated experimentally in a dedicated surging-flow wind tunnel. A generalized pressure coefficient was developed that accounts for local static pressure variations due to surging. This critical generalization facilitated direct c...

Noise reduction was achieved on a model helicopter rotor using pulsed dielectric barrier discharge (DBD) plasma actuators mounted at the blades' leading edges. The rotor was mounted on a bi-axial balance in an anechoic chamber, and the following three-point noise-reduction strategy was adopted: first, the blades' collective (geometric) angle was in...

Recently, a fluidic facemask concept was proposed to mitigate the transmission of virus-laden aerosol and droplet infections, such as SARS-CoV-2 (COVID-19). This paper describes an experimental investigation of the first practical fluidic facemask prototype, or “Air-Screen”. It employs a small, high-aspect-ratio, crossflow fan mounted on the visor...

The effect of high-amplitude harmonic surging on airfoil laminar separation bubbles was investigated theoretically, and experimentally in a dedicated surging-flow wind tunnel. A generalized pressure coefficient was developed that accounts for local static pressure variations due to surging. This generalization, never previously implemented, facilit...

A novel low-cost knee exerciser concept, previously assessed under laboratory and sports-related clinical conditions, was evaluated within the context of a developing-world hospital environment. The newly-designed exerciser is of simple and robust design, incorporating a shock-absorber/lever mechanism, and patient testing was carried out without re...

A novel fluidic facemask, or Air-Screen, is described and demonstrated experimentally for protection against virus-laden aerosol and droplet infections, such as COVID-19. The Air-Screen employs a small, high-aspect-ratio, crossflow fan mounted on the visor of a filter-covered cap to produce a rectangular air jet, or screen, in front of the wearer’s...

View Video Presentation: https://doi.org/10.2514/6.2022-0471.vid In this work, high-order Computational Fluid Dynamics (CFD) simulations have been performed for a NACA 0018 airfoil with an active flow control slot at Reynolds numbers between Reynolds number of 125,000 and 250,000 using the GPU-accelerated PyFR solver. Pressure distributions and res...

Flow separation, or stall, in axial flow turbomachinery results in a loss of pressure or compression in the case of fans and compressors, or the loss of power or thrust generation in the case of turbines. Wave-power-based Wells turbines, in particular, suffer so acutely from blade stall during normal operation, that it compromises their viability a...

Quasi-static and dynamic stall control experiments, using dielectric barrier discharge plasma actuators, were performed on a square-tipped aspect-ratio-one wing, with a NACA 0015 profile, under harmonic pitching, at a Reynolds number of 0.3M. Relatively low O(1) and relatively high O(10) pulse-modulated reduced excitation frequencies were introduce...

The pervasiveness of unmanned air vehicles (UAVs), from insect to airline scales, combined with active flow control maturity, has set the scene for vehicles that differ markedly from present-day configurations. Nano and micro air vehicles, with characteristic Reynolds numbers typically less than 10 ⁵ , rely on periodically generated leading-edge vo...

The urgent need for fresh water remains a strong driver for reverse-osmosis (RO) desalination, but the systems are energy-inefficient and produce significant green-house gas emissions. This motivated our research, which introduces and investigates a novel concept for wind-driven RO desalination. It is based on the Coandă effect, applied periodicall...

DBD plasma actuators were implemented on a two-bladed rotor model, and the effect of pulse-modulated actuation was evaluated at tip Reynolds numbers of 100,000 and 150,000. A parametric study showed significant blade drag reduction at pulsation frequencies around unity, a mild sensitivity to reduced frequency, and a small effect of duty cycle. In g...

A wide range of validation experiments were performed in the Technion constricted Arc Plasma heated supersonic wind Tunnel (TAPT) for the purposes of measuring bulk voltage, enthalpy, and tunnel efficiency. These integral measurements were compared with a modified model of the constrictor flow and temperature fields that was previously validated ag...

One-dimensional unsteady Reynolds-averaged Navier-Stokes computations were performed for oscillatory transitional and turbulent pipe flows and the results were validated against existing experimental data for a wide variety of oscillatory Reynolds and Womersley numbers. An unsteady version of the Johnson-King model was implemented with optional nea...

Dielectric barrier discharge (DBD) plasma actuators were implemented on the leading edges of Wells turbine impeller blades for the purpose of controlling leading-edge separation, and the turbine performance was recorded. Pulse-modulated perturbations at dimensionless frequencies near unity produced remarkable increases in performance. Under massive...

A novel positive displacement, high pressure, vertical axis wind pump (HP-VAWP) was evaluated for the application of stand-alone high-pressure reverse-osmosis desalination and drip irrigation systems. The direct interface between a vertical axis wind turbine (VAWT) and a positive displacement pump, that delivers a constant volume of liquid per revo...

A hybrid Large Eddy Simulation (LES) / Actuator Line Model (ALM) computational approach is employed to simulate the turbulent wake of a two-bladed rotor in hover. Validations are performed for the Knight and Hefner two-bladed rotor case with comparisons to previous LES predictions using the immersed boundary method, as well as hybrid RANS-LES using...

The well-known and popular Beddoes–Leishman dynamic stall model was assessed for its ability to predict dynamic stall control by leading-edge steady slot blowing. Lift coefficient examples for both a leading-edge staller (NACA 0012) and trailing-edge staller (NACA 0018) were considered. Much like the original model that employs static data or compu...

Many engineering applications involve lifting surfaces (wings) that oscillate about in pitch, heave, and/or stream-wise (referred to as surge). For small pitch amplitudes, under the stall angle of attack, the lift force will respond linearly to such oscillations. However, at sufficiently large amplitudes, unsteady flow separation and dynamic stall...

This research investigates a novel concept in small-scale wind-energy generation. Instead of rotating turbine blades, the present concept produces reciprocating motion that is achieved by alternately activating boundary layer control (slot blowing) on opposite sides of a circular cylinder. This is commonly called the Coandă-effect, and here we depl...

An experimental stability and transition investigation of a centrifugally unstable wall-jet blown from a slot over a circular Coandă cylinder was conducted using flow visualization and particle image velocimetry. Clear and unambiguous observations of spontaneously generated stationary streamwise structures, never previously observed, were analyzed...

Dynamic stall often occurs under conditions of simultaneous unsteady pitching and surging (e.g., rotorcraft and wind turbines), but many models employ a dimensionless time base that implicitly assumes that surging is superimposed, in a quasi-steady manner, on dynamic pitching. An unsteady wind tunnel was used to examine this assumption, where a tec...

Dynamic stall is exploited to produce positive torque on a large-chord/radius-ratio (c∕R) vertical-axis wind turbine. An elementary kinematic analysis shows that the blades experience large variations in angle of attack that produce deep dynamic stall, combined with large relative dynamic pressure variations, that are phase shifted by approximately...

Experiments were conducted to simultaneously quantify the voltage distribution and heat losses along the length of an arc-plasma wind-tunnel constrictor employing air and argon gas. The objective was to generate a detailed data set and make it available electronically to the scientific community. High-spatial-resolution voltage measurements allowed...

Experiments were conducted to simultaneously quantify the voltage distribution and heat losses along the length of an arc plasma wind tunnel constrictor employing air and argon gas. The objective was to generate a detailed data set and make it available electronically to the scientific community. High spatial resolution voltage measurements allowed...

An elementary kinematic analysis of large chord-radius ratio (c/R) vertical axis wind turbines was performed and a small two-bladed model turbine (0.4c/R0.6) was built and tested. The analysis shows that these machines operate at low blade-speed/wind-speed ratios and hence the blades experience large changes in angle of attack, that produce dynam...

This research involved the generation of an unprecedented data set on the Technion Arc Plasma Supersonic Wind Tunnel, where simultaneous voltage distribution and heat losses are measured in real time. Under certain conditions, the voltage exhibited a wave-like pattern along the constrictor. This was attributed to either an unstable arc geometry in...

https://arc.aiaa.org/vc/flowcontrol

Flow in the wake of a sphere has been studied for at least the last hundred years. The three-dimensional (3-D) flow structure has been observed many times using dye visualization and prior to the direct numerical simulations by Johnson & Patel ( J. Fluid Mech. , vol. 378, 1999, pp. 19–70), its structure at a Reynolds number of approximately 300, wa...

The 3D flow field in the wake of a tethered sphere (D=6 mm, m ∗ =7.77, L ∗ =4.1) undergoing vortex induced vibrations (VIV) in a water tunnel, was measured using tomographic particle image velocimetry (tomo-PIV). The flow field was investigated for four different reduced velocities ranging between 3.6<U ∗ <13.2, corresponding to Reynolds numbers ra...

The rebreathing of expired air, with high carbon dioxide and low oxygen concentrations, has long been implicated in unexplained Sudden Infant Death Syndrome (SIDS) when infants are placed to sleep in a prone (facedown) position. This study elucidates the effect of aerodynamic parameters: Reynolds number, Strouhal number and Froude number, on the pe...

Pulse-modulated dielectric barrier discharge plasma actuators are applied to the problem of flow separation on a Hermes 450 unmanned air vehicle V-tail panel. Risk-reduction airfoil experiments were conducted followed by full-scale wind tunnel tests. Silicone-rubber based actuators were calibrated and subsequently retrofitted to both the airfoil an...

Pulsed dielectric barrier discharge (DBD) plasma actuators were evaluated for the control of flow separation from the V-tail panels of a well-known unmanned air vehicle, using lightweight high-voltage generators. Following risk-reduction airfoil experiments, full-scale experiments were performed on an unswept tapered V-tail panel at typical takeoff...

A simplified model was developed for the purpose of providing rapid solutions of the flow within plasma arc constrictors. The validity of the model was established by comparing its results to both significantly more complex models and experimental data. For turbulent flows, the main heat transfer mechanism in the medium is due to eddy viscosity and...

Measurements of the lift coefficient history obtained from a NACA 0018 airfoil undergoing pitching maneuvers with blowing-type active flow control were used to obtain low-dimensional models for control. Disturbance models for the lift response to pitching at different steady-state blowing amplitudes were identified based on a variation of the Goman...

Experiments indicate that vortices trailing finite wings can be perturbed by periodic wing pitching, leading to rapid dissipation and bursting. To illustrate the perturbation mechanism, Betz vortex rollup relations are combined with the Theodorsen theory for unsteady lift response. A sinusoidal pitch motion on a rigid elliptic planform wing is comp...

V-tail configurations are common on unmanned aerial systems (UAS), but the tail panels suffer from flow separation, resulting in loss of control during crosswind takeoff and landing. Implementing spanwise dielectric barrier discharge (DBD) plasma actuators with silicone rubber as a dielectric material both on an airfoil and on a full-scale Hermes 4...

In this work, Computational Fluid Dynamics (CFD) simulation of active ow control via steady blowing is carried out on a thick symmetric airfoil (NACA-0018) at low Reynolds numbers (65000 - 250000). Within this Reynolds number range, the flow over the airfoil is characterized by leading edge separation, including the presence of a Laminar Separatio...

A device for use in extracting energy from an incoming fluid flow is presented. The device comprises: an oscillator assembly mounted on a base, the oscillator assembly comprising: a main body for exposing to an incoming fluid flow; and a joining element attached to the main body and configured for anchoring the main body to the base and enabling os...

The death of an apparently healthy infant during sleep, commonly referred to as Sudden Infant Death Syndrome (SIDS), is the main cause of death for infants between one and twelve months of age.1 A leading hypothesis is that exhaled air is confined to the vicinity of the nostrils, leading to rebreathing of exhaled CO2, causing death by asphyxia.2 We...

Dynamic stall on both horizontal axis and vertical axis wind turbine blades is accompanied by simultaneous changes in pitch and surge, but this simultaneous effect has never been documented. Using a unique unsteady wind tunnel, synchronous oscillations in angle of attack and flow speed were considered on two prototypical wind turbine blades. At a s...

An aerodynamic load control concept termed “adaptive blowing” was successfully tested on a NACA 0018 airfoil model at Reynolds numbers ranging from 150,000 to 500,000. The global objective was to eliminate lift oscillations typically encountered on wind turbine blade sections. Depending on the jet momentum flux, steady blowing from a control slot i...

Wind-tunnel experiments were conducted on two symmetric airfoils with different thickness ratios as a test of the ability of Goman–Khrabrov-type models to predict the lift coefficient history during pitching maneuvers. The primary difference between the two airfoils was that the thin airfoil had no hysteresis in its lift curve during quasi-steady m...

The current paper investigates the aeroelastic modelling of large, flexible multi- MW wind turbine blades. Most current performance prediction tools make use of the Blade Element Momentum (BEM) model, based upon a number of simplifying assumptions that hold only under steady conditions. This is why a lifting line free vortex wake (LLFVW) algorithm...

The transient flow of a two-dimensional wall-jet over a circular cylinder, following rapid initiation and termination, was investigated experimentally. Unsteady surface pressures and unsteady pressure-sensitive paint were used to gain a basic understanding of the flow physics. Jet initiation produced a starting vortex, upstream of which the Coandă...

A theory based upon linearized governing equations is developed that describes the operation principles and design parameters for low-speed wind tunnels with longitudinal freestream oscillations. Existing measurements made in unsteady wind tunnels are shown to be consistent with the theory and targeted validation experiments performed in a variable...

A combined theoretical and experimental investigation was carried out with the objective of evaluating theoretical predictions relating to a two-dimensional airfoil subjected to high amplitude harmonic oscillation of the free stream at constant angle of attack. Current theoretical approaches were reviewed and extended for the purposes of quantifyin...

A control system is presented for controlling operation of a vertical axis wind turbine (VAWT) for generating energy from an incoming fluid flow. The control system comprises at least one flow affecting arrangement associated with at least one blade of the VAWT and a control unit connected to said flow affecting arrangement, the flow affecting arra...

An experimental investigation was carried out to quantify the aerodynamic lift acting on a NACA 0018 airfoil subjected to combined pitching and surging and to compare the results to established theories. A dedicated unsteady wind tunnel was employed that produces large surge amplitudes, and airfoil loads were estimated by means of unsteady surface...

This paper presents a basic theory and validation experiments for unsteady low-speed wind tunnels of the blow-down-type. Linearized governing equations are developed and show that the tunnel frequency and width is proportional to the mean tunnel free-stream velocity and inversely proportional to the test section length and the square of the exit-ar...