Emad Gharaibah

Unsteady Separation Control on Wind Turbine Blades using Fluidic Oscillators

Fluidic oscillators are actuators that are essentially constituted of a flow vane with no moving parts. They are very effective in generating an oscillating velocity field, and because of their robustness and potential to meet most application requirements they have been thoroughly investigated in previous years. In this work fluidic oscillators have been embedded in an airfoil representative of the outboard sections of wind turbine blades, and subsequently tested at full-scale Reynolds numbers 2.0·10 6 ≤ Re ≤ 4.8·10 6 in the laminar wind tunnel at the University of Stuttgart. The effects of the unsteady actuation on the lift and drag strongly depend upon Re, the level of actuation, and the state of the airfoil surface. However, strong improvements have been obtained throughout the whole testing envelope, with relative lift increase spanning from a minimum of 10 to over 60% and substantial stall margin extension. In addition, employing fluidic oscillators strongly reduces the suction surface boundary-layer thickness and the unsteadiness of the mean flow velocity.

Unsteady Separation Control for Wind Turbine Applications at Full Scale Reynolds Number

he drive towards lowest possible cost of electricity (CoE) for wind turbines calls out for blade aero technologies that can be used to maximize the energy yield while minimizing cost, loads and noise constraints. Aero performance is typically constrained by need to minimize extreme loads in off-design (parked or otherwise), which result from airfoil stall at high angles of attack. Separation and stall control via vortex generator jets, created by different actuators – either steady or unsteady pneumatic (i.e., pressurized air), or zero-net mass flow like synthetic jets or plasma actuators, is an enabling technology to overcome these limits on high yield blade designs. With available separation control, blades with aggressive aerodynamic designs with low-solidity blades and/or operations at higher wind classes can become feasible.

A Numerical Model of Dispersed Two Phase Flow in Aerated Stirred Vessels based on Presumed Shape Number Density Functions

Based on the two fluid model and the population balance equation for bubble size, a novel approach is proposed for the numerical simulation of disperse two phase flow, e.g. in aerated stirred vessels. The bubble size distribution is represented with a number density function of presumed shape,which makes possible the partial solution of the population balance equation in a pre-processing step. The pre-processing is not computationally expensive, so the number of bubble size groups maybe chosen sufficiently large to represent the evolution of the size distribution due to break-up and coalescence of bubbles very accurately. In the subsequent CFD simulation,it is not necessary to solve a set of transport equations for each size group. Instead, the size distribution of the disperse phase is represented by its first two moments, i.e,the mean and variance of bubble diameter. Transport equations for mean and variance are solved by the CFD code,with source terms depending on local flow parameters and bubble size distribution. These source terms are taken from a look-up table, which has been generated in the pre-processing step. p]In the present paper,the over all formulation and solution algorithm of the proposed model is out lined. For a spatially homogeneous system, it is demeonstrated that number density functions of presumed shape can indeed represent accurately the evolution of a bubble size distribution under the action of bubble break-up and coalescence with realistic kernel functions. Furthermore, it is shown that a rather large number of bubble size groups is required to reach discretization independence for the population balance equation. The proposed model has been implemented in a multi-dimensional CFD code,and employed in the simulation of turbulent two-phase(gas-liquid)dispersed flow in an aerate dstirred vessel.