Evaluation of a Vortex Generator Model in Adverse Pressure Gradient Boundary Layers

Abstract

The use of a two-dimensional statistical passive vortex generator model, applied to an adverse pressure gradient boundary-layer flow, is evaluated qualitatively against experimental and fully resolved vortex generator computations. The modeling approach taken here has the advantage of substantially reducing the complexity of including such flow separation control devices in a computational mesh, thus giving the opportunity to carry out faster parametric studies. Additional stresses, originating from the vortex generator model approach, are added as additional turbulent stresses to the mean governing equations instead of resolving vortex structures in the computational domain. The vortex generator model has been applied to allow direct comparison with prior experiments carried out at the Royal Institute of Technology Stockholm. Variations of the vortex generator streamwise position and tests of different vortex generator setups, such as co-and counter-rotational settings, are presented. Distributions of wall-pressure and skin-friction coefficients are used to evaluate the vortex generator model against fully resolved vortex generator data. It is shown that the vortex generator model successfully predicts attached and separated flow states. Moreover, the results illustrate the vortex generator model's capability to predict flow control sensitivity with respect to the streamwise position. Nomenclature c = vortex generator/wing chord length c f = local skin friction coefficient c p = local wall pressure coefficient D = mean distance between neighboring vortex generator pairs d = mean distance between vanes of a single vortex generator pair H 12 = shape factor = h = height K = local airfoil section lift slope l = length Re = Reynolds number r 0 = vortex core radius U i = mean streamwise velocity field u 0 i u 0 j = time/spanwise averaged Reynolds stresses u r = azimuthal vortex-induced velocity field V i y; z = total induced vortex velocity field V r = azimuthal single vortex velocity field x = streamwise coordinate y = wall-normal coordinate z = spanwise coordinate = angle of attack/incidence 1 = modified 1 parameter in differential Reynolds stress transport model = circulation e = circulation per unit width u 0 i u 0 j = time/spanwise averaged vortex stresses = displacement thickness 99 = 99% boundary-layer thickness = momentum thickness = backflow coefficient Subscripts att = attachment point value end = ending coordinate exp = experiment value max = maximum value sep = separation point value start = starting coordinate VG = vortex generator value wall = wall value 1 = freestream value

Experimental investigation of the flow past passive vortex generators on an airfoil experiencing three-dimensional separation

“…Following Stillfried at al. [59], the vortex velocity field, ( , ), is defined as the difference between the time averaged velocity field, , ( , ) = ( , , ) ̅̅̅̅̅̅̅̅̅̅̅̅ , and the mean flow, ( ) =< , ( , ) >. The latter is obtained by spanwise averaging 〈 〉 over the width of the VG pair.…”

Wake Characteristics of Vane-Type Vortex Generators in a Flat Plate Laminar Boundary Layer

“…Significant reduction of reversed flow in separated region of the bump was observed when counter-rotating vortices were generated. Stillfried et al carried out computations of flow on a bump with various rectangular vortex generator arrays in adverse pressure gradient boundary layer flow [15]. They found that vortex generators should be placed some distance upstream of the separation bubble location.…”

Effects of Sweeping Jet Actuator Parameters on Flow Separation Control

“…Reduced efficiency of the SWJ actuators with smaller spreading angle is also consistent with the results about the skew angle of conventional VGJ actuators in the literature. Although there are mixed results about the optimum skew angle of VGJ actuators, an angle between 60° and 90° is reported to be well-suited for separation control studies on an APG ramp [8,25] and variation from 60° to 90° in the skew angle does not cause significant changes in the overall flow features [26]. Although we could not test the SWJ actuators with 180° spreading angle (or 90° skew angle), for the cases tested, and considering the results provided in the literature about the VGJ-skew angle, a spreading angle between 120° and 180° may provide better pressure recovery and thereby may result in better actuator efficiency.…”