Vortex Generators near Shock/ Boundary Layer Interactions

Longitudinal vortex generator applied to heat transfer enhancement of a flat plate solar water heater

The performance of a compressor cascade is considerably influenced by secondary flow effects, like the cross flow on the end wall as well as the corner separation between the wall and the vane. An extensive experimental study of vortex generator application in a highly loaded compressor cascade was performed in order to control these effects and enhance the aerodynamic performance. The results of the study will be used in future projects as a basis for parameterization in the design and optimization process for compressors in order to develop novel nonaxisymmetric endwalls as well as for blade modifications. The study includes the investigation of two vortex generator types with different geometrical forms and their application on several positions in the compressor cascade. The investigation includes a detailed description of the secondary flow effects in the compressor cascade, which is based on numerical and experimental results. This gives the basis for a specific approach of influencing the cascade flow by means of vortex generators. Depending on the vortex generator type and position, there is an impact on the end wall cross flow, the development of the horse shoe vortex at the leading edge of the vane, and the extent of the corner separation achieved by improved mixing within the boundary layer. The experiments were carried out on a compressor cascade at a high-speed test facility at DLR in Berlin at minimum loss (design point) and off-design of the cascade at Reynolds numbers up to Re = 0.6 × 106 (based on 40-mm chord) and Mach numbers up to M = 0.7. At the cascade design point, the total pressure losses could be reduced by up to 9% with the vortex generator configuration, whereas the static pressure rise was nearly unaffected. Furthermore, the cascade deflection could be influenced considerably by vortex generators and also an enhancement of the cascade stall range could be achieved. All these results will be presented and discussed with respect to secondary flow mechanisms. Finally, the general application of vortex generators in axial compressors will be discussed.

Supersonic engine intakes operating supercritically feature shock wave / boundary layer interactions (SBLIs), which are conventionally controlled using boundary layer bleed. The momentum loss of bleed flow causes high drag, compromising intake performance. Micro-ramp sub-boundary layer vortex generators (SBVGs) have been proposed as an alternative form of flow control for oblique SBLIs in order to reduce the bleed requirement. Experiments have been conducted at Mach 2.5 to characterise the flow details on such devices and investigate their ability to control the interaction between an oblique shock wave and the naturally grown turbulent boundary layer on the tunnel floor. Micro-ramps of four sizes with heights ranging from 25% to 75% of the uncontrolled boundary layer thickness were tested. The flow over all sizes of microramp was found to be similar, featuring streamwise counter-rotating vortices which entrain high momentum fluid, locally reducing the boundary layer displacement thickness. When installed ahead of the shock interaction it was found that the positioning of the micro-ramps is of limited importance. Micro-ramps did not eliminate flow separation. However, the previously two-dimensional separation was broken up into periodic three-dimensional separation zones. The interaction length was reduced and the pressure gradient across the interaction was increased.

An overview on heat transfer augmentation using vortex generators and nanofluids: Approaches and applications

This paper describes an experimental comparison of two passive approaches for controlling the shock interaction with a turbulent boundary layer: low-profile vortex generators and a passive cavity (porous wall with a shallow cavity underneath). This investigation is the first known direct comparison of the two methods wherein the advantages and disadvantages of both are revealed. The experiments were conducted with a normal shock wave in an axisymmetric wind tunnel. The shock strength (M = 1.56-1.65) was of sufficient magnitude to induce a large separation bubble, thus causing substantial boundary-layer losses. The low-profile vortex generators were found to significantly suppress the shock-induced separation and improve the boundary-layer characteristics downstream of the shock. However, the suppression of the separation bubble decreased the extent of the low total pressure loss region associated with the lambda foot shock system which results in a lower mass-averaged total pressure downstream of the shock. The passive cavity substantially reduced the total pressure loss through the shock system (and thus wave drag) by causing a more isentropic compression over a larger lateral extent. However, the boundary-layer losses downstream of the shock were significantly increased.

This paper deals with a three dimensional numerical study of heat transfer and pressure drop for fin and tube bank heat exchanger with a vortex generator. A new type of vortex generator, curved semi-wavy vortex generators (CSWVGs) is introduced and compared its performance. The computational domain consists of a small portion between two adjacent fins and air is used as the working fluid. The vortex generators are punched out from the fin surface. the analysis show that with the use of new vortex generators convective heat transfer rate has been increased by increasing the solid to fluid interaction through delaying the boundary layer separation, directing the working fluid to the wake region and bringing vortices into the working fluid. The most favourable position and geometric parameter of the new CSWVGs are found by studying the average Nusselt number and friction factor at various Reynolds ranging from 800 to 2000.

The numerical simulation was carried out to investigate mechanism of the heat transfer enhancement in the fin-and-tube heat exchangers. As known, the vortex generators (VGs) were widely used to improve the thermal performance with bad flow resistance characteristics and led to bad comprehensive performance. This paper aims to expound the mechanism of thermal hydraulic characteristics and explore the effect of VGs position on the comprehensive performance. Three types of fins (type 1, type 2, and type 3) were discussed in this paper. The j factor, f factor, and performance evaluation (PEC) of three types of VGs in different positions were discussed and compared. Based on the numerical results, a detailed description of the effect of three types of VGs on the heat transfer performance and flow resistance characteristics was presented at different Reynolds number in the range between 1300 and 2000. In addition, local velocity distribution, local temperature distribution, and local pressure drop distribution were analyzed and discussed. And the effect of VG angle on the thermal performance and flow resistance was presented. It can be concluded that the main heat transfer occurred in the region before the tube, and the wake region behind the tube was harmful to improve the thermal performance and reduce the flow resistance. Besides, VG in the wake region was obviously beneficial to the enhancement of the thermal performance with less energy loss.

An experimental investigation was undertaken to measure the effect of various configurations of low-profile vortex generator arrays on the flow in a diffusing S-duct. Three parameters that characterize the vortex generator array were systematically varied to determine their effect: (1) the vortex generator height; (2) the streamwise location of the vortex generator array; and (3) the vortex generator spacing. Detailed measurements of total pressure at the duct exit, surface static pressure, and surface flow visualization were gathered for each vortex generator configuration. These results are reported here along with total pressure recovery and distortion coefficients determined from the experimental data. Each array of vortex generators tested improved total pressure recovery. The configuration employing the largest vortex generators was the most effective in reducing total pressure recovery. No configuration of vortex generators completely eliminated the flow separation that naturally occurs in the S-duct, however the extent of the separated flow region was reduced.

The aim of this study was to investigate in detail the flow field resulting from transonic shock wave / boundary layer interactions under control conditions. Several control techniques have been investigated: 1) active control consisting in sucking a part of the boundary layer flow through a slot, 2) hybrid control, which is a combination of a passive control cavity placed underneath the shock region and a suction slot (or cavity) located downstream, and 3) control by a local deformation of the surface, the so-called bump concept. In the experimental part of the study, flow surveys have been executed with a two-component LDV system, including mean velocity and Reynolds tensor component measurements. Results show that active control by slot suction produces an increase in wave drag, but a significant decrease in friction drag, especially when the slot is located just downstream of the shock. The tendencies are inverted when bump or hybrid control is applied, the bump control having nevertheless a moderate effect on the friction drag. Moreover, active control by a slot efficiently diminishes the turbulence level downstream of the interaction domain. The aim of the theoretical study was to discuss the problems involved in the modeling of the vertical velocity component at the wall in the control region and of the effect of control on the turbulent field. The RANS research solver Nasca has been used for numerical simulations. Both, the algebraic Baldwin-Lomax model and the transport equation [k-e] Chien model were investigated, the latter having shown deficiencies in the prediction of the channel flow. On the other hand, Poll’s law and the Bohning-Doerffer law give a similar and fair reproduction of the vertical velocity at the wall in the control region.

Flow separation control and performance evaluation of an asymmetric diffuser using vortex generators

PurposeFlow separation on wings, blades and vehicles can be delayed or even suppressed by the use of vortex generators (VG). Numerous studies, documented in the literature, extensively describe the performance of triangular and rectangular VG winglets. This paper aims to focus on the use of non-conventional VG shapes, more specifically an array of trapezoildal-perforated VG tabs.Design/methodology/approachIn this study, computational fluid dynamic simulations are performed on an inline array of trapezoidal VG with various dimensions and inclination angles, in addition to considering perforations in the VG centers. The methodology of the present numerical study is validated with experimental data from the literature.FindingsThe performance and the associated flow structures of these tested non-conventional VG are compared to classical triangular winglets. For the proposed non-conventional trapezoidal VG, at the onset of stall, a 21% increase of lift over drag on the airfoil is observed. The trapezoidal VG enhancement is also witnessed during stall where the lift over drag ratio is increased by 120% for the airfoil and by 10% with respect to the triangular winglets documented in the literature.Originality/valueThe originality of this paper is the use of non-conventional vortex generator shape to enhance lift over drag coefficient using three-dimensional numerical simulations.

Flow fields around asymmetrical micro vortex generators in supersonic flow

The influence of the local effects of rarefaction on the prediction of the main parameters typical of Shock Wave - Boundary Layer Interaction are presented and analyzed. Slip flow boundary conditions have been implemented in CIRA CFD code H3NS, and results have been compared with available wind tunnel experiments and DSMC results. Finally, an experimental wind tunnel test case, planned in CIRA Plasma Wind Tunnel facility, has been analyzed in order to check possible local effects of rarefaction.

Modeling arrays of passive vortex generators (VGs) pairs, mounted in the fully turbulent boundary layer of a flat plate and generating streamwise counterrotating vortex structures is the object of this investigation. Usually, a sound computational fluid dynamics (CFD) investigation requires an adequate grid with a corresponding large number of grid points around such VGs in order to obtain an accurate solution of this flow case. This, in turn, leads to a time-demanding grid generation which often comes along with lots of practical challenges during the creation. An effective way to get around this time-consuming process is to introduce a way to model these flow separation devices statistically and, by that, to add their statistical physical effects to the governing equations rather than resolving their geometries in the computational grid. KTH, the Royal Institute of Technology Stockholm, and FOI, the Swedish Defence Research Agency, have developed a computational tool for statistical VG modeling that makes it possible to simulate and add the additional statistical effects of passive VGs in wall-bounded flows, whereas the need for a local mesh refinement is no longer required. This approach for the modeling of passive VGs turbulent flow separation devices is presented using a two dimensional Navier-Stokes flow solver. Computational results for fully three dimensional resolved VGs as well as experimental results are evaluated and compared to this statistical VG model approach. It is shown that the VG model approach gives very promising results that compare well to results from CFD and experiments. Nevertheless, the influence of the vortex structures on the flow and on the turbulent stresses decays quicker than it is observed in experiments. Further-more, the statistical VG model is evaluated for asymmetric diffuser flow and compared to experimental data with as well as without VGs. It is shown that the VG model is capable of predicting the influence of the VGs on decreased and vanished separation of this highly sensitive flow case, giving a better pressure recovery at the diffuser's outlet.

Circulation control can greatly enhance lift on a xed wing. However, the introduction of a high-speed Coanda jet is theorized to signi cantly increase noise, which is a growing concern for a variety of potential circulation control applications. The sound produced by a circulation control airfoil is measured using a phased microphone array and freeeld microphones in an open-jet anechoic wind tunnel. The phased array maps reveal dominant high-frequency noise sources at the sidewall-trailing edge interfaces. These three-dimensional sources prohibit the accurate measurement of two-dimensional circulation control trailing edge noise by a single microphone. Hence, an integrated phased array spectrum is computed and compared with a theory that identi es three potential noise sources: the interaction of the freestream boundary layer with the slot lip edge, the interaction of boundary layer turbulence with the rounded trailing edge, and the interaction of the jet with the surrounding slot geometry. The theory requires detailed ow eld information, including friction velocities, boundary layer thicknesses, displacement thicknesses, and local mean velocities. These parameters are estimated using particle image velocimetry measurements. A quantitative comparison of the measured and theoretical spectra reveal large di erences. The model predicts that the interaction of the freestream boundary layer with the slot lip is the dominant noise source mechanism over most frequencies.