# Dr Gioacchino Cafiero

## Academic and research departments

Centre for Aerodynamics and Environmental Flow, Department of Mechanical Engineering Sciences.### Biography

Dr. Cafiero covers the position of Lecturer in Aerospace Engineering in the Department of Mechanical Engineering Sciences at the University of Surrey. His research interests range from the fundamental study of free shear flows, with particular emphasis on the turbulence dissipation scaling and its implications, to passive and active methodologies to enhance heat and mass transfer in wall-bounded and free shear flows. In particular, Dr. Cafiero developed a large experience over the years on the use of continuous and synthetic jets for flow control for several applications, such as convective heat transfer enhancement, skin friction drag reduction, and applications to the automotive sector, such as near wake manipulation of commercial road vehicles. Over the years Dr. Cafiero has developed a strong background in Particle Image Velocimetry (PIV) in all its declinations (planar, stereoscopic, and volumetric), providing also some relevant improvements to the technique and to compute highly accurate statistics from planar and volumetric data.

### Areas of specialism

### Previous roles

### Affiliations and memberships

### Research collaborations

- Ecole Central Lille;

- Universidad Carlos III de Madrid;

- TU Delft;

- University of Naples Federico II;

- Politecnico di Torino;

- Imperial College London;

- University of Cambridge - BP Institute.

### My teaching

ENG 3162: Group Design. Aero 1, Aero 2 and Aero - UAS;

ENG 3207: Aerodynamics II.

### Courses I teach on

## Undergraduate

Aerospace Engineering BEng (Hons) or MEng

### My publications

### Publications

*Re*

_{Ä}*=180). A statistical analysis of the near-wall structures demonstrates that the control mechanism acts in a way to reduce them in the forced configuration. It is conclude that the effect of the forcing is such that the near-wall structures merge and become less prone to inducing new structures, thus effectively reducing their number, and consequently the near-wall turbulence activity.*

*Cafiero G., Castrillo G., Greco C.S., Astarita T. (2017) Effect of the grid geometry on the convective heat transfer of impinging jets,International Journal of Heat and Mass Transfer104pp. 39-50 Elsevier*

*Passive methods are recognized as one of the most efficient means to achieve high heat and mass transfer in impinging jets. In a recent study, Cafiero et al. (2014) demonstrated the effectiveness of square fractal grids (SFGs, obtained repeating the same square pattern at increasingly smaller scales) in terms of heat transfer enhancement when locating the grid in correspondence of the nozzle exit section. Indeed, the capability of producing turbulence at multiple scales and the possibility of tuning the peak in the turbulence intensity profile as a function of the grid geometric parameters are both extremely appealing for heat transfer enhancement purposes. In this study, the effect of the grid geometry on the convective heat transfer rate of impinging jets is assessed and discussed. Three main effects are taken into account: the grid thickness ratio (obtained by varying the thickness of the first iteration of the SFG), the effect of the secondary grid iterations and the choice of the initial pattern. It is demonstrated how a larger thickness ratio, which in the present case corresponds to an anticipated location of the peak in the turbulence intensity profile, is beneficial to get a spotted high convective heat transfer rate at short nozzle to plate distances. Either the use of a single square grid, or the choice of a different initial pattern (for example a circular fractal grid) is instead indicated when it is desirable a uniform distribution of the convective heat transfer rate.*

*Cafiero Gioacchino, Discetti S., Astarita T. (2016) Flow field topology of impinging jets with fractal inserts,Springer Proceedings in Physics - Proceedings of the iTi Conference on Turbulence 2014 - Progress in Turbulence VI165pp. 243-248 Springer*

*An experimental investigation of the flow field features of a round air jet equipped with a fractal (FG) and a regular (RG) grid insert impinging on a flat surface is carried out by means of 2D-2C Particle Image Velocimetry (PIV). The results are compared to those for a round jet without any grid (JWT). The test Reynolds number is set to 10, 000. The average flow fields and the turbulent kinetic energy distributions are presented. In particular, the effect of the presence of the fractal grid on the turbulence intensity distribution and on the planar component of the Reynolds stress is analyzed. Some differences between the location of the maximum of the turbulence intensity profile and the data reported in the literature are found. A possible interaction process between the wakes of the grids and the growing shear layer of the jet might be responsible of this discrepancy. A comparison between the flow field and the heat transfer results obtained by the authors in a previous work is also carried out. What is underlined is that both an higher turbulence level and a much stronger axial velocity cause an increment in the heat transfer rate.*

*Cafiero G., Discetti S., Astarita T. (2015) Flow field topology of submerged jets with fractal generated turbulence,Physics of Fluids27(11)115103 AIP Publishing*

*Fractal grids (FGs) have been recently an object of numerous investigations due to the interesting capability of generating turbulence at multiple scales, thus paving the way to tune mixing and scalar transport. The flow field topology of a turbulent air jet equipped with a square FG is investigated by means of planar and volumetric particle image velocimetry. The comparison with the well-known features of a round jet without turbulence generators is also presented. The Reynolds number based on the nozzle exit section diameter for all the experiments is set to about 15 000. It is demonstrated that the presence of the grid enhances the entrainment rate and, as a consequence, the scalar transfer of the jet. Moreover, due to the effect of the jet external shear layer on the wake shed by the grid bars, the turbulence production region past the grid is significantly shortened with respect to the documented behavior of fractal grids in free-shear conditions. The organization of the large coherent structures in the FG case is also analyzed and discussed. Differently from the well-known generation of toroidal vortices due to the growth of azimuthal disturbances within the jet shear layer, the fractal grid introduces cross-wise disturbs which produce streamwise vortices; these structures, although characterized by a lower energy content, have a deeper streamwise penetration than the ring vortices, thus enhancing the entrainment process.*

*Gehlert P., Cafiero G., Vassilicos J.C. (2018) Effect of fractal endplates on the wingtip vortex,Proceedings of the 2018 AIAA Aerospace Sciences Meeting(210059) American Institute of Aeronautics and Astronautics (AIAA)*

*An experimental investigation was performed on the effect of fractal endplates on the wingtip vortex of a NACA 0012 semi span wing at a Reynolds number of 2 x 10*^{5}. The endplates were obtained by introducing three different fractal patterns. Constant temperature anemometry and stereoscopic particle image velocimetry were employed to assess both the local flow properties as well as the spatial organization of the wingtip vortex. The results show that the introduction of a fractal endplate strongly affects both the geometry and the turbulence features of the vortex. In particular, it is found that the fractal geometry weakens the vortex by spreading the turbulent kinetic energy over a broader range of frequencies. We relate this loss of coherence to a faster dissipation of the vortex, thus paving the way to the employment of fractal endplates to reduce the hazard associated to such flow features.

*Cafiero G., Vassilicos J.C. (2019) Non-equilibrium turbulence scalings and self-similarity in turbulent planar jets,Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences475(2225) The Royal Society*

*We study the self-similarity and dissipation scalings of a turbulent planar jet and the theoretically implied mean flow scalings. Unlike turbulent wakes where such studies have already been carried out (Dairay et al. 2015 J. Fluid Mech. 781, 166-198. (doi:10.1017/jfm.2015.493); Obligado et al. 2016 Phys. Rev. Fluids 1, 044409. (doi:10.1103/PhysRevFluids.1. 044409)), this is a boundary-free turbulent shear flow where the local Reynolds number increases with distance from inlet. The Townsend-George theory revised by (Dairay et al. 2015 J. Fluid Mech. 781, 166-198. (doi:10.1017/jfm.2015.493)) is applied to turbulent planar jets. Only a few profiles need to be self-similar in this theory. The self-similarity of mean flow, turbulence dissipation, turbulent kinetic energy and Reynolds stress profiles is supported by our experimental results from 18 to at least 54 nozzle sizes, the furthermost location investigated in this work. Furthermore, the non-equilibrium dissipation scaling found in turbulent wakes, decaying grid-generated turbulence, various instances of periodic turbulence and turbulent boundary layers (Dairay et al. 2015 J. Fluid Mech. 781, 166-198. (doi:10.1017/jfm.2015.493); Vassilicos 2015 Annu. Rev. Fluid Mech. 95, 114. (doi:10.1146/ annurev-fluid-010814-014637); Goto & Vassilicos 2015 Phys. Lett. A 3790, 1144-1148. (doi:10.1016/j.physleta. 2015.02.025); Nedic et al. 2017 Phys. Rev. Fluids 2, 032601. (doi:10.1103/PhysRevFluids.2.032601)) is also observed in the present turbulent planar jet and in the turbulent planar jet of (Antonia et al. 1980 Phys. Fluids 23, 863055. (doi:10.1063/1.863055)). Given these observations, the theory implies new mean flow and jet width scalings which are found to be consistent with our data and the data of (Antonia et al. 1980 Phys. Fluids 23, 863055. (doi:10.1063/1.863055)). In particular, it implies a hitherto unknown entrainment behaviour: the ratio of characteristic cross-stream to centreline streamwise mean flow velocities decays as the -1/3 power of streamwise distance in the region, where the non-equilibrium dissipation scaling holds.*

*Castrillo G., Cafiero G., Discetti S., Astarita T. (2016) Blob-enhanced reconstruction technique,Measurement Science and Technology27(9)094011 IOP Publishing*

*A method to enhance the quality of the tomographic reconstruction and, consequently, the 3D velocity measurement accuracy, is presented. The technique is based on integrating information on the objects to be reconstructed within the algebraic reconstruction process. A first guess intensity distribution is produced with a standard algebraic method, then the distribution is rebuilt as a sum of Gaussian blobs, based on location, intensity and size of agglomerates of light intensity surrounding local maxima. The blobs substitution regularizes the particle shape allowing a reduction of the particles discretization errors and of their elongation in the depth direction. The performances of the blob-enhanced reconstruction technique (BERT) are assessed with a 3D synthetic experiment. The results have been compared with those obtained by applying the standard camera simultaneous multiplicative reconstruction technique (CSMART) to the same volume. Several blob-enhanced reconstruction processes, both substituting the blobs at the end of the CSMART algorithm and during the iterations (i.e. using the blob-enhanced reconstruction as predictor for the following iterations), have been tested. The results confirm the enhancement in the velocity measurements accuracy, demonstrating a reduction of the bias error due to the ghost particles. The improvement is more remarkable at the largest tested seeding densities. Additionally, using the blobs distributions as a predictor enables further improvement of the convergence of the reconstruction algorithm, with the improvement being more considerable when substituting the blobs more than once during the process. The BERT process is also applied to multi resolution (MR) CSMART reconstructions, permitting simultaneously to achieve remarkable improvements in the flow field measurements and to benefit from the reduction in computational time due to the MR approach. Finally, BERT is also tested on experimental data, obtaining an increase of the signal-to-noise ratio in the reconstructed flow field and a higher value of the correlation factor in the velocity measurements with respect to the volume to which the particles are not replaced.*

*Cafiero G., Greco C.S., Astarita T., Discetti S. (2016) Flow field features of fractal impinging jets at short nozzle to plate distances,Experimental Thermal and Fluid Science78pp. 334-344 Elsevier*

*An experimental analysis of the flow field generated by an impinging jet equipped with a fractal grid insert located at the nozzle exit is carried out by means of Tomographic Particle Image Velocimetry. The Reynolds number based on the nozzle exit section diameter d is set to 15,000. The presence of the grid leads to a non uniform curvature of the jet shear layer. As a consequence, azimuthal instabilities are triggered in proximity of the nozzle exit section, causing the production of streamwise vortices. Furthermore, the organization of the coherent structures in proximity of the impinged wall is discussed and related to the convective heat transfer distribution. Owing to the presence of counter rotating wall vortices along the diagonals of the on-plate imprint of the fractal grid, a region of minimum in the scalar transfer map can be detected. In addition to that, similarly to the well known vortex rings that characterize the case of round jet without turbulence promoter, the presence of azimuthally coherent structures that might be generated by Kelvin-Helmholtz instability of the jet shear layer is presented and discussed.*

*Cafiero G., Castrillo G., Greco C.S., Astarita T. (2019) On the effects of square-fractal turbulators on the flow field generated by a synthetic jet actuator,Experimental Thermal and Fluid Science102pp. 302-315 Elsevier**In this work the effects due to the introduction of a 3-iterations square fractal grid inserted at the exit section of a synthetic jet actuator are experimentally analysed and discussed. The contributions related to the secondary iterations are assessed comparing the results with those obtained inserting a single-square grid (obtained by removing the 2nd and the 3rd iteration bars). The flow field generated by an axisymmetric synthetic jet is also investigated as a reference.*

*The synthetic jet is produced under three different dimensionless stroke lengths, ? _{0}/D = 7.1, 19.2 and 50, covering flow regimes ranging from the nearly pure vortex to the nearly continuous jet. Results are reported both in terms of time-averaged and phase-averaged statistics. Notwithstanding the different nature of the jet flow, fractal turbulators confirm the interesting feature of an extended range of turbulence production, as already documented for continuous jets.*

*Phase-averaged results show that the vorticity field generated by the presence of the grid interacts with the vortex produced in the jet shear layer, causing its azimuthal distortion. This phenomenon can be prevented in the single-square grid case at lower ? _{0}/D values, but it is still clear when the fractal one is located at the nozzle exit section.*

*Regardless of the inlet conditions, the outer vortex position scales with the stroke length. However, the introduction of the grid shows an effect on the lifetime of the vortex, with a faster dissipation in the fractal grid case.*

*Cafiero G., Castrillo G., Greco C.S., Astarita T. (2019) On the effects of square-fractal turbulators on the flow field generated by a synthetic jet actuator,Experimental Thermal and Fluid Science102pp. 302-315 Elsevier*

*In this work the effects due to the introduction of a 3-iterations square fractal grid inserted at the exit section of a synthetic jet actuator are experimentally analysed and discussed. The contributions related to the secondary iterations are assessed comparing the results with those obtained inserting a single-square grid (obtained by removing the 2nd and the 3rd iteration bars). The flow field generated by an axisymmetric synthetic jet is also investigated as a reference. The synthetic jet is produced under three different dimensionless stroke lengths, L0/D=7.1,19.2 and 50.0, covering flow regimes ranging from the nearly pure vortex to the nearly continuous jet. Results are reported both in terms of time-averaged and phase-averaged statistics. Notwithstanding the different nature of the jet flow, fractal turbulators confirm the interesting feature of an extended range of turbulence production, as already documented for continuous jets. Phase-averaged results show that the vorticity field generated by the presence of the grid interacts with the vortex produced in the jet shear layer, causing its azimuthal distortion. This phenomenon can be prevented in the single-square grid case at lower L0/D values, but it is still clear when the fractal one is located at the nozzle exit section. Regardless of the inlet conditions, the outer vortex position scales with the stroke length. However, the introduction of the grid shows an effect on the lifetime of the vortex, with a faster dissipation in the fractal grid case.*

*Cafiero G., Ceglia G., Discetti S., Ianiro A., Astarita T., Cardone G. (2014) On the three-dimensional precessing jet flow past a sudden expansion,Experiments in Fluids55(2)1677 Springer Verlag*

*A circular jet flow past an abrupt expansion under some conditions switches intermittently between two states: quasi-axisymmetric expansion and gyroscopic-like precessing motion. In this work, an experimental investigation into the self-excited precessing flow generated by a 5:1 expansion of a round jet in a coaxial cylindrical chamber is carried out by means of tomographic particle image velocimetry. The experiments are performed on a jet issued from a short pipe at a Reynolds number equal to 150,000. Proper orthogonal decomposition (POD) is applied to extract information on the organization of the large coherent structures of the precessing motion. The application of this technique highlights the dominance of three modes: the most energetic two are associated with the jet precession; the third one is representative of the axial motion. An estimate of the precession probability based on the modal energy obtained from the application of POD is proposed. The precession frequency is extracted using a low-order reconstruction (LOR) of a subset of the POD modes. The reconstructed flow field topology obtained by the LOR highlights an underlying mechanism of swirl generation in proximity of the inlet nozzle; the phenomenon is closely related to the interaction between the entrainment in the far field and the recirculation regions in the near field. The application of a stability criterion shows that the self-induced swirl flow results to be unstable. The instability is responsible for the generation of helicalshaped vortices in the near field, even though the dominant feature for the unconfined jet issued from the same nozzle is the axisymmetric ring-vortices generation.*