Usama Muhammad Zahid

Let us consider a plane channel flow where the walls are coated by a film of ferrofluid, namely a viscous suspension of magneto-sensitive nanoparticles (see Figure 1). Recent investigations show promising results about the drag reduction effects of equipping walls with a ferrofluid film under the action of a static magnetic field [1,2]. The applications of such phenomenon can be seen in mechanical (seals and gaskets, turbulence control, bearings, and lubrication) and biomedical (drug delivery, blood vessel stents, blood transport, and Magnetic Resonance Imaging [3]) perspectives. Experimental observations revealed that at high Reynolds numbers, the ferrofluid interface develops wavy perturbations that can increase in amplitude, eventually leading to ferrofluid detachment. The wavelength and amplitude of these interface waves can be adjusted by the intensity of an applied spanwise-oriented magnetic field and are influenced by the thickness of the ferrofluid film. As a result, it is possible for the main flow to be turbulent while the wavy interface remains stable. The problem of drag reduction when the main flow is turbulent has never been considered. [1] Dev, A. A., Dunne, P., Hermans, T. M., & Doudin B., (2022). Fluid drag reduction by magnetic confinement. Langmuir, 38 (2):719-726. [2] Kögel, A., Völkel, A., & Richter, R., (2020). Calming the waves, not the storm: Measuring the Kelvin–Helmholtz instability in a tangential magnetic field. JFM, 903, A47. [3] Kole, M., & Khandekar, S., (2021). Engineering applications of ferrofluids: A review, J. Magn. Magn. Mater. (537): 168222.

The primary objective of the project is to identify the conditions necessary for maintaining the stability of a ferrofluid film and to optimize the drag reduction when the main flow is turbulent.

ANALYTICAL: Linear stability analysis in laminar flow conditions. The analysis will be extended considering turbulent flow conditions. EXPERIMENTAL: Investigation of the origin of drag reduction either associated with the slip condition at the ferrofluid interface and with the effect of progressive waves (Fukagata et al. [4]). The experiments are carried out at BOKU University (Vienna, Austria) collaborating with the group leaded by Prof. Markus Holzner. NUMERICAL: Direct numerical simulation of the flow within the ferrofluid film which cannot be observed optically because of ferrofluid opacity. [4] Fukagata, K., Iwamoto, K., & Hasegawa, Y. (2023). Turbulent drag reduction by streamwise traveling waves of wall-normal forcing, Annu. Rev. Fluid Mech. 56:1.

This research contributes to the knowledge of ferrofluid film dynamics in channel flows, especially under turbulent conditions. By optimizing the stability and drag reduction effects, the findings can enhance the efficiency of mechanical systems and improve biomedical applications involving ferrofluids.